SUSTAINING WATER SUPPLIES WHILE RESPONDING TO OPERATIONAL REQUIREMENTS AT DE BEERS VENETIA MINE Gary Brown1 and Philip Erasmus2 1Golder Africa Associates, PO Box 6001, Halfway House, 1685. 2De Beers Venetia Mine, PO Box 668, Musina, 0900. ABSTRACT The intrepid novelist Rudyard Kipling’s portrayal of the Limpopo River as “the great, grey-green, greasy Limpopo River, all set about with fever trees …” does not reflect the real challenges De Beers Venetia Mine faces in abstracting raw water for their open cast kimberlite operation, located in the remote arid northern extremities of South Africa, approximately 80 km west of Musina and some 500 km north of Johannesburg, with the area presenting a mean annual precipitation value of 350 mm and annual evaporation rates in the order of 2 500 mm. Construction of Venetia Mine began in 1990, was officially opened in August 1992 and had reached full output in 1993. Venetia Mine is South Africa’s largest producer of diamonds. Production statistics for 2002 reflect total tonnages mined of 24,3 million of which 4,71 million tons of ore was processed and 5,08 million carats recovered. Despite the large catchment and many tributaries, the largest being the Shashe River, feeding the Limpopo River, the river is usually dry for most of the year. Ninety percent of the river’s runoff occurs from December to April. The Mine’s commitment to practising sound environmental management principles is reflected in its proud list of environmental orientated achievements. Despite these challenging conditions and strict statutory and self imposed restrictions to water abstraction for supply to the Mine, the Mine has since commissioning and with increased production not exceeded the overall water abstraction permit volumes and has demonstrated acute ability in integrating the supply of water for its operations between surface flow conditions in the Limpopo River and the ground water source within the Greefswald and Schroda aquifer. Ten years later and added pressure from the effects of the 2000 floods, as well as market driven demands for diamonds, increased production at Venetia is again being contemplated. With changes in legislation being enforced, Venetia Mine has carefully considered all water availability options available to ensure that the water sources available to the surrounding area are sustained. THE ORIGINAL WATER SUPPLY SCHEME Eight possible sources of water were investigated during the 1980 feasibility studies, of which three were selected as the most probable: the Glen Alpine Dam on the Mogalakwena River, the Limpopo River upstream of the Shashe confluence on the farm Samaria and the Limpopo downstream of the Shashe confluence on the farm Greefswald. As part of the mining Environmental Impact Assessment (EIA), other potential water sources were also investigated: a wellfield some 20 kilometres to the south of Alldays; subsurface water on the farm Venetia and the construction of a surface water storage dam in the Kolope River on Venetia. This search for a sustainable water supply spanned a decade, culminating in July 1989 with the formulation of a joint venture between the Department of Water Affairs and Forestry (DWAF) and De Beers for the development of a wellfield at Greefswald, the land being owned by the State and 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 the mining rights by De Beers. Figure 1. Locality of Venetia Mine. Following this decision, De Beers purchased the adjacent farm Schroda to utilise the ground water previously used on this property for crop irrigation to an alternative use. The farm was also converted into a 4 000 hectare conservation area. Figure 2. Aerial view of Greefswald and Schroda areas. The water availability from this source was deemed to be adequate to meet the original water demand estimation of 4,2 million cubic metres of water per annum. THE INTEGRATED ENVIRONMENTAL MANAGEMENT (IEM) APPROACH Once the DWAF had undertaken to develop the Limpopo Water Scheme, an Environmental Task Group (ETG) was established in 1989. The DWAF had also proposed the IEM procedure for the project; which integrates environmental considerations into all stages of project development. The ETG would be responsible for producing EIA’s that would determine environmental consequences of drawing water from the Greefswald and Schroda aquifers and of developing associated infrastructure. The group’s duties included providing specialist information, evaluating environmental reports and incorporating findings and recommendations into the planning and management of the project. Members of the ETG included people with relevant expertise from DWAF, De Beers, Anglo American Corporation, the Provincial Administration’s Nature Conservation Division, the South African Defence Force (deployed at Greefswald at the time) and a representative from the local Farmer’s Association. A multi-disciplinary team of earth scientists was established, bringing together ecologists, hydro-geologists, hydrologists, limnologists, archaeologists and palaeontologists. Ecological and hydrological baseline studies commenced in 1990 under the management of the ETG with proposed actions rated against environmental factors in a scoping exercise. The most sensitive and vulnerable of these interactions proved to be the impact of abstraction on the riparian forest and perennial river and refuge pools. The riparian forest on Greefswald is one of the few remaining examples of this vegetation type to be found in South Africa with agricultural development destroying much of the forest elsewhere along the Limpopo River. This high conservation status of riparian forest on Greefswald thus made it essential that it should not be threatened by the excessive lowering of the water table as a consequence of water abstraction. Subsequently the ETG set ground water drawdown limits for the wellfield as design parameters: one of two metres in the refuge pools and one of four metres within the riverine forest on the banks. These limits were measured from a pre-pumping baseline of the dry season peak of 1989. Remote sensing techniques were also deployed to provide information on the natural and agricultural influences on the riparian forests. It was discovered that the abstraction of 4,2 million cubic metres abstracted per annum from the aquifer only represented 6,25 % of the total water consumption between the confluence of the Shashe and the weir at Beit Bridge some 80km downstream of Greefswald. In addition, the use of 2 million cubic metres of water for irrigation at Schroda was discontinued once De Beers had purchased the farm. As a result, only an effective 2,2 million cubic metres of water would be taken from the aquifer for the project. EARLY TECHNICAL INVESTIGATIONS To develop a sustainable water supply scheme for Venetia, an understanding was required of the relationship between abstraction from the aquifer, the water drawdown and the duration of no surface flow in the Limpopo River. A computerised ground water model of the aquifer was developed to obtain initial assessments of the aquifer performance after which a pilot abstraction scheme was implemented in June 1990 to refine the model. Calibration and extension of the model allowed for greater accuracy in drawdown predictions. The 25-year-old records of flow events in the Limpopo River provided useful information on the anticipated duration of no surface flow events and the volumes of water available for recharging of the aquifer. The results of these early studies confirmed that the water demand for the Venetia project would amount to a small percentage of the total water demand along the Limpopo River from the Shashe confluence to the weir at Beitbridge. And that the zone of influence would not extend to areas utilised by other users, either upstream or downstream. It was concluded that the aquifer would recharge adequately during surface flow events and that these flow events would occur frequently enough at suitable magnitudes, provided that no further surface water impoundments were developed upstream of the Shashe confluence. During 1993 the mining operation was extended from a five-day operation to continuous operation, increasing the water demand up to an initial extended volume of 1,9 million cubic metres of water. The concluding studies revealed that this scenario would exceed the ETG’s drawdown limits, necessitating the need to develop a storage facility to store floodwater abstracted during surplus flow periods. In addition, the dam would serve as a supply of water during extended periods of drought when water levels within the Limpopo River could decline to below allowed abstraction rates A 4,08 million cubic metre capacity off channel storage dam for floodwater was designed and constructed on the farm Schroda during 1993. 4500000 27000 4000000 Dam full capacity level 24000 3500000 21000 3000000 18000 2500000 15000 2000000 12000 cubic metres 1500000 9000 1000000 6000 500000 3000 pumped & seepage cubic metres 0 0 JUL SEP FEB JUN FEB JUN DEC APR OCT APR MAR NOV MAR MAY MAY AUG JAN02 JAN03 Pumped to Mine Seepage returned Stored Volume Figure 3. Typical monitoring of the OCS Dam utilisation. INVENTORY OF THE ORIGINAL INSTALLED SYSTEM Greefswald: • 9 production boreholes capable of delivering 864 m3/hour • 6 floodwater boreholes capable of abstracting 1 080 m3/hour • transfer pumpstation capable of pumping 1080 m3/hour and 32 km of 600 mm diameter pipeline to Venetia Mine Schroda: • 12 production boreholes capable of delivering 882 m3/hour • 4 floodwater boreholes capable of delivering 900 m3/hour • transfer pumpstation capable of pumping 954 m3/hour and 6,3 km of 350 & 500 mm diameter pipeline to Greefswald transfer pumpstation • off channel storage dam with capacity of 4 million cubic metres OPERATING PHILOSOPHY Greefswald was initially selected as the prime water source, resulting in the majority of water abstraction arising from the highly porous alluvial deposit located in this area.