Water Sources, Lower Virgin River Basin: Evidence for Deep Groundwater Contribution from Geochemical Tracer Analyses

In the Basin and Range Province deep carbonate groundwater has been suggested as a significant source to many overlying alluvial groundwater basins. Notwithstanding, our knowledge of the deep groundwater is limited because data from such considerable depths are costly and scarce, the geology is complex, and the wells in the alluvial groundwater, which provide most available data, are unevenly distributed.

Lower Virgin River Basin presents an opportunity for testing geochemical tracers in diagnosing deep groundwater contribution to basin-fill aquifers. In this study, historical and newly collected samples from springs, rivers, and wells tapping basin-fill groundwater were classified using major ions and total dissolved solids. Classification is based on a combination of Multiple Discriminant Function and Cluster Analysis allowing for unbiased, reproducible results. Using geologic and hydrologic information and from ordination of the classes based on Na⁺, K⁺, SO₄^2-, and Cl^- ions, six hydrochemical classes are characterized as recharge and discharge end-members and used to define potential sources and movement of water through the basin. Flow dynamics are tested using box-and-whisker plots of deuterium and oxygen isotopic data. The isotopic data reveal that potential recharge waters have significantly high isotopic values whereas the discharge waters have low values. A Wilcoxon Rank Sum test of deuterium isotopic data confirms the difference between recharge and discharge end-members is significant. Except for the class containing floodplain groundwater and surface water, linear and non-parametric regressions, combined with Spearman Correlation, show no significant relationship between isotopic and chloride data. A bivariate plot of deuterium versus oxygen shows that recharge and discharge end-members have significantly different gradients. These data support conclusions that the deep groundwater is an important source to the alluvial groundwater, except for floodplain groundwater, and that combining the classification approach with chemical and isotopic tracers is a powerful tool for diagnosing deep groundwater.