Acid Rock Drainage (ARD)
In many instances, finding the source and transport pathways of Acid Rock Drainage (ARD) cannot be solved by ordinary characterization or monitoring methods. These standard methods, which include drilling and groundwater sampling, may not have the sufficient lateral resolution to pin-point the problem area. Given that Acid Rock Drainage is a critical and time sensitive issue for the mining industry, HGI has several geophysical techniques and survey programs that are available to aid in the process of solving this challenging problem.
Acid Rock Drainage is a common environmental problem at abandoned mine sites in the United States and poses problems for local watersheds, including streams, soil, groundwater, and aquatic life.
ARD Hazards
Two primary methods are typically used in an Acid Rock Drainage investigation: High Resolution Resistivity HRR™ and Residual Potential Mapping RPM™ . Both techniques have been tested and proven for over 20 years through successful surveys throughout the mining industry. ARD targets are typically well suited for HRR™ and RPM™ techniques because ARD is considerably more electrically conductive than the host material through which it is flowing. Results are commonly used to direct a targeted drilling program for remedial action.
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Locate Subsurface Fluid Flow Pathways
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Delineate Vertical and Horizontal Extent of Acid Rock Drainage
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Optimize Locations for Remediation Efforts
Example:
Acid Rock Drainage Landusky-Zortmann Reclaimed Gold Mine, Montana
An electrical resistivity survey was completed at the Landusky mine, in a historic mining district dating from the 1880s. The survey consisted of 15 lines on the surface of the reclaimed Suprise pit, Queen Rose pit, and the region immediately south of Swift Gulch. Additionally, wells and seeps were used by energizing electrodes in direct contact with groundwater to increase the sensitivity of the resistivity method at depth. The survey was conducted to locate potential acid rock drainage pathways that are contaminating Swift Gulch. The results below show that the lowest resistivity values were coincident with the Queen Rose pit. Furthermore, the low resistivity feature appeared to trend northeast along a known fault, consistent with the geologic understanding of the site. A scatter plot of resistivity values versus total dissolved solids (TDS) showed a strong correlation (R2=0.85).
Results from an electrical survey at the Landusky-Zortmann Reclaimed Gold Mine, Montana
Acid Rock Drainage Hazards
Acid Rock Drainage is a common environmental problem at thousands of abandoned mine sites in the United States and elsewhere. Whether naturally or anthropogenically caused, ARD poses problems for local watersheds, including streams, soil, groundwater, and aquatic life. The most severe environmental problems typically stem from low pH, sulfide-induced water and the mobilization of heavy metals from pits, leach pads, and waste rock piles. Although iron and aluminum typically are the main dissolved species in ARD, others such as cadmium, lead, zinc, and copper may also be present.
The creation of ARD has been well established. ARD is caused by weathering and oxidation of ore minerals, such as pyrite and pyrrhotite. When these sulfide minerals are exposed to water and oxygen, oxidation and hydrolysis reactions produce sulphuric acid and free hydrogen ions, acidifying the water. Open pit mining and the subsequent formation of pit lakes may further enhance the process by providing an available source for water infiltration and mobilization of the acid water.
Once offsite, the ARD may require expensive treatment options to mitigate against further degradation of the watershed. These options are limited to the invasive capture and control of groundwater, through pump-and-treat, reactive barriers, or funnel and gate systems. Regardless of methodology, a proper characterization is necessary to understand the source and direction of ARD. Geophysical characterization, and in particular electrical resistivity, is an attractive option to begin understanding the flow of ARD based on price and coverage. A resistivity survey will likely cost less than a couple of completed groundwater monitoring wells. If these wells were sited incorrectly, then the geophysical survey more than pays for itself.