Optimising mine pollution control measures

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SRK News | Issue 55: Mine Water Management

Sheila Imrie, Principal Scientist
Simon Lorentz, Principal Hydrologist

Palabora Copper (PC) operated an open pit to recover copper ore (and secondary metals) from 1964 until 2002, and has since mined the ore by underground block caving.

Portions of the mine complex, particularly the tailings impoundments and magnetite stockpiles, are located within 1km of Kruger National Park (KNP). The primary Waste Rock Dump and Return Water Dam are located within 500m of the Selati River, a tributary of the Olifants River, which also flows through KNP. PC has installed a comprehensive network of groundwater and surface water sampling sites to monitor the impacts of their activities and has implemented some remedial actions. However, subsurface contaminant flow does move across the KNP boundary.

The objectives of this project were primarily to update the conceptual and numerical groundwater model and subsequently to assess the current and proposed remedial options. Conducting Guelph permeameter tests on an existing tailings impoundment. However, SRK applied a multidisciplinary approach to assess not only groundwater, but also surface water and vadose zone pathways to identify and quantify contaminant migration. Key results from the study include:

Upper & Lower Aquifer Interaction.
The site is underlain by a semi-confined Upper Aquifer and a confined Lower Aquifer, almost entirely driven by flow within dyke contact zones. These aquifers have not been previously defined in borehole logs, and screens or open holes through both aquifers exist. With statistical analysis and studying old borehole logs, however, water quality and levels in ‘dyke contact’ boreholes showed very different signatures to those of ‘granite only at depth’ boreholes. Many of the proposed pollution control boreholes have previously been sited within dyke contacts where yields are highest. However, this SRK project proved that the Lower Aquifer is largely protected from contamination in the Upper Aquifer by a naturally upward vertical hydraulic gradient and distant recharge zone.

If the Lower Aquifer is pumped for ‘groundwater remediation’, however, the Aquifers’ drawdown may reverse this vertical hydraulic gradient, inadvertently spreading the contaminant plume, and unnecessarily pumping uncontaminated water from the Lower Aquifer.

Preferential Pathways for Contaminant Flow.
Contaminant flow near the source is often fairly diffuse and only later converge into sandy drainage channels flowing towards the rivers. Thus, the cost-effectiveness of the currently proposed pumping of the Upper Aquifer requires re-evaluation to prevent selecting boreholes with poor long-term yield rates and localised contaminant mass capture. The drainage channels form the prime location for effective cut-off trenches and pollution control boreholes.

Surface Water and Interflow Contribution to River Loads.
The contributions of sulfate to the Selati River reveal a significant discrepancy between groundwater and the loads from flow and water quality in the river. We assumed a significant sulfate load from sporadic, rainfall induced discharges. However, this mechanism needs to be confirmed before remedial measures can be recommended, necessitating further study during 2017.

With the application of SRK’s fresh eyes, critical data review and crossdisciplines (geohydrology, hydrology, hydropedology) and office collaboration, various traditional concepts were challenged, thus bringing to light important new conceptual thinking, backed up by numerical modelling, and forming a key-pin of future remediation management and associated studies at the site.

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