A Review of the Technical Approaches Used for TMDLs Development in Mississippi

Abstract. A TMDL (Total Maximum Daily Load) is the calculation of the maximum amount of a pollutant that a waterbody can receive and still safely meet water quality standards. Section 303(d) of the Clean Water Act and the EPA’s Water Quality Planning and Management Regulations require states to identify and list water quality limited waterbodies, those not meeting state water quality standards, within their boundaries, to prioritize them, and to develop and document TMDLs for the pollutants of concern. In many cases, mathematical models are used during TMDL development, such as to evaluate the relationship between load reduction and compliance with water quality standards. A wide range of models have been used, ranging from simple mass balance calculations to complex integrated watershed, hydrodynamic, and water quality models. While there is considerable guidance on models that may be used in the TMDL process, there is relatively little documentation summarizing which models have been used. A study was conducted to identify the different technical approaches that have been used for TMDL development in Mississippi. The identification is based upon reviewing 253 TMDL reports approved by EPA, which are available on the MDEQ’s web page. The water quality modeling approach most commonly used, as reported in 131 surveyed documents (52% of the sample), was the estimation of the TMDL using a simple mass balance equation. Subsequently, for sediments, in 45 of the surveyed TMDLs (18% of the sample), TMDLs were estimated based on reference sediment yields, or targets for each level ecoregion III within Mississippi. STREAM, NPSM, and WASP were used in 33, 16, and 13 TMDL reports (i.e. 13%, 6%, and 5%, respectively). The remaining 6% of the surveyed documents (15 TMDLs) reported the use of AWFWUL1 (3 TMDLs), BATHTUB (3 TMDLs), flow duration curves (3 TMDLs), total toxicity approach (3 TMDLs), AWFWIV1 (1 TMDL), linear regression model (1 TMDL), and QUAL2E (1 TMDL). The type of model used varied with, among other factors, the pollutant of concern and complexity of the waterbody.