PRESENT AND FUTURE IMPACTS OF WATER USE ON URANIUM CONCENTRATIONS IN NORTHERN NEW MEXICO AQUIFERS

Northern New Mexico depends heavily on aquifers for drinking water and, like much of the arid southwest, there are serious concerns and present and future water availability. Current estimates of water supply assume that water quality will remain constant over time. This study seeks to determine the extent to which this assumption is valid with respect to heavy metal contamination.

Historical and modern trends in groundwater chemistry in the Española Basin, a sub-basin of the Rio Grande Rift, suggest that significant uranium contamination occurs locally in some areas. Isotopic analyses demonstrate that the source of the contamination is natural. Through water sampling, analysis, and modeling we are attempting to determine the mechanisms controlling the present spatial distribution of the uranium contamination and to predict the impact of current and future groundwater production on contaminant concentrations.

Preliminary data analysis and modeling results suggest that both alkalinity and redox conditions are important factors in controlling spatial distribution of uranium in groundwater. Reactive transport modeling suggests that a simple model of groundwater flow, coupled with a kinetically-controlled silicate dissolution and uranium equilibrium reactions are consistent with generalized large-scale spatial trends. Local variations in uranium concentrations are apparently heavily influenced by redox conditions.

Forty years of data from one wellfield in the basin clearly suggest that water quality deteriorated after several decades of pumping. The mechanism(s) responsible for the measured increases in uranium and arsenic concentrations are unknown; possibilities include either a purely physical mechanism such as enhanced extraction of deeper, older waters or a geochemical mechanism such as changes in alkalinity or redox states that affect trace element solubility. We present reactive flow modeling results at the regional scale which identify those locations within the aquifer which are the most vulnerable to future deterioration of water quality, and suggest how current estimates of water supply should be adjusted to reflect possible deterioration in quality.