171-Watershed Water Quality Attainment Using TMDL—A Delaware USA Review.

Abstract

The impaired waters of the watershed streams and/or tributaries, etc., in the USA would be included in the Section 303 (d) List of the Clean Water Act (CWA), and a Total Maximum Daily Load (TMDL) needs to be developed and promulgated under the regulatory requirements of 40 CFR Part 130—Water Quality Planning and Management. The 40 CFR Part 130 Section 130.7—Total Maximum Daily Loads (TMDL) And Individual Water Quality-Based Effluent Limitations, establishes the contaminant discharge limitations toward a sustainable-attainment of the state and federal set watershed water quality standards. A TMDL analysis was accomplished using a hydrodynamic and water quality model Generalized Environmental Modeling System for Surface waters (GEMSS), to verify the effectiveness of prescribed point and nonpoint source load reductions to meet the TMDL objectives. A new and improved GEMSS model, a union of 1-dimensional streams and the 3-dimensional river and bays, was used to project water quality conditions as a result of point and nonpoint source load reductions. The point and nonpoint source nutrient reduction loads prescribed in the 1998 TMDL analysis of the State of Delaware's Indian River, Indian River Bay, and Rehoboth Bay, were applied to the entire watershed and water quality effects were examined. Modeled nutrient concentrations were compared to water quality standards and nutrient 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.