Using the radium quartet to quantify submarine groundwater discharge and porewater exchange

The specific ingrowth rates of different radium isotopes make them valuable tracers to distinguish processes occurring at different temporal scales. Here we demonstrate the use of the radium quartet (223Ra, 224Ra, 226Ra and 228Ra) to differentiate flows of submarine groundwater discharge and porewater exchange to a coastal embayment (Alfacs Bay, NW Mediterranean Sea), based on the assumption that these processes occur on different time scales. In order to evaluate the seasonal dynamics of groundwater and porewater inputs to the bay, we conducted three seasonal samplings at Alfacs Bay, during which samples for Ra isotopes were collected from bay waters, groundwater springs, porewaters and irrigation channels. Activities of short-lived Ra isotopes in the bay showed a strong seasonality, (e.g. average 224Ra activities in summer (∼32 dpm 100 L−1) up to 4 times higher than in winter (∼8 dpm 100 L−1)). In contrast, the activities of the long-lived Ra isotopes were fairly constant throughout the year (e.g. activities of 226Ra were ∼16 and ∼14 dpm 100 L−1 in summer and winter, respectively). The relatively short exposure to sediments of recirculation fluxes resulted in porewaters significantly enriched in short-lived Ra isotopes relative to the long-lived ones (e.g. 224Ra = 1100-1300 dpm 100 L−1; 226Ra = 17-99 dpm 100 L−1), whereas coastal groundwaters were enriched in all the Ra isotopes (e.g. 224Ra = 120-150 dpm 100 L−1; 226Ra = 200-400 dpm 100 L−1). The distinct signatures of different sources allowed us to construct seasonal Ra mass balances to estimate both groundwater discharge, which ranges from (40 ± 60)·103 m3·d−1 in summer to (310 ± 200)·103 m3·d−1 in winter, and porewater exchange fluxes, ranging from (1200 ± 120)·103 m3·d−1 in summer to (270 ± 40)·103 m3·d−1 in winter. Whereas the seasonal variability of groundwater inputs is likely governed by the terrestrial hydraulic gradient, a qualitative evaluation of the drivers of porewater exchange suggested that the strong seasonality of the seawater recirculation inputs is likely mediated by seasonal cycles on the activity of benthic infauna. Ra isotopes are thus valuable tracers to differentiate fluxes of both submarine groundwater discharge and porewater exchange, allowing a more accurate evaluation of the fluxes of freshwater and solutes to coastal ecosystems, as well as their implications for coastal biogeochemical cycles.