TMDL MODELING OF DELAWARE'S INLAND BAYS

Water quality monitoring and assessment studies suggest high concentrations of nitrogen and phosphorus within the Delaware Inland Bays located in Sussex County in southern Delaware. These nutrients are essential for both plants and animals of the Inland Bays. However, in large quantities they may negatively impact the ecology of the bays. Some symptoms of nutrient over enrichment are excessive macroalgae growth (sea lettuce and other species), phytoplankton blooms (some potentially toxic), large daily swings in dissolved oxygen levels, loss of submerged aquatic vegetation (SAV), and fish kills.Therefore, DNREC in 1998 adopted a Total Maximum Daily Loads (TMDL) regulation for nitrogen and phosphorous for the estuarine portions of the Indian River, Indian River Bay, and Rehoboth Bay. The 1998 TMDL Regulation, which required significant reduction of nutrient loads from point and nonpoint sources, did not include the Little Assawoman Bay or the freshwater streams and ponds which were on the State's 303(d) list of impaired waters. In this study, the efficacy of the load reductions called for by the 1998 TMDL Regulation for meeting water quality standards in the impaired waters was examined. In addition, the TMDLs for Little Assawoman were established.This analysis was accomplished using a hydrodynamic and water quality model called the Generalized Environmental Modeling System for Surface waters (GEMSS) constructed by J. E. Edinger, Associates, to verify the effectiveness of prescribed point and nonpoint source load reductions to meet the TMDL objectives. The model improved upon the 1998 TMDL analysis by including the connection between the Indian River to Little Assawoman Bay via the Little Assawoman Canal, as well as by joining the surrounding associated streams and ponds on the State's 303(d) list. The new model, a union of 1-dimensional streams and the 3-dimensional river and bays, was then used to project water quality conditions as a result of point and nonpoint source load reductions.Modeled concentrations were compared to water quality standards and nutrient target values. Using the GEMSS model, the point and nonpoint source nutrient reduction loads prescribed in the 1998 TMDL analysis of the Indian River, Indian River Bay, and Rehoboth Bay, were applied to the entire watershed and water quality effects were examined. As required under the 1998 TMDL report, all nutrient point source loads were reduced to zero. Nutrients, chlorophyll a, and dissolved oxygen levels averaged over the summertime critical time period in the streams and ponds within the Inland Bays watershed were compared to target values. Similarly, nutrients and dissolved oxygen concentrations in the tidal portions of the Inland Bays averaged during the critical time period were compared against standards while chlorophyll a was compared against its target values. The results of the model runs showed that implementation of the load reductions required by the 1998 TMDL Regulation to the entire watershed would result in achieving all applicable water quality standards and target values.