This study investigates the effectiveness of direct current electrical resistivity as a tool for assessing ground water/surface water interactions within streams. This research has shown that patterns of ground water discharge can be mapped at the meter scale, which is important for understanding stream water quality and ecosystem function. Underwater electrical resistivity surveys along a 107-m stream section within the Burd Run Watershed in South Central Pennsylvania identified three resistivity layers: a resistive (100 to 400 Omega m) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Omega m) middle layer corresponding to residual clay sediments, and a resistive (100 to 450 Omega m) bottom layer corresponding to the carbonate bedrock. Tile probing to determine the depth to the bedrock and resistivity test box analysis of augered sediment samples confirmed these interpretations of the resistivity data. Ground water seeps occurred where the resistivity data showed that the residual clays were thinnest and bedrock was closest to the streambed. Plotting the difference in resistivity between two surveys, one conducted during low-stage and the other during high-stage stream conditions, showed changes in the conductivity of the pore fluids saturating the sediments. Under high-stream stage conditions, the top layer showed increased resistivity values for sections with surface water infiltration but showed nearly constant resistivity in sections with ground water seeps. This was expressed as difference values less than 50 Omega m in the area of the seeps and greater than 50 Omega m change for the streambed sediments saturated by surface water. Thus, electrical resistivity aided in characterizing ground water discharge zones by detecting variations in subsurface resistivity under high- and low-stream stage conditions as well as mapping subsurface heterogeneities that promote these exchanges.
Self-potential (SP) is the method that everyone knows about but nobody seems to appreciate. Out of more than 850 papers published in the Symposium for the Application of Geophysics to Environmental and Engineering Problems (SAGEEP) between 1988 and 2001, 63 included SP as a key word, but most mentioned it only in passing. This is surprising when you consider that SP is nonintrusive, fast, and inexpensive, requiring little more than a voltmeter and a few nonpolarizing electrodes, and that environmental geophysical surveys are typically low-budget operations. What is doubly surprising is that less than a handful of the 850 SAGEEP papers discussed SP anomalies of electrochemical origin, even though electrochemical potentials associated with ore bodies are by far the most important source of SP in the mining industry.The majority of environmental papers that focused on SP as the primary geophysical method discussed the mapping of seepage in dams, embankments, leaky containment ponds, and other sources of streaming potential. Two possible reasons for this apparent lack of enthusiasm for SP among environmental geophysicists are electrical noise and difficulties with interpretation.Exploration geophysicists are familiar with SP noise sources such as telluric currents, electrode drift, topographic effects associated with streaming potentials, photovoltaic potentials, and changes in soil composition, moisture, and vegetative cover. But environmental sites add power lines, buried utilities (some cathodically protected), grounded fences and equipment, corroding scrap metal, and other man-made sources to the list of undesired voltages. For instance, we have observed an electric rail system create an SP interference 1 km away; an interference from electric ore trains more than 20 km from the survey area; and a data logger's large increase in the SP noise levels between about 8 A.M. and 4:30 P.M., coinciding with the day shift at a nearby plant.Effective 60-Hz notch filters in modern handheld digital voltmeters eliminate most cultural noise. In most cases the SP signal levels are one or two orders of magnitude above background noise.
[1] Low-permeability sediments situated at or near the sediment-water interface can influence seepage in nearshore margins, particularly where wave energy or currents are minimal. Seepage meters were used to quantify flow across the sediment-water interface at two lakes where flow was from surface water to groundwater. Disturbance of the sediment bed substantially increased seepage through the sandy sediments of both lakes. Seepage increased by factors of 2.6 to 7.7 following bed disturbance at seven of eight measurement locations at Mirror Lake, New Hampshire, where the sediment representing the greatest restriction to flow was situated at the sediment-water interface. Although the veneer of low-permeability sediment was very thin and easily disturbed, accumulation on the bed surface was aided by a physical setting that minimized wind-generated waves and current. At Lake Belle Taine, Minnesota, where pre-disturbance downward seepage was smaller than at Mirror Lake, but hydraulic gradients were very large, disturbance of a 20 to 30 cm thick medium sand layer resulted in increases in seepage of 2 to 3 orders of magnitude. Exceptionally large seepage rates, some exceeding 25,000 cm/d, were recorded following bed disturbance. Since it is common practice to walk on the bed while installing or making seepage measurements, disruption of natural seepage rates may be a common occurrence in nearshore seepage studies. Disturbance of the bed should be avoided or minimized when utilizing seepage meters in shallow, nearshore settings, particularly where waves or currents are infrequent or minimal.Citation: Rosenberry, D. O., L. Toran, and J. E. Nyquist (2010), Effect of surficial disturbance on exchange between groundwater and surface water in nearshore margins, Water Resour. Res., 46, W06518,
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