Quantifying groundwater discharge to Texas Coastal Bend estuaries

Recent studies have provided evidence that submarine groundwater discharge is a significant source of water and dissolved nutrients to the coastal ocean. The chemical implications of these studies are especially important because, relative to surface water, groundwater is typically enriched in many compounds including nitrogen bearing nutrients. Therefore by affecting the supply and relative proportions of essential nutrients, direct groundwater discharge has the potential to influence phytoplankton populations and estuarine ecosystems as a whole. Another potential submarine discharge that may occur in the shallow restricted waters of the Texas coast is leakage of oil-field brine. Such leakage also has important ecological implications similar in some ways to groundwater discharge. The studies in this dissertation concern improving the methods and techniques used in measuring submarine discharges. Using the Texas Coastal Bend as a study area I have: (1) conducted a detailed evaluation of submarine discharges to Nueces Bay and (2) compared indications of submarine discharge between Nueces, Baffin, and Copano Bays. These investigations use a combination of geochemical and geophysical techniques. The geochemical methods are based primarily on measurements of naturally-occurring dissolved Ra isotopes in samples of bay, river, ocean, and groundwater. The geophysical methods employ electrical resistivity profiling to look for evidence of groundwater movement within the bay bottom sediments. Results show that dissolved radium concentrations within Nueces Bay are among the highest observed in coastal estuaries. Geochemical analysis and geophysical surveys indicate that both groundwater and leakage of oil-field brine are potential submarine inputs. Samples from Nueces, Copano, and Baffin Bays show that the seasonal increase in dissolved 226Ra activity for Nueces Bay is substantially larger than that of the other two bays. This increase is not readily explained by either evaporation or riverine supply. These results clearly suggest that the Ra supply to Nueces Bay is unusually large. For Nueces Bay, the most relevant differences between the three bays that might account for this are (1) the proportionally larger salt marsh and (2) the higher density of petroleum wells and pipelines. Though submarine groundwater discharge is not to be ruled out, leakage of oil-field brine is strongly indicated.