23 Hydrogeology (Posters)

Sunday, 20 March 2011
1:30 PM - 5:30 PM
Grand Ballroom/Urban Room

HYDROLOGIC AND GEOPHYSICAL STUDIES AT THE HUGHES BOREHOLE: ACID MINE DRAINAGE PRECIPITATING FROM A FLOWING ARTESIAN WELL

Paper number: 23-3
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Located 2 km west of Cassandra, near Portage, PA, the Hughes borehole discharges large volumes of acid-mine drainage (AMD) onto the surface from a flooded, abandoned deep mine. This flowing artesian discharge is a significant contributor of AMD to the Little Conemaugh River basin. Discharge in the range of 800 to 3000 gpm adds an average of 8,300 pounds of pollutants to the river (PA DEP, Watershed Restoration Action Strategy, Subbasin 18E). At the 6-acre site surrounding the borehole, over 40 years of AMD has caused water quality degradation and metal precipitation on the surface. These precipitates consist largely of iron, manganese, sulfates, and aluminum compounds and have formed a layer with a maximum thickness of 1 meter. These solids have been deposited at an approximate rate of 3 cm per year since 1968, when the borehole seal failed. Water quality studies indicate elevated concentrations of iron and sulfates and low pH.

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.