Date of Award
Master of Science in Geology
Reinhard K. Frohlicj
Surface methods of electrical resistivity measurement are used to detect a layer of salt-polluted groundwater within a crystalline bedrock aquifer. Fractured, schistose bedrock overlain by a 15 ft (4.6m) thickness of jointed till has been polluted by runoff from a storage facility for road salt in Little Compton, Rhode Island. Conductivity measurements in two bedrock monitoring wells on the site confirm the existence of highly mineralized groundwater in the bedrock aquifer. Interpretations of two vertical electrical sounding (VEST) curves obtained slightly up-gradient topographically from the pollution source show that a 160-177 ft (49-54 m) thickness of bedrock is polluted while the entire thickness of till is relatively unpolluted. Interpretations of four other VES curves obtained slightly down-gradient from the pollution source show that the till layer is polluted, but the polluted bedrock layer in undetectable. Where the till polluted, the till’s bulk resistivity apparently is sufficiently reduced to suppress the effect of a polluted bedrock layer. While the suppression phenomenon is a major obstacle to the use of resistivity methods in areas of surficial pollution, in outlying areas where high concentrations of mineralized groundwater have flowed more rapidly through the bedrock aquifer than through the surficial aquifer, resistivity methods may be more efficient than random drilling for detecting bedrock pollution. The bulk resistivities interpreted for polluted bedrock compare favorably with published laboratory measurements on rock samples. A calculated bedrock formation factor of 77 is used in conjunction with Archie’s law to obtain a quantitative measure of the pollution. Two other resistivity methods, horizontal profiling and AB rectangle mapping, did not provide conclusive evidence of bedrock pollution where the overlying till was also polluted. However, an AB rectangle map over unpolluted till shows a resistivity contour pattern similar to the fracture orientation observed in local bedrock outcrops. With further research and the development of a computer program to perform the numerous calculations, the AB rectangle method could prove to be an effective method for the placement of bedrock monitoring wells.
Sanders, David Patrick, "Resistivity Methods Applied to Pollution Detection in a Crystalline Bedrock Aquifer at Little Compton, R.I." (1983). Open Access Master's Theses. Paper 2028.