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disinfected. After disinfection, a follow-up sample will be collected and analyzed for Total <br />Coliforms. <br />The addition of a sodium hypochlorite as a disinfectant is not advisable unless necessary. <br />Adding sodium hypochlorite to the groundwater adds the risk of forming disinfection by- <br />products, which are regulated contaminants under the Safe Drinking Water Act. If sodium <br />hypochlorite is used as a disinfectant, a neutralization procedure will be required before injection <br />can occur. <br />C. Concerns related to increased oxygen content of A2 sandstone groundwater as it <br />is pumped to the surface during the aquifer-pump test. The third concern related to potential <br />changes in water quality is that an increase in dissolved oxygen could occur when the A2 <br />sandstone groundwater is pumped to the surface and exposed to the atmosphere. The proposed <br />injection well is screened within a uranium roll-front ore deposit in the A2 sandstone. <br />Groundwater in confined aquifers has a very low oxygen content. When groundwater from the <br />A2 sandstone is pumped to the surface, it will be exposed to atmospheric conditions, which are <br />higher in oxygen than conditions present within the A2 sandstone groundwater. Uranium in a <br />roll-front ore deposit is recovered by the injection of a highly oxidizing lixiviant solution. The <br />uranium becomes dissolved in the lixiviant, which flows to production wells that pump the <br />uranium-laden lixiviant to the surface for uranium recovery. Commenters familiar with this <br />process are concerned that increasing the dissolved oxygen content of the A2 sandstone <br />groundwater before reinjection would mobilize uranium and other associated constituents in the <br />ore deposit surrounding the proposed injection well upon reinjection of the groundwater. <br />Commenters also expressed concern that introducing oxygen into the ore body would start the <br />decay process of uranium. There was also concern that injection would result in well fouling <br />which would contaminate the groundwater. <br />EPA Response: <br />The low level of dissolved oxygen in the groundwater to be reinjected under the UIC Class V <br />Final Permit will not result in mobilizing uranium in the subsurface, because the dissolved <br />oxygen will be quickly overcome by the ambient reducing conditions of the aquifer once the <br />groundwater is reinjected. The chemical components of the aquifer matrix contribute to these <br />ambient reducing conditions. Two of these chemical components are iron and sulfur. The <br />concentrations of iron and sulfur have been measured in the aquifer matrix from borehole <br />samples collected at the proposed injection well location. Oxygen reacts readily with iron and <br />sulfur and will preferentially combine with them over uranium. To verify that the dissolved <br />oxygen present in the groundwater prior to reinjection would not result in mobilization of <br />uranium upon reinjection, EPA directed Powertech to <br />1. provide calculations to demonstrate that the iron and sulfur present in the injection zone <br />at the location of the proposed injection well will act as a preferred oxygen sink <br />compared with uranium, and <br />2. provide a calculation on how far away from the injection well reinjected groundwater <br />would travel before the dissolved oxygen in the groundwater is consumed by the iron and <br />sulfur present. <br />Page 6 of 24