1. 10th Inkaba yeAfrica/!Khure Africa (AEON) Conference/Workshop
Lord Milner Hotel, Matjiesfontein - Karoo
29 September – 3 October 2014
ESTIMATING THE DECANT RATE AT A REHABILITATED OPENCAST MINE WHERE NET GROUNDWATER INFLOW OCCURS
François Fourie1
1. Institute for Groundwater Studies, University of the Free State
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2. OUTLINE The Problem Site Information
Estimating the Decant Rate by Using a Water Balance Approach
Investigating Preferential Pathways for Groundwater Ingress
Validating the Results by Constructing a Salt Balance

3. Estimating the decant rate
The Problem
Pre-mining
Estimating the decant rate
post-rehabilitation
Post-rehabilitation
Decant rate ≈ Recharge through spoils

4. The Problem (Cont.)
Recharge through the rehabilitated spoils of backfilled opencast mines is strongly dependent on a number of factors, including:
Surface topography of the pits (runoff, ponding)
Success of rehabilitation (compaction of spoils, topsoil cover, vegetation)
The water make of rehabilitated opencast mines is often estimated by assuming an average recharge value through the spoils.
Average recharge values through spoils (expressed a percentage of the MAP):
17% (Wates, Meiring and Barnard, date unknown)
15-25% (Usher, 2003)
20% (Vermeulen, 2003)
15-20% (Vermeulen, 2006)
14-20% (Hodgson et al., 2007)

5. The Problem (Cont.)
For the colliery under investigation, a recharge value of 20% results in an estimated decant rate of approximately 4 ML/day.
This volume is smaller than the volume of water that the colliery currently has to deal with.
This means that:
the recharge percentage is too low, and/or
GWIn > GWOut
The current study aims to estimate the decant rate by making use of:
measured quantities (rainfall, pumped volumes, dam water levels)
estimated quantities (evaporation from open water bodies, sprayers)

6. Site Information
Located in Mpumalanga, near the boundary of quaternary sub-catchment, QSC1, near the confluence of the river systems draining this sub-catchment and an adjacent sub-catchment, QSC2.
78% of the surface area of the two sub-catchments is at elevations higher than the decant level of the colliery.

7. Site Information (Cont.)
Water Reticulation within Pit A: Schematic Plan View

8. Site Information (Cont.)
Water Reticulation within Pit A: Cross-sectional View

9. Site Information (Cont.) Water Levels in Boreholes within Pit A

10. Site Information (Cont.) Compartmentalisation of Pit A

11. Estimating the Decant Rate

12. Estimating the Decant Rate (Cont.)
With the assumption that that the individual compartments are at their decant levels, we have for the dams within the pit:
Volume stored
Net volume actively displaced by pumping
Volume actively removed from the system

13. Estimating the Decant Rate (Cont.)
(January to June 2012) ∆P
(January to June 2012)
∆S - ∆P = m3

14. Estimating the Decant Rate (Cont.)
Decant rate calculated at 4.90 ML/day for period January to June 2012.
However, low rainfall (232 mm) experienced during this period.
Assuming recharge through the spoils of 20% of the rainfall, yields a volume of Ml/day.
This means that 2.23 Ml/day not accounted for.
Even if the recharge percentage is too small, it is clear that GWIn > GWOut .

15. Preferential Pathways for Groundwater Ingress
Magnetic survey perpendicular to corridors and partial corridors
Magnetic Profiles

16. Preferential Pathways for Groundwater Ingress (Cont.)

17. Validating the Results with a Salt Balance
Salt balance for years of average rainfall:
Sulphate generation of 7.36 kg/ha/day, in good agreement with:
Hodgson and Krantz (1998): – 10 kg/ha/day
Usher and Vermeulen (2006): kg/ha/day

18. Conclusions
The decant rates at rehabilitated opencast collieries could be significantly underestimated if the decants are assumed to be recharge-driven without consideration for the possibility of net groundwater inflow.
Underestimation of the decant rates will lead to flawed water management strategies which could result in adverse environmental impacts

19. Conclusions (Cont.)
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