Hydrogeochemical and mineralogical factors influencing uranium in background area groundwater wells: Grants, New Mexico

Abstract

Lower San Mateo Creek Basin (SMCB), New Mexico Groundwater in the alluvial aquifer of the lower SMCB has a high concentration of total dissolved solids, alkalinity, and dissolved uranium at concentrations above USEPA limits; the source of the water quality was investigated through evaluation of geology, mineralogy, and geochemistry of sediments and groundwater. Surface-expressing bedrock in the larger Basin is dominated by uranium-bearing units such as the Morrison Formation and Dakota Sandstone; erosion of these units contributed sediment to Quaternary alluvium valley fill in the Basin with heterogeneous distribution of uranium. The potential for uranium-bearing alluvial fill to affect the groundwater quality was investigated upgradient of the Grants Reclamation Project (GRP), a former uranium milling facility in the lower SMCB, by hydrogeochemical and geophysical methods. Uranium is primarily associated with fine-grained materials (clay/silt) and is labile depending upon sediment geochemical conditions. Natural occurrence of uranium minerals in SMCB alluvial sediments here results in concentrations in groundwater greater than the USEPA water quality standards; as such, site-specific standards are established and supported by the findings of this work. Regional groundwater systems, derived from weathered mineralized bedrock, require careful evaluation of water quality to understand background conditions; uranium in groundwater results from hydrogeochemical processes at the interface of fine and coarse grained sediments within the aquifer.