CONTINUOUS SEDIMENT RESISTIVITY PROFILING WITH SYNOPTIC DISSOLVED 226RA, 228RA, 224RA, 223RA AND SURFACE SALINITY MEASUREMENTS DETECT AND CHARACTERIZE SUBMARINE DISCHARGES TO NUECES BAY, TEXAS

A synoptic geophysical and geochemical survey was used to investigate the nature and spatial distribution of submarine discharges to upper Nueces Bay, Texas, where previous dissolved Ra measurements suggested a submarine Ra input to the bay. The 17 kilometer survey used continuous resistivity profiling to estimate the vertical and horizontal variation of electrical conductivity in the bay bottom sediments to a total depth of 8 m. Continuous measurements of surface water salinity, temperature, dissolved O₂, and 28 point samples for dissolved ²²⁶Ra, ²²⁸Ra, ²²⁴Ra, ²²³Ra activity were taken during the resistivity survey. The survey found vertical fingers or plumes of high sediment/porewater conductivity extending up through 7 meters of bay bottom sediments into the surface water within 100 m of prominent surface salinity and dissolved Ra maximums (²²⁶Ra>60 dpm/100 L). At these locations there were also peaks in water temperature and lows in dissolved O₂. In one of these areas submerged petroleum pipelines were evident in the conductivity profile but not in the area of the bay where the sediment conductivity and surface water anomalies were strongest and most correlated. These results strongly suggest either submarine brackish groundwater discharge or the leakage of oil field brine from submerged petroleum pipelines and wells. This study demonstrates the application and usefulness of sediment resistivity profiling to submarine groundwater investigations and shows how it can be used as part of a sequence of large scale chemical tracer assessments and detailed synoptic surveys to localize discharge areas and more comprehensively characterize submarine discharges.