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University of Wyoming


Understanding Pollutants Within Natural Systems

To effectively monitor changes in pollutants of most concern, it is extremely important to understand how these pollutants typically behave in natural systems and, in particular, in specific watersheds. Equally important is an understanding of how these pollutants might respond to specific BMPs. Many of the pollutants BMPs are intended to address, such as sediment and nutrients, are found naturally in surface and groundwater, and they only become problems at high concentrations that impair the beneficial uses of these water bodies. An effective monitoring program explicitly considers how these materials may change as they move from a source into the ground or surface water. This includes an understanding of how a given pollutant may be introduced or mobilized within a watershed, how it moves across the land and within surface or groundwater, and the transformations that may occur during this process.

Before any successful monitoring or modeling takes place, consider the following:

  1. What is the pollutant of concern? Will the monitoring focus on a chemical or physical pollutant (such as phosphorus or sediment), a characteristic of the water (such as temperature), or a living organism (such as bacteria or a type of algae)?

  2. If the focus is on a particular pollutant, is its source clearly known (point or nonpoint) and is it understood how the pollutant is transported from the source to the water? As one very simple example, pollutants may be found in multiple forms (dissolved, absorbed, adsorbed, or particulate). Dissolved forms may move easily in groundwater or subsurface flows, while particulate forms will only be transported in surface runoff.

  3. What BMPs are available to address the pollutant(s) of concern, and how will this affect a monitoring program? BMPs may be quite site-specific or may cover quite extensive areas. Consider also that BMPs may have unintended consequences that should be monitored, such as increased infiltration that reduces surface runoff but could increase subsurface or groundwater contamination.

  4. What is the pollutant’s fate and transport within a given watershed? How is the pollutant processed and transformed in transit to a water body and once it is in the water? For example, is it taken up by plants and transformed to a different form, does it readily adsorb to soil particles, is it in a particulate form that may settle out before reaching a water body, is it extremely soluble and does it move anywhere water goes, or does it float on the surface?

  5. How will natural variability in the pollutant of concern be accounted for? Likewise, how will natural variability in climatic conditions affect streamflow and temperature? It is important to understand other natural changes in the system throughout a season. For example, during fall and winter, organic materials break down and release dissolved nutrients, so a nutrient monitoring plan should anticipate this natural increase.

  6. Long-term changes in a watershed might mask or affect the response to a BMP implementation. Some monitoring approaches are better than others at controlling for changes in land uses, periods of drought, recent introduction of invasive species, or other long-term impacts.

Monitoring in a variable world:

BMPs are designed and implemented to address water quality concerns or impairments. As the ability of a specific BMP to mitigate water quality impairment in a given situation is often unknown or uncertain, it is imperative to assess the effectiveness of a BMP in addressing a specific issue. In BMP effectiveness monitoring, the objective is typically to assess and/or demonstrate the impact of the BMP on mitigating a water quality issue of concern. The ease with which this is done will depend on the magnitude of this impact relative to background conditions and background “noise,” such as natural variability of flow in the intermountain west. The range of natural variability in the system must be taken into consideration because this variability may mask any change resulting from BMP implementation. As a general rule, more frequent samples are needed in a highly variable system, but, by targeting the sampling timing, the monitoring program may be greatly improved. Section 10 contains additional information on handling natural variability in detecting change.

Dramatic changes in chemical concentrations or physical properties of water often occur naturally, so it is important to understand this variability in a system. A common mistake in monitoring programs is to interpret these natural changes as having resulted from human impacts. In particular, a common mistake is to interpret a short-term naturally occurring reduction in a chemical concentration as a response to a BMP. See Section 11 for more information on how to interpret data.

Predictable seasonal changes:

Depending on the specifics of the impairment, monitoring may not be necessary at all times of the year. Many of the chemical, physical, and biological changes in a stream are a result of streamflow, so it is particularly important to understand the hydrology of the watershed and water body. Some pollutants of concern may only be transported during baseflow when the system may be more dominated by groundwater, while others are transported only during snowmelt or storm events. Concentrations may vary dramatically during the year depending on location and stream system, geology and geographic location, or the watershed.

The impairments in a water body may not be apparent during all seasons, so it may not be necessary to sample year round. For example, intensive monitoring of water temperature during the winter is not useful if the problem is high temperatures during summer low-flow conditions. As another example, total annual loads of sediment are often delivered primarily during spring runoff and storm events, in which intensive monitoring during baseflow may not be worth the time or money.

Daily changes:

Daily (diel) changes in sunlight may directly affect plant and animal behavior and air and water temperature. For example, drifting of aquatic macroinvertebrates can be significantly different between night and day. Behaviors of fish and some zooplankton are also driven by light conditions. Plants respond significantly to night and day change. Photosynthesis by aquatic plants may increase pH during the day. Oxygen, on the other hand, may drop significantly at night because of plant respiration in the absence of photosynthesis. Flow may also change between night and day, as increased daytime temperatures result in increased snowmelt and downstream flows.

Storm events:

Many nonpoint source particulate pollutants (such as sediment and E. coli) may be transported into waterways primarily during storm events or snowmelt periods that generate surface runoff. In some systems, these short-term, episodic conditions may be the most critical periods to monitor. See Section 7 (Protocols) for more information on monitoring approaches such as continuous monitoring or flow-triggered monitoring devices that can be used in situations such as these.

Management changes:

River systems are subject to sudden changes as a result of different types of river management. In the West, irrigation diversion and return flows may result in significant changes in river flow or concentrations of sediment, nutrients, and salts. Upstream reservoirs may result in modified runoff flows, releases to support downstream irrigation, or releases for power generation. Be aware of these upstream influences when designing a monitoring plan.