IMPROVED METHODS FOR CALCULATING THE TMDL MARGIN OF SAFETY

The Margin of Safety (MOS), designed to account for uncertainty in TMDL calculations, is a required element of a TMDL. It is recognized (e.g. NRC 2001) that the Margin of Safety component is not currently being addressed in a rigorous manner. The Water Environment Research Foundation (WERF) sponsored research to improve the TMDL process; the work described here represents the aspect of that research devoted to improved methods for calculating the Margin of Safety.The problems that presently inhibit the application of the Margin of Safety were investigated and divided into three categories:Limited practical experience in defining TMDL uncertaintyAbsence of policy regarding the desired degree of protectionPractical limitations of data poor situationsThis project then defined the attributes of an improved Margin of Safety approach that would meet the statutory requirements of the TMDL rules, as well as address the problems addressed above:Account for uncertainty in TMDL calculationsConsider degree of protection:Address data limitation/implementation concerns:Be feasible to conduct on a widespread basisAn improved method for incorporating the MOS into the TMDL process was developed to adhere to the above attributes. The method consists of first defining the desired level of protection to be provided by the MOS, which requires policy input. The second step consists of selecting between a range of implicit and explicit approaches for determining the MOS. Guidance is provided on explicitly defining the MOS associated with implicit approaches. Finally, the implementation feasibility of the resulting MOS must be considered, with a potential outcome of the process being an Adaptive Management approach to the TMDL.