Groundwater and surface water quality in a coastal bay with negligible fresh groundwater discharge: Arraial do Cabo, Brazil

Abstract

Submarine groundwater discharge (SGD) is an important mechanism governing the flux of materials to the coastal region. The loading of nutrients to coastal regions occurs through terrestrial outflow, atmospheric deposition and exchanges with the ocean. In this study, multiple tracers of groundwater input (223Ra, 224Ra, 226Ra, 228Ra, salinity, Ba, Si, U, δ(18O) and δ(2H)) were used to determine the amount of nutrients contributing to the submarine groundwater discharge in the Arraial do Cabo region, which is located on Brazil's southeastern coast. This region is a summer resort, and its population increases by a factor of ten during vacation periods. The nutrient concentrations peaked at 1700μmolL−1 for NO3− and 900μmolL−1 for NH4+ in groundwater wells close to the beachfront; however, no elevated nutrient concentrations were observed in the coastal seawater. This finding was credited to the existence of a physical barrier which hinders the existence of freshwater SGD and also due to the groundwater pumping existent on almost all buildings close to the shoreline. Seepage experiments demonstrated the presence of SGD, which represents recirculated seawater, based on the δ(18O) and δ(2H) levels and is about 0.06m3m−2day−1. Ra was enriched in groundwater samples (1.2±1.0Bqm−3 for 223Ra and 12.2±9.2Bqm−3 for 224Ra, 95% confidence interval) compared to seawater samples (0.077±0.013Bqm−3 for 223Ra and 0.66±0.17Bqm−3 for 224Ra, 95% confidence interval).