High 226Ra and 228Ra activities in Nueces Bay, Texas indicate large submarine saline discharges

We have investigated submarine groundwater discharge to Nueces Bay (Texas) using naturally occurring Ra isotopes. Dissolved Ra activities in Nueces Bay are among the highest observed in coastal estuaries; as great as 2600 dpm m− 3 for 228Ra and 1000 dpm m− 3 for 226Ra. Using a combination of salt and Ra mass balances, we demonstrate that river discharge and bay bottom sediments cannot supply the Ra needed to balance tidal export. In the case of 226Ra there is an additional source of 218 × 106 ± 105% dpm day− 1 which is 9 times the maximum supply from bay bottom sediments and 50 times the Ra supplied by the Nueces River. A groundwater flux of 310,000 m3 day− 1 is required to supply the needed 226Ra, based on the measured maximum Ra activity of local groundwater. Though as little as 10% of this flux may be advecting terrestrial groundwater this would still represent 160% of the Nueces River discharge. This makes it unlikely that groundwater discharge alone is supplying all of the additional 226Ra. Oil-field brine could potentially account for the remainder. Leakage of 6290 m3 day− 1 of oil-field brine from the submerged petroleum wells and pipelines within the bay could supply all of the needed 226Ra. Such large fluxes of brackish groundwater and oil-field brine could significantly affect bay nitrogen budgets, salinities, and dissolved oxygen concentrations and should be considered when determining the freshwater inflow requirements for Nueces Bay and similar estuaries.