Hydrogeological processes and near shore spatial variability of radium and radon isotopes for the characterization of submarine groundwater discharge

Abstract

Inadequate characterization of variability in natural tracers of submarine groundwater discharge (SGD) introduces large errors into tracer-derived SGD estimates. To address this gap, we investigated spatial variability in the natural SGD tracers 223Ra, 224Ra, 226Ra, 228Ra and 222Rn using a high-density array of piezometers and seepage meters over a nearshore area where discharging groundwater transitioned from fresh to saline. Seepage meters and piezometers were used to sample groundwater and to quantify fluxes and salinity. A series of spatial patterns was distinguished beyond the normal salinity impact on Ra activity. The discharge in the interface between saltwater and freshwater was characterized by higher activities of the longer-lived isotopes (226Ra and 228Ra), while areas dominated by benthic exchange had higher activities of the shorter-lived isotopes (223Ra and 224Ra). Spatial differences in Ra activities were associated with variation in salinity and residence time in the aquifer and were indicative of underlying hydrogeological processes. At the freshwater-saltwater interface, fresh discharge is driven by terrestrial hydraulic gradients and saline discharge is driven by density gradients; both result in long residence times in the aquifer. Benthic exchange of saltwater has shorter residence times, much less than required to enrich long-lived isotopes and reach secular equilibrium. The highest activities of 222Rn were detected in the fresh discharge zone, and the lowest activities in areas dominated by benthic exchange. Direct sampling at discharge points allowed comparison between groundwater collected from inland wells and the water discharging to the bay, and indicated significant differences in the activities despite the short distance from wells to the discharge area. Substantial spatial variability in Ra and Rn was observed on the meter scale, with trends that reflected groundwater origin and residence time. Inadequate characterization of variability, trends, and fluxes introduces large errors into tracer-derived estimates of submarine groundwater discharge. Thus, the study of submarine groundwater discharge with radioactive tracers requires adequate hydrogeological knowledge to identify processes that have strong impacts on Ra and Rn activities and associated fluxes to the ocean.