Improved Consideration of the Margin of Safety in Total Maximum Daily Load Development

Abstract

The margin of safety (MOS), designed to account for uncertainty in calculations, is a required element of a total maximum daily load (TMDL). It is widely recognized that the margin of safety component is not currently being rigorously addressed throughout the TMDL program. This work investigates the existing barriers to rigorous consideration of the margin of safety, and presents a method designed with attributes to address the existing problems. Rigorous application of the margin of safety is presently inhibited by: (1) limited practical experience in defining TMDL uncertainty; (2) absence of policy regarding the desired degree of protection to be provided by the MOS; and (3) data-poor/high-uncertainty situations with MOS values so large as to make implementation impractical. The method provided here 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. The final step considers the implementation feasibility of the resulting MOS, with a potential outcome of the process being an adaptive management approach to the TMDL.