Groundwater Discharge and Salinity Sources to an Impaired Major River in a Semi-Arid Coastal Region: Nueces River, Texas

Abstract:

Nueces River, an impaired stream located on the South Texas Gulf coast area, has shown water quality degradation due to to increased salinity levels in areas adjacent to the Calallen saltwater reservoir dam. This study investigates the role of submarine groundwater discharge in delivering increased salt contents to the river and how the subsurface hydrology is affected by the presence of a salt barrier (i.e. saltwater dam) which separates the tidal and non-tidal parts of the Nueces river basin. Thus, a combination of resistivity profiling and elemental and stable isotope geochemistry methods has been applied to portions of the river located downstream (tidal) and upstream (non-tidal) of the dam. Preliminary data show that salinity levels gradually increases at the river bank indicating that groundwater is likely a source of solutes to the river in the upper, non-tidal portion. The presence of vertical upwelling of conductive groundwater plumes is also revealed by marine resistivity profiles collected along the river. Different sampling during the spring and summer of 2014 show higher concentration values of major ions (i.e., calcium, magnesium, sodium, potassium, chloride, etc.) and salinity of pore water for the upstream river at several locations while it remains relatively constant for bottom- and surface water. In addition, because the groundwater and porewater have slightly lower pH values, a shift to more acidic surface water accompanied by some increases in dissolved major ion concentrations and salinity suggest that groundwater might represent a source of increased salt content in the upper portion of the river. On the other hand, downstream dissolved major ion concentrations generally decrease in pore- and bottom water from spring to summer and are correlated with decreases in salinity while surface water becomes more saline with an increase in major ions. Therefore, these preliminary data indicate different hydrology systems of the two portions of the river, the likely different sources of groundwater discharge and that terrestrial groundwater discharge rather than recirculated water is a source of increased salinity in the upper portion of the river while it may supply freshwater to the lower part.