COLORADO DISCHARGE PERMIT SYSTEM ( CDPS)

SUMMARY OF RATIONALE DENVER INTERNATIONAL AIRPORT CDPS PERMIT NUMBER COS 000008, DENVER COUNTY TABLE OF CONTENTS ++ I. TYPE OF PERMIT ................................................................................................ 1 II. FACILITY INFORMATION................................................................................ 1 III. RECEIVING STREAM......................................................................................... 2 IV. FACILITY DESCRIPTION.................................................................................. 3 V. PERFORMANCE HISTORY............................................................................... 7 VI. TERMS AND CONDITIONS OF PERMIT..................................................... 13 VII. REFERENCES...................................................................................................... 24 VII. CHANGES AFTER PUBLIC NOTICE............................................................. 25 ++

I. TYPE OF PERMIT New Industrial  Stormwater II. FACILITY INFORMATION A. Facility Type: Individual Industrial, > 10 acres Fee Category: Category 24, Subcategory 2 Annual Fee: $ 317 B. Primary SIC Code: 4581  Airports C. Legal Contact: Bruce Baumgartner Manager of Aviation Denver International Airport Airport Office Building, 10th Floor 8500 Pena Boulevard Denver, Colorado 80249 6340 ( 303) 342 2200 D. Facility Contact: Janell Barrilleaux Environmental Services Denver International Airport Airport Office Building, 7th Floor 8500 Pena Boulevard Denver, Colorado 80249 6340 ( 303) 342 2730 E. Facility Location: NW3 Sec. 21, T2S, R65 W, 6th P. M. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 2, Permit No. COS 000008 II. FACILITY INFORMATION F. Monitoring Points/ Outfalls: E Outfall  Concrete detention structure at flood control pond T 239 on Third Creek approximately 100 yards upstream of DIA's boundary ( 39.895900 lat/ 104.745944 long)  Sub basin C+  monitoring station on Third Creek, west of runway 34R ( 39.515750 lat/ 104.422252 long)  Second Creek  Second Creek immediately north of Pena Boulevard, near the west end of runway 7/ 25, south of the west end of runway 7/ 25 ( 38.37264 lat/ 104.733040 long)  Runway 35R  the outfall to Box Elder Creek immediately downstream of the runway ( 39.854736 lat/ 104.640416 long)  Runway 26  the outfall to Box Elder Creek at the east end of runway 8/ 26, just inside the security fence ( 39.877168 lat/ 104.613424 long)  Hayesmount  the outfall to the Denver Hudson Canal, east of the Fire Training Facility, on the north side of runway 8/ 26, between taxiways R7 and R8 ( 39.879640 lat/ 104.653216 long)  Barr Lake Drainage  outfall to Barr Lake Drainage channel, just west of runway 34, between taxiways F9 and F10 ( 39.892236 lat/ 104.685328 long)  Runway 7/ 25, the outfall to Second Creek at the southwest corner of the runway ( 39.840152 lat/ 104.727008 long)

III. RECEIVING STREAM A. Identification, Classification and Standards 1. Identification: Discharges to Second Creek and Third Creek ( which are in segment 16 of the Upper South Platte River Basin), which flow to the South Platte River, Segment 15 of the Upper South Platte River Sub basin of the South Platte River Basin. ( Discharges to Second and Third Creeks can also be diverted to Barr Lake and/ or the O'Brien Canal, which are part of the Farmer's Reservoir and Irrigation Company ( FRICO) system.)

Discharges to Hayesmount Tributary ( which is in segment 3 of the Middle South Platte River Basin), which flows to Segment 1 of the Middle South Platte River Sub basin of the South Platte River Basin.

Discharges to Box Elder Creek ( which is in segment 5 of the Middle South Platte River Basin), which flows to Segment 1 of the Middle South Platte River Sub basin of the South Platte River Basin.

2. Classification: Segment 16 of the Upper South Platte River Basin and Segment 3 of the Middle South Platte River Basin are designated as Use Protected, and are classified for the following uses: Recreation, Class 1 a; Aquatic Life, Class 2 ( Warm); and Agriculture. Applicable numeric standards for the segments are listed below, in accordance with the above classification as found in Regulation No. 38 , Classifications and Numeric Standards for the South Platte River Basin ( 5 CCR 1002 38).

Physical and Biological D. O. = 5.0 mg/

pH = 6.5  9.0 s. u. Fecal Coliform = 200/ 100 ml E coli = 126/ 100 ml Segment 5 of the Middle South Platte River Basin is designated as Use Protected, and is classified for the following uses: Recreation, Class 2; Aquatic Life, Class 2 ( Warm); and Agriculture. Applicable numeric standards for the segments are listed below, in accordance with the above classification as found in Regulation No. 38 , Classifications and Numeric Standards for the South Platte River Basin ( 5 CCR 1002 38). COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 3, Permit No. COS 000008 III. RECEIVING STREAM Physical and Biological D. O. = 5.0 mg/

pH = 6.5  9.0 s. u. Fecal Coliform = 2000/ 100 ml E coli = 630/ 100 ml In addition, interim organic pollutant standards from the Basic Standards, section 31.11( 3) Awater and fish @ classification table apply to Barr Lake. As part of a water quality study ( see Section V. B. 2 of the Rationale), volatile and semi volatile analyses were conducted at several points. All were nondetects except for 2,4 dimethlyphenol, which is a typical laboratory blank residue contaminant. Therefore, it was determined that none of these parameters are an issue with this facility, and so they are not dealt with in the permit. However, a periodic resampling ( once every permit term, for the renewal application) will be required.

3. Impacts on Downstream Water Supplies or Other Receiving Waters:

The stream segments downstream of the discharge are not classified as drinking water supplies. Of all the receiving waters, the discharge into Third Creek is of main concern. ( Most of the runoff from the industrialized areas at Denver International Airport ( DIA) drains to Third Creek.) Third Creek flows for about two miles northwest of the airport, until it reaches the O= Brian Canal. From there, the flow can be diverted to either Barr Lake, or the Denver Hudson Canal. Both canals are owned by FRICO. ( See Figure 1, Part I. G of the permit.) Barr Lake is classified for the same uses as listed above. However, it is also a wildlife sanctuary, and so attention must be given to any potential impacts from the discharge to Third Creek.

IV. FACILITY DESCRIPTION A. Industry Description 1. Type of Industry: The facility is Denver International Airport ( DIA), which opened in February, 1995. During 1997, DIA handled 490,000 aircraft operations and over 34 million passengers, making it the sixth busiest airport in the country. Airport activity is projected to grow to 45 million passengers and 650,000 aircraft operations annually by 2010.

There are currently 50 tenants leasing facilities from DIA which have industrial activities exposed to stormwater. These include airlines, air cargo lines, aircraft services, and car rental agencies. The following activities occur at one or more of these leaseholds at DIA:

 aircraft deicing, fueling, maintenance, washing, painting/ stripping, lavatory services  building and ground maintenance  cargo handling  chemical storage and handling  equipment cleaning/ degreasing, fueling, maintenance, storage  fuel storage  material storage  outdoor apron cleaning  vehicle fueling, maintenance, storage, washing  waste storage and disposal In addition, three tenant leaseholds are covered by separate general stormwater permit certifications, and will not be covered under this permit: the Aircraft Services International Group fuel farm, and the United Airlines and Continental Airlines maintenance hangars. To the extent that other tenant leaseholds and facilities obtain coverage under separate stormwater permits in the future, they also will not be addressed in this permit. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 4, Permit No. COS 000008 IV. FACILITY DESCRIPTION Stormwater Permit Coverage: Stormwater discharges from airports that have vehicle maintenance, fueling, equipment cleaning and/ or airport deicing are required to be covered by a Colorado Discharge Permit System ( CDPS) permit in order to be discharged to Waters of the State. This coverage may be obtained under either a general or individual CDPS Stormwater Discharge Permit.

DIA was previously covered under the general permit for Stormwater Discharges Associated with Heavy Industrial Activity, certification number COR 020267. However, given the size and complexity of the site, past violations, and special requirements in terms of monitoring and management practices, the Division determined that coverage under an individual stormwater permit ( including numeric action levels and mandatory Best Management Practices) would provide more environmental control, and was more appropriate for this site. The general permit certification will be inactivated once the individual permit goes into effect.

Aircraft Deicing Fluids ( ADF): DIA, like most larger airports, seasonally utilizes chemicals for aircraft and runway deicing to enhance the safety of air travel and airport operations. The open air application of these chemicals to aircraft and pavement surfaces can result in migration of deicing fluids to the storm sewer system and eventual discharge to receiving streams. The primary aircraft deicing fluid ( ADF) used at DIA is propylene glycol ( PG). Propylene glycol exerts a significant oxygen demand that can deplete dissolved oxygen ( DO) in receiving waters, threatening aquatic life. DIA also currently uses potassium acetate for runway, taxiway and apron area deicing. During the 1997 1998 deicing season, DIA used nearly one million gallons of PG based ADF for aircraft deicing. The use of these chemicals at DIA is expected to increase with the growth in aircraft operations.

Aircraft Deicing: Limited deicing, which is allowed at the A and C concourse gates, is defined as the minimum level of deicing required to ensure that it is safe to taxi the aircraft to the full deicing site. Full deicing typically uses a higher quantity of deicing fluid, based on the airline= s and pilot= s judgment, such that the aircraft is safe for takeoff. Full deicing is normally restricted to the designated deicing areas shown in Figure 2, Part I. G of the permit. See discussion, Section VI. B. 2. c. 1 of the Rationale. Figure 2 shows only those areas where deicing is allowed as of the issuance date of this permit. Future modifications to deicing areas during the permit term will not result in a change to Figure 2, but rather will be reflected in DIA's SWMP.

There are several types of ADF used at DIA. Type 1 is the deicer used at least 90% of the time, for both limited and full deicing. Types 2 are 4 are anti icers, which are thicker than deicers. A small amount of an anti icer is used after limited and full deicing, to prevent additional ice buildup. Type 2 is rarely used at DIA, as Type 4 is considered to provide better performance. All decisions on which types of deicers to use and when are made by the airlines.

Federal Regulations: There are currently no specific federal guidelines or effluent limits in place for this type of facility. EPA has conducted a nationwide study to determine the feasibility and necessity of such guidelines, and has determined not to promulgate industry specific effluent limits or guidelines for aircraft deicing at this time. Any changes in federal guidelines will be handled by an amendment to the permit, as necessary. ( See Section VI. E. 1 of the Rationale.)

2. Facility Systems Description: The discharges covered by this permit originate from the storm sewer system.

a. Clean and Dirty Water Systems: DIA operates a state of the art ADF collection, retention and recycling system. The system known as the Adirty water system@ separately conveys spent ADF with stormwater from authorized deicing areas ( e. g., deicing pads and concourse gates) to retention ponds ( also called industrial stormwater ponds) for subsequent recycling, or treatment by the Metro Wastewater Reclamation District ( Metro). Stormwater from other portions of the airport property are conveyed to Second Creek, Third Creek, Box Elder Creek, and other drainageways via the Aclean water system.@

The majority of the deicing fluids are applied to passenger aircraft at dedicated deicing pads located just west of each of the three concourses. ADF contaminated runoff from these pads is collected and, if in sufficient concentration, is conveyed to the spent glycol storage tanks located adjacent to the recycling plant, for subsequent treatment and offsite reuse. Approximately 20 to 35 percent of the ADF used at DIA is recycled. If the concentration of deicing fluid is too low for cost effective recycling, then this stormwater is directed to the West Pond ( 002) for pumping to Metro for treatment and disposal. The stormwater from deicing at the gates on the west COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 5, Permit No. COS 000008 IV. FACILITY DESCRIPTION ramp area is directed to the Northwest Pond ( 002) and the runoff from the east ramp is directed to the Northeast Pond ( 001).

Full deicing is allowed at B Concourse. Fluid from this activity may be collected for recycling into Pond 003A, adjacent to the recycling facility. This pond, however, is now also being used to collect runoff from the West Airfield Diversion System, and is therefore only partially available for full recycling of runoff from Concourse B. A new high capacity pond system is under construction to serve the West Airfield Diversion System, which will allow Pond 003A to more effectively contain full gate deicing at B Concourse for recycling ( see Compliance Schedule, Section VI. E. 4. a of the Rationale). When not being recycled, B Concourse runoff goes to pond 001 and/ or pond 002.

Similarly, the South Cargo Apron, General Aviation Ramp, and other authorized locations are served by industrial stormwater ponds for collection of deicer contaminated runoff, for pumping to Metro. From time to time, when the concentration in ponds 001, 002 and 005 ( South Cargo) is high enough to permit economical recycling, water from these ponds is trucked to the recycling facility. Pond 004, serving the General Aviation area, has never been pumped to Metro, as the deicing taking place on the General Aviation ramp has been very limited, and so evaporation has been sufficient to regenerate pond capacity. ( See Figures 3 and 4, Part I. G of the permit, for a stormwater management flow schematic and a contaminated stormwater system process schematic.)

From time to time, various utility vaults, sumps and manholes surrounding the concourses and on other locations around the airfield, become filled with water. For maintenance reasons, this water must be removed periodically. The estimated annual volume is about 100,000 gal. This water frequently contains glycol, and may also contain fuel or other constituents. The pumped water is handled as per BMP # 7 ( see Section VI. B. 2. c of the Rationale).

Despite the recycling/ collection system, during initial operations at DIA it was discovered that underdrains along the runways and taxiways were providing a conduit for some ADF and pavement deicers to reach the clean water system. This was due to ADF dripping off the aircraft in areas outside of the collection system, or being wind blown to such areas during application. ADF that escapes the dirty water system has been termed Afugitive ADF.@

The discharge of relatively large quantities of fugitive ADF to Third Creek resulted in the Division and DIA entering into a Compliance Order on Consent. It also resulted in many changes to the management of the ADF contaminated runoff, including an expansion of the collection system. ( This is discussed further in Section V. B. 2 of the Rationale.)

The capacity of the current dirty water system to handle storm events has two primary limitations. First, the recycling plant can process up to 25 gallons per minute ( gpm) for stormwater containing at least a 10 percent concentration of PG. The processing rate is reduced at lower ADF concentrations because more water must be removed to achieve the target finished PG concentration. A 4.2 million gallon retention pond and four 420,000 gallon tanks provide storage. However, if deicer contaminated stormwater flows exceed storage and recycling plant processing capacity, then the runoff is directed to the West Pond.

A second limitation to the dirty water system capacity is the amount of deicer contaminated stormwater that can be discharged to the Metro Wastewater Reclamation District (" Metro"). Metro limits BOD5 contributions from DIA to a daily maximum of 9 tons and a 30 day average limit of 7 tons per day, based on Metro= s capacity to handle high BOD loads without violating their own discharge permit. ( DIA monitors COD and then calculates the BOD loading. COD monitoring is used because results can be obtained in two hours, whereas monitoring for BOD5 takes five days to process.)

Five ponds, ranging in size from 3.6 to 13.7 million gallons, currently provide storage of ADF contaminated stormwater flows such that they can be pumped to Metro at a controlled rate, consistent with BOD5 loading limitations. Flows in excess of pond storage and pumping capacity would overflow to Third Creek and other receiving drainages. This has occurred several times. An additional 22 million gallons of new industrial stormwater pond capacity is being constructed. Also, another 5.7 million gallon pond cell will be constructed before the new runway is operational. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 6, Permit No. COS 000008 IV. FACILITY DESCRIPTION b. Site Drainage System: The Third Creek basin drains most of the western and most of the southern portions of the airport operations area. Third Creek sub basins A and B drain the south cargo area, the general aviation area and the parking structures. Third Creek sub basins C and D drain the west portion of all three concourses, including the dedicated deicing pads, and the glycol recycling facility. Third Creek sub basin E drains the north campus area ( flight kitchens, mail facility, etc.). No aircraft de icing occurs in this sub basin. ( Sub basin E is not the same as the monitoring point " E Outfall," which is the outfall from pond T 239 to Third Creek near DIA's boundary. See description, Section II. F of the Rationale.) Another drainage, flowing directly to Barr Lake, drains the far northern segment of the west airfield. No aircraft deicing occurs at DIA in this basin.

DIA had constructed temporary retention ponds in the Third Creek drainage within airport property to capture ADF that escapes the dirty water system. This was done as an interim measure while facility improvements were being designed and constructed ( see Compliance Schedule, Section VI. E. 4. a of the Rationale). These ponds were referred to as the C,

> D and C+ ponds ( see Figure 5, Part I. G of the permit). The C and D ponds included permanent headwalls and sluice gates to control flow rates. The ponds were used to hold contaminated stormwater from those sub basins until it could be pumped to one of the industrial ponds, for eventual disposal to Metro. DIA also constructed an aeration system in the C pond in an attempt to reduce BOD levels and raise DO in the ADF contaminated stormwaters. In addition, in late 1997, DIA constructed the West Airfield Diversion System to divert contaminated runoff ( tributary to Ponds C and D), that was going to the clean water system, to the dirty water system instead.

Ponds C, D and C+ had to be drained, and the dam at C+, the headwall at D, and the aeration system at C, had to be removed to allow for construction of a new runway and the new containment ponds ( see Compliance Schedule, Section VI. E. 4. a of the Rationale). The headwall at D will be replaced once the construction is completed. Once the runway becomes operational, the C and D ponds cannot be used routinely for retention, due to the potential hazard to aircraft caused by a wildlife attractant. The ponds will only be used in emergency situations.

DIA has completed a water quality study of the facility, which outlines several more improvements to be made in the system, including a 60% increase in the capacity of the dirty water pond system. These improvements are discussed in more detail in the Compliance sections of the Rationale. It is not known at this time whether or not these additional measures will be sufficient to prevent future impacts to water quality from the discharge.

A monitoring station, referred to as C+, is being installed downstream of the confluence of flow from C and D ponds.

c. Future Infrastructure: A sixth runway is now under construction. Some of the earthwork for construction was completed as part of overall airport construction in the early 1990s. Completion of the runway was then deferred. The remainder of the construction was begun in Spring 2001 and will continue through 2003.

A new deicing pad (` J') is under construction east of the existing C pad. This is to augment existing pad capacity, and is expected to be completed in late 2001.

3. Chemicals Used: Table IV 1 lists the main chemicals used at the facility that may be in the discharge.

Table IV 1  Chemical Additives Chemical Name Purpose Constituents of Concern Propylene Glycol based deicing fluid Ice removal and prevention on aircraft BOD, COD, corrosion inhibitors, fire retardants Ethylene Glycol Antifreeze for vehicles BOD, COD, corrosion inhibitors, fire retardants Potassium acetate Airfield pavement anti icer/ deicer BOD, COD Magnesium chloride Roadway anti icer/ deicer Chloride Sand Pavement traction Sediment Salt Pavement deicer Sodium chloride E 36 Liquid Runway Deicer Runway/ taxiway anti icer/ deicer Potassium acetate, BOD, COD COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 7, Permit No. COS 000008 Oil and Grease Vehicle, aircraft lubrication, maintenance Oil and Grease Jet Fuel Aircraft fueling Total Petroleum Hydrocarbons ( TPH)

V. PERFORMANCE HISTORY A. Monitoring Data 1. Discharge Monitoring Results: Table V 1 summarizes the effluent data reported at the designated sampling points as required by the stormwater permit for DIA, from 10/ 27/ 97 to 9/ 20/ 00, based the Annual Reports for that period. Where only one or two sample results are available, the average was not calculated.

Table V 1  Self Monitoring Results Sub basin A Sub basin C Parameter No. of Samples Reported Concentrations Avg/ Min/ Max No. of Samples Reported Concentrations Avg/ Min/ Max Flow, MGD 5 0.20/ 0.07/ 0.51 4 0.06/ 0.01/ 0.15 BOD5, mg/

15 65/ 4/ 334 10 188/< 2/ 784 TSS, mg/

24 97/ 10/ 415 22 266/ 6/ 4520 Oil & Grease, mg/

23 2.3/ 6/ 12a/ 22 2.2/< 5/ 25 pH, s. u. ( range) 24 6.8 10.0 22 6.7 8.1 Propylene Glycol, mg/

24 27/< 1/ 336 22 51/< 1/ 540 Ethylene Glycol, mg/

22 0.5/< 1/ 10 21 1/< 1/ 8 Total Petroleum Hydrocarbons, mg/

11 0.3/< 1/ 3 11 < 1/< 1/< 1 Total Phosphorus, as P, mg/

12 0.20/ 0.04/ 0.67 11 0.64/< 0.02/ 4.75 Total Kjeldahl Nitrogen, as N, mg/

12 1.5/< 0.3/ 3.6 11 7.5/ 0.4/ 28.4 Chloride, mg/

11 49/ 5/ 132 7 1389/< 1/ 4500 Fecal Coliform, #/ 100 ml 2 /< 4/ 340 2 / 9/ 180 Nitrate/ Nitrite, as N, mg/

12 1.18/ 0.06/ 4.12 11 0.5/ 0.02/ 1.13 Total Organic Carbon, mg/

15 48/ 7/ 195 10 93/ 8/ 284 Acetate, mg/

6 5/< 5/ 28 5 34/< 5/ 120 a/ All results were below 15 mg/

, except for one result at 457 mg/

; see explanation below. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 8, Permit No. COS 000008 V. PERFORMANCE HISTORY Sub basin D E Outfall Parameter No. of Samples Reported Concentrations Avg/ Min/ Max No. of Samples Reported Concentrations Avg/ Min/ Max Flow, MGD 3 0.25/ 0.06/ 0.47 5 1.38/ 0.02/ 6.42 BOD5, mg/

12 432/ 8/ 1050 13 39/< 2/ 434 TSS, mg/

21 98/ 5/ 1060 24 98/ 3/ 812 Oil & Grease, mg/

20 4.1/< 5/ 44 24 1.8/< 5/ 21 pH, s. u. ( range) 21 6.0 8.5 24 6.9 8.0 Propylene Glycol, mg/

21 230/< 1/ 850 24 10/< 1/ 238 Ethylene Glycol, mg/

20 0.3/< 1/ 3 23 0.1/< 1/ 2 Total Petroleum Hydrocarbons, mg/

11 < 1/< 1/< 1 12 < 1/< 1/< 1 Total Phosphorus, as P, mg/

11 0.17/< 0.02/ 1.33 12 0.1/< 0.02/ 0.27 Total Kjeldahl Nitrogen, as N, mg/

11 3.0/ 0.5/ 12.1 12 0.9/< 0.03/ 1.90 Chloride, mg/

8 17/< 1/ 73 9 215/ 56/ 576 Fecal Coliform, #/ 100 ml 2 / 6/ 200 2 / 11/ 160 Nitrate/ Nitrite, as N, mg/

11 0.44/< 0.02/ 0.86 12 1.99/< 0.02/ 15.8 Total Organic Carbon, mg/

12 205/ 6/ 392 13 23/ 5/ 150 Acetate, mg/

3 33/ 9/ 79 6 < 5/< 5/< 5 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 9, Permit No. COS 000008 Runway 35R Second Creek Parameter No. of Samples Reported Concentrations Avg/ Min/ Max No. of Samples Reported Concentrations Avg/ Min/ Max Flow, MGD 0  0

BOD5, mg/

9 65/ 3/ 401 11 101/ 4/ 434 TSS, mg/

19 31/ 3/ 192 13 444/ 21/ 2840 Oil & Grease, mg/

19 2.4/< 5/ 12 13 0.9/< 5/ 12 pH, s. u. ( range) 19 6.5 8.1 13 6.9 8.1 Propylene Glycol, mg/

17 5/< 1/ 52 13 < 1/< 1/< 1 Ethylene Glycol, mg/

19 < 1/< 1/< 1 13 0.2/< 1/ 1 Total Petroleum Hydrocarbons, mg/

9 < 1/< 1/< 1 8 < 1/< 1/< 1 Total Phosphorus, as P, mg/

9 0.21/ 0.05/ 0.49 8 0.7/ 0.10/ 2.64 Total Kjeldahl Nitrogen, as N, mg/

9 1.2/< 0.01/ 6.40 8 1.4/ 0.8/ 3.0 Chloride, mg/

8 6.6/ 5/ 24 8 1339/ 32/ 730 Fecal Coliform, #/ 100 ml 0  0

Nitrate/ Nitrite, as N, mg/

9 0.66/ 0.07/ 1.73 8 0.42/ 0.04/ 0.80 Total Organic Carbon, mg/

9 42/ 6/ 244 9 72/ 6/ 220 Acetate, mg/

6 37/< 5/ 220 4 63/< 5/ 204 2. Additional Discharge Monitoring Results: Table V 2 summarizes the additional COD monitoring required at E Outfall for DIA, from 3/ 97 to 12/ 99.

Table V 2  Self Monitoring Results  E Outfall  COD, mg/

Month # of Samples Avg. Min. Max. Std. Dev. # over 100 mg/

b/

Mar 97 17 1359 212 3230 897 17 Apr 97 15 634 114 2848 766 15 May 97 33 81 0 198 41 12 Jun 97 28 73 16 260 63 4 Jul 97 31 64 4 156 32 3 Aug 97 31 42 0 190 34 2 Sep 97 30 35 8 112 24 1 Oct 97 28 77 15 301 75 8 Nov 97 30 256 31 1008 258 18 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 10, Permit No. COS 000008 Dec 97 29 61 0 159 34 2 All 97 272 196 0 3230 443 82 Jan 98 31 33 11 71 14 0 Feb 98 28 36 7 56 13 0 Mar 98 30 44 17 66 12 0 Apr 98 30 122 19 423 107 10 May 98 31 47 17 121 19 1 Jun 98 30 32 0 58 14 0 Jul 98 31 27 0 78 20 0 Aug 98 31 37 14 128 24 1 Sep 98 30 35 12 64 13 0 Oct 98 31 35 12 65 13 0 Nov 98 29 29 5 67 16 0 Dec 98 30 36 3 71 15 0 All 98 362 42 0 423 42 12 Jan 99 29 34 8 143 24 1 Feb 99 28 35 19 61 9 0 Mar 99 31 31 9 61 11 0 Apr 99 30 57 1 150 39 5 May 99 31 73 15 334 70 3 Jun 99 30 29 10 53 11 0 Jul 99 30 37 10 77 14 0 Aug 99 29 38 15 92 18 0 Sep 99 30 30 5 92 20 0 Oct 99 31 23 4 49 10 0 Nov 99 30 24 6 56 12 0 Dec 99 27 25 4 39 9 0 All 99 356 36 1 334 30 9 b/ See Discussion of Limitations, Section VI. B. 2. b of the Rationale. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 11, Permit No. COS 000008 V. PERFORMANCE HISTORY B. Compliance With Terms and Conditions of Previous Permit 1. Discussion of Monitoring Data: The Oil and Grease sample for 6/ 5/ 98 showed a result of 457 mg/

. The permittee believes this result is inaccurate, and is probably a lab error, since no sheen or odor was observed during the sampling event.

See Section VI. B. 2. b of the Rationale for a further discussion of the monitoring data.

2. Permit Conditions: On May 27, 1994, DIA was issued permit certification number COR 020267 under the general permit for Stormwater Discharges Associated with Heavy Industrial Activity. The permit states, in part, that " Stormwater discharges from industrial activities shall not cause or threaten to cause pollution, contamination or degradation of State waters." It also states that " Except as provided . . . all discharges authorized by this permit shall be composed entirely of stormwater."

The first documented violation of the permit was on April 7, 1996, when small quantities of PG were discovered in Third Creek. A subsequent investigation by airport and State Health Department officials determined that the ADF had dripped off airplanes as they taxied from de icing pads and approached taxiways and runways for takeoff. The fugitive ADF then drained into the airport= s underground drainage system located along the edges of the runways and taxiways and during subsequent rain storms or snowmelt washed into Third Creek.

A second incident occurred on March 5, 1997, when 102 gallons of ADF were spilled after a valve failed. Some of the ADF, which subsequently mixed with approximately 3,000 gallons of storm water, made its way to Third Creek. Damage to Third Creek, in terms of a lack of insect and invertebrate life, and a black tarry substance in the stream bed, was observed following this incident. Subsequent observations by staff from the Water Quality Control Division ( the " Division") indicated that the damage was not permanent. According to a report commissioned by the Farmers Reservoir and Irrigation Company, prepared by Dr. John Stednick, and dated May 28, 1997, "[ w] ater quality measurements at Third Creek at O= Brian and at Barr Lake suggest that no propylene glycol had reached these points."

In the months following that incident, there were numerous reports of glycol or glycol by products in Third Creek, again due to the ADF dripping off aircraft during taxiing and take off. The primary indication was a strong odor in the creek associated with the breakdown of glycol.

The violations resulted in the Division and DIA entering into a Compliance Order on Consent, on August 5, 1998. The order provided for a $ 273,800 civil penalty. This included a $ 29,000 cash fine ( based on economic benefit), and a $ 244,800 violations penalty, which DIA proposed to offset with three Supplemental Environmental Projects ( SEPs). At a 2: 1 ratio ($ 2 spent for every 1$ of the penalty), the total cost of the SEPs was $ 489,600. Division policy requires that SEPs not be used for immediate permit compliance. The agreed upon SEPs were designed to address future compliance, by predicting future problems and proposing solutions. The SEPs included:

a. A detailed study and evaluation of the airport= s industrial waste management system, to identify all sources of contamination which may be entering state waters ( including Box Elder and Second Creeks, as well as Third Creek) and impacting water quality. The report on this study was submitted to the Division on September 30, 1998.

b. A two year monitoring program for the wells which have the potential to be impacted by glycol in Third Creek. This addressed concerns that the drinking water supplies of downstream residents might be adversely affected by any glycol reaching the creek. The program was completed at the end of 1999, with no observed impacts to the monitored groundwater wells.

c. A study to examine water quantity/ stormwater drainage issues impacted by DIA, and how these impacts can be mitigated. The report on this study was submitted to the Division on February 26, 1999. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 12, Permit No. COS 000008 V. PERFORMANCE HISTORY While negotiations on the order were proceeding, DIA initiated several temporary and permanent improvements to their system. The main permanent change was a $ 2.6 million project which modified the storm drainage system, so that runoff and fugitive ADF from the underdrain system in the vicinity of the de icing pads would generally be diverted to DIA= s dirty storm water system, and then into the Metro Waste Water Treatment system. The project was completed in December, 1997. It has eliminated the majority of the problem, as the 1998 sampling results in Table V 2 demonstrate; however, additional measures are still needed.

More detailed investigations of the problem revealed another concern in the design of the deicing pads west of the concourses. Numerous utility corridors and vaults are routed directly beneath the pads. Elevated concentrations of PG and COD observed in the utility corridors suggested that ADF may be entering the corridors through utility manholes on the pads. Seepage through joints in the pads= concrete may also contribute to migration of ADF to the clean water system, either via the utility corridors or more directly via seepage from under the concrete at the edges of the pads, ramps, and aprons.

To address this issue, DIA sealed all manholes on the pads, and sealed concrete joints at the pads and around the concourses. Sealing of the manholes has met with only limited success, due to the occasional need to access these manholes. These actions appear to have helped reduce the amount of seepage into the subgrade in the 1997 1998 deicing season, in turn reducing the amount of ADF observed in the underdrains. The vaults are also pumped out on a periodic basis, with the captured fluids being recycled onsite or pumped to Metro. In the first quarter of 1999, DIA connected selected utility conduits into the West Airfield Diversion System, which drains to the glycol collection system.

The result of these measures has been a substantial decrease in the amount of ADF reaching Third Creek, and a subsequent decrease in the levels of COD reported ( see Table V 2). The Water Quality Study ( SEP # 1) resulted in numerous recommendations for further system improvements, as follows:

1. diversion system capacity improvements 2. drain selected utility manholes and underdrains to dirty system ( completed) 3. contain surface runoff at east end of Concourse B commuter gates 4. construction of additional contaminated snow dumps 5. paving of selected landscaped islands west of the west deicing pads 6. contaminated snow management 7. revise city plowing procedures 8. increase tenants= awareness Recommendation 1," diversion system capacity improvements" will be implemented by constructing a new industrial stormwater containment pond and upgrading the capacity of the West Airfield Diversion System, as stated in the Compliance Schedule ( see Section VI. E. 4. a of the Rationale).

Recommendation 2, " drain selected utility manholes and underdrains to the dirty system" has already been completed.

The need for implementing Recommendation 3, " containment of runoff at east end of B Concourse commuter gates", will be further analyzed. Due to infrastructure changes east of this ramp area, this recommendation may no longer be pertinent.

Recommendations 4, 6 and 7, regarding construction of a contaminated snow dump, contaminated snow management and revision of City plowing procedures, will not be needed. This is because the landscaped islands surrounding the deicing pads are used by DIA Maintenance for storage of contaminated snow from the pads, This contaminated snowmelt will flow, through the ADF Management System, to the new industrial stormwater ponds.

A training program is being developed to implement recommendation 8, " increase tenant awareness." ( Additionally, a meeting was held in April, 2001 to improve overall tenant awareness of their environmental responsibilities.)

The Division reserves the right to require additional measures to be implemented, if the currently planned improvements are not sufficient to protect water quality. However, the Division recognizes that it is infeasible to completely eliminate fugitive ADF from site runoff. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 13, Permit No. COS 000008 V. PERFORMANCE HISTORY Additionally, SEP # 3, the water quantity/ stormwater drainage study described above, recommended construction of a regional detention pond near the DIA property boundary for flood control purposes. Pond T 239 and the associated concrete detention structure were completed in 2000. This is now the official monitoring point for the " E Outfall".

VI. TERMS AND CONDITIONS A. Stormwater Management Plan The purpose of a Stormwater Management Plan ( SWMP) is to identify possible pollutant sources to stormwater and to set out best management practices that, when implemented, will reduce or eliminate any possible water quality impacts. A SWMP shall be developed for the overall site, and for each tenant covered by this permit, and submitted to the Division. SWMPs shall be prepared in accordance with good engineering practices. ( The plan need not be completed by a registered engineer.)

The plan shall identify potential sources of pollution which may reasonably be expected to affect the quality of stormwater discharges associated with industrial activity from the facility. In addition, the plan shall describe practices which will be used to reduce the pollutants in stormwater discharges associated with industrial activity at the facility and to assure compliance with the terms and conditions of this permit. DIA must implement the provisions of their SWMP as a condition of their permit.

DIA submitted a SWMP to the Division in May, 1994. This submittal also included SWMPs prepared for each tenant with industrial activities exposed to stormwater. The general SWMP has since been updated, and was re submitted to and reviewed by the Division in draft form. The revised SWMP will be resubmitted in final form within 30 days after the effective date of this permit, to take into account any new requirements imposed by the permit. The Division reserves the right to review the SWMP and require additional measures to prevent and control pollution.

The SWMP shall include the following items, at a minimum:

1. Industrial activity description 2. Site map 3. Stormwater management controls, including:  Name of the SWMP administrator  Identification of potential pollutant sources and Best Management Practices  Sampling information  Preventive maintenance  Good housekeeping  Spill prevention and response procedures  Employee training  Identification of discharges other than stormwater 4. Comprehensive inspections 5. Consistency with other plans ( See Part I. C of the permit for a more detailed description of SWMP requirements.) COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 14, Permit No. COS 000008 VI. TERMS AND CONDITIONS B. Determination of Action Levels 1. Action Levels a. The following action levels will apply at Outfall E, and are discussed in Section VI. B. 2.

Table VI 1  Action Levels Parameter Limitation Rationale COD, mg/

100  action level c/ Best Management Practices/ Best Professional Judgment b. The following action levels will apply at monitoring station C+, and are discussed in Section VI. B. 2.

Table VI 2   Action Levels Parameter Limitation Rationale COD, mg/

200   action level c/ Best Management Practices/ Best Professional Judgment c/ Daily Maximum The permit also contains a provision that exceedances caused by severe natural conditions ( such as tornadoes, widespread flooding, etc.), if adequately documented, shall not be subject to enforcement action by the Division. However, notification and follow up reporting to the Division will be required.

2. Discussion of Action Levels, Best Management Practices ( BMPs)

a. Applicable Federal Effluent Guidelines and Standards: No federal guidelines have been promulgated for this type of facility at this time ( see Section IV. A. 1, Federal Regulations).

b. Discussion of Limitations: The " action levels" for COD as listed above are not effluent limitations. They indicate that, when COD exceeds this level, DIA must implement additional Best Management Practices ( BMPs) to supplement the routine BMPs already in use. These may include the reasonably available measures as defined in Section VI. B. 2. c. d/, and/ or the BMPs 9 and 10 described in Section VI. B. 2. c of the Rationale.

The permit contains a compliance schedule for the construction and implementation of system improvements.

The action levels for COD were derived from several sources, and are consistent with the information presented in EPA's recent study of the ADF industry. Daily COD monitoring at outfall E has shown a dramatic drop in COD levels from before the West Airfield Diversion System was constructed to after the diversion was operational ( see Table V 2). This is ascribed to the various BMPs undertaken during that time, as well as the diversion. In general, field experience and review of the data indicate that the system is functioning satisfactorily when the COD level is below 100 mg/

at E Outfall. However, there have been situations where the COD level is greater than 100 mg/

, and no odor or other impacts associated with high COD have been observed. Alternately, there have been situations where a COD level of slightly more than 100 mg/

has been reported, concurrent with operational difficulties. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 15, Permit No. COS 000008 VI. TERMS AND CONDITIONS Another consideration is what the COD levels might be in a natural setting, with no human induced impacts occurring. A review was done of COD data from stream reaches in Colorado that are similar to Third Creek. ( The data are from the STORET system, a nation wide data system which includes monitoring data collected by the Division, the U. S. Geological Survey, and other entities.) The characteristics used for sample stations were: non urbanized areas, minimal slope, with open areas, and an intermittent or low instream flow. The data indicate a wide variability in COD levels. The majority of sample results were under 100 mg/

; however, there were also numerous instances of levels above 100 mg/

, with a high reading of 280 mg/

. ( The data are available in the permit file.) In natural settings, COD can be caused by a number of sources, such as animal droppings, decaying vegetation, and other organic material.

Thus, while COD has proven to be a very useful indicator in general, it is possible for abnormally high readings to exist even when there are no causes directly attributable to DIA activities. Therefore, the Division has decided to use a multiple approach to numeric limits. The data demonstrate that, in general, the COD level at E Outfall under normal operating circumstances is below 100 mg/

. If the COD level exceeds 100 mg/

at E Outfall, the potential for noncompliance exists, and additional BMPs must be implemented. However, since there is the possibility that slightly elevated COD levels are due to factors not related to DIA operations, such exceedances would not be considered a permit violation. However, failure to implement additional BMPs in this case would be a violation. Although the naturally occurring COD levels may be much less than 100 mg/

, this action level is based upon a Best Professional Judgment ( BPJ) determination by the Division for a technology based level.

A review of the data from the C+ pond indicates that COD levels are lower than the action level of 200 mg/

most of the time. The COD action level at C+ is also based on Best Professional Judgement, and represents a temporary benchmark level. Benchmark levels are the pollutant concentrations which, when exceeded, the Division has determined represent a level of concern. The benchmarks also provide an appropriate level to determine whether a facility's stormwater management measures are being successfully implemented. For DIA, the 200 mg/

action level is appropriate for the current operating conditions, but may need to be revised when the stormwater management measures are fully complete ( April 1, 2002) and the Receiving Water Study results are known ( see Section VI. E. 2 of the Rationale). As is the case for the action levels at E Outfall, when COD concentrations at C+ exceed 200 mg/

, DIA must implement additional BMPs to supplement the routine BMPs already in use. These may include the reasonably available measures as defined in Part I. D. 2 of the permit The action levels are based on a combination of: an analysis of the potential instream, naturally occurring conditions, the aesthetic impacts, and a Division based Best Professional Judgment determination for a technology based level, i. e., the achievable levels based on the stormwater management technology which is technically and economically feasible for the site. The Division has determined that DIA is utilizing a state of the art facility in terms of glycol management, in comparison with other airport sites in the nation. This determination is supported by EPA's recent study of the ADF industry. EPA concluded, in part, that if all airports across the country achieved as high a percentage of ADF collection as DIA, the result would be a significant positive impact on water quality across the nation. Thus, this level of glycol management and treatment at DIA is considered, at this time, to satisfy the requirement for a best practical, available, and economically achievable technology for establishing the COD levels in this permit. Once any final technology based levels have been determined by EPA, the Division will evaluate these levels in relation to the limits in this permit.

The Division considered setting an effluent limit based on the Dissolved Oxygen ( DO) Water Quality Standard ( WQS) for Third Creek ( 5.0 mg/ minimum), but has found that further study is warranted before an appropriate limit can be established. ( See discussion, Section VI. E. 2 of the Rationale.) Although the WQS for Segment 16 of the Upper South Platte River remain in effect, they were originally derived from a 1976 EPA publication and were not based on historic ambient water quality data from the DIA watersheds.

Data from DIA over the past two years show that low DO concentrations can routinely occur even when no impacts from DIA are present. DO levels, especially in intermittent streams, can vary widely based on a number of factors, including flow rate, pooling, temperature, stream bed characteristics, direct rainfall, and degree of turbulence, in addition to organic loading. In most cases, it would be difficult to ascribe DO levels solely to DIA activities. As COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 16, Permit No. COS 000008 VI. TERMS AND CONDITIONS discussed in Regulation No. 31.7( 1)( b), natural conditions can cause or contribute to allowable exceedances of numeric WQS. Since there is no clear idea of what causes these fluctuations, or if they are naturally occurring, it is not reasonable to impose an effluent limit and hold DIA responsible for the DO levels in the creek, at least until the creek is better understood. Therefore, DIA will conduct an in depth study of Third Creek ( see Section VI E. 2 of the Rationale), with a focus on DO. The Division will evaluate the completed study, and consider the application of appropriate effluent limits via a permit amendment.

In the meantime, the Division has extensive COD data available, indicating that deicing/ anti icing activities typically have a direct bearing upon the instream COD levels. Thus, COD measures the potential impact that DIA activities have on the stream, with reasonable accuracy ( see above discussion). Using a COD limit provides a far greater correlation between instream levels and DIA activities, and so is the limiting factor at this time.

An effluent limitation for PG is not included in the permit. Although PG is the main source of oxygen depletion in Third Creek, it can begin to break down relatively quickly. High levels of oxygen demand can remain even when no PG is present. Monitoring data from the past two years indicates that the levels of PG can fluctuate greatly, without a corresponding fluctuation in the COD levels. Thus, it is not considered a reliable indicator of water quality impacts. Monitoring is still required on a monthly basis, however, to provide information on when elevated levels may be present and under what circumstances.

c. Best Management Practices ( BMPs): A wide variety of BMPs are used at DIA for managing ADF, as listed in their SWMP. Including a BMP in the SWMP means that it becomes a permit requirement, and must be carried out in order to remain in compliance with the permit. Usually, BMPs as listed in a SWMP can be amended as needed, and are sent to the Division for review. The following BMPs, among others, are included in DIA= s SWMP. These BMPs are also listed in the permit ( meaning that they cannot be changed without a permit amendment or modification, as opposed to only a SWMP modification). ( Note that in the case of pond 004 ( General Aviation), these will apply only when discharge to Metro begins from that pond. This has not yet occurred.)

1) Full deicing activities are normally restricted to the areas at the site identified in Figure 2. Under special situations, Airport Operations may grant permission for full deicing at the A and C concourse gates where it is not normally allowed. This would be necessitated by operational considerations of mechanical or safety issues. For example, if the pads are at capacity so that some planes do not have access, at a time when there is heavy traffic flow, overcrowding can create a hazard at the ramp and gate areas. In addition, there can be mechanical malfunctions whereby planes cannot get to the pad. Full deicing at A and C concourse gates will only be allowed when authorized by the Airport Manager with concurrence by Environmental Services. Such concurrence would only be given if pond capacity were available to handle the extra loading. Historically, this has happened on less than 0.5% of deicing event departures. Full deicing at any other areas is prohibited.

Figure 2 shows only those areas where deicing is allowed as of the issuance date of this permit. Future modifications to deicing areas during the permit term will not necessarily result in a change to Figure 2, but rather must be reflected in DIA's SWMP.

2) To the maximum extent practicable, ADF mixed with stormwater runoff collected from the three west deicing pads is recycled at the on site recycling facilities. However, if the concentration of spent deicing fluid received by the facility is too low for recycling ( due to technical or economic factors), the fluid is instead pumped to Metro for disposal.

3) Each of the industrial stormwater ponds has the capability to discharge to Metro. All the ponds but one are constructed with two cells, such that efficient pumping to Metro from one cell and inflow to the other can occur concurrently. Since DIA= s Metro contribution permit limits the amount of BOD that can be sent to Metro, the concentration of BOD in the pumped water must be determined, so that pumping rates can be established. DIA shall maintain analytical equipment on site to allow efficient coordination of pumping rates with concentrations. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 17, Permit No. COS 000008 VI. TERMS AND CONDITIONS 4) Level monitoring procedures will be used to allow accurate and timely monitoring of industrial stormwater pond levels. DIA environmental staff will observe the level of each pond at least daily during periods of significant precipitation or periods of significant runoff. Observations of pond levels will be conducted at least weekly during periods of dry weather.

5) Since the capacity of the industrial stormwater ponds is limited, the levels in the ponds shall be managed to maintain the maximum practicable capacity available at all times by pumping to Metro up to the BOD permit limit, or to the pumping capacity of the pumps, whichever is less.

DIA environmental staff will determine which industrial stormwater pond has the highest priority for pumping to Metro. Factors to be considered in setting the pumping priority include the pond levels and their rates of change, the ponds= COD concentrations, and climactic conditions.

6) Daily logs of observations will be maintained by DIA environmental laboratory/ field staff and periodically reviewed by the environmental engineering/ scientist staff.

7) If any of the various utility vaults, sumps and manholes surrounding the concourses and on other locations around the airfield become filled with water which must be pumped out, the pumped water is inspected, tested and disposed of as appropriate. It will not be discharged into the clean water system without prior Division approval.

8) In order to avoid unnecessary hydraulic load to the industrial stormwater ponds, thus reducing the possibility of overflow, DIA personnel will have the option of closing the inlet valves to the industrial stormwater ponds and diversion system valves during times when runoff to the pond is clean ( COD at 30 minute digestion time is < 75 mg/

) . This is expected to occur in late spring and summer. Periodic sampling of the discharge for COD shall be required, at a frequency which, based on site and weather conditions, reflects DIA's best judgment in ensuring that discharges greater than 75 mg/

do not occur. Additional sampling will be required at each instance of such closures, as outlined in Part I. B. 1. c of the permit; however, Division notification is not required.

9) In general, DIA must utilize all elements of the glycol management system. However, in the event that an industrial stormwater pond is nearing capacity, but the water flowing into the pond is not clean, and all other reasonably available and applicable measures d/ have been employed to prevent environmental impacts from this situation, DIA personnel will have the option of closing the inlet valve to avoid overfilling and overflowing the pond. Prior to making this decision, DIA personnel will determine the relative COD concentration of the water in the pond versus the water currently flowing to the pond ( based on a 30 minute digestion period), and close the inlet valve if the pond contents are determined to be more concentrated. Division notification will be required within 8 hours of valve closure. Additional sampling will be required at each instance of such closure, as outlined in Part I. B. 1. c of the permit.

10) In the event that an industrial stormwater pond is nearing capacity, and the water in the pond has a lower COD concentration than the water flowing to the pond, and all other reasonably available and applicable measures d/ have been employed to prevent environmental impacts from this situation, DIA shall have the option to release water from the pond to the clean system, in order to maintain or increase capacity for the higher concentration inflow. The COD reading from the pond cell contents must then be less than the COD reading of the pond inflow ( based on 30 minute digestion period) for release to begin. Sampling of pond contents and inflow will continue at least every 4 hours throughout the discharge operation, with the COD of pond contents remaining less than that of the inflow for discharge to continue. Additional sampling will be required at each instance of such closures, as outlined in Part I. B. 1. c of the permit. Division notification must be made within 8 hours of starting release. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 18, Permit No. COS 000008 VI. TERMS AND CONDITIONS 11) Regardless of the volume of water in the industrial stormwater pond, or of pond inflow conditions, if the water in the pond cell is below 75 mg/

COD, DIA shall have the option to release it to the clean stormwater system. Sampling must continue at least every 4 hours while release continues, with the same sample results being required. Additional sampling will be required at each instance of such discharge, as outlined in Part I. B. 1. c of the permit. Division notification will be required within 8 hours of starting release.

d/ Reasonably available measures can include, but are not limited to, additional small scale aeration, requesting Metro to allow increased pumping volume on a temporary basis, or other forms of water management. These particular measures are not required in every case; unique situations can arise at DIA that require unique solutions. It is the permittee= s responsibility to choose the most appropriate measures, and then to provide adequate documentation and justification as to the reasoning behind and efficacy of those measures.

BMPs # 8 through 11 allow the permittee a great deal of freedom in operating their collection system. They are designed to reduce, to the greatest extent practical, the COD concentration of water flowing to the clean stormwater system, and to minimize the amount of water diverted from the natural waterway. It should be recognized that the COD samples are not necessarily representative of the entire pond contents nor of all pond inflow. These decisions must usually be made very quickly, under demanding conditions. BMP # 8 will probably be the most commonly employed of these options. The conditions required for implementation of BMPs # 10 and 11 are very unlikely, but they are included for completeness. These BMPs are considered Atime critical decisions,@ therefore the 30 minute COD digestion period is used, although it is somewhat less accurate than the 2 hour digestion period. Full documentation and justification will be required by the permittee whenever BMPs # 9 through 11 are used. COD readings are observed in Third Creek significantly above 50 mg/ l when no impacts from DIA are present. The action level is 100 mg/ l. Therefore, 75 mg/ l is considered a reasonable intermediate COD value to consider " clean" for the purposes of BMPs # 8, 9 and11 as discussed above. Sampling and analytical methods can also result in considerable statistical variation in measured COD values from the same flow stream. For this reason , for COD results of less than 100 mg/

, if subsequent COD measurements from a particular sample point vary by less than 10% from the initial COD measurement at that point, it can be considered for the purpose of valve control during operation of these BMPs that the COD has not changed.

d. Antidegradation: Since the receiving water is Use Protected, an antidegradation review is not required pursuant to section 31.8( 2)( b) of The Basic Standards and Methodologies for Surface Water.

C. Monitoring 1. Effluent Monitoring:

a. Effluent monitoring will be required at Sub basin A outfall, Sub basin C outfall, Sub basin C+ monitoring station, Subbasin D outfall, and E Outfall to Third Creek, as shown in Table VI 4. Refer to the permit for locations of monitoring points. However, sampling at Sub basin C outfall and Sub basin D outfall will only be required once the new sampling equipment has been installed, or by December 31, 2001, whichever comes first. See Discussion, Section VIII. 5 of the Rationale. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 19, Permit No. COS 000008 VI. TERMS AND CONDITIONS Table VI 4  Monitoring Requirements  Sub basin A outfall, Sub basin C outfall, Sub basin C+ monitoring station, Sub basin D outfall, E Outfall to Third Creek Parameter Measurement Frequency Sample Type Flow, MGD Quarterly Instantaneous or Continuous Chemical Oxygen Demand, mg/

Daily e/ Grab Oil and Grease, mg/

Monthly Grab Oil and Grease, mg/

Weekly Visual pH, s. u. Monthly Grab Total Suspended Solids, mg/

Monthly Grab Acetate, mg/

Monthly Grab Propylene Glycol, mg/

Monthly f/ Grab Ethylene Glycol, mg/

Quarterly Grab BOD5, mg/

Quarterly Grab Total Petroleum Hydrocarbons ( Jet Fuel), mg/

Quarterly Grab Total Phosphorus, as P, mg/

Quarterly Grab Nitrate plus Nitrite, as N, mg/

Quarterly Grab Total Kjeldahl Nitrogen ( TKN), as N, mg/

Quarterly Grab Chloride, mg/

Quarterly Grab e/ The daily monitoring for COD is used as a process control, and is only required when flow conditions and other factors, in the judgment of DIA personnel, will allow representative sampling. While representative monitoring should be performed if reasonably possible, operational difficulties, such as weather or personnel restraints, will also be considered when reviewing compliance with the requirements. f/ The samples should be taken during or following a deicing event, at such time to be as representative of the discharge from the deicing event as possible. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 20, Permit No. COS 000008 VI. TERMS AND CONDITIONS b. Effluent monitoring will be required at Second Creek ( near the west end of runway 7/ 25, north of Pena Blvd), and Runway 35R ( east side, north end), as shown in Table VI 5. Refer to the permit for locations of monitoring points.

Table VI 5  Monitoring Requirements  Second Creek, Runway 35R Parameter Measurement Frequency Sample Type Chemical Oxygen Demand, mg/

Weekly g/ Grab Dissolved Oxygen, mg/

Weekly g/ i/ Grab pH, s. u. Weekly g/ Grab BOD5, mg/

2X/ Month g/ Grab Acetate, mg/

2X/ Month g/ Grab Propylene Glycol, mg/

2X/ Month g/ h/ Grab Oil and Grease, mg/

2X/ Month Visual Oil and Grease, mg/

Monthly Grab Total Suspended Solids, mg/

Monthly Grab Flow, MGD Quarterly Instantaneous or Continuous Ethylene Glycol, mg/

Quarterly Grab Total Petroleum Hydrocarbons ( Jet Fuel), mg/

Quarterly Grab Total Phosphorus, as P, mg/

Quarterly Grab Nitrate plus Nitrite, as N, mg/

Quarterly Grab Total Kjeldahl Nitrogen ( TKN), as N, mg/

Quarterly Grab Chloride, mg/

Quarterly Grab g/ Samples may be taken on a monthly basis during June, July and August of each year. h/ The samples should be taken during or following a deicing event, at such time to be as representative of the discharge from the deicing event as possible. i/ DO sampling will not be required in stagnant water; if no observable flow exists, no sampling is necessary. If sampling conditions are such ( e. g., very low flow) that non representative results are expected, such conditions shall be recorded and reported. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 21, Permit No. COS 000008 VI. TERMS AND CONDITIONS c. Effluent monitoring will be required at E Outfall, Sub basin C+ monitoring station and Third Creek at 112th Ave., as shown in Table VI 6. Refer to the permit for locations of monitoring points.

Table VI 6  Monitoring Requirements  E Outfall to Third Creek, Sub basin C+ monitoring station, Third Creek at 112th Ave.

Parameter Sample Location Measurement Frequency Sample Type Dissolved Oxygen, mg/

E Outfall to Third Creek, Third Creek at 112th Ave. Daily j/ Grab Dissolved Oxygen, mg/

Sub basin C+ monitoring station 3X/ Week j/ Grab j/ DO sampling will not be required in stagnant water; if no observable flow exists, no sampling is necessary. If sampling conditions are such ( e. g., very low flow) that non representative results are expected, such conditions shall be recorded and reported.

2. Dry Weather Monitoring: Monitoring is also required whenever, during the inspection of outfalls, the DIA staff observes or suspects an illicit discharge. The required parameters, which are based on the industrial activities within the drainage area, are listed in Part I. B. 1. b of the permit. Additional parameters may be added to the initial screening, depending upon the nature of the discharge and attendant conditions.

3. Changes to Monitoring Requirements: Several of the parameters for which monitoring is required are not believed to be significant sources of pollution at this facility. For the sampling points listed in Table VI 4 and/ or Table VI 5, the sampling frequencies for Oil and Grease, pH, Acetate, Chloride and Total Suspended Solids could be reduced in the event that DIA can provide adequate justification, based on each of the following items:

a. The permittee can provide at least two years of sampling data for the facility ( for each discharge point where sampling is required), for each of the parameters listed above. ( The samples must be from at least two different calendar years, and taken in 1998 or after.) A summary of all available monitoring data should be included in the amendment request, whether or not it was previously submitted to the Division.

b. The industrial activities at the site ( such as materials handling and storage, chemical use, waste disposal practices, erosion controls, and other types of industrial activities as defined in Part I. B. 5. b of the permit) have not changed since the samples were taken ( and are not expected to change), in any way that could have an adverse impact on stormwater quality.

c. The sample results for the parameters listed above do not demonstrate any problems with stormwater quality. The permittee may compare their data with other water quality data, such as EPA's list of stormwater permit benchmarks from the multi sector permit, or the federal Nationwide Urban Runoff Program ( NURP) data.

The Division will make a determination on the merits of the amendment request within 45 days of receipt. If the request is deemed adequate, the permittee will be notified and their permit amended to change the sampling requirement for the outfall( s) in question, for the remainder of the permit term. If the request is not considered adequate, the permittee shall be notified of the reason for the Division's determination. The permittee will also be able to re petition the Division after another year of sampling data has been obtained.

D. Reporting 1. Discharge Monitoring Report: DIA must submit a Discharge Monitoring Report ( DMR) on a monthly basis to the Division. This report should contain the required summarization of the test results for the parameters shown in Tables VI 4, VI 5, VI 6 of the Rationale and Part I. B. 1 of the permit. See the permit, Part I. B. 2. for details on such submission. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 22, Permit No. COS 000008 VI. TERMS AND CONDITIONS 2. Annual Report: The permittee will be required to submit an Annual Report, covering October 1 through September 30, on the overall compliance with the SWMP. The Annual Report will be due to the Division on or before November 28 of each year. An Annual Report form will be provided by the Division. ( See Part I. F. 1 of the permit for a list of Annual Report requirements.)

3. Additional Reporting: DIA must continue to submit a monthly status report on the status of glycol management at DIA, at least until the construction compliance schedule items have been completed. After that time, the monthly status report shall be required until such time as the Division determines that the increased reporting frequency is not necessary, based on DIA= s compliance history. ( See Part I. F. 2 of the permit.)

4. Reporting to Municipality: Any permitted facility discharging to a municipal storm sewer shall provide the municipality with a copy of the permit application, Annual Reports, and/ or Discharge Monitoring Reports, upon request. A copy of the SWMP shall also be provided to the municipality upon request.

5. Special Reports: Special reports are required in the event of a spill, bypass, or other noncompliance. See Part II. A. of the permit for reporting requirements.

E. Additional Terms and Conditions 1. Aircraft Deicing Fluids ( ADF) Toxicity: ADF formulations contain various additives to enhance the desired properties of the deicing medium. These include AdPack ( corrosion inhibitors, surfactants, and buffers) and 4( 5) methylbenzotriazole ( MEBT) ( fire retardant) as well as others. It is suspected that MEBT is detrimental to soil borne organisms that biodegrade propylene glycol ( PG) and that AdPack contains constituents that are toxic to aquatic organisms. Due to the potential negative effects of ADF additives ( AdPack and MEBT) in general, it is desirable to ascertain the impact of such additives on the State Waters involved with the discharge.

EPA has recently conducted an effluent limitation guideline study on aircraft deicing. The study includes an assessment of the need for various testing, including toxicity. The final report was released at the end of 1999, and concluded that no ADF effluent limits or guidelines would be developed at this time. Additionally, a subcommittee of the Society of Automotive Engineers ( SAE), which sets all specifications required of ADF, is working with the Air Transport Association ( ATA) on approving environmental testing requirements and procedures ( for deicing anti icing and runway deicers), and on testing with the intent of obtaining baseline data and subsequently considering toxicity data. The deicer manufacturers often change the formulations, which are proprietary, and DIA does not have control over the specific formulations that the different airlines use ( beyond a general provision that propylene glycol be used instead of ethylene glycol).

Therefore, the Division has determined that it would not be productive at this time to require DIA to conduct a similar study. If federal effluent limits or guidelines are adopted during the term of this permit, they would be incorporated in the permit by amendment. The Division retains the right to require that DIA conduct a follow up study on the impacts of the discharge of ADF on Third Creek, if needed. Such a study could include accumulative impacts, probable dilution factors, the environmental fate of the additives, and an aquatic organism assessment of affected areas.

2. Receiving Water Study: DIA has developed a work plan to conduct a receiving water study to support development of permit conditions and water quality standards for dissolved oxygen. This work plan details the physical, chemical and biological data to be collected and analyzed to derive permit conditions and limitations. It contains a description of data needs, proposed sampling, modeling efforts, project schedule, and study deliverables. This study will be a multi year study designed to address system constraints, permit limits, water quality standards ( dissolved oxygen) and the use classification of receiving waters which surround DIA.

3. Signatory and Certification Requirements: Signatory and certification requirements for reports and submittals are discussed in Part I. F. 6. of the permit. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 23, Permit No. COS 000008 VI. TERMS AND CONDITIONS 4. Compliance Schedules: Except as provided below, compliance with the terms and conditions of this permit, including the Stormwater Management Programs, shall be required by the effective date of the permit.

a. As per the Water Quality Study ( see discussion, Section V. B. 2 of the Rationale), and as modified by various correspondence and the final construction plans ( submitted to the Division on July 12, 2001), DIA will complete the following construction projects by the dates indicated:

1) Paving of selected portions of certain islands adjacent to the west deicing pads for use as contaminated snow dumps and/ or modification of collection structures to collect snowmelt and to collect additional subsurface flows, to be completed by April 1, 2002 k/.

2) Increased capacity of the dirty water conveyance system, and construction of at least 22 million gallons of new containment capacity for the collection of dirty stormwater, to be implemented as indicated in Table VI 7, above.

3) Repair of the existing B Concourse glycol collection system by January 1, 2000 to facilitate additional recycling of deicing fluids from full gate deicing activities ( completed).

Table VI 7  Compliance Schedule  Construction Activity Deadline Notice to Proceed for design contract July 21, 1999 ( completed)

Design complete September 15, 2000 ( completed)

Notice to Proceed for construction contract February 1, 2001 ( completed)

Construction complete and system start up December 15, 2001 Facility fully operational April 1, 2002 k/

k/ This date may be extended by the Division if insufficient precipitation is received by then to allow for a full operational testing of the new facilities.

b. The permittee shall complete the specified activities by April 1, 2003, in accordance with the following schedule of compliance, as approved by the Division.

Table VI 8  Compliance Schedule   Receiving water study Activity Deadline Draft study work plan completed March 30, 2001 ( completed)

Final study work plan completed July 1, 2001 ( completed)

Data collection completed December 31, 2002 Report submitted to Division April 1, 2003 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 24, Permit No. COS 000008 VI. TERMS AND CONDITIONS The permittee has submitted a study work plan to the Division. The plan includes data collection and analysis, reviewing the findings with the Division and EPA, and submitting a final report.

The results of the initial study may indicate the need for additional phases of study; in that case, the additional phases will automatically be included in the permit as compliance schedule items.

For each activity outlined in Tables VI 7 and VI 8, above, the permittee shall be required to submit a report of progress to the Division, no later than 14 calendar days following the identified date.

F. Waste Minimization/ Pollution Prevention Waste minimization and pollution prevention are two terms that are becoming increasingly more common in industry today. Waste minimization includes reducing the amount of waste at the source through changes in industrial processes, and reuse and recycling of wastes for the original or some other purpose ( such as materials recovery or energy production). Pollution prevention goes hand in hand with waste minimization. If the waste is eliminated at the front of the line, it will not have to be treated at the end of the line. The direct benefits to the industry are often significant, both in terms of increased profit and in public relations. This program can affect all areas of process and waste control with which an industry deals. Elimination or reduction of a wastewater pollutant can also result in a reduction in an air pollutant or a reduction in the amount of hazardous materials which must be handled or disposed.

This discharge permit does not specifically dictate waste minimization conditions at this time. The Division does strongly encourage the permittee to continue working on developing and implementing a waste minimization plan. Several industries have already developed plans and found that implementation resulted in substantial savings. Both the Colorado Department of Public Health and Environment and the Environmental Protection Agency ( EPA) have information and resources available. For more in depth information, please contact these agencies.

Kathryn Dolan January 31, 2001 VII. REFERENCES A. Camp Dresser & McKee Inc. Water Quality Study Final Report. Denver: as commissioned by Denver International Airport, 9/ 30/ 98.

B. Colorado Department of Public Health and Environment, Water Quality Control Commission. Regulation No. 31 The Basic Standards and Methodologies for Surface Water ( 5 CCR 1002 31). Denver: CDPHE, as revised 1/ 12/ 1998 and effective 3/ 2/ 98.

C. Colorado Department of Public Health and Environment, Water Quality Control Commission. Regulation No. 38 Classification and Numeric Standards for South Platte/ Laramie/ Republican/ Smoky Hill River Basins. Denver: CDPHE, as revised 11/ 9/ 1998 and effective 12/ 30/ 98.

D. Colorado Department of Public Health and Environment, Water Quality Control Commission. Regulation No. 61 Colorado Discharge Permit System Regulations. ( 5 CCR 1002 61). Denver: CDPHE, as revised 3/ 9/ 99 and effective 4/ 30/ 99.

E. Stednick, John D., Ph. D., Mountain River Associates, Inc. Water Quality Monitoring for Propylene Glycol in Third Creek. Fort Collins: as commissioned by the Farmer= s Reservoir and Irrigation Company, 5/ 28/ 97.

F. U. S. Government, Office of the Federal Register, National Archives and Records Administration. Code of Federal Regulations ( Parts 400 to 424). Washington: 7/ 1/ 96. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 25, Permit No. COS 000008 VIII. CHANGES AFTER PUBLIC NOTICE Comments on the draft permit were received from DIA and the Farmer's Reservoir and Irrigation Company ( FRICO). The comments that resulted in changes to the permit and/ or rationale are discussed below. The Division's response to FRICO's comment letter is available for review in the permit file. In addition, other changes were made by the Division, as discussed below.

1. FRICO pointed out that the classifications for some of the receiving waters had changed. The revised Water Quality Standards ( WQS) have been included in Sections III. A. 1 and III. A. 2 of the Rationale.

2. FRICO expressed concern that full deicing could take place at areas other than those authorized. A statement was added to the permit clarifying that full deicing is only allowed at the areas identified in Part I. D. 2. a of the permit. New areas may be added if included in the SWMP and adequate preventative measures taken ( see discussion under Item 3, below). In addition, the ` special conditions' authorizing full deicing at Concourses A and C were questioned. A fuller explanation of the circumstances and requirements has been added to Section VI. B. 2. c. 1 of the Rationale and Part I. D. 2. a of the permit.

3. DIA asked that the BMPs described in Parts I. D. 2. d and I. D. 2. i. 2 of the permit be deleted. The first one concerned the use of an aeration system at sub basin C. Due to construction of the sixth runway, this basin has been drained and the aeration system removed, so compliance with this item was no longer possible. The second BMP concerned allowing runoff from the dirty water system to bypass the collection ponds if the runoff was relatively clean and sufficient dilution was provided within the receiving stream, thereby reducing the hydraulic load to Metro. However, due to infrastructure changes and constraints, it is not practical for DIA to implement this BMP routinely at this time, as required. DIA will continue to explore other practical means to further reduce unnecessary hydraulic loads to the ponds as the receiving water study and site construction progress. These two BMPs have been deleted from the permit and Rationale, and the facility description amended.

For the BMP in Part I. D. 2. h. ( formerly numbered I. D. 2. i. 1), the draft permit contained requirements that conflicted with the provisions in the final paragraph of that section concerning Division notification. With the elimination of the second part of that BMP, I. D. 2. i. 2 ( see above), which by its nature required close oversight, the permit now makes clear that this BMP, the diversion of clean stormwater (< 75 mg/

) from the dirty water system, does not require Division notification. However, a provision has been added requiring periodic monitoring for COD during the release, instead of just a one time sample.

For the BMP in Part I. D. 2. k ( formerly numbered I. D. 2. l), it has been clarified that Division notification is required.

A condition has been added addressing the variations in COD levels during the exercise of the BMPs in Part I. D. 2. h k. Sampling and analytical methods can result in considerable statistical variation in measured COD values from the same flow stream. A slight variation in COD level is usually insufficient justification for a major operational change, as the impact to the environment would be minimal. Therefore, if subsequent COD measurements from a particular sample point vary by less than 10% from the initial COD measurement at that point ( e. g., first sample result   70 mg/

, second sample result   76 mg/

) , it can be considered for the purpose of valve control during operation of these BMPs that the COD has not changed.

As per the BMP in Part I. D. 2. a, Figure 2 in Part I. G of the permit, shows the areas where full deicing is allowed as of the time of permit issuance. However, these areas can be added to or changed. For the sake of administrative efficiency, the permit has been amended to indicate that such changes can be handled via modifications to the SWMP, rather than a permit amendment. The Division will still require such SWMP changes to be submitted before implementation, and reserves the right to require stricter controls as necessary ( see Part I. D. 3. c of the permit) .

4. In conjunction with the investigation into the relationship between COD and DO in DIA's discharges, the sampling frequency for DO has been increased to daily at E Outfall and at 112th Avenue. ( This eliminates the need for using 75 mg/ l of COD as a trigger for increasing the DO sampling to twice per week.) Sampling for DO three times per week has been added at the monitoring station at C+. It is anticipated that with this increased sampling frequency, samples will often be taken when the flow rate is very low, which can affect the readings. Field notes will include any information on conditions that could cause the samples results to not be representative of the discharge. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 26, Permit No. COS 000008 VIII. CHANGES AFTER PUBLIC NOTICE 5. Based on updated information from DIA, changes were made to the facility description. Sub basins C and D are no longer in use as detention ponds. A sampling station is being installed at C+. Sampling has been temporarily halted at C and D due to access problems caused by the new construction. The original sampling points for C and D are upstream of the current locations. The newer locations were based on the construction of headwalls at C and D. Since these will no longer be used as ponds on a regular basis, the sampling points will be moved back to their original locations. New sampling equipment is being installed at these points, and will be in place by late 2001. Requirements for monitoring at C and D will be temporarily suspended until that time. Monitoring at the new station at C+ will provide the necessary information for permit compliance; the stations at C and D are used primarily for process control. The facility description has been amended to reflect these changes. In addition, the footnote describing " reasonably available measures" ( Part I. D. 1.2, footnote h/) has been changed to delete examples of actions to be taken at C and D, as they are no longer available for the described uses.

Figures 3, 4 and 5 have been updated to reflect changes at DIA since the original figures were prepared.

6. A provision has been added in Part I. B. 2 of the permit, specifying that the requirements for monitoring conditions only apply to required monthly and quarterly sampling. The intent of this section is to ensure that the samples are representative of a typical discharge from a storm event. However, they are not appropriate or useful for the more frequent sampling required ( e. g., daily COD or DO), as such sampling is performed on a routine basis, regardless of precipitation conditions, etc. Part I. B. 2 of the permit has been amended.

7. In the public notice draft, Item 1 under the compliance schedule in Section VI. E. 4. a of the Rationale and Part I. A. 2. a. 1 of the permit contained an incorrect date. Compliance is required by April 1, 2002, not April 1, 2001, as the improvements are tied to the increased capacity of the dirty water conveyance system. These dates have been corrected.

8. References to " sub basin E outfall" have been changed to " E outfall" so as to avoid confusing the outfall of the T 239 pond to Third Creek near DIA's border ( a compliance point), with the actual drainage area referred to as sub basin E, which drains the north campus area ( see description, Section IV. A. 2. b of the Rationale).

9. The compliance schedule for construction of increased capacity in the dirty water conveyance system and containment facilities has been changed. The interim date of November 1, 2001 for completion of construction has been changed to December 15, 2001, at DIA's request, due to construction delays and difficulty in obtaining a building permit. The compliance date for the facility to be fully operational has not changed.

The dates for the receiving water study compliance schedule have been added, and the discussion changed to acknowledge the items that have been completed. See Part I. A. 2. b of the permit.

Kathryn Dolan July 12, 2001 PERMIT AMENDMENT # 1 COLORADO DISCHARGE PERMIT SYSTEM ( CDPS)

SUMMARY OF RATIONALE CITY AND COUNTY OF DENVER  DENVER INTERNATIONAL AIRPORT CDPS PERMIT NUMBER COS 000008 By letters dated October 23, 2001, February 6, 2002, and February 15, 2002, the permittee has requested three changes to their permit regarding compliance schedule items and monitoring requirements.

I. PURPOSE OF AMENDMENT ` A'

Denver International Airport ( DIA) has proposed a reduction in their monitoring program for the number of constituents tested, frequency of testing, and number of sites tested. DIA has amassed a very substantial database for discharges from Sub basin A outfall, Sub basin C outfall, Sub basin C+ monitoring station, Sub basin D outfall, E outfall, Second Creek outfall, and Runway 35R. Data from 1/ 98 through 11/ 01 are summarized in Table 1. ( The past four years of data were used; data collected before the West Airfield Diversion System was operational are not considered to be representative of current conditions.) The data were analyzed, to determine which parameters are clearly not of concern and might be deleted, and of those to be retained, what the sampling frequency should be.

Monitoring  Required Parameters The data show that monitoring for several parameters may be deleted, based on a comparison with EPA's benchmark values. The benchmark values from the NPDES Stormwater Multi Sector General Permit for Industrial Activities are also included in Table 1. The benchmarks are the pollutant concentrations which, when exceeded, EPA determined represent a level of concern. The level of concern is a concentration at which a stormwater discharge could potentially impair or contribute to impairing water quality, or affect human health from ingestion of water or fish.

The grab samples for Oil and Grease have generally shown very low levels. Therefore, the requirement will be for a visual check for Oil and Grease; if a sheen is detected, then a grab sample is required. Acetate, Ethylene Glycol, and Total Petroleum Hydrocarbons ( TPH) have been found to be at low levels or below detection in almost all cases, and so monitoring for these parameters has been deleted. The visual check for Oil and Grease can also be used to detect the potential presence of TPH.

The levels of Nitrate plus Nitrite, as N, were found to be low at Sub basin C, Sub basin D, Second Creek and Runway 35R. Monitoring for Nitrate plus Nitrite at these points has been deleted.

The levels of Total Phosphorus have generally been well below the EPA benchmark of 2 mg/ l, and so monitoring for it has been deleted. The levels for Total Kjeldahl Nitrogen ( TKN) were low at Sub basin E, Second Creek and Runway 35R. Monitoring for TKN is at these points has been deleted.

The levels for Chloride were low at all outfalls except for Sub basins C and E, and Second Creek. ( Chloride is a constituent of Magnesium Chloride, a deicing compound used on the roads around DIA. While runoff from the roadways is not regulated under this permit, Chloride has been shown to cause interference in Chemical Oxygen Demand ( COD) tests, and so monitoring is still required where it has been found at elevated levels.) Chloride data from Sub basin C is not considered to be representative of the current drainage area ( see discussion under Monitoring Location, below), since it no longer includes as large an area of roadway deicing. Roadways make up a large part of the Second Creek drainage, which accounts for the high Chloride levels found there. Monitoring for Chloride at Sub basin E and C+ will continue, at an increased frequency, as a check on the potential for interference in the COD test accuracy. However, any requirements for new/ changed Best Management Practices ( BMPs) for roadway deicing will be handled under Denver's municipal stormwater permit, as it is not related to the industrial activities and areas covered under this permit. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment # 1 Rationale  Page 2, Permit No. COS 000008 Monitoring Locations The Division considered DIA's request to eliminate some monitoring points. The request was based on the idea of using representative discharges. When a facility has two or more outfalls that, based on a consideration of industrial activity, significant materials, and management practices and activities within the area drained by the outfall, the permittee reasonably believes discharge substantially identical effluents, the permittee may test the effluent of one such outfall and report that the quantitative data also apply to the substantially identical outfalls.

Given the size and scope of the facility, the variety of activities that take place, and the facility's compliance history, the Division determined that it was still important at this time to have periodic checks on the stormwater quality at all of the points originally included in the permit. However, DIA has made tremendous improvements to their system in the past few years, and so a reduction in monitoring frequencies is justified ( see below).

The one exception to maintaining the present locations is the requirement to monitor for Dissolved Oxygen ( DO) in Third Creek at 112th Avenue ( about one mile northwest of DIA's boundary). Since this site is not on DIA's property, and incorporates flow from non DIA property, the requirement to monitor at this site will be deleted. However, the Division still recommends that this site be sampled periodically, to give some indication of the potential downstream impacts from DIA discharges.

The location for monitoring at Sub basin C has changed. When the airport originally opened, monitoring was performed at the outfall from Sub basin C. This drainage area was relatively small, consisting of the west half of Concourse A and deicing pad A. A headwall was constructed downstream of C ( creating Pond C), to capture all drainage in Third Creek upstream of that point ( including Sub basins C and A, and roadways and open space as well as industrialized areas). Due to the new pond, the outfall from Sub basin C was frequently underwater. Therefore, sampling was performed at the headwall instead. The data in Table A represent drainage from this larger area. The headwall was removed due to construction on the new Sixth Runway. Thus, the outfall from Sub basin C is again being used as the monitoring point.

Monitoring Frequency The most onerous requirement in the permit, in terms of resources required, has been for daily COD monitoring at five of the seven outfalls, and daily DO monitoring at two outfalls. COD monitoring was originally used as a key measure of permit compliance, in that a correlation was found between the amount of aircraft deicing fluids and the amount of COD present. At that time, the trigger levels were developed based on existing data and Best Professional Judgment, and additional BMPs were being implemented on an as needed basis. Today, many improvements have been made to the ADF management system, and DIA operates the system to the fullest extent possible on a routine basis. Therefore, the ambient data that are collected are not being directly used to make decisions about additional BMPs. ( The primary additional BMP in use at this time is construction of the new pond capacity, which is on going. See Amendment B.)

The permit is based on using COD levels as a trigger for increased Best Management Practices ( BMPs). While there is some initial evidence from the current receiving water study to indicate that this may no longer be the most appropriate permit structure for DIA, the Division has decided not to change that section until the results of the study are final. The study results are expected to lead to numerous changes in the permit, and in the assessment of the potential for water quality impacts from DIA's discharges. This permit amendment will deal with the reductions in monitoring that are clearly justifiable at this time; further reductions may be appropriate in the future based on the study results.

The Division believes that monitoring for COD twice a week at Sub basin C+ and E outfall will be adequate to evaluate COD levels in the receiving waters to the extent necessary. In addition, a provision has been added requiring that at the time of sampling for COD at outfall E and C+, an additional sample sufficient to test for BOD5 must also be collected. If the COD result is greater than 200 mg/ l, then the BOD5 sample must also be analyzed. This is required in order to determine the amount of oxygen demand present when there is the potential for Chloride interference. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment # 1 Rationale  Page 3, Permit No. COS 000008 Monitoring for all parameters at Sub basin A, C and D outfalls has been standardized to require that all remaining parameters be monitored once every two months. This is a decrease from the daily COD monitoring, as that data is no longer used as a process control, and so is not critical to airport operations. For other parameters, the frequency has been increased from quarterly, to simplify the sampling schedule.

Monitoring at Second Creek and Runway 35R has been decreased in frequency. Monitoring at Runway 35 R is now required once every six months; for Second Creek, monitoring is required once per year. Past data have shown low pollution potential from these points, which is due to the low level of industrial activity in the drainage areas.

II. PURPOSE OF AMENDMENT ` B'

DIA has requested a change in the final two dates in the permit's construction compliance schedule.

The permit was originally issued in July, 2001. At that time, the compliance schedule for construction of the new industrial stormwater ponds was based on the best estimates and contract information from DIA staff. Since that time, there have been numerous and substantial delays in the construction:

 There were delays in obtaining the Building Permit for construction of the laboratory and pumping facilities associated with the project, while the process for resolving the fees to be paid to Denver Wastewater and the Metro Wastewater Reclamation District ( MWRD) was being completed.

 The terrorist attacks of September 11, 2001 had impacts on all areas of the airport. DIA has implemented more stringent security requirements ( e. g., badging, fingerprinting and background checks for construction workers) that have resulted in delays to the construction project.

 A portion of the construction requires a 48 inch diameter boring under the runway to install larger pipes to enhance collection capabilities. During the boring, rock was encountered, which led to over two months of delay.

 The majority of the delay is attributed to problems with the construction contractor regarding its choice of means and methods, sequencing of work and staffing problems. DIA has had to terminate the contractor and to work with the bonding company to select a new contractor. In addition, some of the installation work performed by the original contractor was determined to be defective, and so that work has to be removed and replaced.

The Colorado Discharge Permit Regulations ( No. 61.8( 3)) allow the permittee a reasonable opportunity to attain compliance with permit requirements. The Division has determined that DIA has acted in good faith to complete the project as quickly as possible, and that the proposed new schedule is reasonable. It is important to amend the permit as expeditiously as possible, so that DIA is not put in a situation of permit non compliance due to processing delays. However, since the new contractor has not yet been chosen, the new deadlines proposed by DIA are based on an estimated project schedule. It is possible that these dates could change, based on new information presented after this permit is at Public Notice. In that case, the final compliance schedule dates could change by up to three months either way, without a formal Public Notice process. If either of the dates must be changed by more than three months, a new Public Notice will be published.

II. PURPOSE OF AMENDMENT ` C'

The construction compliance schedule originally addressed construction of two new ponds ( 10 million gallons total capacity), and paving of certain grass ` islands' adjacent to the deicing pads and connection of drainage from these islands directly into the glycol collection ponds. DIA has paved some areas west of the deicing pads for use in stockpiling contaminated snow. However, experience with the West Airfield Diversion System since the time of the original project plan indicated a need for increased capacity to collect flows from the grass islands and other areas as well. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment # 1 Rationale  Page 4, Permit No. COS 000008 The project has been significantly expanded and now includes:

 five new re engineered diversion structures  to increase conveyance capacity, 48 inch pipe on the north branch, bored under a runway and taxiways, and 24 inch pipe on the south branch ( to replace existing 12 inch line)  three pond cells with 22.5 million gallons total capacity to accommodate the increased conveyance capacity ( replaces two originally planned ponds)

The compliance schedule task described in Part A. 2. a. 1 of the permit ( paving grass areas) has been amended to read:

1) Modification of collection system to collect snowmelt and additional surface and subsurface flows, to be completed by December 31, 2002.

The new compliance schedule is as follows:

The permittee shall complete the specified construction activities by April 1, 2002 December 31, 2002, in accordance with the following schedule of compliance, as approved by the Division.

Table 1  Compliance Schedule  Construction Activity Deadline Notice to Proceed for design contract July 21, 1999 ( completed)

Design complete September 15, 2000 ( completed)

Notice to Proceed for construction contract February 1, 2001 ( completed)

Pond construction complete and system start up November 30, 2002 Facility fully operational December 31, 2002 a/

a/ This date may be extended by the Division if insufficient precipitation is received by then to allow for a full operational testing of the new facilities.

Pages 5, 6, 7, 8 and 9 of the permit have been changed. Page 7a has been added.

Kathryn Dolan March 7, 2002 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 5, Permit No. COS 000008 Table 1  Summary of Historical DMR Data, 1/ 98   11/ 01 OUTFALL A COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum 10.00 3.10 1.00 5.80 10.00 2.50 0.50 0.50 4.00 0.50 0.04 0.06 0.02 5.00 maximum 604.00 9.70 457.00 10.00 762.00 480.00 336.00 10.00 584.00 3.50 0.67 4.12 18.10 289.00 mean 119.66 5.39 16.62 7.53 125.21 46.94 21.59 1.03 99.40 1.22 0.21 1.27 2.79 78.88 standard deviation 108.76 1.07 77.88 0.75 161.75 118.15 60.33 1.78 150.60 1.29 0.14 1.39 4.23 81.78 coefficient of variation 91% 20% 469% 10% 129% 252% 279% 173% 152% 106% 69% 109% 152% 104%

no. of samples 76 40 34 34 34 17 34 32 20 16 16 16 16 17 % non detects 0% 0% 74% 0% 0% 76% 59% 91% 0% 94% 6% 6% 12% 0%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 OUTFALL C COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum 2.00  1.00 6.70 6.00 2.50 0.50 0.50 1.00 0.50 0.01 0.01 0.40 0.50 maximum 3805.00  25.00 8.10 4520.00 990.00 540.00 8.00 784.00 3.50 4.75 1.13 28.40 4910.00 mean 242.66  3.59 7.39 222.48 144.15 70.34 1.27 210.07 1.63 0.51 0.45 6.03 2049.45 standard deviation 312.97  4.76 0.37 831.56 300.08 162.49 1.88 248.81 1.50 1.15 0.34 8.20 1958.70 coefficient of variation 129%  133% 5% 374% 208% 231% 148% 118% 92% 227% 76% 136% 96%

no. of samples 957 none 28 29 29 10 28 26 15 16 16 16 16 10 % non detects 0%  81% 0% 0% 30% 46% 78% 15% 100% 8% 8% 0% 18%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 6, Permit No. COS 000008 Table 1  Summary of Historical DMR Data, 1/ 98   11/ 01 ( cont.)

OUTFALL D COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum 5.00  1.00 6.00 5.00 9.00 0.50 0.50 8.00 0.50 0.01 0.01 0.50 0.50 maximum 29560.00  44.00 8.50 1060.00 140.00 1160.00 3.00 768.00 3.50 1.33 0.86 6.60 73.00 mean 741.66  5.75 7.23 94.38 59.75 257.31 0.70 375.55 1.40 0.19 0.40 2.08 17.19 standard deviation 1468.99  9.65 0.57 235.85 62.62 346.41 0.64 303.06 1.45 0.40 0.27 1.74 23.84 coefficient of variation 198%  168% 8% 250% 105% 135% 91% 81% 104% 219% 68% 84% 139%

no. of samples 690 none 20 21 21 4 21 20 11 10 10 10 10 8 % non detects 0%  70% 0% 0% 0% 24% 90% 9% 100% 30% 20% 0% 13%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 OUTFALL C+ COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum 1.00 3.40 1.00 7.20 198.00 2.50 0.50 0.50 4.00 3.50 0.21 0.09 2.80 183.00 maximum 2880.00 17.80 10.00 8.00 8540.00 2.50 16.00 0.50 70.00 3.50 0.87 1.47 3.00 210.00 mean 165.64 6.84 2.80 7.54 2126.20 2.50 6.30 0.50 37.00 3.50 0.54 0.78 2.90 196.50 standard deviation 218.30 2.30 4.02 0.31 3600.58 0.00 7.97 0.00 46.67 0.00 0.47 0.98 0.14 19.09 coefficient of variation 132% 34% 144% 4% 169% 0% 127% 0% 126% 0% 86% 125% 5% 10%

no. of samples 1455 168 5 5 5 5 5 5 2 3 2 2 2 2 % non detects 0% 0% 80% 0% 0% 100% 60% 100% 0% 100% 0% 0% 0% 0%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 7, Permit No. COS 000008 Table 1  Summary of Historical DMR Data, 1/ 98   11/ 01 ( cont.)

OUTFALL E COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum 1.00 0.50 1.00 6.90 3.00 2.50 0.50 0.50 1.00 0.50 0.01 0.01 0.02 3.00 maximum 2570.00 12.00 59.00 8.00 1010.00 92.00 164.00 2.00 318.00 3.50 1.05 15.80 5.10 3040.00 mean 71.29 5.96 4.63 7.61 173.58 11.65 8.79 0.55 31.72 2.19 0.16 2.57 1.26 499.00 standard deviation 130.18 2.02 10.32 0.31 274.83 24.04 31.05 0.27 78.15 1.54 0.25 4.20 1.18 812.71 coefficient of variation 183% 34% 223% 4% 158% 206% 353% 48% 246% 70% 152% 164% 94% 163%

no. of samples 1433 342 34 34 33 17 34 32 18 16 16 16 16 15 % non detects 0% 0% 74% 0% 0% 82% 82% 97% 28% 100% 6% 6% 19% 0%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 OUTFALL 112TH COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum 2.00 0.30

maximum 1430.00 17.80

mean 72.91 5.45

standard deviation 107.29 2.26

coefficient of variation 147% 41%

no. of samples 1203 277 none none none none none none none none none none none none % non detects 0 0            none EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale  Page 8, Permit No. COS 000008 Table 1  Summary of Historical DMR Data, 1/ 98   11/ 01 ( cont.)

OUTFALL 35R COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum   1.00 6.50 3.00 2.50 0.50 0.50 3.00 0.50 0.05 0.01 0.01 2.00 maximum   59.00 8.50 192.00 220.00 78.00 0.50 401.00 3.50 0.49 1.73 6.40 3040.00 mean   4.97 7.46 35.62 37.74 6.45 0.50 78.29 2.43 0.22 0.69 1.26 241.77 standard deviation   10.71 0.39 47.96 70.61 17.18 0.00 126.04 1.49 0.14 0.56 1.62 840.79 coefficient of variation   216% 5% 135% 187% 266% 0% 161% 61% 65% 82% 129% 348%

no. of samples none none 29 30 29 17 30 29 14 14 14 14 14 13 % non detects   67% 0% 0% 65% 77% 100% 7% 100% 0% 7% 29% 7%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 OUTFALL SECOND CREEK COD DO Oil and Grease pH TSS Acetate Propylene Glycol Ethylene Glycol BOD5 TPH Total P NO3 + NO2 as N TKN Chloride ( mg/ L) ( mg/ L) ( mg/ L) ( s. u.) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L) ( mg/ L)

minimum   1.00 6.90 10.00 2.50 0.50 0.50 4.00 0.50 0.10 0.04 0.70 1.00 maximum   12.00 8.20 2840.00 204.00 3.00 1.00 434.00 3.50 2.64 1.04 3.00 7030.00 mean   2.09 7.61 349.55 21.50 0.61 0.57 87.31 2.25 0.56 0.51 1.52 1174.92 standard deviation   2.30 0.31 591.88 53.80 0.52 0.18 135.28 1.54 0.69 0.27 0.71 2061.46 coefficient of variation   110% 4% 169% 250% 86% 31% 155% 69% 123% 54% 47% 175%

no. of samples none none 23 23 22 14 23 22 13 12 12 12 12 12 % non detects   83% 0% 0% 71% 96% 95% 7% 100% 8% 8% 8% 0%

EPA permit benchmark 120 none 15 6.0 9.0 100 none none none 30 none 2 0.68 1.5 860 PERMIT AMENDMENT # 2 COLORADO DISCHARGE PERMIT SYSTEM ( CDPS)

SUMMARY OF RATIONALE CITY AND COUNTY OF DENVER  DENVER INTERNATIONAL AIRPORT CDPS PERMIT NUMBER COS 000008 Amendment Fee ( 55% of Annual Fee) $ 7060 By letter dated July 14, 2003, the permittee has requested several changes to their permit regarding action levels, monitoring requirements, best management practices ( BMPs), system capabilities, and Stormwater Management Plan ( SWMP) revision.

I. ACTION LEVELS Denver International Airport ( DIA) has requested that the action levels contained in Part I. A. 1 of the permit be removed. The action levels require the implementation of reasonably available measures and/ or additional BMPs when concentrations of Chemical Oxygen Demand ( COD) in Third Creek meet or exceed the action levels This was based on the premise that the system was not operating optimally when the COD action levels were reached. These action levels were used at a time when additional improvements to the spent aircraft deicing fluid ( ADF) management system were planned but not yet implemented. Some temporary BMPs were being used while design and construction were underway. These BMPs are no longer used, given the facility upgrades. DIA operates the system to optimize collection of spent ADF at all times, regardless of COD concentrations in Third Creek. Therefore, the implementation of additional BMPs based on action level, is no longer appropriate.

In April 2003, DIA completed a Receiving Water Study that concluded that it was not currently possible to establish a reliable correlation of Dissolved Oxygen ( DO) to COD levels in Third Creek. Therefore, COD monitoring on Third Creek will not be used in this permit as a measure of compliance with the DO standard.

There has also been some intent to use in stream COD concentrations as an indicator of the effectiveness of the spent ADF management system. In stream COD concentrations have not been shown to be a reliable indicator of the effectiveness of the system, due to factors such as the dynamic nature of the stream and the commingling with stormwater runoff from other sources, such as municipal activities ( roadway deicing and maintenance, construction, etc.). Therefore, the Division has replaced the action levels with a requirement that DIA provide estimates of the effectiveness of the system in terms of the level of ADF capture being achieved. This information will be used by the Division, together with other information ( annual reports, sampling data, etc.) to determine the effectiveness of the current system and the need for any new or revised BMPs.

II MONITORING REQUIREMENTS DIA requested reduction and clarification of the wet weather monitoring requirements contained in Part I. B. 1. a of the permit.

DIA requested that Second Creek and Runway 35R be eliminated as discharge points for wet weather monitoring on the basis that residual ADF is present very infrequently in these streams, at levels much lower than observed in Third Creek. Therefore, DIA asserts that wet weather monitoring of Third Creek is representative of potential worst case conditions for the discharge of stormwater associated with industrial activity at the site. The permittee further stated that the Receiving Water study adequately characterizes the nature of discharge from these locations. The Division agreed to eliminate Second Creek as a discharge point for wet weather monitoring, due to the low levels of residual ADF historically observed, and the fact that the majority of activities that occur upstream of the outfall are commercial and municipal rather than industrial in nature. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment # 2 Rationale  Page 2, Permit No. COS 000008 Given the size and scope of the facility, the variety of industrial activities that take place upstream of the Runway 35R outfall, and the aquatic life known to exist downstream of the discharge, the Division determined that it was still important to have periodic checks on the stormwater quality at this discharge point.

DIA requested that C+ and E on Third Creek be eliminated as discharge points from the permit and that all monitoring requirements for these locations be removed. The Division agrees that these locations are in stream sample sites, not industrial outfalls, and recognizes that drainage into Third Creek also occurs from other activities on DIA property that are not regulated under this permit ( e. g., public street maintenance, new development, commercial facility management). However, given the size and scope of industrial activities that do occur on DIA property, the resulting industrial stormwater drainage, and the compliance history, the Division determined that it is reasonable for DIA to perform biological monitoring on Third Creek to demonstrate the continued viability of the aquatic life community. DIA is in the process of proposing site specific standards for dissolved oxygen ( DO) to the Water Quality Control Commission ( WQCC) for their consideration. The Division felt that the biological monitoring should be performed following the rulemaking on these new standards. Therefore, the wet weather monitoring requirements at these locations( C+ and E) have been eliminated, while wet weather monitoring requirements at the industrial outfalls have been retained. A requirement to perform biological monitoring in Third Creek at E on DIA property has been added.

Monitoring point E was selected as a future biological monitoring site because the completed Receiving Water Study identified the existence of a persistent fathead minnow population in this area. The Division determined that a one time fish survey should be performed to demonstrate fish community persistence as a means of measuring attainment of the aquatic life beneficial use in this waterbody. The survey shall be conducted according to the following parameters:   Sampling will occur in September to avoid impacting early life stages that would likely be present between April and July, and avoid presence of ice;   Sampling will occur in the reach at monitoring point E using a seine of an appropriate mesh size to capture adult fathead minnow;   Appropriate sampling protocols/ measurements will be performed to allow the calculation of fish density ( number of fish/ square meter);   Captured fish will be weighed and measured before being returned to the stream; and   Fish density and fish measurements will be collected for informational purposes only; the goal of the survey is to determine the presence of fish, which would demonstrate fish community persistence.

DIA requested that the A discharge monitoring point be relocated for safety reasons. The Division concurred with this request. The new location is shown in Figure 5.

DIA requested that the measurement frequency for Third Creek outfalls be reduced from once every two months to quarterly. DIA has amassed a substantial amount of data for the Third Creek industrial outfalls and the receiving water. As described in Section VIII of the Rationale, below, the Division has determined that sampling once every two months is appropriate at this time for the characterization of seasonal differences in storm events that result in the discharge of stormwater associated with industrial activity, in particular aircraft deicing.

DIA requested that the permit state that repeat visits to discharge outfalls are unnecessary if there is no discharge from the outfall to surface waters resulting from a qualifying event. DIA's basis for the request was that the lack of discharge from an event was itself a characterization of the event. DIA noted that many factors could cause a lack of discharge from a qualifying storm event, including infiltration, evaporation, sublimation, and diversion into the spent ADF management system. The Division determined that additional attempts to meet the wet weather monitoring requirements should be made, following a qualifying event that does not result in discharge, as the lack of discharge could simply be due to irregular precipitation patterns. COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment # 2 Rationale  Page 3, Permit No. COS 000008 DIA requested that the requirement that wet weather monitoring samples be taken during the first hour of the discharge be modified to require that the sample be representative of the first flush of storm water runoff. This request was made on the basis that stormwater runoff in the winter months often does not occur until hours or days following the storm event. The language in the permit has been revised in response to this request.

III BEST MANAGEMENT PRACTICES ( BMPs)

DIA requested that the BMPs contained in Part I. D. 2 of the permit be removed, and that such BMPs regarding operation of the spent ADF management system be included in the SWMP. In recent years, DIA has made several requests to the Division for operational flexibility by requesting a variance from the BMPs contained in the permit. Often these requests have been made during or immediately following unique storm events, in response to which DIA has stated that different operating scenarios for the spent ADF management system are appropriate. The Division determined that, while some changes are warranted, it is important that the permit dictate some guiding principals regarding operation of the spent ADF management system. Therefore, several BMPs that apply to the system as a whole have been retained. The Division has agreed to remove other BMPs from the permit that specifically describe operational procedures. Such BMPs or their equivalent will be included in the SWMP. As per Part I. C of the permit, implementation of the BMPs in the SWMP is a permit requirement. The Division will review the revised SWMP, and may require additional measures as needed to prevent and control pollution, based on the SWMP review and/ or additional information received.

IV SYSTEM CAPABILITIES DIA requested that it be explicitly stated in the permit that stormwater associated with industrial activity may contain residual levels of spent ADF that migrate beyond the capture system. DIA also requested that it be stated that the spent ADF management system has a finite capacity, and that stormwater associated with industrial activity, resulting from precipitation events that exceed the capacity of the system, has the potential to be discharged from areas within the capture system. The Division determined that these statements would be more appropriate for inclusion in DIA's SWMP than in the permit.

V SWMP REVISION DIA requested a period of 90 days after the effective date of the revised permit for updating the SWMP in final form. The requirement has been changed from the original 30 day deadline in the permit in response to this request.

VI. CHANGES TO PERMIT A change was made on Page 1 of the permit, to clarify that discharges are also authorized to Barr Lake Tributary and Hayesmount Tributary. On Page 4 of the permit, a change was made to clarify that discharges from Runway 35R flow to Hayesmount Tributary, and not directly to Box Elder Creek.

Pages 1, 4, 5, 7, 7a, 9, 10, 13, 14, 15, 17, 18 and 25 of the permit have been changed. Page 10a has been added ( but does not contain new information).

Kathryn Dolan December 4, 2003 COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment # 2 Rationale  Page 4, Permit No. COS 000008 VII. RESPONSE TO PUBLIC NOTICE COMMENTS The Division received comments on the proposed amendment from the Farmers Reservoir and Irrigation Company ( FRICO). Based on these comments, three changes/ clarifications were made to the amendment:

1. A concern was expressed about the potential impacts on Third Creek from the new Sixth Runway, since it was not fully operational at the time that the Receiving Water Study was conducted. A wet weather monitoring location has been added to evaluate the quality of stormwater runoff from the Sixth Runway. The sample location, within the southern portion of the runway, will be representative of runoff generated from areas beyond the limits of the edge drain collection system ( i. e., beyond the pavement area, in runway/ taxiway infield areas) and will also be representative of any runoff generated from within the collection area, which exceeds the collection capacity of the edge drain system. Figure 5 has been amended to show the new monitoring location. The parameters and monitoring frequency will be the same as for outfalls A, C and D.

2. The items to be included in the ADF capture efficiency annual report were questioned. The reporting elements have been further specified as the total amounts of ADF that are applied, recovered, treated ( via Metro), recycled, and not collected ( i. e., fugitive ADF).

3. A concern was expressed about the reduction in monitoring frequency at outfalls A, C and D, from once every two months to quarterly, and a requirement for Whole Effluent Toxicity ( WET) testing was suggested. The Division has determined that the proposal to decrease the wet weather monitoring may not be sufficient to provide a reasonable characterization of seasonal differences in storm events. The monitoring frequency will remain at every two months for now; this issue may be re examined when the permit is renewed. The merits of WET testing will also be considered at that time.

Kathryn Dolan April 8, 2004