Seasonal dynamics of dissolved Ra isotopes in the semi-arid bays of south Texas

Copano Bay, Nueces Bay, and Baffin Bay, Texas exhibit some of the highest dissolved 226Ra activities observed in coastal waters despite being in a semi-arid region where surface water is scarce. To determine the reason for this, dissolved 226Ra and 228Ra activities were measured in these bays at three periods during the course of their seasonal flushing cycles. CH4 and dissolved Cl/Br ratios were also measured during the final sampling period to independently assess the possibility of oil-field brine leakage and its potential influence on bay Ra activities. Independent of the high bay Ra activities, evidence for oil-field brine leakage was not found, though we cannot discount this possible influence on the Ra budget. Our results do show that all three bays exhibit pronounced seasonal swings in Ra activity that culminate in high absolute values. Mixing model results indicated that supply from submarine groundwater discharge can balance the Ra budgets of these bays. However, the Ra cycle in these systems is not controlled by a single dominant process, but rather the seasonal relationship of riverine Ra supply, submarine groundwater discharge, input from benthic sediments, and direct evaporation. Thus these bays are best described by a Ra cycle where activities are controlled by several processes in a dynamic, seasonal equilibrium. This illustrates the importance of seasonal controls on estuarine Ra cycling in general, but seasonality may be especially important for semi-arid regions where the hydrologic cycle is influenced by episodic freshwater inputs.