USE OF U-234 AND U-238 ISOTOPES TO EVALUATE CONTAMINATION OF NEAR-SURFACE GROUNDWATER WITH URANIUM-MILL EFFLUENT - A CASE-STUDY IN SOUTH-CENTRAL COLORADO, USA

The U-234/U-238 alpha activity ratio (AR) was determined in 47 samples of variably uraniferous groundwater from the vicinity of a uranium mi ll near Canon City, Colorado. The results illustrate that uranium isot opes can be used to determine the distribution of uranium contaminatio n in groundwater and to indicate processes such as mixing and chemical precipitation that affect uranium concentrations, Highly to moderatel y contaminated groundwater samples collected from the mill site and la nd immediately downgradient from the mill site contain more than 100 m u g/l of dissolved uranium and typically have AR values in the narrow range of 1.0-1.06. Other samples from the shallow alluvial aquifer far ther downgradient from the mill contain 10-100 mu g/l uranium and plot along a broad trend of increasing AR (1.06-1.46) with decreasing uran ium concentration. The results are consistent with mixing of liquid mi ll waste (AR approximate to 1.0) with alluvial groundwater of small, b ut variable, uranium concentrations and AR of 1.3-1.5. In the alluvial aquifer, the spatial distribution of wells with AR values less than 1 .3 is consistent with previous estimates of the probable distribution of contamination, based on water chemistry and hydrology. Wells more d istant from the area of probable contamination have AR values that are consistently greater than 1.3 and are indicative of little or no cont amination. The methodology of this study can be extended usefully to s imilar sites of uranium mining, milling, or processing provided that l ocal geohydrologic settings promote uranium, mobility and that introdu ced uranium contamination is isotopically distinct from that of local groundwater.