Groundwater contamination monitoring with multichannel electrical and electromagnetic methods

Abstract

Electrical and ground-based electromagnetic surveys have been carried out at the Ranger minesite in the Northern Territory, Australia to investigate their use in detecting any seepage from structures used to store ore processing tailings. The main aim of this work has been to develop a clearer understanding of any seepage problems at this minesite using a combination of self-potential (SP), direct current (DC) resistivity, induced polarisation (IP), and transient electromagnetic (TEM) methods, with the results being interpreted in conjunction with hydrogeological data. Ultimately, it is aimed to apply an optimal combination of the methods to long-term monitoring of potential seepage. A 64-channel system developed at the Cooperative Research Centre for Australian Mineral Exploration Technologies (CRC AMET) has been used to record simultaneously the response at a number of electrodes with the SP, DC resistivity and IP methods. A 2D array of electrodes was set up to monitor the SP response over an area measuring 200×300 m. The simultaneous measurements enable time-varying telluric noise associated with SP responses to be minimised. In-line array DC resistivity and IP measurements were made efficiently at 10-m station intervals with the multichannel system, e.g. complete Schlumberger soundings centred at 30 separate stations were made in approximately half a day. Data collected in October and December 1998 north of the Ranger minesite tailings dam are the first of a long-term project, in which changes of responses will be monitored. A trend of increasing chargeability is observed towards a fault that is the main path of any seepage from the tailings dam. At this stage, maps of the SP response measured with the 2D array at different times indicate that the results are reproducible, and there is an association of an SP anomaly with the presence of seepage in an intersection of two faults. It appears that the IP and SP methods offer the best possibility for the direct detection of seepage in this case. Furthermore, the DC resistivity and TEM methods help to resolve geological structure relevant to hydrogeological processes that could affect the behaviour of seepage over time.