The Use of Solute-Transport Models to Estimate Time-Varying Nitrogen Loading to Coastal Estuaries In Response to Wastewater-Management Actions, Cape Cod, Massachusetts

The disposal of waste-derived nitrogen within watersheds on Cape Cod, Massachusetts contributes to the eutrophication of coastal waters that can result in degradation of water quality and clarity, algal blooms, and the loss of eel-grass habitat necessary to support healthy ecosystems. The Massachusetts Department of Environmental Protection has initiated the Massachusetts Estuaries Project (MEP) to assist communities in developing wastewater-management strategies that will protect marine ecosystems by determining Total Maximum Daily Loads (TMDLs) for the State’s coastal waters. Cape Cod is of particular concern due to the region’s shallow permeable coastal aquifer, rapidly urbanizing watersheds, ecologically-sensitive estuaries, and reliance on fishing and tourism.

   The MEP uses a GIS-based approach wherein terrestrial nitrogen loads within a watershed are determined by mapping parcel-scale water-use data to the contributing area to the estuary determined using a steady-state groundwater-flow model; these terrestrial loads are used as input into a hydrodynamic model that simulates mixing and tidal-flushing to estimate nitrogen concentrations within the estuary at critical points. At present (2010), TMDLs have been determined for a number of estuaries and many communities are developing wastewater-management strategies to meet these thresholds. The response of estuarine water quality to a set of wastewater-management actions is a function of transport processes within the aquifer system. Although groundwater-flow models can be used to delineate watershed boundaries for a given set of hydrologic conditions, solute-transport models are needed to calculate complex, time-varying nitrogen loads to an estuary in response to a set of sequential wastewater-management actions. Specifically, solute-transport models can incorporate dynamic changes in watershed boundaries, transport time through the aquifer, and physiochemical processes such as dispersion and attenuation.