Geoelectrical Methods Used to Study Polluted Bedrock Aquifers

Geoelectical investigations of fractured bedrock aquifers have been performed in three main study areas: (i) Tiverton, R.I. As part of a Rhode Island Department of Environmental Management study evaluating the hydrogeology of an area in which bedrock wells are contaminated with hydrocarbons. (ii) Johnston, R.I. as a part of a study conducted for Solid Waste Management Co., to evaluate the hydrogeology of the fractured bedrock under a landfill. (iii) Presque Isle, Maine as a part of a study by the Geologic Survey of Maine, to place a high yield well in bedrock for the purpose of irrigation. Remote sensing and geophysical methods were used to locate possible fracture zones in the three areas. Vertical electrical soundings, after Schlumberger, have been made over these suspected fractured zones. Other measurements have been made by the profiling and the AB rectangle method. Theory has been presented that links flow of fluids to flow of direct current through fractured rock. This theory results in an equation for predicting permeability from formation factors, k = α F-r (Katsube and Hume, 1987). Comparisons to hydraulic parameters have been made using the bulk resistivities of the bedrock, as interpreted in Schlumberger depth soundings, and formation factors, calculated with known ground water resistivities. The Johnston, RI study area showed a good relationship between permeabilities, predicted by the formation factor, and hydraulic conductivities, averaged from packer tests. This further resulted in the actual estimating equation of k = 7.53 x 10-6F-1.08. The Maine study area showed a good linear relationship between bedrock resistivity and well yield on a bilogarithm plot. This relationship keeps the general form of the equation presented. Although actual predictions of yield are not possible, area may be ranked from low to high potential yield. Correlations were also made to seismic velocities of the bedrock in the Johnston and Tiverton, RI areas. These comparisons yielded interesting results, suggesting that in areas of wide ranging pore water resistivities, the bulk resistivity and not the formation factor may better describe the relative hydraulic characteristics of the bedrock. Methods have been suggested which would greatly improve and enhance the use of Schlumberger profiling and AB-rectangle techniques. This method involves selecting optimal current electrode spacing using the depth sounding curve. The expected resistivities are calculated, using a computer program, for the AB-rectangle given the model interpretation from a depth sounding. These values are replotted on the depth sounding curve to view the effect of other layers on the measurement. An ideal size of the rectangle may be found using this technique, giving better control of the inherent change in depth with this method. True anomalies may then be calculated by subcontracting the value of the expected resistivity from the measured at the location