Constructed Wetlands

For decades we have understood that the primary reason for the decline in the health of Chesapeake Bay is that too much nitrate and phosphate are being added to the water. Acting as fertilizers, nitrate and phosphate promote the prolific growth of tiny suspended algae. The tiny plants cloud the water and reduce light penetration, thus preventing the growth of desirable bottom-dwelling plants. There are not enough organisms like oysters to eat all the suspended algae, so the short-lived plants die and accumulate on the bottom. In summer, rapid microbial decomposition of the dead plants can lead to anoxia (loss of dissolved oxygen in the water), stressing or even killing animals.

Agriculture is the largest source of nitrate and phosphate to the Bay. In the Northern Neck the nutrients are delivered to local waterways by the discharge of shallow groundwater. Each day more than 2 million liters (approximately 600,000 gallons) of water is discharged from every square mile of the Northern Neck. On average, each liter of water contains about 4 milligrams of nitrate (4 ppm). Moving underground slowly, this massive flow of water is not easily recognized. Runoff, though easily recognized, is a much less significant process unless the plant cover over our porous soils is compromised. This kind of non-point-source pollution by groundwater is difficult to reduce, although nutrient management plans for both nitrate and phosphate, 100 foot strips of trees bordering waterways, and septic system maintenance are effective tools.

Sewage treatment plants are the second largest source of nitrate and phosphate to Chesapeake Bay. Their effluent discharges directly to rivers or to the Bay itself. The technology exists to remove large amounts of nitrate and phosphate from the waste-water stream of sewage treatment plants. Installation of “tertiary” treatment would nearly double the cost of waste-water treatment for most citizens. Obviously, dealing with a point-source of nitrate and phosphate from a pipe is much easier than dealing with widespread polluted groundwater caused by disseminated agricultural and homeowner practices.

There are three stages to sewage treatment, whether it is accomplished by a municipal plant or by an on-site system:

1) Primary treatment (like a septic tank) removes solids so that the effluent water can be oxidized without clogging up the pipes. The solids constitute the sludge we are trying to keep off our fields.

2) Secondary treatment (like a drain-field) oxidizes the water, killing pathogenic (disease-causing) organisms and destroying odoriferous compounds like ammonia and hydrogen sulfide. Nitrogen and phosphorous in the water are oxidized to nitrate and phosphate. Most sewage treatment systems stop with secondary treatment.

3) Tertiary treatment removes most of the nitrate and phosphate from the wastewater stream. We have only recently realized the importance of adding tertiary treatment to the sewage treatment process in order to minimize environmental damage.

Chesapeake Bay Foundation has compiled the discharge from sewage treatment plants in the Bay watershed (www.cbf.org/site/PageServer). How does the Northern Neck rank? Terrible! Four of our five plants (Reedville, Kilmarnock, Warsaw and Colonial Beach) are unacceptable, discharging water containing more than 10 ppm nitrate. Montross, discharging water only slightly higher in nitrate than our groundwater, “needs improvement.”

What should be none? Our wastewater plants are small and contribute miniscule amounts of nitrate and phosphate to the Bay in comparison with large municipalities. That said, it is also true that plants which discharge into small waterways can have a massive effect on that waterway. The Callao sewage treatment plant, presently in the planning stages, will undoubtedly further degrade the water quality in Lodge Creek. The roots of trees and other plants, together with microbial reactions, are currently consuming some of the nitrate and phosphate discharged from scattered drain-fields in the watershed. This will no longer be true once the treatment plant is on-line and the effluent is piped directly into Lodge Creek.

There is a form of tertiary treatment, best adapted to small plants like ours, that is inexpensive and nearly maintenance-free. “Constructed wetlands” are simply shallow, wide, gravel-filled trenches in which wetland plants are established. The gravel beds receive the permitted effluent from secondary sewage treatment by the treatment plant. As the wetland plants grow, they consume some of the nutrients. More important, however, are reactions that take place year-round below the surface of the gravel bed. Microbes convert nitrate to harmless nitrogen gas, just as happens in our natural marshes (and another reason to treasure and improve our marshes.) Inorganic reactions at the surface of the gravel particles remove large amounts of phosphate. One of the best plants to use in constructed wetlands is Phragmites! Phragmites grows fast, crowds out other plants and is not eaten by anything. The attributes that make Phragmites desirable for constructed wetlands are the same ones that make it undesirable in natural marshes.

Good stewardship means minimizing the impact of our wastes on the environment. Maintenance of septic systems, including inspection and tank pump-out (if necessary) is one way for all of us to accomplish this goal. Upgrading our existing sewage treatment facilities to state-of-the-art nutrient removal, especially for plants that discharge directly into small waterways, is also important if we really want to try and improve the water quality of local waterways.