Risk-Based Margins of Safety for Phosphorus TMDLs in Lakes
Publication: Journal of Water Resources Planning and Management
Volume 137, Issue 3
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.
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Acknowledgments
UNSPECIFIEDNortheastern University provided funding for portions of the work summarized in this article. Informal review comments from Bruce Jacobs and Peter Shanahan and review comments from two anonymous reviewers improved the manuscript.
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Information
Published In
Journal of Water Resources Planning and Management
Volume 137 • Issue 3 • May 2011
Pages: 276 - 283
Copyright
© 2011 American Society of Civil Engineers.
History
Received: Feb 25, 2009
Accepted: Jun 5, 2010
Published online: Jun 24, 2010
Published in print: May 1, 2011
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