Mapping the Variability of Groundwater Quality in an Abandoned Tailings Deposit using Electromagnetic Geophysical Techniques

Abstract

A geophysical study was conducted at an abandoned coal mine site in southwestern Indiana in an effort to characterize the spatial variability of groundwater quality and to identify areas that contain high concentrations of total dissolved solids (TDS) and other indicators of acid mine drainage. The study utilized an EM34-3 terrain conductivity instrument to measure the apparent electrical conductivity of the underlying earth. Terrain conductivity is routinely attributed to the electrical conductivity of the underlying material, porosity, moisture content, and the dissolved electrolytes in pore fluid. To interpret the instrument response, terrain conductivity data were compared to field and laboratory chemistry of water samples collected from 27 monitoring wells.

Terrain conductivity values ranged from 17-58 millisiemens/meter over the extent of the study area which included mine refuse, levee material, and natural soils. The contribution of pore water chemistry to the overall terrain conductivity was analyzed by measuring the specific conductance (SpC) of ground water samples which is a reflection of the concentration of TDS. The specific conductance ranged from 1380-5410µmhos/cm at 25° C; where the higher SpC values correspond to a higher concentration of TDS due to pyrite dissolution. A map of the terrain conductivity values indicated that high conductivity values were concentrated in specific areas which will need special attention in remediation plans. The mapping also indicated that the majority of the site contains groundwater with a low SpC and should be amenable to less intensive remediation. A map of the contamination plume based on terrain conductivity values was consistent with a groundwater flow model constructed for this site. A correlation was also observed between subsurface hydraulic conductivity and terrain conductivity measurements (R2=0.66) indicating an instrument response to soil permeability. This study indicates that shallow electrical geophysical exploration can be used to locate groundwater contamination plumes when subsurface hydraulic properties are taken into account.