IN SITU EVALUATION OF NEARSHORE POREWATER EXCHANGE RATES IN FLAMENGO BAY, BRAZIL

As part of a larger IAEA/UNESCO intercomparison study of submarine groundwater discharge measurement techniques in southeastern Brazil, we sampled porewaters from four multilevel piezometers installed along a 45-m long transect normal to shore. In addition, water level recovery rates were estimated using pump tests along a parallel onshore-to-offshore well transect. This coastline is characterized by the Serra do Mar Mountain range, which abuts Flamengo Bay leaving a thin strip of sandy beaches along this rugged Atlantic coast. Tidal ranges measured in the coastal community of Ubatuba adjacent to the bay were about 1 to 1.5 m. The shoreline is steep (~ 0.044) and wave run-up is about 0.3 to 0.6 m above sea level, especially on flood tide. Rn-222 and Cl^-, along with conductivity and temperature, were measured at discrete depth intervals from the piezometers down to 1.5 to 2 m below the sediment-water interface to evaluate the contribution of groundwater to the bay. Additionally, a fluorescien dye solution saturated with SF₆ was injected into one discrete depth interval in each piezometer to evaluate the exchange rate between the upper sediments and the surface waters. Using the SF₆, exchange rates were rapid, on the order of 28 to 184 cm/day, in the upper 1 to 2 m of sediments. Rn-222 and Cl^- in porewaters support a high turnover rate in these sediments. In this well-mixed sediment-porewater regime, terrestrial groundwater inputs were difficult to quantify. Nonetheless, below about 50 cmbsf, porewater salinities were less than 5 ppt in the nearest piezometers to shore, indicating terrestrial groundwater was discharging to the bay.