The Evolution of the TMDL

Since its original inception, the Clean Water Act has included a comprehensive approach to protecting water quality, involving basin-wide assessments and the application of a range of regulatory and non-regulatory strategies to reduce pollution. At the heart of this comprehensive approach is the Total Maximum Daily Load (TMDL), a waterbody-specific pollution discharge limit. The current edition of Water Practice™ is devoted to the evolution of these limits and was derived from the Water Environment Federation specialty conference TMDL 2007. This biennial conference fell, coincidentally enough, on the 35th anniversary of the 1972 Clean Water Act. Just as significantly, it has been about 10 years since the TMDL program really started to accelerate, spurred primarily by litigation that forced states to start developing TMDLs under court ordered schedules.

Intrinsic to the TMDL is the consideration of both the point and non-point pollutant sources, as well as natural sources of the constituent. While the “clean water industry” embraced the implementation of point source controls early, dealing with stormwater and societal practices leading to nonpoint sources remains a challenging management and policy issue for many municipalities and regulators. In keeping with the evolving nature of this area, the TMDL policy and science topics that we find important are both old and new, both long-standing and unanticipated.

In the arena of policy issues, we are still promoting water quality criterion flexibility using Use Attainability Analyses (UAA) – something that was identified as a top research need when WERF conceived its fledgling research program in 1991. This has evolved into promoting more adaptive implementation strategies to better address the natural variability in measuring restoration goals. In this current issue, Freedman et al. elaborate on the particulars of this approach. New to the policy table are fundamental legal challenges concerning what the “Daily” in TMDL really means. 

On the scientific and technical front, issues include developing numeric criteria for nutrients and bacteria contamination. While bacterial criteria are not new, measuring DNA and tracing the source of bacteria is improving, while the control of bacterial sources is getting more creative. For a memorable example, there is a community pilot testing a program for the control of pet waste in homeowner's back yards. In the current issue, Stiles & Tate look at bacterial TMDLs in the context of recreational stream usage and Stringfellow et al. marshal support for the development of a dissolved oxygen TMDL.

Given the delayed advancement of the TMDL program, only lately has there been enough progress to better assess and quantify how states are proceeding. Critically, we still lack a large number of implementation case studies, though our body of knowledge is improving. In this issue, Benham et al. outline some of the studies that are available and discuss what may be learned from them. Overall, WEF’s biannual TMDL conference provides an important overview of the status of the program, allowing water professionals to share needed practical information and interact constructively with their colleagues. As the past Co-chair of the TMDL 2005 and 2007 specialty conferences it has been very rewarding and a privilege to work with the professionals who contribute their time and talents to this organization. In keeping with the success of the TMDL conference, this edition’s selection of papers helps to meld the old with the new and to provide a road map of where we have gone and how we can eventually realize the ambitious clean water goals envisioned over 35 years ago.

Mitchell Griffin, Issue Editor
CH2M HILL