HYDROLOGIC AND GEOPHYSICAL STUDIES AT THE HUGHES BOREHOLE: ACID MINE DRAINAGE PRECIPITATING FROM A FLOWING ARTESIAN WELL
To investigate the subsurface effects of the discharges, we performed geophysical surveys using an R-50 resistivity meter, a Geonics EM-31 probe, and a proton precession magnetometer. The magnetometer mainly detected cultural features at the site, including the borehole casing itself, fencing, and buried debris. The depth of investigation of the EM-31 probe was limited by the highly conductive surface layer. We then conducted resistivity sounding across the entire site, reaching a Wenner array spacing of 31.6 m. Comparative data were also obtained outside the influence of the AMD discharge. We analyzed the data using inversion software and data tables. The results reveal a conductive surface layer 10 m thick with a resistivity of only 90 ohm-m, and a thick, underlying layer with a resistivity of 1600 m. We interpret the layer deeper than 10 m as sandstone bedrock with minimal fracturing, and the surface layer as a thin soil and regolith column beneath the surface cap, with underlying fractured bedrock that has been infiltrated and saturated to a depth of 10 m by conductive, high-TDS fluids from the borehole discharge.