High Resolution Resistivity Mapping of Subsurface Controls on Acid Rock Drainage at an Active Mine Site

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Two dimensional and quasithree dimensional high resolution resistivity (HRR) imaging of a refractory ore stockpile area at an active mine identified several distinct electrically conductive features. A residual potential mapping (RPM) survey delineated the possible flow pathway between two known acid rock drainage (ARD) seeps. Subsequent drilling shows a strong correlation between the resistivity imaging results and the pre-development drainage patterns, the associated clay base fill, and the seeps. In addition, the drilling indicated no pooling of ARD and strongly suggested lateral movement of the ARD along the top of the clay fill layer. This initial interpretation confirms that the clay fill base prevented vertical permeation of the ARD into the bedrock and suggests that the ARD pooling to the east and up gradient of the investigation area is most likely due to a malfunctioning French drain. Introduction A geophysical characterization of subsurface acid rock drainage (ARD) pathways originally associated with refractory ore stockpiles at an active gold mine was completed. Upon removal of one stockpile, the problem temporarily disappeared. The following year more ARD was observed at locations along a haul road that borders the boundary of the area. Much of the currently observed ARD reports to a sump on the opposite side of the haul road. The source and flow paths that convey the ARD were not well understood. The objectives of this geophysical survey program were to characterize the subsurface in and around the area that was thought to contain the source zones and transport pathways for ARD. Because it was anticipated that the ARD would be considerably more electrically conductive than the host material through which it is flowing, electrical geophysics was selected as the investigative tool. Two methods were used in this investigation: high resolution resistivity (HRRTM) and residual potential mapping (RPMTM). HRR data are acquired using a pole-pole array and processed using an algorithm developed by hydroGEOPHYSICS, Inc. (HGI) (Rucker & Fink, 2007). RPM is a mise-a-la-masse electrical potential method that removes the primary field for interpretation (Baldyga & Fink. 2004). The results of the survey were used to direct a targeted drilling program in search of subsurface ARD confirmation for eventual remedial action.