An Assessment of Uranium in Groundwater from Domestic Wells in Southeastern Nebraska, USA

Farzana I. Rahman, University of Nebraska-Lincoln

Document Type Article

Abstract

In this study, I assessed the groundwater quality of drinking water from domestic wells within the Upper Big Blue Natural Resources District (UBB NRD) in southeastern Nebraska. Rural residents in the UBB NRD obtain drinking water from domestic wells that terminate in the High Plains Aquifer, one of the most important groundwater resources in the United States. This aquifer is known to have several potential sources of uranium, a locally common geogenic contaminant that may affect human health.

The two main methodologies used in this study were statistical modeling using kriging and process-based modeling using PHREEQC. Ordinary kriging was used to predict areas that may be at risk for elevated uranium concentrations in groundwater based on historical data from the National Uranium Resource Evaluation (NURE) program and the predictions were validated using the results from 50 domestic well samples collected in this study. A majority of the sites were below the MCL for uranium, but 4 sites were above the MCL, with the highest concentration at 5x the MCL. Nitrate exceeded the MCL for nearly 50% of sites tested. Arsenic was below the MCL in all sites. The overall patterns of uranium occurrence predicted using Ordinary Kriging were in agreement with uranium concentrations measured in groundwater samples from domestic wells. However, the model underpredicted the magnitude of uranium concentrations that we observed in the region, which were as high as five times the limit for uranium in drinking water. Statistical analysis and geochemical provided insights into possible processes that may be contributing to uranium mobilization, which varied among sites where high concentrations of uranium were detected. Uranium concentrations were associated with calcium, magnesium, sulfate, potassium, and total dissolved solids. Geochemical modeling showed that uranyl-carbonate complexes were the dominant species for most sites. Uranium concentrations were also correlated with the saturation indices for rutherfordine, schoepite, barite, gypsum, and anhydrite. Additional site-specific data is required to determine the source of uranium and arrive at a mechanistic understanding of uranium mobilization for each of these particular wells. Future studies would do well to identify a narrow hypothesis and sample strategically, because this is a highly heterogeneous system in time and space, and is difficult to characterize using the data that is available from public data sources.

Advisors: Erin Haacker and Daniel Snow