1. WQCD Response: Biotic Ligand Model Issues 1

2. Overview Biotic Ligand Model (BLM) is a relatively new tool from EPA for developing site-specific criterion-based copper standards. It is the best tool available for the task. Hearing process has provided an opportunity to work closely with stakeholders to gain insights regarding acquisition of input data and interpretation of model output. The Division appreciates the cooperative approach.

3. Status Review ProponentSegmentsStatusComment AuroraUS16iWithdrawnAdditional data needed ParkerCC01Temp ModAdditional data needed Plum CreekUS10aTemp ModAdditional data needed MetroUS15 & MS01aSite-specific stdsAdoption recommended CentennialUS16g & 14Site-specific stdsAdoption recommended UTSDBT02Reviewed stdsLet stand

4. What Makes a Good Proposal? Metro is a good case study Spatial coverage brackets significant hydrologic features and extends far enough to support proposal Temporal coverage spans enough years to incorporate seasonal and hydrologic variability Large sample size improves precision of result Longevity plan will support review of standards at next basin hearing

5. South Platte, Denver to Platteville (9 sites, 42 mi)

6. Results with Confidence Intervals

7. Wrap-up Expecting more copper proposals based on BLM Process should be easier to navigate next time because there is greater clarity regarding expectations for data collection and analysis Future proposals to benefit from: Revised guidance from Division Revised documentation and expanded model output from EPA

8. Significant Hydrologic Features