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 logo here…

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

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

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)

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)

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.

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

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

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

Site Information (Cont.) Compartmentalisation of Pit A

Estimating the Decant Rate

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

Estimating the Decant Rate (Cont.) (January to June 2012) ∆P (January to June 2012) ∆S - ∆P = 882 025 m3

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 2.67 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 .

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

Preferential Pathways for Groundwater Ingress (Cont.)

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): 5 – 10 kg/ha/day Usher and Vermeulen (2006): 7 kg/ha/day

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

Conclusions (Cont.) THANK YOU