Evaluating Interbasin Flow and Groundwater-Surface Water Interactions, Lower Virgin River Basin USA: Unusual Statistical and Graphical Analysis of Stable Isotopes and Solutes

Rocks and basin-fill sediments of the intermountain basin region are complexly folded and faulted making it challenging to study the flow systems in the basins.  Identifying the flow systems in these arid basins is critical for optimizing numerical models used by water managers and stakeholders to sustainably manage water resources, and to prevent adversely affecting existing groundwater and surface water rights, environment, and riparian ecosystems.

In this study, stable isotopic values (δD and δ¹⁸O) and selected chemical solutes data from waters in the study area are used to evaluate the hypotheses of interbasin groundwater flow and interactions between the groundwater and Virgin River in the lower Virgin River Basin.  Unconventionally, linear regression, Spearman correlation test, scatterplot, box-and-whiskers plot, and Wilcoxon Rank Sum test are applied to glean information from δD and δ¹⁸O, and Na, K, SO4, and Cl data of the hydrochemical facies for the study area.  The δD values of the recharge end members from adjacent basins and within the lower Virgin River Basin are high and significantly different from the low δD values of the discharge end members.  Box-and-whiskers plot of δ¹⁸O values, comparing to box-and-whiskers plot of δD values, indicate oxygen isotopic exchange between the discharge groundwater and the aquifer minerals.  The isotopic exchange implies a longer residence time that the younger recharge waters cannot account for.  Correlation test of δD and δ¹⁸O versus Cl, and box-and-whiskers plot of δD, δ¹⁸O, and solutes data indicate the Virgin River + floodplain aquifer is more homogeneous and evaporated than samples of the recharge and discharge end members.  Regionally transported deep carbonate water is invoked to explain the low δD values of the discharge waters.