Baseline Ground Water Quality Conditions at in Situ Uranium Wellfields in Wyoming

Abstract

Interest in the ground water quality distributions at uranium in situ mining wellfields has increased because of the need to address differences in federal and state water quality standards and restoration requirements. There has been considerable discussion about the water quality distributions at individual wellfields because of specific operators' concerns (e.g., modifying a monitoring program). However, this is one of the first overviews of water quality distributions at wellfields in Wyoming. Three water quality parameters (Total Dissolved Solids (TDS), uranium, and radium) were evaluated at nine wellfields. These parameters were selected for information on overall water quality (TDS) and on water quality in the vicinity of the production zones (uranium and radium). Box and Whisker Plots were used to compare the TDS and Uranium concentrations in each production zone and surrounding monitor ring. Visually, the plots indicate that it is difficult to distinguish between TDS concentrations in a production zone and surrounding monitor ring, and the TDS concentrations are all below 1,200 milligrams per liter (mg/l) with the majority below 500 mg/l. In contrast, it is much easier to distinguish the uranium concentrations in a production zone because they are generally much higher than the concentrations in the surrounding monitor ring. This is confirmed by statistical analyses. Even so, the uranium concentrations in the production zone often do not exceed 0.03 mg/l, the Maximum Concentration Limit for uranium established by the U.S. Environmental Protection Agency for drinking water. Frequency histograms of radium concentrations in the production zones show scattered data, and most of the concentrations exceed 100 picoCuries per liter (pCi/l). In contrast, the majority of the radium concentrations in the monitor rings are less than 100 pCi/l, with most less than 20 pCi/l, and the higher concentrations are only in a few wells in the ring (generally in areas where the ore trend extends beyond the mining area). The limited contrast in TDS concentrations, as compared to the generally significant contrasts in uranium and radium concentrations, inside and outside a production zone must be taken into consideration in efforts to harmonize federal and state ground water quality standards and restoration criteria.