Water-Effect Ration (WER)

  • Background
  • Findings
  • Resolution

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Cenla Environmental Science client #2014-09 is a municipal WWTP. The LPDES permit conditions for this facility contains a copper limit on its discharge from outfall 001. CK conducted toxicity testing using Ceriodaphnia dubia to support a Water-Effect Ratio (WER) study for copper. Toxicity tests were used to assess the toxicity of copper in ambient receiving stream waters and the wastewater effluent. The WER determined by conducting side-by-side toxicity tests of synthetic lab water (LDW) and simulated downstream water (SDW) spiked with copper. After adjusting for any hardness differences, the WER for the sample is the lesser of (a) the SDW LC50 divided by the LDW LC50, or (b) the SDW LC50 divided by the documented Species Mean Acute Value (SMAV). The geometric mean of two separate sampling event WERs, performed a minimum of 30 days apart, is the site WER.


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CK performed the testing on samples from two separate sampling events that were 37 days apart. The first event was used to conduct a range finder for copper spiking and for Run #1 of the study. The second sampling event was used for Run #2 of the study. Ceriodaphnia dubia48-hour acute testing was conducted as described in; Streamlined Water-Effect Ratio Procedure for Discharges of Copper, EPA 822-R-01-005, March 2001 and Methods for Measuring Acute Toxicity of Effluents and Receiving Waters to Freshwater and Marine Organisms, Fifth Edition, EPA 821-R-02-012, October 2002.


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The WER result for Total Recoverable copper was 9.38, and the WER result for Total Dissolved copper was 8.02. These results will be used by permit to holder to assist with negotiating a more favorable copper limit in its next LPDES permit.