Radon activity and submarine groundwater discharge in different geological regions of a coastal barrier in Southern Brazil

Abstract

Submarine groundwater discharge (SGD) is still a relatively new topic and when associated with geological techniques like Ground-Penetrating Radar (GPR) it can be considered innovative. The present work aims to better understand the influence of geology in this important process of fluid flow from the continent to the coastal environment, with regards to radon isotope flux and hydrological budgets in Southern Brazil. An area that presents two different stacking patterns—progradational and retrogradational—was chosen and GPR scans were applied to these in an attempt to obtain stratigraphic deposition details. The derived discharge rates were calculated as well as the spatial budgets for the water fluxes from the coastal environment. The water moving across the sediment–water interface in these two geological regions is different, despite having the same main driving forces such as rainfall, tide and wave setup. The water flux is 13 times more expressive in progradational stacking (46.2 ± 2.4 cm/day) and radon activity is correlated with depth due to the different sedimentological layers. In the region dominated by retrogradational stacking, the discharge is lower (3.6 ± 0.1 cm/day) and the average radon activity distribution is lower which is influenced by sediment type. Also, the estimated SGD in each region is 969.3 ± 51.2 × 103 and 59.7 ± 1.7 × 103 m3/day for 70 and 55 km of coastline, respectively. These significant differences (p = 0.004) in radon activities and water fluxes certainly are defined by the geological issues being the result of depositional architecture, permeability and porosity.