2018-04-23_REVISION - M2007031 (4)

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COLORADO Department of Public Health 8 Environment Water Quality Control Division Fact Sheet,Permit No. C00048939 zero low flow, and a mixing zone is not applicable to this discharge, a mixing zone study is not warranted at this time. e. Reasonable Potential Analysis- Using the assimilative capacities contained in the WQA, an analysis must be performed to determine whether to include the calculated assimilative capacities as WQBELs in the permit. This reasonable potential (RP)analysis is based on the Determination of the Requirement to Include Water Quality Standards-Based Limits in CDPS Permits Based on Reasonable Potential, dated December, 2002. This guidance document utilizes both quantitative and qualitative approaches to establish RP depending on the amount of available data. A qualitative determination of RP may be made where ancillary and/or additional treatment technologies are employed to reduce the concentrations of certain pollutants. Because it may be anticipated that the limits for a parameter could not be met without treatment, and the treatment is not coincidental to the movement of water through the facility, limits may be included to assure that treatment is maintained. A qualitative RP determination may also be made where a federal ELG exists for a parameter, and where the results of a quantitative analysis results in no RP. As the federal ELG is typically less stringent than a Limitation based on the WQBELs, if the discharge was to contain concentrations at the ELG (above the WQBEL), the discharge may cause or contribute to an exceedance of a water quality standard. For this permit, the division also used information provided in the US EPA Development Document for Effluent Limitations Guidelines and New Source Performance Standards for the Ore Mining and Dressing Point Source Category, Gold Placer Mine Subcategory May 1988 (the development document for the federal ELG), which identifies various metals and other parameters that can be found in placer mining discharges, as part of the qualitative RP analysis. Metals for which a water quality standard was not identified for the receiving stream were typically not considered further in the qualitative RP analysis. The metals considered from the development document are: arsenic, cadmium, chromium, copper, mercury, nickel, lead, selenium, silver, and zinc. The permittee indicated that no on-site dressing or processing of gold bearing ores is conducted at the Scott Blue II placer mine, and this condition was considered the RP analysis provided below. To conduct a quantitative RP analysis, a minimum of 10 effluent data points from the previous 5 years, should be used. The equations set out in the guidance for normal and lognormal distribution, where applicable, are used to calculate the maximum estimated pollutant concentration (MEPC). For data sets with non-detect values, and where at least 30%of the data set was greater than the detection level, MDLWIN software is used consistent with Division guidance to generate the mean and standard deviation, which are then used to establish the multipliers used to calculate the MEPC. If the MDLWIN program cannot be used the Division's guidance prescribes the use of best professional judgment. For some parameters, recent effluent data or an appropriate number of data points may not be available, or collected data may be in the wrong form (dissolved vs total) and therefore may not be available for use in conducting an RP analysis. Thus, consistent with Division procedures, monitoring will be required to collect samples to support a RP analysis and subsequent decisions for a numeric limit. A compliance schedule may be added to the permit to require the request of an RP analysis once the appropriate data have been collected. For other parameters, effluent data may be available to conduct a quantitative analysis, and therefore an RP analysis will be conducted to determine if there is RP for the effluent discharge to cause or contribute to exceedances of ambient water quality standards. The guidance specifies that if the MEPC exceeds the maximum allowable pollutant concentration (MAPC), limits must be established and where the MEPC is greater than half the MAPC (but less than the MAPC), monitoring must be established. Discussions for RP determinations for each parameter are shown in the parameter Evaluation section below. Page 6 of 17 4300 Cherry Creek Drive S.,Denver,CO 80246-1530 P 303-692-2000 www.colorado.gov/cdphe/wqcd * J H r'7 John W.Hickenlooper,Governor Larry Woik,MD,MSPH, Executive Director and Chief Medical Officer CO