Model Predicted Water Quality Response to Reductions in Inorganic and Organic Nitrogen Loading

摘要

As part of a total maximum daily load analysis of North Carolina's Neuse River Estuary, eutrophication modeling was conducted using a modified version of an existing two-dimensional, laterally-averaged model (CE-QUAL-W2). The model simulated conditions in the estuary for a 43-month period that included two episodes of extraordinarily high nutrient and freshwater loading. Phytoplankton blooms with chlorophyll-a concentrations in excess of 40 μg/l were seen in three of the four years simulated, while bottom water anoxia was seen intermittently in each of the four summers. The calibrated model was used to predict the water quality improvement in the estuary associated with a variety of nitrogen load reductions varying from 5% to 30%. For each scenario, an assumption was made as to how much of the reduction would come from dissolved inorganic, dissolved organic, and particulate organic nitrogen. Water quality improvement was quantified by comparing the predicted chlorophyll-a concentrations for the nutrient reduction scenarios to a case without nutrient reduction. Additional cases were run to investigate the extent to which changes in sediment quality, occurring over several years might produce an additional improvement in water quality by reducing sediment oxygen demand and benthically mediated nutrient recycling. As expected, reduced nitrogen loading produced lower water-column nitrogen concentrations and lower chlorophyll-a concentrations. The magnitude of change in chlorophyll-a concentration was surprisingly small, however, as compared to the magnitude of load reduction. The magnitude of change in chlorophyll-a concentration differed according to the apportionment of nitrogen load reduction. Chlorophyll concentrations were most sensitive to inorganic nitrogen reduction and least sensitive to organic nitrogen load reductions. Including sediment denitrification improved the fit between observed and predicted nitrate, chlorophyll-a, and dissolved oxygen concentrations. Denitrification was also found to produce a negative feedback in the system whereby long-term reductions in sediment organic matter concentrations associated with nutrient reduction produced very slightly higher chlorophyll-a concentrations as compared to corresponding cases that did not consider the changes in sediment organic matter concentrations.