Comparison of Electrical Resistivity Methods for Investigation of Ground Water Conditions at a Landfill Site

Abstract

AbstractTwo electrical methods were used and compared in the Investigation of a solid waste landfill site in the central Coast Ranges of California Terrain conductivity measurements and dipole‐dipole resistivity surveys were used in support of an investigation of the hydrology and geochemistry of the site with main concerns being: 1) establishment of baseline conditions, 2) determination of possible contamination and 3) design of a sampling/ monitoring system. Geophysical data were obtained prior to the start of the drilling program and later integrated with reconnaissance geologic data for the purpose of selecting sites for monitoring wells. Two electrical methods were used: 1) dipole‐dipole resistivity; best suited for determining changes in electrical properties with depth and 2) terrain conductivity; best suited for determining lateral changes in electrical properties at a particular depth. Interpretations of the surveys were used in making inferences about the geometry of the saturated zone, ground water gradients, and relative depths to the saturated zone in areas with highly variable conditions. The data were useful in checking for possible ground water contamination and for extrapolating well data across the site.Electrical properties of rock at low frequencies are dominated by effects due to the presence of pore fluids. Differences in conductivity between saturated and unsaturated rock depend primarily on inherent electrical properties of the rock, porosity and conductivity of the fluid. Electrical detection of subsurface zones of contamination, sometimes referred to as “plumes”, in either the saturated or unsaturated zones, requires the existence of a zone of contrasting electrical behavior produced by the contamination. Such contrasts, or anomalies, can be caused by the presence of increased (higher conductivity) or decreased (lower conductivity) concentration of ions. In areas where materials with contrasting electrical properties (such as sandstone and claystone) are present in the subsurface with irregular geometry, anomalies can be difficult to interpret and additional information about geology and ground water conditions must be relied upon.The geophysical survey for this study began with dipole‐dipole resistivity measurements carried out along orthogonal lines in the landfill area. Subsequently, terrain conductivity measurements were carried out and compared with this data. A brief explanation and comparison of the methods used and the results of the surveys are discussed. Following completion of the surveys, the dipole‐dipole data were modeled with the use of a finite‐element computer algorithm. The models are compared with geological and hydrological data obtained during the drilling phase of the program.