Geophysical Investigations at an Abandoned Mine Site Subjected to Reclamation Using a Fixated Scrubber Sludge Cap

Abstract

A series of geophysical investigations were undertaken to evaluate relationships between sources and areas of groundwater contamination and the effects of reclamation on groundwater quality in part of an abandoned mine land site in the coal-mining district of Indiana. Prior to reclamation, acidic groundwater was perennially draining from a deposit of coarse refuse that was the main source of contaminated water leaving the site. Fixated scrubber sludge (FSS) was used as a capping material to isolate the pyritic refuse from further recharge, and the FSS was in turn capped by a layer of soil material. The buried refuse and associated plume of acidic groundwater could be seen as a low-resistivity layer when an electrode spacing of 16 m was deployed in an Offset Wenner configuration. An electromagnetic terrain conductivity survey was undertaken in conjunction with a magnetometry survey to identify the areas of high metal (pyrite) content and acidic groundwater. A comparison between the apparent conductivity distribution and a map of the prereclamation distribution of mine waste indicated that the plume of acidic groundwater had migrated toward, but not reached, the surface-water outlet of the site. Repeated measurements of soil-moisture profiles from the surface into the buried refuse revealed the FSS layer to be a zone of no moisture change, which indicated that the layer was serving as an effective barrier to further recharge of the aquifer in the buried refuse. Continuous monitoring of water levels in the buried refuse and the soil layer above the FSS confirmed that water was perching on the layer of FSS. Iron concentrations in samples of water collected from the well that was screened in the refuse declined from a maximum of 6,000 mg/L prior to reclamation to <2,000 mg/L three years after the FSS was applied.