NITROGEN LOADING FROM COASTAL WATERSHEDS TO RECEIVING ESTUARIES - NEWMETHOD AND APPLICATION

In this paper we develop a model to estimate nitrogen loading to water sheds and receiving waters, and then apply the model to gain insight a bout sources, losses, and transport of nitrogen in groundwater moving through a coastal watershed. The model is developed from data of the W aquoit Bay Land Margin Ecosystems Research project (WBLMER), and from syntheses of published information. The WBLMER nitrogen loading model first estimates inputs by atmospheric deposition, fertilizer use, and wastewater to surfaces of the major types of land use (natural vegetat ion, turf, agricultural land, residential areas, and impervious surfac es) within the landscape. Then, the model estimates losses of nitrogen in the various compartments of the watershed ecosystem. For atmospher ic and fertilizer nitrogen, the model allows losses in vegetation and soils, in the vadose zone, and in tile aquifer. For wastewater nitroge n, the model allows losses in septic systems and effluent plumes, and it adds further losses that occur during diffuse transport within aqui fers. The calculation of losses is done separately for each major type of land cover, because the processes and loss rates involved differ f or different tesserae of the land cover mosaic. if groundwater flows i nto a freshwater body, the model adds a loss of nitrogen for traversin g the freshwater body and then subjects the surviving nitrogen to loss es in the aquifer. The WBLMER model is developed for Waquoit Bay, bur with inputs for local conditions it is applicable to other rural to su burban watersheds underlain by unconsolidated sandy sediments, Model c alculations suggest that the atmosphere contributes 56%, fertilizer 14 %, and wastewater 27% of the nitrogen delivered to the surface of the watershed of Waquoit Bay, Losses within the watershed amount to 89% of atmospheric nitrogen, 79% of fertilizer nitrogen, and 65% of wastewat er nitrogen. The net result of inputs to the watershed surface and los ses within the watershed is that wastewater becomes the largest source (48%) of nitrogen loads to receiving estuaries, followed by atmospher ic deposition (30%) and fertilizer use (15%). The nitrogen load to est uaries of Waquoit Bay is transported primarily through land parcels co vered by residential areas (39%, mainly via wastewater), natural veget ation (21%, by atmospheric deposition), and turf(16%, by atmospheric d eposition and fertilizers). Other land covers were involved in lesser throughputs of nitrogen. The model results have implications for manag ement of coastal landscapes and water quality. Most attention should b e given to wastewater disposal within the watershed, particularly with in 200 m of the shore. Rules regarding setbacks of septic system locat ion relative to shore and nitrogen retention ability of septic systems , will be useful in control of wastewater nitrogen loading, Installati on of multiple conventional leaching fields or septic systems in high- flow parcels could be one way to increase nitrogen retention. Control of fertilizer use can help to a modest degree, particularly for option al uses such as lawns situated near shore. Conservation of parcels of accreting natural vegetation should be given high priority, because th ese environments effectively intercept atmospheric deposition, Areas u pgradient from freshwater bodies should be given low priority in plans to control nitrogen loading, because ponds intercept much of the nitr ogen transported from upgradient.