A New Focus on Groundwater–Seawater Interactions (Proceedings of Symposium HS1001 at IUGG2007, Perugia, July 2007)

A New Focus on Groundwater–Seawater Interactions (Proceedings of Symposium HS1001 at IUGG2007, Perugia, July 2007). IAHS Publ. 312, 2007, 100-106.

Multi-channel resistivity investigations of the freshwater–saltwater interface: a new tool to study an old problem

PETER W. SWARZENSKI¹, SARAH KRUSE², CHRIS REICH¹& WOLFGANG V. SWARZENSKI³

1 US Geological Survey, St Petersburg, Florida 33701, USA

pswarzen@usgs.gov

2 Dept of Geology, University of South Florida, Tampa, Florida, USA

3 US Geological Survey, retired, 1932 Los Angeles Ave., Berkeley, California 94707, USA

Abstract It has been well established that fresh or brackish groundwater can exist both near and far from shore in many coastal and marine environments. The often permeable nature of marine sediments and the underlying bedrock provides abundant pathways for submarine groundwater discharge. While submarine groundwater discharge as a coastal hydrogeological phenomenon has been widely recognized, only recent advances in both geochemical tracers and geophysical tools have enabled a realistic, systematic quantification of the scales and rates of this coastal groundwater discharge. Here we present multi-channel electrical resistivity results using both a time series, stationary cable that has 56 electrodes spaced 2 m apart, as well as a 120 m streaming resistivity cable that has two current-producing electrodes and eight potential electrodes spaced 10 m apart. As the cable position remains fixed in stationary mode, we can examine in high resolution tidal forcing on the freshwater–saltwater interface. Using a boat to conduct streaming resistivity surveys, relatively large spatial transects can be rapidly (travel speed ~2–3 knots) acquired in shallow (~1–20 m) waters. Sediment formation factors, used to convert resistivity values to salinity, were calculated from porewater and sediment samples collected during the installation of an offshore well in Tampa Bay, Florida, USA. Here we examine the seabed resistivity from sites within Tampa Bay using both stationary and streaming configurations and discuss their overall effectiveness as a new tool to examine the dynamic nature of the freshwater–saltwater interface.

Key words electrical resistivity; coastal hydrogeology; formation factor; submarine groundwater discharge