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Mr. Thomas A. Schreiner April 29, 2007 Page 2 EXCEEDENCE Comment 2. In response to Rule 3.1.7(7)(b)(iii), please clarify how an exceedance of a baseline water quality parameter will be determined, reported to the Division, and mitigated. Response Determination of an Exceedance An exceedence of a baseline water quality parameter will be determined by comparison of the results of the sampling with the baseline water quality standards established for this project. Pursuant to Colorado Water Quality Control Commission (CWQCC) regulations', the baseline water quality standards shall be the greater ofZ: Background concentration computed for each species, determined as the 99% upper confidence limit from the first five quarterly readings taken from the monitoring well prior to start of MRV production mining operations at the site. 2. Agricultural groundwater quality limits, based on the most likely use of groundwater in the area (as the natural TDS and sulfate concentrations exceed drinking water standards). The Agricultural limits are presented in Table 1. ' ,Colorado Water Quality Control Commission, 5 CCR 1002-41: Regulation No. 41-The Basic Standards for Ground Water, March 22, 2005. 2 CWQCC provide guidance for selection of a groundwater standard as follows: 'For ground water classified "Domestic Use -Quality" or "Agricultural Use -Quality," where a table value is exceeded by the background level, the applicable standard for that parameter shall be either 1) the table value or 2) the background level for that parameter. This determination shall be made considering the increased risk to public health, crops, or livestock associated with the background levels, the extent of the exceedance above the table value, the degree to which the pollution is deemed correctable and subject to treatment; and the economic reasonableness of such treatment requirements.' [CWQCC, 2005, 41.5(6)(5)]