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Introduction to TMDLs

West Metro Water Alliance A Path to Clean Water – Understanding TMDLs and Watershed Planning September 21, 2011. Introduction to TMDLs. Diane Spector Wenck Associates, Inc. Outline. What is a TMDL? What are the most common impairments? How are TMDLs calculated?

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Introduction to TMDLs

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  1. West Metro Water Alliance A Path to Clean Water – Understanding TMDLs and Watershed Planning September 21, 2011 Introduction to TMDLs Diane Spector Wenck Associates, Inc.

  2. Outline • What is a TMDL? • What are the most common impairments? • How are TMDLs calculated? • What is the difference between a wasteload and load and why should I care? • Biotic impairments: fish and bugs matter

  3. 1972 Clean Water Act • 1972 Federal Water Pollution Control Act was intended: • “… to restore and maintain the chemical, physical, and biological integrity of the Nation’s waters.”

  4. Minnesota Impaired Waters As of 2008 • 2,575 total impairments • Restored 14 impairments (2 lakes and 12 river segments) • 680 TMDLs in development (excl toxics) • 220 approved TMDLs in implementation In the Upper Mississippi River Basin • 69 rivers and 263 lakes are impaired • 468 TMDL studies needed

  5. Common Impairments-2010 list • 1,774 impairments on 388 rivers and 647 lakes. • 3,049 total impaired waters

  6. What is a TMDL? • A Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards, and an allocation of that amount to the pollutant sources.

  7. TMDL – A Number • WLA = Wasteload Allocation (attributed to existing or future permitted sources) • LA – Load Allocation (attributed to existing or future non-permitted sources) • MOS – Margin of Safety • RC – Reserve Capacity (Future Capacity)

  8. How are TMDLs calculated? Identify and measure or quantify sources of pollutant load • Watershed runoff • Point sources • Septic systems • Lake bottom sediments • Stream sediment oxygen demand • Atmospheric deposition

  9. How are TMDLs calculated? Model watershed hydrology and pollutant loading • Hydrologic and water quality model: P8, XP-SWMM, SWAT, HSPF • Predicted runoff volume based on actual precip • Pollutant load

  10. Modeled Sediment Load by Land Cover

  11. Total Phosphorus Budget Bald Eagle Lake

  12. How are TMDLs calculated? Model lake or stream response • Lake response model: BATHTUB, MNLEAP • Streams: QUAL2K • Inputs: annual runoff volume, load, lake volume, depth, stream morphometry, fitting factors • Adjust factors until model predictions match actual observed data

  13. How are TMDLs calculated? Using calibrated model, reverse calculate maximum load

  14. TMDL: Bald Eagle Lake

  15. What is the difference between Load and Wasteload? • A Wasteload is a permitted source • Industrial point source • Construction permit • MS4s: stormwater • Wasteload allocations are regulated in your NPDES Phase I and II permits • Load allocations are not regulated by permit: internal load, natural processes, atmospheric deposition, rural runoff

  16. Wasteload Allocations • Individual allocations • One or more permitted sources are allocated their own wasteload and reduction • Categorical allocations • Groups of permittees are allocated wasteloads and reductions

  17. Biotic Impairments • Measure with an Index of Biotic Integrity • IBI reflects composition and health of the biotic community in comparison to a reference • Summary of metrics, e.g.: • # target species • % community composition by target species • % or # pollution intolerant or tolerant species • # individuals or species

  18. Biotic Impairments • First step is Stressor Identification study • Often identify stressors other than conventional pollutants • May be a numeric TMDL or surrogate such as flow or impervious cover • May also include non-numeric targets

  19. Shingle Creek Biota TMDL Numeric: • Sediment oxygen demand load allocations, channel reshaping standards • Chloride wasteload allocations Non-numeric: • Specific habitat enhancements • Removal of fish barriers • Desirable flow duration curve

  20. A note on process • The purpose of a TMDL is to identify the load reductions necessary to restore water quality and biotic integrity • Also includes general implementation actions that will likely help meet those targets • Not expected to “solve the problem” • Implementation Plan contains more detailed actions

  21. Thank you!

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