Risk-Based Margins of Safety for Phosphorus TMDLs in Lakes

Abstract

A total maximum daily load (TMDL) under Section 303(d) of the U.S. Clean Water Act is the maximum amount of pollutant that a water body can assimilate without violating applicable water quality standards (WQSs). The TMDL regulatory process includes development of limitations for estimated pollutant sources and an allowance for a margin of safety (MOS). This paper describes a methodology for determining the MOS on the basis of the classical engineering concept of a design safety margin, the difference between the expected capacity and expected load. In risk-based design, the design safety margin is selected to produce an acceptable failure risk over a design period. This principle can be readily applied to MOS determination, as illustrated here by application to phosphorus TMDLs in Massachusetts. The application includes evaluation of adopted MOS values and development of risk-based MOS values at specified levels of exceedance risk. The risk-based MOS values varied widely from the established MOS values, illustrating a type of inconsistency produced by current approaches to selecting the MOS.