Assessing Background Groundwater in the Uranium Mining Belt of Northwestern New Mexico

Northwestern New Mexico has experienced substantial uranium (U) mining and milling since the 1950s. This activity has left a legacy of groundwater highly contaminated with U and other contaminants near closed mills where unlined tailings impoundments were constructed over shallow alluvial aquifers. As efforts proceed to remediate these aquifers, the question of what is natural background arises because that is often the remediation goal. Under the tailings in Grants, New Mexico, U concentrations once exceeded 100 mg/L while downgradient in nearby subdivisions advection from the tailings caused the U concentration to increase to over 0.1 mg/L, or three or more times the drinking water standard of 0.03 mg/L. Comparison of conceptual flow models for pre-milling, mill operations, and post-mill operations shows flow paths have changed substantially due to the discharge of highly contaminated mine dewatering water upstream of the site and the mounding of seepage under the tailings pile. Groundwater advection of infiltrated mine dewatering discharge could not have affected U concentrations at the millsite because it would have taken more than 80 years. Also, U has a high retardation in alluvial soil which would increase the time for transport to almost 400 years, and U attenuates or becomes bound in the soils. Groundwater mounding during millsite operations likely caused a flow reversal in the groundwater just upgradient from the millsite which explains the observed trends in U concentrations in wells above the millsite. Advection and dispersion from the millsite caused U to affect groundwater as much as half a kilometer above the millsite. Historic U concentrations that precede mil site contamination match those observed in unaffected groundwater sampled in regional studies and accurately reflect nearby groundwater concentrations. There is no evidence that natural sources cause any observed spikes in the U concentration.