DEPARTMENT OF WATER AFFAIRS AND FORESTRY BREEDE RIVER BASIN STUDY GROUNDWATER ASSESSMENT Final MAY 2003 Groundwater Consulting Services P O Box 2597 Rivonia 2128 Tel : +27 11 803 5726 Fax : +27 11 803 5745 e-mail : [email protected] This report is to be referred to in bibliographies as : Department of Water Affairs and Forestry, South Africa. 2003. Groundwater Assessment. Prepared by G Papini of Groundwater Consulting Services as part of the Breede River Basin Study. DWAF Report No. PH 00/00/2502. BREEDE RIVER BASIN STUDY GROUNDWATER ASSESSMENT EXECUTIVE SUMMARY The objectives of this study were to assess the significance and distribution of groundwater resources in the Breede River catchment, estimate the amount of abstraction and degree of stress it may be causing and to indicate the scope for further development of groundwater resources. This was achieved by a review of all available literature and obtaining yields and quantities from all significant schemes. The characterisation of important aquifers and assessment of the groundwater balance (recharge versus consumption) allowed for identification of further groundwater potential. The geohydrology of the Breede River catchment is controlled by the occurrence of the rocks of the Table Mountain Group (which form the mountainous areas), the occurrence of high levels of faulting and folding in the syntaxis area of the upper catchment and the variable rainfall, being highest in the mountainous areas in the west. These factors result in a catchment with highest groundwater potential in the west, where recharge, yields and abstraction potential are greatest and the quality is the best. As a result of these factors, the western half of the catchment is also the area with the greatest groundwater use. Total groundwater use in the catchment is around 100 million m3/a, far less than the annual recharge of 640 million m3/a. The majority of this recharge (75%) takes place in the western 29% of the catchment. A significant component of recharge that occurs in the mountains rapidly becomes baseflow that feeds streams and recharges the alluvial aquifer in the valley bottoms. Other recharge mechanisms include throughflow (or upflow) from the TMG to Malmesbury, Bokkeveld and alluvial aquifers and direct precipitation. Fractured and confined to semi-confined aquifers occur within the consolidated rocks of the Breede catchment. In these aquifers, the groundwater is usually under pressure and artesian conditions may be present where the piezometric surface is above ground level. The other important aquifer types found in the catchment are the unconsolidated sand deposits which make up the alluvial aquifers. Groundwater levels in the alluvial aquifers are close to surface and yields of from 10 – 15 l/s are common from shallow (<30m) boreholes supplying irrigation schemes in the valleys. The fractured aquifers are less extensively used in the upper Breede catchment because of difficulty in accessing drilling sites on the rugged terrain. Boreholes in these aquifers are usually deeper than those drilled in the alluvium. The groundwater quality in all aquifers in the upper Breede is generally excellent (<450 mg/l). As one moves down the catchment, the quality deteriorates, particularly in poorly transmissive rocks such as the Bokkeveld and Karoo. A way to increase the groundwater production in irrigation areas is to mix marginal quality groundwater with less saline surface water. There is limited information of chemical indicators other than TDS but it is expected that the alluvial aquifers may have elevated concentrations of nitrate as a result of I:\HYDRO\8718\REPORTS\Groundwater\GROUNDWATER ASSESSMENT.doc MAY 2003 BREEDE RIVER BASIN STUDY : GROUNDWATER ASSESSMENT ii irrigation of vineyards that are situated on most of the alluvium in the valleys. Groundwater is not considered to contribute significantly to the salinisation of the middle Breede. A review of the literature on salinisation in the middle Breede catchment concludes that groundwater contributes as little as 1% - 2% of the salt load to the river. Assessment of the land under irrigation and average water requirements per crop provide an estimate of the total water demand for irrigated agriculture in the Breede catchment. The balance of irrigated land not supplied by irrigation boards gives an indication of water supplied by farmer's own schemes, which includes boreholes. The percentage of irrigation supplies made up by groundwater is estimated at 32% in the upper Breede catchment. Irrigation use of groundwater in the middle Breede catchment is around 18 million m3/a. Groundwater consumption in the lower Breede is predominantly domestic and stockwatering and is estimated to comprise approximately 4 million m3/a. Future groundwater exploitation, especially for any large scale schemes, should be concentrated on the alluviual, Bokkeveld or TMG aquifers of the western part of the catchment. Smaller scale abstraction schemes in the eastern half of the catchment will have to exploit the TMG aquifers, since these are the areas of best recharge and quality. Constraints to increased groundwater use in the middle and lower Breede include saline groundwater and poorly transmissive aquifers. The groundwater contribution to river flow or baseflow in selected sub-catchments was assessed using several hydrograph separation techniques. The results indicate that the baseflow is approximately 30% of mean annual run-off in the upper and middle Breede catchment but only about 10% using the linear interpolation method. The linear interpolation is considered to be more representative of the groundwater contribution to stream flow. Gauged river flow data give higher percentages for the groundwater component of river flow than modelled (WR90) data, possibly indicating the role of irrigation return flow in developed catchment (e.g. the Hex) in the upper Breede. Four groundwater abstraction schemes have been used as examples of potential conjunctive use schemes with costs provided. Groundwater schemes are not able to deliver the volumes of water typical of surface water schemes, however, in the Breede Catchment, potential exists for conjunctive use schemes to maximise basin yield in areas adjacent to large dams. For example, a 64 borehole wellfield on the eastern side of the Rawsonville alluvial aquifer is estimated to be able to deliver around 5 million m3/a at a unit cost of R1,16 / m3. Existing groundwater use is largely by individual farmers irrigating close to the source of groundwater. There is potential for communal schemes in many parts of the catchment but these should be focused where there is potential for new irrigation areas and where the soils are suitable for irrigation. Aquifers that could be exploited for irrigation in various regions of the Breede catchment are indicated. Crops that are not as sensitive to elevated salinity as grapes should also be investigated since groundwater quality in the middle and lower Breede may exclude development of vineyards. I:\HYDRO\8718\REPORTS\Groundwater\GROUNDWATER ASSESSMENT.doc MAY 2003 BREEDE RIVER BASIN STUDY : GROUNDWATER ASSESSMENT BREEDE RIVER BASIN STUDY GROUNDWATER ASSESSMENT CONTENTS Page No. 1. INTRODUCTION ............................................................................................................................1-1 1.1 TERMS OF REFERENCE....................................................................................................1-1 1.2. EXTENT OF STUDY AREA ...............................................................................................1-2 2. GEOLOGY ....................................................................................................................................... 2-1 2.1 DESCRIPTION OF THE GEOLOGICAL UNITS...............................................................2-1 2.1.1 Malmesbury Group.................................................................................................. 2-1 2.1.2 Cape Granite............................................................................................................ 2-3 2.1.3 Table Mountain Group............................................................................................. 2-3 2.1.4 Bokkeveld Group..................................................................................................... 2-5 2.1.5 Witteberg Group...................................................................................................... 2-6 2.1.6 Karoo Supergroup (Dwyka, Ecca and Uitenhage Groups) ...................................... 2-7 2.1.7 Bredasdorp Group.................................................................................................... 2-8 2.1.8 Alluvium and Unconsolidated Quaternary Sediments............................................. 2-8 2.2 STRUCTURE AND TECTONICS .......................................................................................2-9 2.2.1 Saldanian Orogeny................................................................................................... 2-9 2.2.2 Cape Orogeny.......................................................................................................... 2-9 2.2.3 Faulting .................................................................................................................... 2-9 3. CHARACTERISATION OF BASIN HYDROGEOLOGY .........................................................3-1 3.1 AQUIFER TYPES FOUND IN THE BASIN .......................................................................3-1 3.1.1