Sources of Nitrogen Polluting East Falmouth’s Coastal Ponds

 

            Few people think twice when they flush their toilets or spread fertilizers on their lawns. For the most part, this is a natural thing to do. But for those people who are concerned about the health of East Falmouth’s coastal ponds, a second thought is in order.

 

Septics Systems and Lawns:

            Waste-water from septic systems and groundwater leached from fertilized lawns contain relatively large amounts of nitrogen, a nutrient that is so over-abundant in these ponds that it is severely degrading their water quality.

            Septic systems and fertilizers from home lawns contribute more than two thirds of the total amount of nitrogen that enters East Falmouth’s Great, Green, and Bournes Ponds.  In contrast, the amount of nitrogen that enters naturally from rainwater and other atmospheric deposition accounts for less than 30% of the total. The Ashumet Nitrogen Plume, when it begins to affect the ponds, will only add 2-3% more nitrogen to the total load. The plume emanates from the Massachusetts Military Reservation, which for years had treated waste-water in rapid infiltration beds that leached nutrients directly into the groundwater.

 

CMAST Model:

            These numbers come from a model developed by Dr. Brain Howes at the Center for Marine Science and Technology [CMAST] at University of Massachusetts, Dartmouth.  CMAST is one of the environmental consultants hired by the Town in 1998 to work with the Ashumet Plume Citizens Committee to analyze the causes, consequences, and possible remediation of the nitrogen-overloading problems in Great, Green, and Bournes Ponds. The purpose of the model was to quantify the amount of nitrogen entering the ponds from various sources within the watershed, thereby identifying the main culprits causing the degrading water quality and helping to frame remedial action.

            The first step in developing the model was to create a detailed map of how land is used throughout the watersheds of the three ponds. Aerial photographs, assessor maps, and the town’s new Geographic Information System [GIS] database provided the locations of houses and buildings, forests and fields, roads and driveways, golf courses, cranberry bogs, and any other forms or structures that could influence the amount of nitrogen entering the watersheds that feed the coastal ponds.

 

Land Uses:

            Each type of land use discharges a characteristic amount of nitrogen into the groundwater. That nitrogen is introduced either naturally from atmospheric deposition or by being added via septic systems and fertilizer applications.

Atmospheric deposition falls uniformly on all surface features at the rate of 11.1 kilograms per hectacre [kg/ha] or roughly 10 pounds of nitrogen per acre.  How much of that reaches groundwater depends on the surface.  On the salt ponds themselves, the rate is 11.1 kg/ha, of course.  Elsewhere, forests and natural fields absorb most of the nitrogen so only 0.57 kg/ha reaches groundwater.  Conversely, 8 kg/ha reaches groundwater from run-off from roofs and unpaved driveways, while pavement also collects nitrogen from automobile exhausts and wildlife and so feeds groundwater at the rate of 15 kg/ha.

As for septic systems and lawns, the average home in the watershed has three bedrooms, is occupied by less than 2 people on an average annual basis [including year-round and seasonal use] and has a 5,000 square foot lawn.  The nitrogen load from the septic system of each house is 3.34 kg /yr and each lawn [averaging fertilized and non-fertilized usage] contributes another 1.1 kg/yr.  So, in addition to run-off, each of the 6300 homes in the three-pond watershed contributes some 41/2 kilograms or 10 pounds of nitrogen each year to the groundwater.   The nitrogen-contribution rates for golf courses and cranberry bogs are 30 kg/ha and 23 kg/ha, respectively.

            Multiplying these specific discharge rates by the total area under each type of land use yields an estimate of the total amount of nitrogen that enters the watershed each year.

 

Attenuation:

            Not all of the nitrogen that enters the watersheds of the ponds actually makes it into the salt ponds themselves, however. Some nitrogen is lost as it travels from its various sources to the salt ponds. ‘Lost’ can mean converted to gas and released into the atmosphere, bound to soil particles like sediment in the bottom of freshwater ponds and streams, or taken up by plants growing in or near those water bodies.   Those losses collectively are called attenuation.

            Attenuation is particularly significant in the upper portions of the watersheds, north of Route 28. Significant amounts of run-off and groundwater from the upper watersheds collect into freshwater streams and ponds that attenuate incoming nitrogen concentrations by 30-43%, depending on the relative size of the freshwater bodies in each watershed.  Those factors were verified by taking samples of nitrogen concentrations in freshwater immediately north of the salt ponds.  Were it not for such natural filtration, the nitrogen load coming into the three salt ponds from their upper watersheds would amount to 21,650 kilograms per year instead 12,650 kilograms per year.

In contrast, nitrogen that enters the groundwater from sources in the lower watershed of the salt ponds discharges directly into those ponds without significant attenuation.  Because there also are more houses and lawns in the lower watersheds, the nitrogen load from the lower watersheds presently amounts to 26,000 kilograms per year.

 

Future Nitrogen Loading:

            The CMAST model also forecasts how much nitrogen will enter the salt ponds from the same types of sources at full build-out, that is, when all of the lots that can be developed under current zoning laws are actually developed. Adjusting the model to make the forecast was a simple step: change the land-use map from one that reflects current development to what land-use will be like when every buildable lot in the three-pond watershed has a house on it [assumed to be have the same characteristics as today].  Full build-out will increase the number of homes to almost 7,750, an increase of more than 1400 or 22% from today.  Because most of those new homes will be built in the upper watersheds, however, the nitrogen load will not increase proportionately.

            The other change is the impact of the Ashumet Nitrogen Plume.  Based on a model developed earlier by Jacobs Engineering for the US Air Force, the Plume eventually will increase nitrogen loading by 2% and 3% in Great Pond and Green Pond, respectively.  Otherwise, all other estimates of nitrogen impacts from sources like atmospheric deposition, golf courses and cranberry bogs are assumed to be the same as in 1999.

            At full build out, then, nitrogen loading will increase to a total of more than 44,350 kilograms per year, up about 5750 kilograms or nearly 15% from 1999.  The increases by pond will be: 21% for Bournes, 16% for Great and 13% for Green Pond.  Home septic systems and lawns will account for 68% of the nitrogen load in Green Pond, 66% of the nitrogen load in Great Pond and 64% in Bournes Pond.

 

Implications:

            While the future will see further nitrogen overloading and further degradation in water quality, most of the causes of poor water quality already exist in today’s homes and lawns.  If nitrogen loading is going to be reduced sharply enough to restore reasonably healthy ponds, most of the nitrogen content of septic waste will have eliminated and all homes will have to use much less fertilizer. Hooking many homes to a sewage treatment plant that removes nitrogen from the waste-water would reduce the septic waste component, although that will be expensive. Reductions in fertilizers will happen if citizens make the conscious decision to do so and then stick with it; such action will save rather than cost money to achieve, and can accomplished as soon as the reductions occur.

            The Selectmen negotiated an excellent contract with the US Air Force, which has pledged $8.5 million to the Town for measures to offset the Ashumet Nitrogen Plume.  The Plume will add only 2-3% to the nitrogen load, and the funds pledged would pay most costs to sewer enough homes to remove about 5% of the nitrogen load.  Unfortunately, the nitrogen pollution problem is far too large for a 5% reduction to solve.  Last November, Town Meeting endorsed the need to identify more comprehensive solutions, and future articles in this space will examine what those solutions might be.