USGS Groundwater Information: Branch of Geophysics
Rory D. Henderson, U.S. Geological Survey, Storrs, CT and University of Connecticut, Storrs, CT
Frederick D. Day-Lewis, U.S. Geological Survey, Storrs, CT
John W. Lane, Jr., U.S. Geological Survey, Storrs, CT
Charles F. Harvey, Massachusetts Institute of Technology, Cambridge, MA
Lanbo Liu, University of Connecticut, Storrs, CT
Submarine ground-water discharge (SGD) is an important source of nutrients and pollutants to ecologically sensitive estuaries. We observe SGD and the movement of freshwater/saltwater interfaces by measuring two physical properties that differ between the coastal aquifer and estuarine waters — temperature and electrical resistivity. SGD from the fresh coastal aquifer is colder in the summer than estuarine waters, and warmer in the winter. Additionally, fresh ground water is more resistive than high salinity estuarine water. We evaluate the utility of two geophysical methods, fiber-optic distributed temperature sensing (FO-DTS) and marine electrical resistivity (MER) to observe SGD and the freshwater/saltwater interface in submarine sediments. Recent advances in FO-DTS technology allow for measurement of temperature with spatial resolution of 1 m, temporal resolution on the order of 1 min, and temperature precision of 0.01 °C, although tradeoffs exist between temporal resolution and precision. The FO-DTS instrument used in this study is based on analysis of Raman scatter. Raman scattering occurs as light interacts with the fiber materials, producing backscatter energy that is temperature-dependent (anti-Stokes scattering) and temperature-independent (Stokes scattering). The ratio of the magnitudes of the anti-Stokes to Stokes scattering depends exponentially on the fiber temperature. MER is a well documented approach to observing subsurface electrical resistivity contrasts associated with porewater salinity, porosity, or lithology. A 48-electrode cable with 1-m spacing was used; full surveys including reciprocal data were conducted in about 50 min. Our study includes two field deployments at Waquoit Bay National Estuarine Research Reserve (WBNERR), East Falmouth, Massachusetts. The first deployment was a reconnaissance effort in May-June 2006 to observe the areal distribution of SGD. This deployment used a 1.3-km fiber-optic cable deployed in a 80 by 60-m grid The cable was weighted and submerged into the bay sediment to a depth of several centimeters. From the FO-DTS data, we inferred a zone of tidally-driven, near-shore, fresh SGD with minimal shore-parallel variation at Waquoit Bay. We returned to the site for a second deployment in June-July 2007, which used FO-DTS in tandem with MER. We instrumented a single transect extending offshore about 50 m. The cables were weighted and permanently installed at a depth of approximately 0.5 m to ensure replication of data collection geometries on return visits to understand seasonal variability. The FO-DTS temperature time-series data at near-shore locations are dominated by a semi-diurnal (tidal) signal, whereas the temperature at off-shore locations are dominated by a diurnal signal (day/night heating and cooling). MER surveys produced high-resolution time-lapse tomograms, providing insight into the temporal variations of the subsurface freshwater/saltwater interface. The tomograms show a high-resistivity zone near the shore at low tide, indicative of fresh SGD, and consistent with the FO-DTS results.
Final copy as submitted to American Geophysical Union for publication as: Henderson, R.D., Day-Lewis, F.D., Lane, J.W., Jr. Harvey, C.F., and Liu, Lanbo, 2008, Characterizing tidally influenced submarine ground-water discharge in an estuary using fiber-optic distributed temperature sensing and marine electrical resistivity [abs.]: EOS Transactions, American Geophysical Union, Spring Meeting Supplement, Joint Assembly, 27-30 May 2008, Fort Lauderdale, Florida, Invited.