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Elandsfontein, Western Cape: groundwater predictive numerical modelling

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A4   |   Letter

SRK News | Issue 55: Mine Water Management

Sheila Imrie, Principal Hydrogeologist

The proposed Elandsfontein phosphate open pit mine is located on the south-western coast of South Africa, 8km inland from the environmentally sensitive Langebaan Lagoon, and bordering the West Coast National Park. SRK was commissioned to develop the numerical groundwater model for the design of the dewatering scheme. This sparked several subsidiary projects including geochemical analysis, engineering design for dewatering, and modelling and design of the mine pit.

The project has successfully overcome many challenges, including the hydrogeological setting which features an upper aquifer (where the ore body resides) composed of quartz grains and calc-arenites from marine deposits. Under it is a few-metre-thick unit of ‘running’ quartzose sands, a strong preferential pathway for groundwater flow, as local pumping tests proved. A confining clay aquitard, is situated below this sand, and under this an alluvial aquifer, comprising basal gravels/quartz sediments and gravel-filled palaeochannels with an upward hydraulic pressure head bearing on the aquitard. In this framework, SRK Cape Town undertook:

  • Early consultations on hydrogeological and modelling approaches with SRK Cardiff, who had undertaken similar dewatering models for a phosphate mine in the Republic of Congo.
  • Consultations with SRK Cape Town’s team to address clay heave concerns from the lower aquifer, and slope stability issues within the upper aquifer.
  • Integration of the hydrogeological model and the geotechnical stress analysis models.
  • Detailed groundwater modelling and calibration of the hydrogeological system.

The South African Department of Water Affairs demanded that all water abstracted should be routed through a closed system and re-injected into the aquifer down-gradient (towards the lagoon) to mitigate potential reduction of baseflow. Challenges included:

  • Rendering mine planning data (from others) into usable formats for the hydrogeological model.
  • Calculating and modelling the continually altering flow rates for dewatering and re-injection.
  • Controlling mounding at the re-injection site and managing the mounded water that partially flows back to the pit area.
  • Considering potential geochemical alterations and risks within the closed (but vigorously pumped) dewatering system.
  • Providing design input to the engineering design team (before drilling, dewatering and re-injection boreholes).

The mine is located in a highly sensitive environment, with multiple stakeholders and media actively challenging the continuance of the mine. SRK was instructed to:

  • Develop clearly defined model objectives up-front to control complexities related to the sensitivity of the project and client expectations.
  • Manage and respond to extensive external review and queries.
  • Present clear and defensible explanations of the groundwater assessment and model results at challenging public stakeholder meetings.

As of February 2017, pit mining the overburden and dewatering (and reinjection) has begun. The mine is meeting tight timelines and regularly consulting SRK during implementation. Dewatering success depends on this continuing close relationship.

SRK North America