Evolving Spectral Induced Polarization (SIP) Response of Mine Waste Rock during Humidity Cell and Metal Leaching Experiments

Mining activities can generate significant amounts of waste rock that are disposed into large waste rock piles (WRPs). The reactive sulfidic minerals still residing within the rock can interact with oxygen and water to generate and release toxic acid mine drainage (AMD) to the environment. Knowledge on evolving WRP characteristics, and its generation and release of AMD, is highly desirable. Traditional approaches for such information can come from geochemical monitoring wells or periodic core sampling within the WRP; however, these are costly and invasive approaches that suffer from limited sampling density. An attractive alternative is non-invasive geoelectrical methods, particularly spectral induced polarization (SIP), which exhibits strong potential due to the expected conductivity and polarization signatures of AMD leachate and waste mineralogy. However, very few studies, if any, have measured SIP responses from evolving waste rock. The objective of this study is to investigate temporal SIP responses from waste rock subjected to oxygen and water influx. Waste rock samples from three WRPs in Nova Scotia, Canada, were placed in leaching columns and humidity cells instrumented with SIP. Each sample is subjected to constant water influx, and cyclic water and air influx over several months, with periodic measurements of SIP and water quality (i.e., acidity, sulfate, EC, metals). Preliminary leaching results show the real conductivity in the waste rock is gradually decreasing as the AMD source depletes, with only small changes in the imaginary conductivity. In the humidity cells, the real and imaginary conductivity first increases, as additional AMD is generated, before gradually decreasing over time. In addition to the support of evolving water quality, the mineralogy and acidity of the post-experiment waste rock will be compared to the pre-experiment. This study highlights the potential of SIP to provide valuable information on WRP evolution.