2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 4
Presentation Time: 1:30 PM-5:30 PM


CABLE, Jaye, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, MARTIN, Jonathan B., Department of Geological Sciences, University of Florida, 241 Williamson Hall, P.O. Box 112120, Gainesville, FL 32611-2120 and STIEGLITZ, Thomas, School of Mathematical & Physical Sciences, James Cook University, Townsville, QLD 4811, Australia, jcable@lsu.edu

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.