Simple Tools for Water Quality Modeling and TMDL Development

Water quality models are being used to determine pollutant loads to water bodies and for developing Total Maximum Daily Loads (TMDLs). Watershed, stream, and reservoir water quality models include a wide range of models that can be applied to virtually every water body type (including estuaries, bays, lakes, bayous, reservoirs, major rivers, streams, and ponds), pollutant type (from nutrients, dissolved oxygen [DO], and bacteria to PCBs, metals, pH, and pesticides) and source type (including urban, agricultural, mining, and forestry). Modeling selection and criteria are based on the water body type and behavior, pollutants and processes of concern, source types, and other defining factors identified during the characterization of the watershed. The vast majority of these models are complex and require a significant amount of input data and time. When modeling water bodies with limited data, simple modeling tools such as load duration curves, BATHTUB, and QUAL2K are more effective. Load duration curves are a simple and accurate method of determining existing and allowable pollutant loads in streams. Because the approach establishes loads based on a representative flow regime, it inherently considers seasonal variations and critical conditions attributed to flow regimes. BATHTUB is a steady state model that predicts eutrophication response in lakes and is commonly used for simulating nutrient loads. QUAL2K is a one-dimensional steady state river and stream water quality model widely used for simulating dissolved oxygen and pH. Load duration curves, BATHTUB, and QUAL2K are water quality models that are efficient and simple to use, and provide adequate results in simulating pollutant loads, loading capacity of water bodies and pollutant load reductions required to meet water quality standards.