Groundwater Resource Assessment Of The Malmani Subgroup Dolomites Within The Olifants River Water Supply Scheme

The 2011 Olifants River Water Supply Scheme (ORWSS) Reconciliation Strategy recommended that the Malmani Subgroup dolomites along the Limpopo-Mpumalanga escarpment be investigated as a potential groundwater resource for input into the ORWSS. The Department of Water and Sanitation - Directorate: Water Resource Planning Systems (DWS D: WRPS) in turn initiated a 2-year project that began in mid- 2016 to develop a feasibility plan for the groundwater resource development of the Malmani Subgroup dolomites within the ORWSS, with the main aims of the project being: 1) to secure groundwater as a long- term option to augment the water supply to the ORWSS by optimising surface water-groundwater conjunctive use; and 2) to determine the artificial recharge potential of the dolomitic (and/or other) aquifers within the ORWSS. The ~2000 m thick, Late Archaean (~2.6-2.5 billion year old) Malmani Subgroup is comprised of stromatolite-bearing dolomites and limestones (i.e. chemical sediments including chert, with some local clastic shale and quartzite), and forms part of the Chuniespoort Group (lower Transvaal Supergroup) with the overlying banded ironstones of the Penge Formation, and mudstones, dolomites and limestones of the Duitschland Formation. The Malmani Subgroup dolomites (and Transvaal Supergroup as a whole) have undergone deformation, fracturing/faulting and dyke intrusion by a range of tectonic events (including the Bushveld Complex intrusion and slumping, Vredefort meteorite impact, “Transvaalide fold-and-thrust belt”, Pan African Orogeny, Gondwana breakup and current East African Rift development), which have resulted in the development of a high yielding (>10 l/s sustainable yields and transmissivities of ~500-2500 m2 /day per borehole in the vicinity of large regional faults/fractures or dolerite intrusions) fractured dolomitic karst aquifer. Quaternary alluvial deposits (of up to 30-40 m thickness) also occur within valleys incised into the Malmani Subgroup at Fertilis (Mohlapitse River), Penge (Olifants River and associated tributaries), Ga-Maditsi (Steelpoort River), and along the Ohrigstad, Blyde and Treur River valleys. Groundwater quality within the Malmani Subgroup dolomitic aquifers in the ORWSS area is generally good (EC of <70 mS/m), however poorer water quality can be present (e.g. elevated EC, nitrates and trace metals) as a result of contamination from human settlements, agricultural irrigation, mining, and recharge from contaminated surface water e.g. the Olifants and Steelpoort Rivers. Current work completed/being undertaken as part of the project includes: identification of two preliminary regional hydrogeological targets and twelve related wellfield target zones (WFTZ); hydrocensus of selected DWS NGA and GRIP boreholes within these two preliminary targets; re-testing of selected high yielding GRIP boreholes at constant discharge rates of 20-25 l/s, and re-analysis of existing GRIP Malmani Subgroup data; macrochemical and dissolved trace metal analysis of groundwater chemistry from tested and drilled boreholes; development of a regional groundwater balance model to determine the groundwater potential per WFTZ; surface-groundwater interaction and artificial recharge assessments (the latter focusing on alluvial deposits overlying the Malmani Subgroup dolomites); identification of potential wellfield sites within the WFTZs based on structural analysis, measured aquifer parameters, groundwater potential and geophysics; numerical groundwater modelling; and drilling/testing of exploration/monitoring boreholes within selected wellfield sites.