Laserfiche WebLink
The input parameters for the calculations include: <br />1. the estimated volume of groundwater that will be pumped to the surface during the <br />aquifer pump test and reinjected, <br />2, the maximum amount of oxygen that could potentially become dissolved in the estimated <br />volume of groundwater as it is exposed to atmospheric oxygen upon being pumped to the <br />surface during the aquifer pump test, and <br />3. the concentrations of iron and sulfur analyzed from the sample of A2 sandstone at the <br />location of the injection well. <br />Powertech's calculations demonstrated that any oxygen dissolved in the groundwater will <br />preferentially react with iron and sulfur rather than uranium, and the concentrations of iron and <br />sulfur present in the ore deposit will consume the amount of dissolved oxygen in the reinjected <br />groundwater by the time the injectate has moved 2.5'feet from the injection well. EPA consulted <br />with an expert in geochemistry at the US Geological Survey to confirm that Powertech's <br />assumptions and calculations were correct. <br />The mobilization of uranium during in-situ leaching recovery is accomplished by the <br />injection of a highly oxidizing lixiviant. A very strong oxidizer is needed to overcome the <br />natural reducing condition of the aquifer from the injection well to the recovery well. Flow of <br />the oxidizer in the subsurface must be maintained by a combination of injection pressure at the <br />injection wells and pumping at the recovery wells in order to maintain mobilization of the <br />uranium. Under the UIC Class V Final Permit the groundwater will be injected at zero injection <br />pressure at the injection wellhead. <br />EPA consulted with the US Geological Survey uranium group to address the comment <br />about oxygen initiating radioactive decay in the uranium ore body. Oxygen is not material to the <br />initiation of radioactive decay. Radioactive decay of naturally occurring uranium isotopes <br />uranium-238, -235 and -234 is a time dependent process: the half-life of these decay <br />processes are 4.47 billion years (U-238), 704 million years (U-235) and 245,500 years (U-234). <br />What is oxygen dependent is the mobility of uranium in groundwater. In more oxygenated <br />environments, uranium is a mobile species and becomes more highly concentrated in <br />groundwater, and with increasingly reducing conditions uranium is fixed in various mineral <br />species such as uraninite, lowering its concentration in groundwater. <br />Well fouling refers to a build-up of minerals precipitating on a well screen over time. It <br />is a common problem with all types of wells. Well fouling does not create a water quality <br />problem; it is more of an issue for flow of water into the aquifer for injection wells or out of the <br />aquifer for water supply wells. Well fouling will not be an issue for the short duration of the <br />proposed aquifer-pump test. <br />3. Comments related to concerns about the communication between aquifers resulting <br />from the improper closure of historic exploration boreholes. Commenters raised the issue of <br />historical documents related to uranium exploration drilling that include information on plugging <br />and abandonment of exploration drill holes in the vicinity of the proposed injection well. These <br />documents raise questions concerning the potential for leakage between the Upper Fox Hills <br />Formation and the overlying Laramie Formation. Since the primary purpose of the UIC Class V <br />Permit is to prevent the contamination of the Laramie Formation groundwater, commenters <br />Page 7 of 24