Wastewater Problem? Just Plant a Marsh


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No one is suggesting that the plant kingdom isn't doing its fair share. All those leafy creatures are photosynthesizing their virtual hearts out producing the oxygen that keeps the rest of us going. They filter the air, and they prevent erosion by slowing the speed of incoming raindrops and by holding the dirt together with their root systems. Plants provide timber for houses and the furniture that fills them, pulp for paper (a tree died so you could read this column) and flowers for hospital patients. Tens of millions of acres produce the grains that feed our cattle, hogs and chickens.

For ourselves, plants are both sustenance and beauty, in either order. The news is that the plant world can do lots more for us: it can become an industrial partner, one that can clean up our mess.

The concept is known as phytoremediation, phyto being the Greek word for plant. Microbes are already being used to clean up such nasty things as oil spills, under the more general category called bioremediation (Smithsonian, April 1993). Now artificial marshes are being planted to further cleanse the effluent from sewage treatment plants or to make the water draining from abandoned mines less lethally acidic. Specially selected species are being planted on land that has been contaminated with one or more heavy metals. Some plants will accumulate the metals--which tend to be very toxic--in their tissue. These may not only solve the problem but may become a cash crop: in the case of zinc and cadmium, the plants harvested from just one acre could be worth several hundred dollars. Still other plants will remove metals from the soil, convert them into gases and release them into the atmosphere (though this presents its own risks).

Metals are not the only contaminants that plants can help clean up. They can accumulate or break down organic (carbon-containing) compounds. Poplars planted in strips will stop plumes of underground water contaminated with petroleum. The trees will also accumulate the hydrocarbons that make up the petroleum, removing them from the ground. Poplars, and a number of other plants, can also break down compounds like TNT (left behind when the Army burns or detonates obsolete munitions) into harmless, inert compounds. There are even plants that will accumulate radioactive nuclides: Science News reported last year that sunflowers floating on rafts with their roots dangling in the water are being used to remove cesium 137 and strontium 90 from a pond at Chernobyl. The plants themselves become radioactive waste, which can be handled, and the water is cleansed. It costs $6 or less per thousand gallons of water, far less than the more "advanced" technologies that exist.

In most cases, these processes are more complicated. Suppose, for example, you have a piece of land that is saturated with lead. In normal soil, no plant will take up much of that lead. But if you amend the soil with a substance that will bind to the lead, the resulting compound will be taken up nicely by Indian mustard (Brassica juncea). Environmental Science and Technology recently reported that on a New Jersey site where batteries once were made, the lead was nearly gone after just one summer of this treatment.

The potential cost savings are attractive. Cleaning dirt on-site can cost $10 to $100 per cubic meter. Taking it off-site to clean can cost three times as much. Tending the plants doing phytoremediation comes to about a nickel a cubic meter. The financial incentive can also lead to environmental largess. New York City has decided to spend nearly $1 billion to protect the Catskill watershed that naturally purifies most of its drinking water. A treatment plant to purify the water would cost $4 billion to $6 billion to build.

What attracts me most, however, is the idea of turning a messy problem into a garden. Take acid mine drainage. It contaminates streams from Pennsylvania to Alabama. The process of mining exposes pyrite (fool's gold) to air and water. Iron and sulfur in the mineral are transformed into iron-laden sulfuric acid. The acid then dissolves other metals--such as manganese and aluminum--from the surrounding rock. Water from rainfall and underground streams washes these contaminants into streams, turning them bright orange and red. The acidity is so strong that it kills fish and aquatic vegetation. The coal industry has been spending more than $1 million per day to treat the effluent with alkaline chemicals, such as lime. But the late, lamented Bureau of Mines had a better idea. The bureau began encouraging the planting of artificial wetlands, using primarily the common cattail Typha latifolia, in places where the acid drainage would go through them. A typical site costs $20,000 to build. It can reduce cleanup costs by $20,000 to $60,000 a year compared with the industry's usual practices.

Here's my favorite example of plants coming to the rescue. According to the trade journal Land and Water, the Indian Creek Nature Center in Cedar Rapids, Iowa, was in a bind. They had planned for an annual attendance of fewer than 10,000 visitors a year and they were getting more than 40,000. The septic system was overloaded. The center is located at the confluence of Indian Creek and the Cedar River, surrounded by flood-prone land, which meant that a conventional leaching field was not possible. They decided to build wetlands instead. An engineering firm donated the services of an engineer who had designed wetland systems for years. Foundations put up the money, the Cedar Rapids Parks Department did the grading, and members of the Cedar Rapids Garden Club volunteered to buy and plant a wetland garden.

The waste stream first goes to a conventional septic system. But then the effluent goes through three basins. The first two are filled with pea gravel. Water does not reach the surface, but wetland plants extend their roots into the dirty water. The combined surface area of the gravel and the root systems of the plants provides a substrate for the bacteria that break down the sewage. The water, now clean, then flows into a third basin, a pond, which has inadvertently become the biggest attraction at the center.

That description is not very inviting. But consider what you would see if you were there. The first basin is planted with cattails and bulrushes. In the second are growing arrow arum, blue and yellow iris, water plantain, cardinal flower, great blue lobelia, ironweed, swamp milkweed, and sweet and marsh blazing-star liatris. In the pond itself are water plantain, arrow arum, sweet flag, marsh marigold, lizard's tail and arrowhead. Along its banks are slender goldenrod, grass-leaved goldenrod, pale purple coneflower, yellow coneflower, sky-blue asters and shrubs. It's really just a sewage treatment plant, and yet I find myself wanting to be there.


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