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U.S. Department of the Interior
An autonomous, electromagnetic seepage meter to study coastal groundwater/surface-water exchangePW Swarzenski1, M Charette2 and C Langevin3
1USGS - St Petersburg, FL
2WHOI - Woods Hole, MA
3USGS - Miami, FL
IntroductionThe bi-directional exchange of groundwater with coastal surface waters may influence not only coastal-water and geochemical budgets, but may also impact and direct coastal ecosystem change (D'Elia, et al., 1981; Valiela, et al., 1990; Burnett et al., 2003). For example, the widespread discharge of nutrient-enriched submarine groundwater into an estuary or lagoon may contribute directly to the onset and duration of eutrophication (Bokuniewicz, 1980; Giblin and Gaines, 1990), as well as the development of harmful algal/bacterial blooms (Laroche et al., 1997). Most often, this submarine groundwater discharge (SGD) (defined here as a composite of meteoric, connate and sea water) occurs as hard-toconstrain diffuse seepage (Figure 1), rather than as focused discharge either through vent or collapse features (Swarzenski et al., 2001). As a result, quantifying SGD rates has remained difficult for both oceanographers and hydrologists alike. This report describes an adaptation of an old tool, the Lee-type manual seepage meter (Lee, 1977), with a state-of-the-art electromagnetic flow meter that enables rapid, autonomous, bi-directional measurements of fluid exchange rates across the sediment/water interface (Rosenberry and Morin, 2004). When such measurements are coupled and interpreted with surface and groundwater pressure, salinity and temperature data, as well as other complementary measurements such as excess watercolumn 222Rn activities, then realistic groundwater/surface-water exchange rates can be obtained in dynamic coastal environments (Swarzenski et al., 2004).
Principles of an EM Seepage Meter
Distinct advantages of the EM seepage meter as a tool to study submarine groundwater discharge include i) lack of moving parts and thus less down time, ii) AC or DC power options, iii) ease of calibration prior to operation, iv) very rapid measurement of a wide range of groundwater/surface-water exchange rates that can span at least three orders of magnitude, and v) rapid sampling rate (typically one measurement per minute), well suited for dynamic coastal environments where tidal forcing, wind, and currents can complicate groundwater/surfacewater exchange.
Field Testing the EM Seepage Meters
Bench calibration studies
A USGS-WHOI intercalibration experiment in Everglades National Park, FLTo assess the utility of the EM seepage meters in environments with very low groundwater/surface-water exchange rates, we installed our EM seepage meters in highly organic bottom sediments of Bottle Creek, a distal reach of the Shark River Slough, Everglades National Park, Florida, in August and October, 2003. Directly adjacent to our EM seepage meter sites, we also installed a dye-dilution type seepage meter (Sholkovitz et al., 2003) that can autonomously record bi-directional rates of groundwater/surface-water exchange. During a 5-day intercalibration experiment, the EM and dilution seepage meters produced average exchange rates very close to one another, 2.3 cm day-1 and 2.4 cm day-1, respectively. These encouraging results exemplify the potential of the EM seepage meter, even in challenging environments. Results from an instrumented shallow well proximal to our seepage meter site as well as from the dilution seepage meter suggest a gradual hydraulic response to our well/meter installation. In low-permeability sediments such as the peat deposits of Bottle Creek, one can expect that the lengthy equilibration time due to meter installation will compromise initial EM readings. In higherpermeability sediments, this initial equilibration time may be much lower, ~ 30 min. (Rosenberry and Morin, 2003).
Sarasota Bay, FLTo contrast the Bottle Creek site, we also deployed our EM seepage meter at one site in Sarasota Bay, Florida (June, 2004), where groundwater/ surface-water exchange rates are expected to be much enhanced by physical and hydrogeologic characteristics unique to these coastal waters. At this site, during the seepage meter deployment, water levels fluctuated by about 80 cm (Figure 5) and correlated reasonably well (R2 = 0.49; Figure 6) with 10-min. complied groundwater/surfacewater exchange rates (average = 15.2 cm day-1). A strong correlation between submarine groundwater discharge rates and surface-water levels implies that groundwater/ surface-water exchange is controlled more by tidally and density-driven sea water recirculation, rather than by groundwater discharge. Our results thus suggest that submarine groundwater discharge at this site in Sarasota Bay is influenced by both physical and hydrogeological forces.
AcknowledgmentsThe authors thank our collaborators who made this work possible: Bill Burnett (FSU), Keith Halford (USGS), Chris Reich (USGS), Marc Stewart (USGS), Brian Blake-Collins (ETI), and the engineers at Quantum Engineering Corp.
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