About 25 km south of Haifa in northern Israel, Dor is the site of an ancient port where ongoing archeological research has traced evidence of diverse civilizationsincluding Phoenicians, Israelites, Persians, and Greeksback to at least 3,000 years before the present (see the Tel Dor Project). The town lies on the Carmel coastal plain, which has undergone large fluctuations in sea level during late Holocene time. For example, at the Dor Beach study site, an old Arabic well that today is tidally inundated and thus defunct was situated about 7.5 m from the coast line in 1915 and still operational; the well's history implies that sea level at Dor Beach has risen more than 10 cm in just one century. A sea-level record constructed from geologic, geomorphologic, and archeological data indicates mean-sea-level fluctuations of about 1 m above and below present sea level during the past 3,000 years (Science, 1984, v. 226, p. 831-832; see URL http://www.sciencemag.org/cgi/content/abstract/226/4676/831).
With funding from the United States-Israel Binational Science Foundation, the USGS Submarine Ground-Water Discharge project has conducted studies at Dor since 2004, using various methods to better understand the role of the area's subsurface geology in submarine ground-water discharge and the proportion of recycled seawater in this discharge. The March 2006 fieldwork focused on shallow coastal hydrogeology at Dor Beach and its control on the exchange of submarine ground water with Mediterranean seawater.
The two-way exchange of coastal ground water with seawater is a ubiquitous phenomenon driven by both marine and terrestrial processes. On the marine side, this exchange is affected by water-level fluctuations, such as waves, tides, and storms, as well as by density differences between various water masses. On the terrestrial side, the flow of ground water toward the sea is affected by the underlying geologic framework and seasonal hydrologic cycles. Although the global volume of freshwater delivered to the sea by submarine ground-water discharge is typically estimated to be much less than that of freshwater delivered by river discharge, the potential of submarine ground water to carry contaminants and excess nutrients into coastal waters makes it an important influence on the nearshore environment.
Swarzenski and his colleagues examined the shallow coastal hydrogeology at Dor Beach by using time-series measurements (measurements taken at regular time intervals) of both radon-222 (222Rn) concentrations and multi-electrode electrical resistivity. Resistivity measurements detect pore-water conductivity on the basis of variations in electrical resistance; ground water generally has lower salinity and lower conductivity (higher resistivity) than seawater. Measuring pore-water resistivity with electrical cables towed behind small boatscalled streaming resistivity profilinghas proved to be a useful technique (for example, see Sound Waves article, "Progress in Delineating Submarine Ground-Water Discharge in Delmarva Coastal Bays"). At Dor Beach, however, the scientists used a new technique: a stationary 112-m long, 56-electrode marine cable laid out in several configurationsshore parallel, shore perpendicular, and shore diagonalacross Dor Beach and into the adjacent lagoon (see map). Pore-water resistivity was measured for about 24 hours (to cover a couple of tidal cycles) in each configuration. The measurements made at Dor enabled the scientists to construct detailed profiles of the subsurface freshwater/saltwater interface and its subtle response over time to tides and other forcing factors. Such data can provide unique information about the extent and rates of submarine ground-water discharge. Before the fieldwork at Dor Beach, the stationary method for collecting marine resistivity data had been used only in Florida (see Sound Waves article, "USGS and Florida State University Scientists Collaborate on Submarine-Ground-Water-Discharge Study in the Northern Gulf of Mexico.")
Time-series measurements of 222Rn concentrations in the adjacent coastal water column complemented the resistivity data. 222Rn has been shown to be a particularly effective tracer of sediment/water exchange processes, including submarine ground-water discharge, because this isotope is commonly much more concentrated in ground water than in surface water, is chemically inert, and radioactively decays at a rate (half-life = 3.82 days) comparable to the time scales of many coastal processes. At Dor Beach, the scientists measured 222Rn concentrations in the water column nearly continuously for 4 days from a boat anchored about 40 m from shore (see map). Concurrently, they collected a continuous record of the temperature, salinity, and depth of the water column.
The 222Rn data were modeled to yield flow rates across the sediment/water boundary, which ranged from about 0 to 30 cm per day (mean = 7.1 cm per day), depending on the tidal range. Such results suggest that the underlying hydrogeologic framework at Dor is favorable for substantial submarine ground-water discharge. Extrapolating the mean estimate across a 100-m-wide coastal zone implies a submarine-ground-water-discharge rate of about 7.1 m3 per day per meter of shoreline, an estimate in good agreement with that derived from the resistivity data. The 222Rn data further indicate that the source of the discharging ground water is a mixture of mostly fresh ground water derived from upland kurkar (shore-parallel sandstone ridges interpreted as lithified sand dunes) and recycled seawater.
The recent fieldwork produced detailed information about submarine ground-water discharge at Dor Beach and demonstrated the value of combining geochemical-tracer studies with stationary high-resolution resistivity measurements. These results have identified the varying geologic controls on coastal-aquifer exchange processes and give coastal-resource managers the information they need to better calculate the amount of nutrient input to these coastal systems.
The full citation for the scientific paper about the recent study at Dor Beach is:
Swarzenski, P.W., Burnett, W.C., Greenwood, W.J., Herut, B., Peterson, R., Dimova, N., Shalem, Y., Yechieli, Y., and Weinstein, Y., 2006, Combined time-series resistivity and geochemical tracer techniques to examine submarine groundwater discharge at Dor Beach, Israel: Geophysical Research Letters, v. 33, L24405, doi:10.1029/2006GL028282 [URL http://www.agu.org/pubs/crossref/2006/2006GL028282.shtml].
For additional information about this and other USGS submarine-ground-water-discharge investigations, visit the Submarine Groundwater Discharge project.
in this issue:
Submarine Ground-Water Discharge at Dor Beach, Israel