Groundwater a Source of Water for the Deep South
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GROUNDWATER A SOURCE OF WATER FOR THE DEEP SOUTH Roger Parsons 1, John Coetzee 2 and Chris Wise 2 1Parsons and Associates Specialist Groundwater Consultants, PO Box 2606 Somerset West 7129. Tel (021) 855-2480. E-mail: [email protected] 2Jeffares and Green (Pty) Ltd ABSTRACT Adequate supply of water to the southern suburbs of Cape Town, including Fish Hoek, Simon’s Town and Noordhoek, is increasingly coming under threat as these suburbs expand and their demand for water increases. Upgrading and expanding existing water reticulation pipelines through Muizenberg to these areas is possible, but will be expensive and disruptive. Alternative sources of water were hence considered, including the construction of a dam at Brooklands above Simon’s Town and development of local groundwater resources. This paper describes the potential for developing groundwater resources to increase a secure supply of water to these areas. INTRODUCTION The area south of Clovelly – including Fish Hoek, Noordhoek, Simon’s Town, Scarborough and Kommetjie – currently obtains water from water resources above Simon’s Town (Lewis-Gay Dam, Kleinplaas Dam, and Rawson Dam) and via a pipeline running through Muizenberg and Kalk Bay. A dam site at Brooklands was identified some 30 years ago as a potential future source of water to augment existing supplies (Figure 1). In response to a land claim – parts of which would be flooded should the dam be built - the City of Cape Town investigated whether the Brooklands Dam site was still required. To be able to make an informed decision regarding the reservation of the land for construction of the Brooklands Dam at some point in the future, the City of Cape Town commissioned a study to investigate water resource and supply options for the area colloquially referred to as the Deep South. The study included an economic evaluation of various water resource and supply options, assessed the security of supply for the Deep South area and attempted to ascertain related environmental and social issues. This paper describes prevailing groundwater conditions and the feasibility of developing groundwater as a source of water for the area. WATER DEMAND AND SUPPLY Based on 1996 census data, the population of the Deep South is in the order of 50 000 (Figure 1). In addition to the established residential component and the naval base in Simon’s Town, a large portion of the study area comprises the Cape Peninsula National Park. The Average Annual Demand (AADD) for the Deep South area in 2001 was estimated at almost 17.0 ML/d (Jeffares & Green, 2003)(1). This includes un-metered usage (7%) and background losses (8%). It was projected the population of the area would increase from 50 000 in 1996 to almost 100 000 by 2030. After taking account of increased service delivery and effects of water demand management, Jeffares & Green (2003)(1) predicted the AADD in 2027 would be in the order of 33 ML/d, with a peak daily demand factor of 2.2. The maximum yield of the existing water supply system was estimated to be about 30 ML/d and should be sufficient to meet project water demand until 2027. However, concerns about the age and fragility of the pipeline (risk of failure) and security of supply (reliance on a single pipeline) necessitate establishment of additional sources of water. Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 –6 May 2004 ISBN: 1-920-01728-3 Cape Town, South Africa Produced by: Document Transformation Technologies Organised by Event Dynamics Settlements Noordhoek Kommertjie Fish Hoek Glencairn Ocean View Simon’s Town City of Cape Town Scarborough Deep South Dams f Groundwater Unit 4 Units f Lewis Gay Unit 3 f Kleinkrans proposed Brooklands Fish Hoek Primary Aquifer Noordhoek Jackson Primary Aquifer f Rawson Unit 2 f f Unit 1 Figure 1. Locality map showing major settlements, dams and the extent of groundwater units in the Deep South. The Deep South currently receives its water supply from a pipeline passing through Muizenberg and Clovelly to Simon’s Town and from smaller dams above Simon’s Town (i.e. the Lewis Gay Dam, Kleinplaas Dam, Rawson Dam and Jackson Dam). Water received from the Muizenberg-Clovelly pipeline is predominantly fed from Theewaterskloof Dam via the Blackheath or Faure Water Treatment Plants. Raw bulk water from the Lewis Gay Dam and Kleinplaas Dam is treated by the City of Cape Town at the Brooklands Water Treatment Plant. Currently, the pipeline yields about 13 ML/d and the Lewis Gay and Kleinplaas Dams collectively yield 3.3 ML/d. Raw bulk water from the Rawson Dam and Jackson Dam, owned by the South African Navy, either serve as strategic storage or is treated by the navy for their own consumption. WATER RESOURCE OPTIONS Prior to establishment of the Unicity in 2001, the responsibility of water supply was a function of individual local municipalities. The Brooklands Dam site was identified as a potential source of water by the Simon’s Town Municipality in 1968 (Ninham Shand, 1969) (2). The municipality intended to construct the dam once the purchase of water from the then Cape Town City Council exceeded the cost of developing the dam. Following restructuring of local government in 1997, the Cape Metropolitan Council took over responsibility for bulk water supply to the area. Based on an investigation into the immediate need for the proposed Brooklands Dam, it was decided the Muizenberg-Clovelly pipeline would be used to meet growing water demand. A series of booster pumps were then installed to increase the flexibility of the reticulation system. A series of land claims and subsequent adjudication by the Lands Claims Commission prompted further investigation into the need for the Brooklands Dam. If the land claims are successful, and if the Brooklands Dam is not required, the possibility exists of returning this land to the claimants. Water demand in the area had to be assessed as well as potential sources of future water supplies. Additional water supply options considered included: • construction of the proposed Brooklands Dam • construction of a new supplementary water supply pipeline from Muizenberg • development of groundwater supplies • desalination • various combinations of the above GROUNDWATER CHARACTERISTICS Geology The Deep South is underlain by granitic rocks of the Cape Granite Suite and sedimentary rocks of the Table Mountain Group (TMG). Fractured and faulted rocks of the TMG form significant secondary aquifers in the Western and Eastern Cape (Pietersen and Parsons, 2001)(3), capable of yielding large quantities of good quality groundwater on a sustainable basis. The contact between the TMG and underlying granites is also considered to be an important target for drilling. Two significant occurrences of Tertiary to Recent unconsolidated sand deposits also occur in the area, which form locally significant primary aquifers. Depth to groundwater is generally a function of elevation above sea level. In lower lying areas, the groundwater piezometric surface is relatively close to surface, but in higher lying areas, groundwater levels may be in the order of 20 to 50 m below surface. Very little groundwater level data were available during the study and it was hence not possible to assess groundwater flow directions nor the compartmentalisation of aquifer units in the area of interest. Numerous faults occur in the study area, which generally lie in a northwest – southeast direction (Figure 1). Presence of these faults suggests a good potential for drilling high yielding boreholes as faults and fractures enhance the ability of geological formations to transmit groundwater. On the basis of the position of the faults, those parts of the study area underlain by hard rock aquifers were divided into 4 separate geohydrological units (Figure 4). Similarly, unconsolidated deposits form two locally significant primary aquifers in the Deep South, one in the Fish Hoek Valley and the other in Noordhoek (Figure 1). Aquifer Yield To be able to provide a preliminary assessment of the potential of groundwater for further development, it was assumed estimation of recharge would provide a measure of the sustainable yield of the groundwater system. Mean Annual Precipitation (MAP) in the area varies between 630mm/a in Fish Hoek and 1300mm/a on top of the mountains. Recharge of the TMG Aquifers is difficult to quantify without adequate data, but 15% MAP is generally accepted as a reasonable conservative estimate in high rainfall areas (Parsons, 2001)(4). Based on work in Atlantis (Fleisher, 1993)(5) and in the Cape Flats (Vandoolaeghe, 1990)(6), recharge for the primary aquifers was assumed to be 30% of MAP. In calculating recharge, all areas within 1 km of the coast were excluded. Because of the threat of saline intrusion, production boreholes would not be placed in this zone. The potential yield of the six delineated geohydrological units is presented in Table 1. Table 1. Potential yield of groundwater units in the Deep South. Groundwater Units Recharge (ML/d) Unit 1 11.0 Unit 2 12.6 Unit 3 8.3 Unit 4 8.0 Fish Hoek primary 8.5 aquifer Noordhoek primary 2.2 aquifer Total 50.6 It is unlikely this potential yield will ever be fully realised. For environmental reasons, large-scale groundwater abstraction from the primary aquifers is unlikely to be approved. Haupt (2002, pers.comm.)(7) estimated the exploitation potential of the TMG Aquifer would be in the order of 50% (8) of the harvest potential calculated by Baron et al. (1998) . Based on past performance, the long-term yield of individual boreholes drilled into TMG Aquifers range between 5 and 10 L/s per borehole. It is thus estimated a well-sited and managed borehole is capable of yielding between 0.5 and 1.0 ML/d.