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Point of Compliance Well Investigation <br /> <br /> <br /> <br />AECOM <br />12 <br /> <br />4. Conclusions <br />AECOM conducted a groundwater investigation at Colowyo Mine to better understand alluvial <br />groundwater chemistry downgradient of the Mine and to identify suitable locations for <br />permanent point of compliance wells. The study objectives were as follows: <br />1. To understand spatial trends in alluvial groundwater chemistry within the Wilson, Taylor, <br />and Good Spring Creek drainages; <br />2. To establish site-specific background concentrations for alluvial groundwater using data <br />from Colowyo’s upgradient and downgradient monitoring wells; <br />3. To define comparison values for assessing groundwater compliance using a combination <br />of site background concentrations and table value standards from the Water Quality <br />Control Commission’s Regulation No. 41 – The Basic Standards for Ground Water (5 <br />CCR 1002-41); and <br />4. To identify permanent compliance well locations on land owned by the Mine where the <br />alluvial groundwater chemistry would generally meet compliance standards, in <br />accordance with Regulation 41 and Rules 4.05.13(1)(b)(i)(C) and 4.05.13(1)(b)(ii) of the <br />Rules and Regulations of the Colorado Mined Land Reclamation Board for Coal Mining <br />(1980 et seq.), which allow many different factors to be considered when siting <br />permanent compliance wells. <br />The study objectives were achieved by installing 16 temporary monitoring wells in the alluvium <br />associated with Wilson, Taylor, and Good Spring creeks, and sampling the temporary wells over <br />a one-year period to characterize the groundwater chemistry. The temporary well concentration <br />datasets were evaluated by comparing the results to site-specific background values as well as <br />Regulation 41 groundwater standards. <br />Two different approaches were used to estimate background concentrations for the Mine’s <br />alluvial groundwater. The first approach relied on concentration data from upgradient wells A-6 <br />and A-8 to calculate site-specific comparison values. AECOM used ProUCL to calculate upper <br />tolerance limits (UTLs) for the upgradient well dataset with 95 percent confidence. The second <br />approach used was to estimate existing ambient water quality in the downgradient Mine area <br />prior to January 31, 1994, as prescribed in Section 41.5.C.6.b.i.A of Regulation 41. Under this <br />method, AECOM compiled pre-1994 concentration data from the NGSW and Gossard Well <br />locations, and again developed UTLs for the combined dataset with 95 percent confidence. <br />Results of the statistical evaluation show that for several constituents, the UTLs calculated are <br />higher than the most stringent groundwater threshold contained in Regulation 41. <br />Evaluation of the temporary well concentration datasets revealed two main findings: <br />1. Contrary to our initial hypothesis, the temporary monitoring well data showed that alluvial <br />groundwater chemistry tends to be better closer to the Mine Permit boundary and <br />decreases downgradient. This is especially true for sulfate and TDS, where the highest <br />average concentrations occurred at downgradient locations such as POC-14 and <br />POC-16. <br />2. Manganese, sulfate, and TDS were the constituents most likely to exceed comparison <br />thresholds in the temporary monitoring wells.