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SUNDAY MINE COMPLEX-HYDROGEOLOGIC REPORT <br /> minerals such as gypsum, anhydrite, halite, and nahcolite possibly accompanied by cation <br /> exchange (calcium in place of sodium) are likely controlling water chemistry in the Top Rim unit. <br /> This process may account for the presence of sulfate and the lack of calcium at well MW-SUN1 <br /> compared to upgradient well locations. The source of the salt minerals may be within the Top <br /> Rim unit itself, but it is possible that salts were introduced to the Salt Wash during structural salt <br /> intrusions associated with the Paradox Member of the Hermosa Formation prior to a complete <br /> breach of the Gypsum Valley anticline. The dominant presence of sodium and potassium in the <br /> groundwater may suggest similar sources. It is common in deep aquifer systems, where little to <br /> no aquifer refreshing occurs from surface or near surface recharge, that downgradient locations <br /> are more saline due to the higher residence time that has occurred. This would be expected in <br /> the Salt Wash Member in the study area and can explain why overall TDS is greater at the well <br /> MW-SUN1 location. <br /> Water chemistry can vary dramatically from strata to strata within the Salt Wash Member as <br /> demonstrated in underground wells installed by Denison in the West Sunday Mine (Denison, <br /> 2012). Monitoring wells screened in the Top Rim unit showed distinctly different chemistry than <br /> a well screened in an underlying mudstone and siltstone unit. The mudstone/siltstone unit <br /> showed a TDS concentration 10 times greater than the Top Rim Sandstone unit. The two <br /> sampled wells that were screened in the Top Rim Sandstone unit had arsenic, molybdenum, <br /> uranium, and vanadium concentrations and gross alpha activities significantly greater than the <br /> well screened in the siltstone unit. Whereas the siltstone unit showed consistently higher <br /> concentrations of other metals including calcium, chloride, cobalt, copper, lithium, magnesium, <br /> manganese, potassium, sodium, sulfate, and zinc. It should be considered, however, that any <br /> geologic media that is recently saturated, as may be the case with these sediments, may not <br /> reflect water chemistry that represents chemical equilibrium. It may require multiple pore <br /> volumes of groundwater displacement and a significant amount of residence time to reach this <br /> equilibrium. <br /> The overall lack of minor and trace metals in the Top Rim Sandstone of the Salt Wash as shown <br /> in the data is a characteristic worth noting, but is not surprising considering the relatively high <br /> pH of the groundwater. Metals may exist in the formation but may not be soluble under these <br /> conditions. <br /> Table 10-5 presents the constituents in the groundwater samples that exceed Colorado <br /> BSGWs. The main constituents of concern are considered to be uranium and the radiological <br /> Western Water& Land, Inc. 47 <br />