TEM Methods for Mapping Acid Drainage

Example Research Project supported by the ASEG Research Foundation

Julie Elders: TEM Methods for Mapping Acid Drainage

Host Institution: Monash University

Supervisor: Jim Cull and B McPhail

Contact: benandjules@bigpond.com

Project Summary

Acid drainage generated by the Loy Yang coal mine overburden pile poses a potential environmental risk to the surrounding fauna, flora, and surface waters, as well as the underlying groundwater system. The overburden pile hosts a heterogeneous distribution of coal and sand and clay rich sediments. The generation of acid drainage is primarily attributed to trace concentrations of pyrite that are capable of generating low pH, acidic effluent upon exposure to atmospheric oxygen. Over the past decade, evidence of acid drainage in the overburden pile has been manifested in the form of low pH (2.5-4.5) surface seepage on the western batters.

A multidisciplinary approach that incorporated geophysical, hydrogeological, and geochemical techniques was employed over the west end of the overburden. A coincident-loop, time-domain electromagnetic (TEM) survey was conducted to identify regions of elevated electrical conductivity within the survey area. The acquired data were interpreted in three ways:

1. TEM mapping was used to define regions of potential acid drainage,
2. Conductivity-depth inversion of the individual line data was conducted to identify the depth to the water table, and
3. Geochemical water sampling results were compared to apparent conductivity variations to help quantitatively interpret the TEM response.

Results from the TEM survey identified two regions of elevated electrical conductivity on the western batters of the overburden pile. The position of these regions correlates with surface seeps present immediately west of the anomalies. The layered-earth model developed from the TEM data has successfully determined the depth to the water table. The model correlates well with the hydrogeological data and, in particular, has identified a surfacing of the water table in two regions where seepage is evident. Additionally, the results indicate that the depth of the water table increases to the east and to the north. This interpretation is consistent with the south-westerly slope of the natural ground surface and the overburden pile. Finally, it was possible to identify a constant relationship between the TEM data and the electrical conductivity of the pore water, which indicates that the geophysical anomalies identified within the overburden pile may indeed reflect regions of high pore water conductivity, which are likely to correlate to acid drainage contaminated waters.

The successful correlation of the TEM data to both the hydrogeological and geochemical data available within the survey area, indicates that TEM can successfully be used to identify regions of acid drainage in heterogeneous environments such as those encountered in spoil and tailings piles from mining operations.

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