Hydrogeology and geochemistry of near-shore submarine groundwater discharge at Flamengo Bay,Ubatuba, Brazil |
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Authors: | June A. Oberdorfer Matthew Charette Matthew Allen Jonathan B. Martin Jaye E. Cable |
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Affiliation: | 1. Department of Geology, San Jose State University, One Washington Square, San José, CA 95192-0102, USA;2. Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institute, Clark 439, MS#25, Woods Hole, MA 02543, USA;3. Department of Geology, University of Florida, P.O. Box 112120, Gainesville, FL 32611-2120, USA;4. Coastal Ecology Institute, Louisiana State University, 1209 Energy, Coast and Environment Building, Baton Rouge, LA 70803, USA |
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Abstract: | Near-shore discharge of fresh groundwater from the fractured granitic rock at Flamengo Bay, Ubatuba, Brazil, is strongly controlled by the local geology. Freshwater flows primarily through a zone of weathered granite to a distance of 24 m offshore. In the nearshore environment this weathered granite is covered by about 0.5 m of well-sorted, coarse sands containing pore water with sea water salinity, with an abrupt transition to much lower salinity once the weathered granite is penetrated. Further offshore, low-permeability marine sediments contain saline porewater, marking the limit of offshore migration of freshwater. Freshwater flux rates based on tidal signal and hydraulic gradient analysis indicate a fresh submarine groundwater discharge of 0.17–1.6 m3/day per m of shoreline. Dissolved inorganic nitrogen and silicate are elevated in the porewater relative to seawater, and appeared to be a net source of nutrients to the overlying water column. The major ion concentrations suggest that the freshwater within the aquifer has a short residence time. Major element concentrations do not reflect in situ alteration of the granitic rocks, possibly because the alteration occurred prior to development of the current discharge zones, or because of large volumes of water discharge in this high rainfall region. |
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Keywords: | submarine groundwater discharge porewater geochemistry nutrient fluxes |
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