In the Pilbara region of Western Australia, management of waste rock includes backfilling of mined out pits. Where the backfill is expected to become inundated following groundwater rebound, flow-through conditions are likely to develop. An understanding of the potential for solute release under these conditions is important when assessing the possible impacts backfilled pits could have on the post-closure groundwater quality.
SRK’s geochemists have a range of leach tests available in their toolbox for use in assessing the leaching potential of mine wastes. These include conventional, nationally and internationally recognised tests, and niche tests tailored to address the specific needs of a project.
An example of a niche test is the saturated column test, designed to assess solute leaching under anoxic conditions that are representative of inundated backfill. SRK has applied this method to a range of sample types, including: Low sulfur, but metalliferous, samples, and high sulfur, acid generating samples, which had previously been subjected to ‘accelerated’ weathering and oxidation in free draining columns.
The inflowing solution was actual groundwater collected from the project site, since interaction with solutes present in the groundwater is another key consideration in determining leaching potential from backfilled materials.
The test results demonstrated a number of geochemical trends that may be expected under saturated conditions, such as:
- Readily soluble salts containing major species such as sulfate were progressively flushed over multiple pore volume exchanges. Such salts were present in significant quantities in the high sulfur wastes that had been ‘weathered’ prior to inundation.
- Dissolution of sparingly soluble minerals released low concentrations of contaminants, such as arsenic, boron, barium, cobalt, manganese, thallium and uranium on an ongoing basis.
- Some elements (e.g. arsenic, manganese) were shown to be very sensitive to dissolved oxygen levels in the inflow solution – interpreted as changes in source mineral solubility in response to redox or oxidation reduction conditions. Within inundated backfill, oxygen concentrations are expected to be low; assessing the applicable redox conditions will be an important factor in determining leaching behaviour for some elements.
- Sorption of some solutes (including antimony, molybdenum, selenium and zinc) from the inflowing groundwater was observed.
