Total Maximum Daily Load (TMDL) program David Croxton U.S. Environmental Protection Agency Watershed Unit Manager– EPA Region 10
What is a TMDL? A Total Maximum Daily Load (TMDL) is a calculation of the maximum amount of a pollutant that a water body (or segment) can receive and still meet the state/tribe’s water quality standards. TMDLs are required by the Clean Water Act when a water body’s designated uses are determined to be impaired or threatened. The pollutants causing or expected to cause the impairment are what are limited under a TMDL.
What is a TMDL? (cont’d) A TMDL is the sum of the following: Waste load allocation (WLA). Total amount of the pollutant from existing point sources (e.g., sewage treatment plant, industrial facility, stormwater). Load allocation (LA).Total amount of pollutant from existing nonpoint sources and natural background (e.g., farm runoff, atmospheric mercury). Margin of safety (MOS).The TMDL calculation must include a margin of safety that takes into account any lack of knowledge concerning the relationship between effluent limits and water quality.
What is a TMDL? (cont’d) Regulations also require the consideration of Seasonal Variation. How does the variation of the seasons affect loading, streamflow, and biological processes? Critical Conditions for Stream Flow and Loading. E.g., critical low flow is often represented by the 7Q10 flow (statistical estimate of the lowest 7-day average flow expected to occur once every ten years) and conservative meteorological conditions (90th percentile). Reasonable Assurance. For TMDLs that allocate loads to point sources, there must be reasonable assurance that non-point source reductions will in fact occur.
TMDL Allocation Explicit MOS Point Source 1(WLA) Reserve Capacity Point Source 2(WLA) Allocation Example Natural Background Point Source 3(WLA) NPS Source Category 3(LA) NPS Source Category 1 (LA) NPS Source Category 2(LA) 5
From Washington Technical Advisory Group Built Intrastructure Report (Dec. 2010) – Appendix C
What will be Success for this Pilot? Overall our goal is to determine the science needs to support potential future changes into TMDL analyses. Learn/adapt Begin with one program to learn which impacts are most relevant and identify knowledge gaps; adapt this process for other programs Identify needed tools, data sources, models, etc., and where they are available Federal partnerships in the region, e.g., C3, will be useful Climate Science Center and Climate Decision Support Center, Landscape Conservation Cooperatives Identify policy issues and decisions
Potential Policy Issues Is climate change part of “background” conditions that aren’t assigned to any source category to address (e.g., lower flows)? How much can the “margin of safety” be used to account for climate change uncertainty versus uncertainty of the models themselves? Can /should we leverage more stringent limits or move to watershed-scale permits for point sources, to get more investments in green infrastructure in the watershed?
Lake Champlain Phosphorus TMDL 2002 – Approved by EPA 2008 – Complaint filed by Conservation Law Foundation 2010 – EPA signs Settlement Agreement