The effluent of treated fly ash from a coal-fired power plant located on the Ohio River periodically exceeded its NPDES acute permit limit for hexavalent chromium of 15 g/L. The increased levels of hexavalent chromium in the effluent are a recent occurrence which were likely due to changes in coal blends burned in the generating units. Ohio EPA determined the use designation of the receiving stream (Limited Resource Water) was being attained and a one-year biomonitoring program of the effluent detected no acute toxicity to Ceriodaphnia dubia or Daphnia magna. The water-effect ratio (WER) procedure was selected to develop a site-specific criterion maximum concentration (CMC) for hexavalent chromium for the effluent's receiving stream. WER procedures followed those described in EPA's "Interim Guidance on Determination and Use of Water-Effect Ratios for Metals" (1994). Site water used in the WER determinations was undiluted effluent since the receiving stream originates at the discharge point of the outfall. 48-hour acute D. magna and 96-hour acute fathead minnow toxicity tests were selected as the primary and secondary tests, respectively for use in three seasonal WER determinations. The Final WER determined from three WER determinations was 7.29. Based on this WER study, the site-specific CMC for this site is converted to 109.35 g/L. After completing negotiations with Ohio EPA and Region V, Ohio EPA established a site-specific CMC for hexavalent chromium of 59 g/L. The site-specific CMC was selected because it was the highest concentration that would not affect the CCC for hexavalent chromium in the next receiving stream (Ohio River).
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