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Understanding WLA Implementation in NPDES Permits: A Primer for TMDL Developers

Understanding WLA Implementation in NPDES Permits: A Primer for TMDL Developers. U.S. Environmental Protection Agency. Module Roadmap. What is the regulatory context for implementing TMDL requirements in NPDES permits? How are effluent limits developed from wasteload allocations (WLAs)?

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Understanding WLA Implementation in NPDES Permits: A Primer for TMDL Developers

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  1. Understanding WLA Implementation in NPDES Permits: A Primer for TMDL Developers U.S. Environmental Protection Agency

  2. Module Roadmap • What is the regulatory context for implementing TMDL requirements in NPDES permits? • How are effluent limits developed from wasteload allocations (WLAs)? • How might other NPDES permit components be affected by a TMDL? • What are some technical and policy challenges related to implementing TMDLs in NPDES permits?

  3. What is a TMDL? • Greatest amount of loading that a water can receive without violating water quality standards (i.e., loading capacity). [40 CFR 130.2(f)] TMDL =SWLAi + SLAi + MOS SWLAi: Sum of waste load allocations for existing and future point sources SLAi: Sum of load allocations for existing and future nonpoint and background sources MOS: Margin of safety

  4. Typical Components of a TMDL • WLAs set pollutant loading cap for point sources • LAs set pollutant loading cap for nonpoint sources • Reserve capacity sets aside allocation for future development • Margin of Safety (MOS) allocation accounts for uncertainty Reserve Capacity Point Source #1 MOS Point Source #2 Nonpoint Source #2 Nonpoint Source #1

  5. EPA Recommended Elements in a TMDL Submittal TMDL Process 1. Description of waterbody, POC pollutant sources, and priority ranking Problem Understanding TMDL Target Identification Source Assessment 2. Water Quality Standards and numeric WQ target* • Loading Capacity- Linking WQ and Sources* (including critical conditions*) Linkage between Loading and Waterbody Response 4. Load Allocations (LAs)* 5. Wasteload Allocations (WLAs)* 6. Margin of Safety (MOS)* 7. Seasonal Variation* Stakeholder Involvement & Public Participation Allocation Analysis 8. Reasonable Assurances+ 9. Monitoring Plan+ 10. Implementation Plan+ Implementation and Monitoring Plan TMDL Report and Submittal 11. Public Participation* * Required by regulation (40 CFR 130.7) + Recommended through guidance From Guidelines for Reviewing TMDLs under Existing Regulations issued in 1992 (May 20, 2002): http://www.epa.gov/owow/tmdl/guidance/final52002.html

  6. NPDES Statutory and Regulatory FrameworkCWA section 301(a) and 40 CFR 122.1(b) • All point sources • Discharging pollutants • Into waters of the United States Must obtain an NPDES permit from EPA or an authorized state, territory, or tribe

  7. What is a Permit? • A permit is a license that is • issued by the government • granting permission to do somethingthat would be illegal in the absence of the permit (e.g., driver’s license) • There is no right to a permit and it is revocable for cause (e.g., reckless driving) • An NPDES permit is a license to discharge and can be revoked for cause

  8. Federal and State Roles • Before NPDES program authorization: • EPA issues permits • EPA conducts compliance and monitoring activities • EPA enforces • State must grant or waive CWA section 401 certification • After NPDES program authorization: • State implements as above • No CWA section 401 certification is required • EPA oversees program administration

  9. Permit Components Components of All Permits Industry-Specific Components Municipal-Specific Components Cover Page Effluent Limitations • Effluent Guidelines • Case by Case Limitations • Secondary • Equivalent to Secondary Technology-based Water Quality-based Monitoring and Reporting Special Conditions Additional Monitoring / Special Studies • Pretreatment • Municipal Sewage Sludge • CSOs Best Management Practices Compliance Schedules Standard Conditions

  10. Types of NPDES Permits—Individual • 1 application submitted 1 permit issued • Appropriate where facility-specific permit conditions (e.g., effluent limitations, management practices, monitoring and reporting) are needed Application Individual Permit

  11. Types of NPDES Permits—General • Authorized at 40 CFR 122.28 • 1 permit issued many Notices of Intent submitted • Appropriate where • multiple, similar sources (e.g., same category, similar process) within the same geographic area require permit coverage • sources have similar discharges and would require the same or similar permit conditions • Permit must identify: • area of coverage • sources covered • NOI process NOIs General Permit

  12. TMDLs and Permit Components Components of All Permits Industry-Specific Components Municipal-Specific Components Cover Page Effluent Limitations • Effluent Guidelines • Case by Case Limitations • Secondary • Equivalent to Secondary Technology-based Water Quality-based Monitoring and Reporting Special Conditions Additional Monitoring / Special Studies • Pretreatment • Municipal Sewage Sludge • CSOs Best Management Practices Compliance Schedules Standard Conditions

  13. Relationship Between WQ Standards and Effluent Limitations Numeric WQ Criteria • Magnitude • Duration • Frequency • Narrative WQ Criteria • Effluent Limitations • Magnitude • Averaging Period Permit writers calculate end-of-pipe water quality-based effluent limitations where necessary to ensure that water quality standards are attained in the receiving water.

  14. Where do TMDLs fit in?

  15. 40 CFR 122.44(d)(1)(vii) (vii) When developing water quality-based effluent limits under this paragraph the permitting authority shall ensure that: • The level of water quality to be achieved by limits on point sources established under this paragraph is derived from and complies with all applicable water quality standards; and • Effluent limits developed to protect a narrative water quality criterion, a numeric water quality criterion, or both, are consistent with the assumptions and requirements of any available wasteload allocation for the discharge prepared by the State and approved by EPA pursuant to 40 CFR 130.7.

  16. What is a Wasteload Allocation? • Wasteload Allocation (WLA): the portion of a receiving water’s loading capacity that is allocated to one of its existing or future point sources

  17. WLA from a TMDL • WLA = portion of the receiving water’s TMDL allocated to a specific point source

  18. Calculating WQBELs from WLAs • Calculating Effluent Limits • TMDLs are often comprised of load allocations for non-point sources as well as WLAs point sources • Loadings can include more constant discharges from point sources and dynamic discharges from non-point sources due to, for example, stormwater events • It is important to determine how the WLA is expressed in the TMDL

  19. Calculating WQBELs from WLAs • Assume the daily loading for the TMDL is equivalent to a maximum daily limit (MDL) as expressed in an NPDES permit. • So, starting from the WLA in the TMDL, what is the process to calculate the MDL that would go in a NPDES permit? • The MDL is developed as follows: • The WLA is used to calculate the long term average (LTA). • The MDL is calculated from the LTA.

  20. Calculate LTAs CV = 0.6 WLA = 50 mg/L = 99th percentile value LTA = 50 mg/L x _______ = 16.05 mg/L WLA multipliers [0.52 - z] e CV 95th 99th percentile percentile 0.1 0.853 0.797 0.2 0.736 0.643 0.3 0.644 0.527 0.4 0.571 0.440 0.5 0.514 0.373 0.6 0.468 0.321 0.7 0.432 0.281 0.8 0.403 0.249 0.9 0.379 0.224 1.0 0.360 0.204 1.1 0.344 0.187 1.2 0.330 0.174 1.3 0.319 0.162 1.4 0.310 0.153 1.5 0.302 0.144 1.6 0.296 0.137 1.7 0.290 0.131 1.8 0.285 0.126 1.9 0.281 0.121 2.0 0.277 0.117

  21. Calculate the MDL LTA multipliers ] [ z - 0.5 2 • MDL • MDL = 99th percentile value • CV = 0.6 • MDL = 16.05 mg/L x • = 50 mg/L e 95th 99th CV percentile percentile 0.1 1.17 1.25 0.2 1.36 1.55 0.3 1.55 1.90 0.4 1.75 2.27 0.5 1.95 2.68 0.6 2.13 3.11 0.7 2.31 3.56 0.8 2.48 4.01 0.9 2.64 4.46 1.0 2.78 4.90 1.1 2.91 5.34 1.2 3.03 5.76 1.3 3.13 6.17 1.4 3.23 6.56 1.5 3.31 6.93 1.6 3.38 7.29 1.7 3.45 7.63 1.8 3.51 7.95 1.9 3.56 8.26 2.0 3.60 8.55

  22. Steps in Developing Chemical-SpecificWQBELs from Aquatic Life Criteria Step 1: Determine Wasteload Allocation (WLA) from Aquatic Life Water Quality Criterion Step 2: Calculate Long-Term Averages (LTAs) and Select Lowest Step 3: Calculate Maximum Daily Limit (MDL) and Average Monthly Limit (AML)* *Other averaging periods used where appropriate (e.g., instantaneous maximum and instantaneous minimum for pH)

  23. WLA from TMDL Widgets-R-Us Nonpoint Source ABC, Inc. WLA Smalltown POTW Nonpoint Source Acme Gadgets Water body of Concern

  24. WLA from a TMDL • WLA = portion of the TMDL allocated to a specific point source

  25. WLA for a Single Facility • Local water quality impacts • Single point source • Other sources considered in “background” • Determine the maximum allowable discharge (i.e., the WLA) for the single facility

  26. Determining WLAs Mass-Balance Equation: QsCs + QdCd = QrCr • Q = Flow (mgd or cfs) • C = Pollutant concentration (mg/l) • Mass = [Concentration] [Flow] • To determine the WLA: QrCr-QsCs Qd WLA = Cd =

  27. Calculating WQBELs from WLAs • WLAs represent a point source’s allocated share of the waterbody’s pollutant loading and should, as appropriate, be translated into a WQBEL for that source • WLAs • derived directly from water quality criteria through TMDLs, watershed analyses, or facility-specific analyses • might be expressed as daily allocation or have the same duration as criteria • WQBELs • derived from WLAs using EPA or state-specific limit development procedures • must be consistent with assumptions used to derive applicable WLAs [40 CFR 122.44(d)(1)(vii)(B)] • typically have different duration/averaging period than WLAs

  28. Calculating WQBELs from WLAs • Follow permitting authority procedures to calculate WQBELs from WLAs • permitting authority regulation, policy, or guidance • EPA’s Technical Support Document for Water Quality-based Toxics Control (TSD) • uses a statistical analysis that assumes effluent data follow a lognormal distribution

  29. Steps in Developing Chemical-SpecificWQBELs from Aquatic Life Criteria Step 1: Determine Wasteload Allocation (WLA) from Aquatic Life Water Quality Criterion Step 2: Calculate Long-Term Averages (LTAs) and Select Lowest Step 3: Calculate Maximum Daily Limit (MDL) and Average Monthly Limit (AML)* *Other averaging periods used where appropriate (e.g., instantaneous maximum and instantaneous minimum for pH)

  30. Key Terms • Lognormal Distribution: the probability distribution of any random variable whose logarithm is normally distributed • Relative Frequency: the fraction or ratio of the number of observations in a category or class to the total number of observations Lognormal Distribution Relative Frequency 0 Concentration

  31. Key Terms (continued) • Long-term Average (LTA): for a continuous random variable (in our case, pollutant concentration), the value at which the area under the distribution curve to the left of the value equals the area under the distribution curve to the right of the value • Coefficient of Variation (CV): a statistical measure of the relative variation of a distribution or set of data (in our case, pollutant concentrations) calculated as the standard deviation divided by the mean

  32. Calculate LTA • Assume a lognormaldistribution of effluent data • WLA is never to be exceeded • Characterize never to be exceeded by an upper bound value(e.g., WLA is the 99th percentile concentration on the lognormal effluent distribution) • Knowing the WLA and a (coefficient of variation) CV, calculate the LTA

  33. Distribution that Meets WLA Requirement Goal: Reduce Concentration Below WLA Relative Frequency X 0 WLA Concentration

  34. Characterize Distribution by LTA and CV Relative Frequency CV X 0 WLA LTA Concentration

  35. Calculate LTAs WLA multipliers 2 [ • Chronic • CV = 0.6 • WLA(chronic) = 1.5 mg/L = 99th percentile value • LTA(chronic) = 1.5 mg/L x _______ • = 0.79 mg/L ] 0.5  - z e CV 95th 99th percentile percentile 0.1 0.922 0.891 0.2 0.853 0.797 0.3 0.791 0.715 0.4 0.736 0.643 0.5 0.687 0.581 0.6 0.644 0.527 0.7 0.606 0.481 0.8 0.571 0.440 0.9 0.541 0.404 1.0 0.514 0.373 1.1 0.490 0.345 1.2 0.468 0.321 1.3 0.449 0.300 1.4 0.432 0.281 1.5 0.417 0.264 1.6 0.403 0.249 1.7 0.390 0.236 1.8 0.379 0.224 1.9 0.369 0.214 2.0 0.360 0.204

  36. Steps in Developing Chemical-SpecificWQBELs from Aquatic Life Criteria Step 1: Determine Wasteload Allocation (WLA) from Aquatic Life Water Quality Criterion Step 2: Calculate Long-Term Averages (LTAs) and Select Lowest Step 3: Calculate Maximum Daily Limit (MDL) and Average Monthly Limit (AML)* *Other averaging periods used where appropriate (e.g., instantaneous maximum and instantaneous minimum for pH)

  37. Calculate MDL and AML • Use the lognormal distribution to calculate the MDL and AML from the lowest LTA • uses upper-bound estimates for both MDL (e.g., 99th percentile) and AML (e.g., 95th percentile) • ties AML to planned frequency of monitoring • allows comparison to technology-based limits

  38. Calculate MDL and AML from the LTA CV Relative Frequency 0 LTA AML MDL Concentration

  39. Step 4: Calculate MDL and AML LTA multipliers ] [ z - 0.5 2 • MDL • MDL = 99th percentile value • CV = 0.6 • MDL = 0.58 mg/L x • = 1.8 mg/L e 95th 99th CV percentile percentile 0.1 1.17 1.25 0.2 1.36 1.55 0.3 1.55 1.90 0.4 1.75 2.27 0.5 1.95 2.68 0.6 2.13 3.11 0.7 2.31 3.56 0.8 2.48 4.01 0.9 2.64 4.46 1.0 2.78 4.90 1.1 2.91 5.34 1.2 3.03 5.76 1.3 3.13 6.17 1.4 3.23 6.56 1.5 3.31 6.93 1.6 3.38 7.29 1.7 3.45 7.63 1.8 3.51 7.95 1.9 3.56 8.26 2.0 3.60 8.55

  40. [z - 0.52 n n Step 4: Calculate MDL and AML LTA multipliers ] e AML AML Value = 95th percentile CV = 0.6 Number of = 8 (assume 2x/wk-Samplessampling) AML = 0.58 mg/L x = 0.80 mg/L 95th percentile 99th percentile CV n=1 n=2 n=4 n=8 n=30 n=1 n=2 n=4 n=8 n=30 0.1 1.17 1.12 1.08 1.06 1.03 1.25 1.18 1.12 1.08 1.04 0.2 1.36 1.25 1.17 1.12 1.06 1.56 1.37 1.25 1.18 1.08 0.3 1.55 1.38 1.26 1.18 1.09 1.90 1.59 1.40 1.27 1.13 0.4 1.75 1.52 1.36 1.25 1.12 2.27 1.83 1.55 1.37 1.18 0.5 1.96 1.66 1.45 1.31 1.16 2.68 2.09 1.72 1.48 1.23 0.6 2.13 1.90 1.55 1.38 1.19 3.11 2.37 1.90 1.59 1.28 0.7 2.31 1.94 1.65 1.45 1.22 3.56 2.66 2.08 1.71 1.33 0.8 2.48 2.07 1.75 1.52 1.26 4.01 2.96 2.27 1.83 1.39 0.9 2.64 2.20 1.85 1.59 1.29 4.46 3.28 2.48 1.96 1.44 1.0 2.78 2.33 1.95 1.66 1.33 4.90 3.59 2.68 2.09 1.50 1.1 2.91 2.45 2.04 1.73 1.36 5.34 3.91 2.90 2.23 1.56 1.2 3.03 2.56 2.13 1.80 1.39 5.76 4.23 3.11 2.37 1.62 1.3 3.13 2.67 2.23 1.87 1.43 6.17 4.55 3.34 2.52 1.68 1.4 3.23 2.77 2.31 1.94 1.47 6.56 4.86 3.56 2.66 1.74 1.5 3.31 2.86 2.40 2.00 1.50 6.93 5.17 3.78 2.81 1.80 1.6 3.38 2.95 2.48 2.07 1.54 7.29 5.47 4.01 2.96 1.87 1.7 3.45 3.03 2.56 2.14 1.57 7.63 5.77 4.23 3.12 1.93 1.8 3.51 3.10 2.64 2.20 1.61 7.95 6.06 4.46 3.28 2.00 1.9 3.56 3.17 2.71 2.27 1.64 8.26 6.34 4.68 3.43 2.07 2.0 3.60 3.23 2.78 2.33 1.68 8.55 6.61 4.90 3.59 2.14

  41. Calculated WQBELs • We calculated the following WQBELs: Maximum Daily Limit = 1.8 mg/L Average Monthly Limit = 0.80 mg/L

  42. Recap of WLA to WQBEL Examples • Understand your state’s procedures for calculating WQBELs • Not all states follow EPA’s TSD methodology • WLAs – are derived directly from water quality criteria • Expression - concentration or load • Duration - daily, monthly or even annual • Both can vary by TMDL • Most WQBELs for continuous, non-municipal discharges are expressed as average monthly limits and maximum daily limits

  43. What challenges arise when implementing WLAs from TMDLs in an NPDES permit?

  44. NPDES Effluent Limitations and TMDLs: Implementation Challenges • Different WLAs for the same point source • Seasonal or annual WLAs • Varying discharge and receiving water flows • WLAs in general permits • New and expanding discharges

  45. Different WLAs for the Same Point Source Widgets-R-Us Nonpoint Source ABC, Inc. WLA? Receiving Water Smalltown POTW Nonpoint Source Acme Gadgets Water body of Concern

  46. Different WLAs for the Same Point Source (continued) • WQBELs must be derived from and comply with ALL applicable water quality standards 40 CFR 122.44(d)(1) vii) When developing water quality-based effluent limits under this paragraph the permitting authority shall ensure that: • The level of water quality to be achieved by limits on point sources established under this paragraph is derived from and complies with all applicable water quality standards

  47. NPDES Effluent Limitations and TMDLs: Implementation Challenges • Different WLAs for the same point source • Seasonal or annual WLAs • Varying discharge and receiving water flows • WLAs in general permits • New and expanding discharges

  48. Seasonal or Annual WLAs • Continuous discharges • 40 CFR 122.45(d) requires average monthly and average weekly or maximum daily limits as the norm • Use TSD procedures (modified as appropriate) to calculate effluent limits from WLAs < 30 days duration • EPA has made exceptions where WLAs are longer-term • “Chesapeake Bay Annual Nutrient Limits Memorandum” http://www.epa.gov/reg3wapd/npdes/pdf/ches_bay_nutrients_hanlon.pdf

  49. Seasonal or Annual WLAs (continued) • Annual or seasonal WLAs could be implemented as effluent limits • Annual or seasonal loading requirement • Annual or seasonal average concentration • Supplement with limits or milestones that have shorter averaging periods as appropriate • See Draft Options for Expressing Daily Loads in TMDLs (USEPA 2007) http://water.epa.gov/lawsregs/lawsguidance/cwa/tmdl/upload/2007_06_26_tmdl_draft_daily_loads_tech-2.pdf • Use TSD procedures (modified as appropriate) to calculate effluent limits from WLAs < 30 days duration

  50. Seasonal or Annual WLAs (continued) • Non-continuous discharges • 40 CFR 122.45(e) requires that discharges be particularly described and limited considering, as appropriate • Frequency • Total mass • Maximum rate of discharge of pollutants • Prohibition or limitation by mass, concentration, or other measure

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