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GeoScience Services <br />2263 is rngston Road <br />GrandJanctian, CO SIS03 <br />(970) 31¢3356 <br />leading edge of the plume. As noted in a previous comment response, the model used a <br />hydraulic conductivity value for the Summerville Formation that was neazly three orders <br />of magnitude higher than actual values measured in laboratory tests. If this laboratory <br />hydraulic conductivity value was incorporated into the model, then the extent of the <br />subsurface plume would be significantly less than the plume extent predicted using a <br />conservative hydraulic conductivity value. <br />The water table for the JD-6 and JD-8 mine azeas are not incorporated in the revised cross <br />sections attached to this report. This is because there is no information on the depth of <br />water at the site. However, based on the limited area up gradient of the mine sites <br />available for recharge and the relatively low recharge rate from precipitation, the <br />likelihood of saturated conditions in the underlying Entrada, Kayenta, or Wingate <br />Formations is minimal. The exposed faces directly to the northeast do not provide <br />adequate exposure to collect sufficient quantities of water to establish saturated <br />conditions in the more permeable units. For compazison purposes, the SM-18 mine is <br />down gradient of the Uncompahgre Plateau where similar units are exposed at high <br />elevations over a lazge area. Higher precipitation rates on the elevated plateau allows <br />sufficient quantities of recharge that result in the water table underlying the SM-18 mine <br />site. Even if there were saturated conditions underlying the ID-6 and JD-8 mine site, the <br />predicted plumes emanating from the waste piles from these mines still does not reach the <br />upper most potential aquifer (Entrada Sandstone) in the area. <br />Of the four mine sites, JD-9 presents the most complex hydrogeologic system due to the <br />perched water in the sandstone units of the Salt Wash. The geologic units underlying JD- <br />9 are similaz to the other three mine sites. Applying the model predictions to JD-9 <br />indicates that the plume would not reach the Entrada Sandstone. Since the waste pile is <br />located on the shales and mudstones of the Brushy Basin Member of the Morrison <br />Formation, the vertical extent of the predicted plume would be significantly less than for <br />the other mine sites. Species of concern could potentially enter perched water zones in <br />the sandstone units of the Salt Wash Formation but background water quality data <br />suggests that the impact would be negligible. See the following comment on water <br />quality for the Salt Wash sandstones. <br />Please supply the Division with the background TDS value for the perched aquifer. <br />For SM-18, JD-6 and JD-8, there is no evidence of perched water in the azea as indicated <br />by drilling or mining activities. In the Uravan area, the U. S. Department of Energy <br />(1994) states, "there is no direct evidence of the presence of a saturated zone in the Salt <br />Wash prior to the mining and disposal activities on Club Mesa." For the JD-9 mine, <br />water samples from the mine located in the upper Salt Wash sandstone report TDS values <br />that range from 1100 mg/1 to 1292 mg/1(Cotter Corp., 1978). Technical reports for the <br />U.S District Court (1986) report total dissolved solids for the Salt Wash on Club Mesa <br />range from 1,388 to 48,914 mg/I. The secondary drinking water standard as of May, <br />1990 reports a mcl of 500 mg/1 for total dissolved solids (Mackenthun and Bregman, <br />4 <br />