Surface Water/Groundwater Interaction: A Forensic Approach to Salinity in an Alluvial Aquifer

The Brazos River Alluvium aquifer is a minor aquifer in central and east Texas under water table conditions. It is an underutilized resource, and is being considered as a future supplemental water source. However, variability in salinity occurs throughout the Brazos River Alluvium aquifer and the source of this variability is unclear. The objective of this study is to characterize salinity variability in the northern segment of the Brazos River Alluvium aquifer and evaluate potential salinity sources. A common supposition is elevated salinity levels in the aquifer are the result of groundwater-surface water interactions between the aquifer and the Brazos River, which also exhibits a high salinity. However, other salinity sources include irrigation, evapotranspiration, and brine contamination from historic oil and gas activities. To characterize the variability of aquifer salinity, water samples collected throughout the study area were analyzed for ions, specific conductance, and stable isotopes of hydrogen and oxygen. The specific conductance of the northern segment of the Brazos River Alluvium aquifer ranges from 390 to 1,640 mS/cm and averages 916 mS/cm. The average specific conductance the Brazos River in the northern segment is 1108 mS/cm. The spatial distribution of specific conductance in the aquifer shows that specific conductance changes rapidly over short distances. In multiple instances specific conductance values double over the course of a few hundred meters. Specific conductance profiles show stratification in wells, and data loggers have been implemented to monitor changes in specific conductance over time. Preliminary data show that the aquifer and river appear to be ionically and isotopically distinct, and that groundwater-surface water interactions between the aquifer and Brazos River is not likely the major source of aquifer salinity. However, irrigation with water from the Brazos River could potentially indirectly affect the salinity of the aquifer.