2015-01-29_REPORT - C1981018 - Laserfiche WebLink

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2.2.1.7 Whole Effluent Toxicity (WET) Testing No WET testing was conducted during the water year since there was no discharge through any outfall carrying this requirement. A history of WET testing at Deserado Mine follows below: BME started the whole effluent toxicity (WET) testing program beginning in the fourth quarter of 1989. The mine water effluent was found to be toxic to the test species. Accelerated testing was conducted but the toxicity to the species continued. In a meeting with Colorado Water Quality Control Division (WQCD) staff on January 31, 1992, the BME staff reiterated the biomonitoring events, and various measures taken to identify the toxicants and to reduce the toxicity. BME took the following steps to identify potential toxicants. • In the early stages of WET testing, the associated H2S in the effluent was deemed to be a potential toxicant. However, a WET testing done by ENSR Consulting in June 1990 after removing H2S by aeration still showed toxicity. This result indicated that some other factor(s) also contributed to the toxicity. In September, 1990, ENSR Consulting was directed to conduct a "toxicity characterization" study to identify the primary cause of the toxicity. This study considered pH effects, volatile organic carbon, metals chelation, aeration, total dissolved solids (TDS) and ammonia. The lab determined high TDS (chlorides) as the primary cause of the toxicity. However, a recent review shows no correlation between LCso values and chloride or TDS levels. BME took the following measures to reduce toxicity of the effluent. • On -site aeration structures were installed for H2S removal before discharging the effluent. • A pilot test was conducted to identify a flocculent for removing high levels of oil and grease from the effluent. • The main water source of the mine was changed from the high TDS( >5000 ppm) alluvial well water to a significantly lower TDS (500 ppm) surface lagoon. 11