Investigating Sources of Selenium in Shallow Groundwater in the Las Vegas Valley

Field: Environmental Science (Physical Sciences)

Selenium concentrations above EPA safe levels are of potential concern in various parts of the Las Vegas Valley (Valley) in both water and sediment media. Previous studies suggest that the source of elevated selenium in the Valley is possibly resurfacing groundwater. Shallow groundwater flows into Las Vegas Wash wetlands, and therefore, it has important environmental implications as it affects the quality of tributaries and surface water. However, to date, there have not been any studies to determine the origins of selenium in Las Vegas Valley shallow groundwater. Identifying and understanding the source, spatial distribution and mobility of selenium regarding the chemical constituents of the shallow groundwater and geology of the shallow groundwater wells are important to support development of effective approaches to control the release of selenium into groundwater in the Valley, and thereby reduce high selenium concentrations in certain tributaries that flow to the Wash. The objective of this study was to identify the source of selenium in shallow groundwater of the Valley through analyzing shallow groundwater water quality data and the geology of the Valley. A detailed analysis of chemical constituents of the shallow ground water and geology of the wells in the Valley helped assess relationships of selenium concentrations to both the overall chemical composition and the geologic zones of the Valley. Results of the study suggest that selenium concentrations in the shallow groundwater is correlated with Quaternary alluvium (Qa) (significant Pearson correlation at the p = 0.01 level (2-tailed)) for the major ions: Na+ (0.88), K+ (0.87), SO42- (0.86), NO3--N (0.74), TDS (0.84), EC (0.87), indicating that selenium is associated with salinity in the Qa zone. However, in the Quaternary to Tertiary consolidated sediment zone (QTs), correlations among Se and mentioned parameters were not significant. Anions, HCO3- and CO32- did not have a strong positive correlation with any of the major anions and cations indicating that even though carbonate minerals are present in the geology of the Valley in both Qa and QTs sediments, the weathering of the carbonate rock and minerals or the dissolution and leaching of the carbonate minerals and mobilization in the groundwater of the Valley are not occurring as much as some other major ions in the shallow groundwater of the Valley. Anions, HCO3- and CO32- did not have a strong positive correlation with Se, indicating that the carbonates do not co-occur with Se in the shallow groundwater of the Valley. Principal component analysis (PCA) performed on the physicochemical parameters of the shallow groundwater wells showed that nearly half of the total variations (46.7%) in the first pricnipal component (the dimension that spans the most variation across the data) were influenced by: diameter of the well, Na+, Mg2+, K+, Ca2+, Cl-, SO42-, NO3--N, F-, EC, TDS, B, Mo and As and Se. PCA performed on the major ions, EC, TDS and Se of Qa and QTs showed that Se was one of the influencing paramaters in the first principal component of Qa; however, in QTs, Se did not have a high loading and was not strongly contributing to the variations of the first pricinpal component. Pearson correlation with t-test analysis performed on electrical conductivty (EC) , selenium (Se) and boron (B), arsenic (As) and molybdenum (Mo) in the Qa and QTs zones showed that salinity is associated with Se, B, and As in the Qa zone and only associated with B in the QTs zone. A redox speciation analysis performed on selenium, demonstrated that selenium speciation in shallow groundwater is most likely present in the most oxidized and mobile form, that is selenate, indicating that salinity is associated with the most mobile form of Se (selenate). Spatial analysis of the iso-concentration maps showed that the zones of high concentration and generally the distribution of EC, total dissolved solids (TDS), hardness, SO42-, and Se across the Valley appear to be consistent across EC, TDS, hardness and sulfate, indicating that the spatial distributions of the mentioned parameters are similar in the Valley. Initial estimated rates of transport of water in various shallow groundwater wells with different geology were assessed in this study and ranged from minimum of 0.02 feet/10 year to maximum of 11.50 feet/10 year, pointing to a generally slow groundwater movement in the shallow groundwater wells throughout the Valley. The analysis and interpretations of this study suggest that the local hydrogeological processes that have caused high salinity in groundwater in areas of the Las Vegas Valley have also led to high concentrations of selenium, and that selenium concentrations are affected by the salinity, and geology of the sites.