An innovative approach to aquifer investigation
Hamersley Iron recently implemented an innovative aquifer testing programme in support of possible future mining below the water table at the Marandoo operation in the Pilbara region of Western Australia. Such future activity would require extensive pumping for mine dewatering, which could affect regional groundwater flows. Groundwater forms the major usable water source in the Pilbara, where average rainfall is below 300 millimeters per year and highly seasonal. Permanent (non-seasonal) surface water bodies are rare and their use is often restricted due to their central importance within Aboriginal culture.
Historically, mining operations in the Pilbara discharge excess water by pumping into existing creek systems. However, evaporative losses often exceed 50 per cent, and ecological experts have recently raised concerns that the constant flows provided by dewatering discharges have significantly altered the natural ecosystem and interfered with its ability to tolerate seasonal dry periods.
When the Marandoo mine began evaluating the impacts of mining below the water table, and investigating how dewatering would be accomplished, re-injection was considered as a means of conserving valuable groundwater resources. There was also concern about the degree to which the deep aquifer was connected with an ecologically important shallow aquifer located nearby, and the potential impacts of dewatering on the forest ecosystem in Karijini National Park, adjacent to the mine, which depends on the shallow aquifer.
To manage the risks effectively, the project team set out to examine the interconnection between the deep and shallow aquifers and the orebody. The testing programme, carried out in October through November 2004, entailed a prolonged period of continuous pumping (44 days) from bores on the mining lease and incorporation of an extensive monitoring network. Simultaneously, the pumped water was re-injected into the deep aquifer system underlying the national park. If re-injection to the deep aquifer resulted in a corresponding rise in the shallow aquifer's water level, that would indicate a degree of interconnection between the two layers. This testing plan addressed stakeholder concerns by avoiding the need to discharge pumped water into surface drainage, and by conserving the fresh groundwater resource that would otherwise be lost during a standard pump test.
Monitoring results showed that, during the re-injection, there was no apparent rise in the shallow aquifer level trends, indicating that there is little connection between the shallow and deep aquifers in the area. Dewatering of the deep aquifer to allow for below-the-water-table mining therefore is not expected to affect the ecosystem associated with the shallow aquifer. The pumping trial also provided extremely valuable data to allow calibration of a numerical ground water model for the project area, subsequently used to plan the dewatering efforts. As a result of the Marandoo trial, aquifer re-injection is now being considered as a tool for investigation and long term excess water management at a number of sites across the Pilbara region.
Back to top