Approaches to Defining TMDL Margins of Safety

One of the most important elements of a Total Maximum Daily Load (TMDL) is defining a reasonable margin of safety (MOS). The MOS is required by statute to “take into account any lack of knowledge.” It serves to protect beneficial uses from uncertainties in the data or calculations used to link pollutant sources to water quality impairment. Published guidance identifies two approaches for defining a MOS. An explicit MOS quantifies an allocation amount separate from other load and wasteload allocations. With an explicit MOS, numeric TMDL targets are established at more protective levels than the analysis results indicate. Explicit MOS can also incorporate reserve capacity for unknown population growth or effects of climate change on water quality. An implicit MOS is not specifically quantified but consists of statements of the conservative assumptions used in the analysis approach. A wide range of approaches are used to define an implicit MOS.In this study, we reviewed several States' established TMDLs and compiled a list of different MOS approaches used. Results of the review show that a wide range of approaches are used to define both explicit and implicit MOS. Definition of a critical condition used to define an implicit MOS is given special consideration, due to the guidance stating how critical conditions should be used in TMDLs. Seasonality specifications are also examined separately, based on TMDL guidance. We examine the other commonly undefined elements of an implicit MOS, including safety factors incorporated into setting numeric water quality standards.Further evaluation of MOS approaches for TMDLs of four beneficial use impairments is presented: pathogen indicators, toxic substances, eutrophication, and high stream temperature. Case studies for each impairment type are presented demonstrating approaches for quantifying an implicit MOS. The estimated quantified implicit MOS are evaluated in comparison with the implicit conservative assumptions.Defining a MOS adequate to protect beneficial uses involves not only an understanding of the TMDL analysis uncertainty, but also policy decisions on the level of protection needed. Due to increased pressure to establish TMDLs, analyses are often conducted with insufficient data. The uncertainty of the analysis needs to be communicated to policy makers and the public so that risks to the resource are well understood. This study recommends TMDL analyses provide a more explicit MOS by presenting quantified uncertainty analyses of the conservative assumptions used. The study also recommends that additional national guidance on defining MOS be developed for TMDL practitioners.