An updated water balance for the Grootfontein aquifer near Mahikeng

An updated water balance for the Grootfontein aquifer near Mahikeng JE Cobbing1* AEON/ESSRI: Africa Earth Observatory Network / Earth Stewardship Science Research Institute, Faculty of Science, Nelson Mandela University, Summerstrand, Port Elizabeth, 6031, South Africa 1 ABSTRACT The Grootfontein Aquifer, part of the important North West dolomite aquifers, supplies about 20% of Mahikeng’s domestic water needs. Over-abstraction caused the large natural spring draining the aquifer to disappear in 1981, and groundwater levels have since fallen nearly 30 m in the vicinity of the former spring. Analysis of water levels and a water balance using recent assessments of groundwater abstractions confirm past work describing the hydrogeological functioning of the aquifer, and suggest that current abstractions need to fall by between 19 and 36 ML/day (7 and 13 Mm3/a) to bring the aquifer back into longterm balance. Continued over-abstraction at Grootfontein implies increasing risk to Mahikeng’s water supply, and illuminates the larger challenge of ensuring groundwater use in the North West dolomites that is sustainable and in the public interest. Keywords: Groundwater, North West dolomites, Mahikeng, over-abstraction, irrigation INTRODUCTION The Grootfontein aquifer is of particular interest because it is well studied hydrogeologically, and it is part of the domestic water supply of Mahikeng, the capital of North West Province. The decades of hydrogeological studies at Grootfontein contrast with its poor management, evidenced by falling groundwater levels (see below). The management of the aquifer is the collective outcome of a diverse and complex series of interacting issues, rather than being primarily a question of hydrogeological data. However, the hydrogeology provides a fundamental basis or ‘envelope of possibility’ for management, and its description is necessary and important. South Africa’s dolomite aquifers are amongst the highestyielding and most important aquifers in the country (Barnard, 2000). The dolomites of North West Province (Fig. 1), known as the North West dolomites, hold around 5 000 Mm3 of water (about the same as the Gariep Dam), and are recharged at a rate of about 300 Mm3/a (Stephens and Bredenkamp, 2002). The North West dolomites are divided into a number of discrete units or ‘compartments’ by igneous dykes and faults (Meyer, 2012), making them a patchwork of semi-autonomous aquifers rather than a single hydraulic entity. Under natural conditions, rainfall recharges these compartments / aquifers, and they drain via springs, seeps and wetlands. Some of the springs draining North West dolomite compartments are large and important – for example, the Molopo Eye near Mahikeng and the Maloney’s Eye near Mogale City are the sources of the Molopo and Magalies Rivers, respectively. Both have long-term average flow rates exceeding 30 ML/day or 11 Mm3/a (DWS NGA data; Vahrmeijer et al., 2013). Over-abstraction in some of the dolomite compartments is a growing problem, threatening domestic supplies, irrigated agriculture and environmental services. Whilst most dolomite groundwater is used for irrigation, hundreds of thousands of people also depend on it for domestic water supply. It also supports many springs, wetlands * To whom all correspondence should be addressed.  +1 202 361 7865; e-mail: Received 17 February 2017, accepted in revised form 5 December 2017. http://dx.doi.org/10.4314/wsa.v44i1.07 Available on website http://www.wrc.org.za ISSN 1816-7950 (Online) = Water SA Vol. 44 No. 1 January 2018 Published under a Creative Commons Attribution Licence and associated ecosystems. Bodibe, Lichtenburg, Itsoseng, Ventersdorp, Mahikeng, Ottoshoop and Zeerust, amongst other towns, all rely mainly on dolomite groundwater for municipal water supplies. Mahikeng is one of the largest and most important groundwater-dependent towns in South Africa. Mahikeng’s water demand is about 50 ML/day (18.3 Mm3/a). About 20 ML/day (7.3 Mm3/a) of this water comes from the Setumo Dam, on the Molopo River to the west of Mahikeng. Groundwater from the Molopo Eye spring and from boreholes in the Grootfontein aquifer supply the other 30 ML/day (11 Mm3/a) – roughly 20 ML/day (7.3 Mm3/a) from the spring and 10 ML/day (3.7 m3/a) from the boreholes. Both the Molopo Eye spring and the Grootfontein aquifer are located in the North West dolomites, about 30 km east of Mahikeng. The Setumo Dam depends on intermittent flows from the Molopo River (which originates at the Molopo Eye), and on leaks and effluent return flows from Mahikeng. The dam is therefore partly dependent on North West dolomite groundwater too. Before the early 1980s the Grootfontein aquifer used to discharge naturally at a large spring, the Grootfontein Eye. This spring used to be the primary water source of Mahikeng (and nearby Mmabatho). Municipal officials drilled boreholes around the Grootfontein Eye in the 1970s to augment the municipal water supply. Irrigation from the Grootfontein aquifer also grew rapidly, starting in the 1960s. These combined groundwater abstractions led to the spring disappearing in October 1981, after which groundwater levels in the Grootfontein aquifer fell. Today the water table in the vicinity of the old Grootfontein Eye is more than 28 m below ground level. This fall in groundwater level has led to some of the municipal boreholes failing, and it now also threatens some of the irrigation boreholes. The volume of water that the remaining Grootfontein boreholes yield to Mahikeng has fallen by more than 60% over the past 10 years to today’s approximately 10 ML/day (3.7 Mm3/a) (DWS, 2014). Overabstraction in the Grootfontein aquifer continues today, and it is likely that domestic supply abstractions will decline further. To compensate, Mahikeng increasingly relies on flows from the Molopo Eye spring (located in a different dolomite compartment to the north of Grootfontein), and on the 54 Figure 1 Regional overview (boundaries after Holland and Wiegmans, 2009) Table 1 Hydrogeological data used Data type Source and description Groundwater levels DWS National Groundwater Archive (NGA) data for 34 water-level measuring stations in and around the Grootfontein aquifer. Confirmed by dipmeter measurements in the vicinity of the former eye. Former Grootfontein Eye flows Literature review, personal communications. The Grootfontein Eye has not flowed since 1981, and no single continuous record of its former flow was obtained. Public water supply borehole pumping rates Literature review, personal communications, DWS pumping data obtained for 2015. Irrigation borehole pumping rates Literature review, personal communications, estimates derived by private consultants obtained for use in WRC Project K5/2429 (Eales, 2015). WARMS license data DWS WARMS database representing licensed amounts at Grootfontein. Actual use may be higher or lower. Major and minor ion chemistry Sixteen samples were col (...truncated)