LINKING GROUNDWATER DISCHARGE TO INCREASED SALINITIES IN A SEMI-ARID COASTAL AREA

Nueces River (NR), located on the South Texas Gulf coast area, one of the main sources of freshwater to nearby communities and to the Nueces Bay, is threatened by water quality degradation, specifically pertaining to increased total dissolved solids (TDS) and salinity concentrations in areas adjacent to Hazel Bazemore Park (HBP) and Calallen saltwater dam. Given the arid nature of south Texas embayments, it is conceivable that groundwater inflows may play a significantly role in delivering contaminants to surface water. Geophysical methods, elemental and stable and radiogenic isotope geochemistry of two portions of NR, downstream (tidal) and upstream (non-tidal) of the Calallen dam, are implemented in this study to link sources of salinity to different recharge sources. Preliminary magneto-telluric data and water quality parameters collected from HBP indicate that TDS gradually decreases at the river bank suggesting the possibility of groundwater-surface water interaction through diffusion and dispersion. Land and marine resistivity soundings collected along the river indicate the presence of vertical conductive groundwater upwelling into the river. Sampling during fall 2013 and spring and summer 2014 show an increase in specific conductance (SC) and radon (²²²Rn) concentrations of porewater (average SC: 6,122 to 7,458 mS/cm and ²²²Rn : 239 to 4,040 Bq/m³) for the upstream NR while surface water concentrations increase from fall 2013 to spring 2014 and are the lowest in summer 2014 (SC: 974; 1,461 and 780 mS/cm and ²²²Rn : 337; 367 and 156 Bq/m³). A shift to more acidic pH values is observed with increases in ²²²Rn and SC hinting at groundwater as a source of increased salinity. Downstream porewater ²²²Rn concentrations are increasing from fall 2013 to summer 2014 but are not correlated with increases in SC suggesting less saline groundwater discharge. For this portion, the highest ²²²Rn (4,592 Bq/m3) is correlated with the lowest SC (6,367 mS/cm). Surface water ²²²Rn and SC are decreasing from fall 2013 (²²²Rn : 476 Bq/m3; SC: 11,303 mS/cm) to summer 2014 (²²²Rn : 57 Bq/m3; SC: 687 mS/cm). These preliminary analyses indicate that groundwater discharge is a source of increased salinity in the upper portion of the river while it may supply freshwater to the lower part.