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 River as “the great, grey-green, greasy , 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 , 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.

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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. Water abstracted from the Schroda wellfield was to augment the supply from Greefswald.

In the absence of long-term base-line data, the success of the Venetia water supply scheme relied upon the timeous resolution of environmental concerns. The riparian forest along the Greefswald aquifer is one of the last 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 level of conservation focus on the riparian forest thus made it essential that it should not be threatened by the lowering of the water table as a consequence of water abstraction.

To this end monitoring programmes and mitigation measures were developed and included in the operating regime developed for the wellfields.

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Figure 4. Typical water drawdown monitoring at Greefswald Aquifer – relative to baseline.

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Figure 5. Typical water drawdown monitoring at Schroda Aquifer – relative to baseline.

Pilot studies revealed that the widely distributed tree species, Croton megalabotrys (Large Fever Berry), was a sensitive indicator of soil moisture status. This species was therefore used as a plant moisture stress indicator and as a measure of vegetation dynamics within the forest. Plant moisture stress is assessed according to the water content in leaves while vegetation dynamics are measured by determining survival in different age classes of the indicator species.

The Department of Environmental Affairs and Tourism from the Limpopo Province monitor four sites, two control sites outside the wellfields and two within, at regular intervals.

Restricted abstraction is enforced on increasing recorded plant moisture stress levels with three operating constraints identified: • Limit 1: 13 bar – Maximum abstraction 300 000 m3 per month • Limit 2: 20 bar – Maximum abstraction 200 000 m3 per month • Limit 3: 23 bar – Maximum abstraction 100 000 m3 per month

The mine subsequently adopted to discontinue all pumping from the Greefswald wellfield if the 23 bar limit is reached.

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Figure 6. Typical monitoring data on drawdown levels and plant moisture stress levels.

Three permanent refuge pools were identified in the Limpopo adjacent to Greefswald. Two are situated upstream of the wellfield and were to serve as controls since they are unaffected by water abstraction. The third pool, Poacher’s Corner, lies between the Greefswald and Schroda wellfields and would be affected by the water scheme.

These pools provide habitat for various fish and other aquatic species when there is no surface flow. This aquatic life re-colonises the river when it begins to flow once more.

Water quality along the aquifer is also monitored to ensure that recharge from the saline inland water is not effected, particularly at Schroda were there are old irrigated croplands with high ground water salinity values adjacent to the river. To date there has been no significant indication of recharge arising from inland.

TEN YEARS LATER

Water abstracted for use on the Mine has remained relatively consistent with the mining production rates and ongoing monitoring of drawdown levels and plant moisture stress levels have resulted in the water abstraction works remaining within its operating parameters.

Figure 7. Annual water demand presentation.

The 2000 floods experienced in the northern regions of the country did however have its influence on the installed wellfield system with the borehole and related infrastructure being damaged. Ironically it was over this period that the Mine’s production water demand was being supplied from the off channel storage dam, originally intended for the supply of water to the mine during drought periods. Several boreholes and interconnecting pipework along the Schroda wellfield were lost in the flood, necessitating the need to review the wellfield configuration.

The flood also had a significant impact on the pools in the river, with the three permanent pools being removed through the scouring and deposition of alluvial material.

A fully integrated and interactive telemetry driven control system has been implemented for the control of water abstraction from the boreholes, and is linked in-line to the water demand on the Mine. Hence the demand for water on the Mine will call for water that will activate the system in terms of the operation of transfer and borehole pumps.

This system, which allows for both fully automatic control and manual re-setting of actions, not only ensures that a borehole pump will automatically stop once the water level in the borehole reaches a pre-determined cut-off level, but also allows for remote access to accept faults detected by the system, hence reducing the arduous routine of call outs for reported faults. The system also ensures that the correct mode of abstraction is selected, continuously records selected water levels, drawdown levels and flowmeter readings and provides feedback on maintenance related issues with a dedicated clock feature that records, reports and reminds on maintenance requirements.

One of the features planned for the upgrading of the system is to enable predictions of plant moisture stress levels by correlating drawdown levels.

Recent studies conducted on the restoration of the Schroda wellfield concluded that the Schroda wellfield could provide as much water as the Greefswald wellfield provided that the abstraction operating control was reverted from individual borehole level control to the overall water level drawdown in the aquifer.

In addition, new borehole construction methods adopted increased the delivery of water from boreholes and has allowed the installation of surplus flow boreholes along the banks of the river and not in the riverbed as previously applied. Computer modelling of this aquifer has confirmed that none of the drawdown constraints will be reached. SHORT TERM WATER REQUIREMENTS

With the fluctuating demands from the business sector, future mining production requirements are assessed regularly. This in turn requires a certain degree of flexibility to the support infrastructure to a mining operation.

In terms of the provision of water for Venetia Mine’s short-term future production increase requirements; planning is at present being completed. Considering the added requirements of the recently promulgated Water Act 36 of 1998, the approach adopted is to integrate the existing registered water use volumes with improved internal water management strategies, without the need to apply for additional water abstraction volumes.

Initial planning is around the utilisation of the surplus flow allocated pumping volumes by integrating this abstraction with the abstraction of ground water during normal operating conditions. This is expected to reduce the demand for water during normal operating conditions on the wellfields at Greefswald and Schroda, which in turn will reduce the potential impact of water abstraction along this area.

Furthermore, the capitalisation of surplus water that collects in the pit and on the slimes dam during big storm events is being considered. A system is being developed on the Mine to store this “opportunistic” water and will provide greater leverage on reducing the call for water from the wellfields.

The planning has involved the DWAF, DEAT and local stakeholders including the Pontdrift and Weipe Farmers Associations as well as SA National Parks Board’s Vembe Dongola Park that is presently under development. This interaction has also seen the formulation of the Vembe Water User Association, which is in the process of finalising its application for registration as a Water User Association

In addition, De Beers are constantly developing technology to reduce water demand on their kimberlite treatment processes, with recent developments indicating a reduction of water demand in the order of 50 percent of present treatment plant process water requirements.

These improvements include the inclusion of dry treatment phases that have traditionally been wet treatment phases and the reduction of slurry water that is collected in the process and pumped to traditional slimes dam where a large percentage of water is lost due to evaporative and infiltration effects.

LONGER TERM WATER REQUIREMENTS

The short term planning being completed at present is expected to address the needs of the open cast mining operation. It is envisaged that Venetia Mine would need to revert to underground mining operations by 2014, with a subsequent reduction in the Mine’s ore extraction rate expected.

The long term water requirements at Venetia Mine, post open cast operation, are therefore expected to be greatly reduced on present water demands.

REFERENCES

1. Anglo American Corporation Corporate Communications Department, “The Venetia Balance – Diamonds, Water and Environmental Responsibility”. 2. De Beer GCO, “Plant Moisture Stress Monitoring of the Limpopo Riparian Vegetation: Greefswald 37 MS”. 3. Grond E, “The Legal and Environmental Aspects Around Venetia Mine’s Water Usage”. 4. Sadowitz SB, Anglo American Civil Engineering Department, Report CED/015/92, “De Beers Consolidated Mines Ltd – Venetia Mine – Wellfield Water Supply Operation Manual.” 5. Venetia Mine, “Off Mine Water Abstraction Report” (Monthly).