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Session 8: Alternate TMDL Endpoints

Session 8: Alternate TMDL Endpoints. Virginia Tech Biological Systems Engineering Department DEQ TMDL Modeling Workshop; Richmond November 19-20, 2013. Objective. To illustrate several procedures used to derive sediment TMDL endpoints in Virginia: Reference Watershed Approach

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Session 8: Alternate TMDL Endpoints

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  1. Session 8: Alternate TMDL Endpoints Virginia Tech Biological Systems Engineering Department DEQ TMDL Modeling Workshop; Richmond November 19-20, 2013

  2. Objective • To illustrate several procedures used to derive sediment TMDL endpoints in Virginia: • Reference Watershed Approach • The Disaggregate Method • The All-Forest Load Multiplier (AllForX) Approach

  3. Sediment is a major pollutant that results in non-attainment of the Aquatic Life Use standard EPA’s TMDL program requires pollutant load reductions that will be protective of beneficial uses of water, such as fishing, swimming, and drinking water There is no water quality standard for sediment in Virginia So, we need a way of defining acceptable stream sediment loads to know when we have achieved our water quality goals Why do we need to define in-stream sediment endpoints?

  4. Stream transport of sediment is a natural condition, which has been increased by the presence of humans Load from an all-forested watershed (“pristine” conditions) Load resulting from a reasonable level of BMP implementation (Nutrient Trading baseline or WIP) Load from a comparable watershed that is non-impaired (reference watershed approach) Load developed from multiple non-impaired watersheds with linkage to stream biology (AllForX approach) How much sediment is desirable?

  5. The Reference Watershed Approach

  6. Used in place of a numeric standard • Used to define the TMDL Load • TMDL Load = adjusted modeled load from the TMDL Reference Watershed Reference Watershed Approach

  7. Reference Watershed Selection • Non-impaired • Same eco- or subeco-region • Comparable land use distribution

  8. TMDL Target Load Assumption: Reducing load in the impaired watershed to the target TMDL load will restore the benthic community. Example Benthic TMDL Impaired Load Non-impaired Toms Brook Watershed TMDL Reference Watershed

  9. Toms Brook TMDL Reference Watershed Hays Creek (HYS) Toms Brook (TMB)

  10. Unit Area Load: • Calculate UAL for TMDL reference watershed (kg/ha-yr) • Multiply by the area of the impaired watershed (ha) • Area-Adjusted Load: • Scale the TMDL reference watershed to the area of the impaired watershed while maintaining its original land use distribution • Model the target TMDL load directly from the area-adjusted TMDL reference watershed (kg/yr) Calculation of Target TMDL Load

  11. Area-Adjustment HYSadj 4,252 ha HYS 20,789 ha TMB 4,252 ha

  12. Urban watersheds – few good references, if any. Watersheds with minor impairments – natural variability may obscure the need for load reduction. Selected reference watersheds have higher simulated loads than impaired watersheds. Reference loads could be too protective. Disadvantages to the Reference Watershed Approach

  13. The Disaggregate Method Approach

  14. Planning procedure • Relates Chesapeake Bay Watershed Model (CBWM) output with local land use data • Consistent with the Bay TMDL and WIPs • Produces reasonable target sediment loads • For biological impairments • At the sub-watershed level Disaggregate Method Intro

  15. Existing loads simulated with the CBWM 2009 scenario • Downstream TMDL loads identified in estuarine model • For each of 92 tidal segments • States identify types and extents of BMPs upland to achieve target loads • Watershed Implementation Plans (WIPs) • Upstream (EOS) loads simulated for the WIPscenario to meet downstream (DEL) TMDL loads in the Bay • Assumption: EOS loads that address downstream impairments will also address upstream impairments Chesapeake Bay TMDL Modeling

  16. Locate TMDL Sub-watershed in Relation to CBWM Model Segments Long Meadow Run watershed River Segment Land-River Segments

  17. CBWM Data • Edge-of-stream unit-area loads, e.g. tons/ac-yr • By landuse and model segment • Distribution of landuses within larger groups • Sub-watershed data • Local landuse • Match local and CBWM landuse groups • Baseline (2009) and Target (WIP) scenarios • WIP scenario Sediment Load = TMDL Disaggregate Method Overview

  18. Matching Landuses in DM Identify common land use groups. Preserve the areas of land use groups from the local level. Apply the CBWM distribution of categories within each group.

  19. Obtain Local Landuse • CBWM – Statistical Basis (Land-River Segment) • Local – Spatial Basis (Sub-watershed)

  20. Sub-watershed Load Calculation

  21. Sub-watershed Scenarios and Target Load

  22. Downstream reductions may not be sufficient to address the upstream impairment. Does not link local biological condition with sediment endpoint. Disadvantages to the Disaggregate Method Approach

  23. All-Forest Load Multiplier (AllForX) Approach

  24. Biology: Aquatic Life Use standard • Virginia Stream Condition Index (VSCI) ≥60 • Sediment: No in-stream WQ standard • Relationship between sediment and the in-stream biology • Measures: • Biology = VSCI • Sediment = All Forest Load Multiplier Basis for Setting Sediment Endpoints

  25. All Forest Load Multiplier (AllForX) Existing Condition All Forest Condition Existing Load All Forest Load AllForX =

  26. Impaired and Comparison Watersheds

  27. AllForX Regression B C A 13.64 B = AllForX value used for the TMDL; AC = the 80% Confidence Interval (shown in green); TMDL = AllForXB x All-forest load; MOS = (AllForXB – AllForXA) x All-forest load.

  28. Identify 3-6 nearby DEQ bio-monitoring stations that are non-impaired Simulate existing and all-forest loads, then AllForX Calculate average VSCI scores Develop regression between VSCI and AllForX Identify AllForX at the point where VSCI = 60 (AllForX60) Calculate 80% confidence intervals around AllForX60 (AllForXLCL,AllForXUCL) TMDL = All-Forest load x AllForX60 MOS = (AllForX60 – AllForXLCL)/AllForX60 AllForX Procedure Summary

  29. Correlation with Habitat Metrics

  30. AllForX Derivation of TMDLs and MOSs

  31. More directly links biological metrics with sediment Is not dependent on a single reference watershed Allows for a quantification of the margin of safety (MOS) in the TMDL equation Advantages of the AllForX Approach

  32. Impairment is for Aquatic Life Use • Implementation is planned around reductions of the surrogate stressor(s), e.g. sediment • The association between the stressor and the biology relies somewhat on best professional judgment • Identification of most probable stressor(s) • Selection of reference watersheds • Selection of modeling endpoint • The TMDL will be met • Not when the sediment load is achieved • When biological monitoring results in VSCI ≥ 60 • May require adaptive management to achieve that result The Bottom Line

  33. Choosing Appropriate Reference Watersheds or Conditions

  34. Biologically non-impaired In the same geographic vicinity, preferably in the same physiographic region and drainage basin Preferably a mixed landusewatershed (not all-forested) 1st to 4th order streams Ideally, a statewide pool of reference watersheds could be developed from which an appropriate subset could be selected for comparison with any given impaired watershed Reference Watershed Characteristics

  35. Questions?

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