Application of Load Allocation Tools for the Development Of Total Maximum Daily Loads (TMDLS) for Bacteria in Texas: Comparison Between Empirical and Dynamic Modelling Approaches

The development of TMDLs in Texas to address recreational use impairments has employed several different approaches. The two primary tools used in the quantification, allocation and source identification of loadings to impaired waterbodies include load duration methods (basic) and dynamic modeling with HSPF (complex). These methods both require the acceptance of assumptions concerning the interactions of bacteria with the physical environment. Each of the methods has unique strengths and weaknesses which primarily result from the inherent complexity of each approach. The use of HSPF affords a higher degree of resolution in defining specific sources within the watershed while the load duration methods only provide the sources at the most basic level of the TMDL. Examples of the application of each tool in nonurban watersheds in Texas indicate that both methods produce expected high percentages of bacteria sources arising from nonpoint sources. Both methods also result in reduction requirements of more than 50% to meet water quality standards. The selection of either approach should address uncertainty as well as consider the objectives of the TMDL and magnitude of the problem. Involvement of stakeholders will be essential in the development and application of the tools and increase the probability that the TMDL and subsequent implementation will be successful.