USE OF NUMERICAL MODELS TO ESTIMATE NITRATE LOADING TO COASTAL ESTUARIES, CAPE COD, MASSACHUSETTS

The discharge of waste-derived nitrate into coastal waters near Cape Cod, MA, has had adverse effects, such as enhanced algal growth, degradation of water quality, and loss of eel-grass habitat, on ecosystems in several estuarine systems. Water-resources managers need modeling tools to estimate current nitrate loads to estuaries as well as future loads resulting from wastewater-management actions such as sewering and centralized wastewater disposal. Currently (2006), terrestrial loads of nitrate to the estuaries are estimated by using a mass-balance summation of loads in which watershed boundaries are delineated by particle-tracking in three-dimensional ground-water flow models, and nitrate inputs are estimated from parcel-scale water-use data. The estimation of nitrogen loads is an effort separate from the regional flow modeling and requires additional post-modeling, GIS-based analyses. In 2005, the U.S Geological Survey evaluated the utility of solute-transport methods in estimating nitrate loads to estuaries in central Cape Cod. This approach integrates flow and transport and allows for the direct estimation of loads by the model. Spatially variable nitrate input to a three-dimensional solute-transport model was estimated from the parcel-scale water-use data; nitrate was assumed to be transported conservatively through the aquifer. For a simple steady-state case, the solute-transport and mass-balance methods yielded similar loads to the estuaries. This result indicates that the detailed water-use data used in the mass-balance method could be adequately represented in the solute-transport model. The use of solute-transport models in evaluating complex, sequentially implemented actions to manage wastewater showed that the method has several advantages over mass-balance methods: 1) the direct simulation rather than post-processing analysis of continuous time-varying loads for sequential wastewater-management actions, 2) the ability to account for changing watershed boundaries in response to changing hydraulic stresses, and 3) the simulation of dispersive transport of mass in the aquifer.