Groundwater contamination monitoring with multichannel electrical and electromagnetic methods

Electrical and ground-based electromagnetic surveys have been carried out a t the Ranger minesite in the Northern Territory Australia to investigate th eir use in detecting any seepage from structures used to store ore processi ng tailings. The main aim of this work has been to develop a clearer unders tanding of any seepage problems at this minesite using a combination of sel f-potential (SP), direct current (DC) resistivity, induced polarisation (IP ), and transient electromagnetic (TEM) methods, with the results being inte rpreted in conjunction with hydrogeological. data. Ultimately, it is aimed to apply an optimal combination of the methods to long-term monitoring of p otential 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 200x300 m, The simultaneo us measurements enable time-varying telluric noise associated with SP respo nses 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 ma de 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 pro ject, in which changes of responses will be monitored. A trend of increasin g chargeability is observed towards a fault that is the main path of any se epage from the tailings dam. At this stage, maps of the SP response measure d with the 2D array at different times indicate that the results are reprod ucible, and there is an association of an SP anomaly with the presence of s eepage in an intersection of two faults. It appears that the IP and SP meth ods offer the best possibility for the direct detection of seepage in this case. Furthermore, the DC resistivity and TEM methods help to resolve geolo gical structure relevant to hydrogeological processes that could affect the behaviour of seepage over time. (C) 2001 Published by Elsevier Science B.V .