WATER RESOURCE PROTECTION POLICY IMPLEMENTATIONS
RESOURCE DIRECTED MEASURES FOR PROTECTION OF WATER RESOURCES
APPENDIX GW1: CASE STUDIES AND WORKED EXAMPLES OF INTERMEDIATE RDM DETERMINATION
Senior Author: Roger Parsons, Parsons and Associates
Editor: Lizette Guest, Guest Environmental Management
Guy Pegram, Pula Strategic Resource Management
Heather MacKay, Department of Water Affairs and Forestry
Date: 24 September 1999
CASE STUDY – CATCHMENT G21B ATLANTIS
0. The town of Atlantis, 50 km north of Cape Town, has been supplied with water from the Atlantis Aquifers since 1976. Exploration work, wellfield development and ongoing monitoring and re-evaluation has resulted in the aquifer being reasonably well understood.
A good conceptual model of the site has been developed by the CSIR (Fleisher and Eskes, 1992; Wright, 1991; Fleisher, 1990; Tredoux et al., 1989). This in turn led to development of a numeric model. The area has also been investigated by DWAF (Bredenkamp and Vandoolaeghe, 1982; Vandoolaeghe and Bertram, 1982) and the IGS (Muller and Botha, 1986). The level of confidence at which the groundwater component of the IRD assessment can be defined, therefore, is ‘high’ to ‘very high’.
1. The total area of Catchment G21B is 304 km2 (see map1). The catchment was divided into three homogeneous response units (Unit 1, Unit 2 and Unit 3). Division was based on geological and geohydrological characteristics and the 1 : 500 000 scale general geohydrological map of the area (DWAF, 1995). Units 1 and 2 comprise primary aquifer systems where essentially unconsolidated sand deposits overly greywackes and shales of the Malmesbury Group. Aquifers in Unit 3 comprise weathered and fractured rocks of the Malmesbury Group.
2. Three distinct geohydrological region types were identified. A sea water geohydrological region extending 2 km in from the coast line was demarcated in Units 1 and 2 while the Witzand and Silwerstroom wellfields were demarcated as high volume abstraction geohydrological regions. Remaining areas were classified as terrestrial vegetation geohydrological regions. However, it is questionable whether the natural vegetation (coastal fynbos) obtains water from the aquifer. Natural fynbos and strandveld vegetation probably obtain water from unsaturated soil and upper sandy horizons (Scott, pers.comm., 1999).
3. Water levels have been monitored regularly since development of the groundwater abstraction scheme. Water level contour maps are available for each geohydrological region. Extensive groundwater quality monitoring has also been carried out. Current geohydrological conditions do not differ significantly from the reference conditions.
Groundwater quality in the catchment is essentially good with EC typically being in the order of 100 mS/m. The water displays a range of characters. Younger water tends towards a Ca HCO3 character while older water tends to a NaCl character. Water abstracted from the wellfields is hard and requires treatment before distribution.
4. The present status category of the High Volume Abstraction geohydrological region in Unit 1 was set at D as large volumes of groundwater are abstracted, but no signs of over abstraction are evident. Total present abstraction amounts to about 60% of the estimated groundwater allocation.
Groundwater levels in the vicinity of the two wellfields have dropped by 5 m to 7 m over the past 20 years, but have now stabilised (Tredoux, pers.comm., 1999). Affected areas measure 4.5 km2 (Witzand wellfield) and 3 km2 (Silwerstroom wellfield) respectively. This amounts to about 2.5% of the total catchment area.
No visible signs of any impact caused by groundwater abstraction are visible. It is pertinent to note the Silwerstroom spring is still flowing. This is in spite of continued groundwater abstraction from the Silwerstroom wellfield during the past 22 years.
The present status of the remainder of the catchment was classified either as ‘A’ or ‘B’ as limited groundwater abstraction takes place elsewhere in the catchment. Groundwater usage in Catchment G21B amounts to some 8.3 x 106 m3/a and was estimated as follows:
Table 1: Estimated groundwater usage
(x 106 m3/a)
In assessing the performance of the Atlantis aquifer and validity of this IRD assessment, artificial recharge activities in the catchment need to be considered. Approximately 2 x 10 6 m3/a good quality water is artificially recharged into the Atlantis aquifer at Pan 7, located 2 km east of the Witzand wellfield (Murray and Tredoux, 1998). Poorer quality water is recharged via a series of coastal recharge basins located in the sea water geohydrological region in Unit 1.
5. A management class ‘b’ for the high volume abstraction geohydrological regions requires that appropriate monitoring continue and the IRD assessment be reviewed within 3 years. Outside of these areas, the management class was set at ‘a’ and review is thus required within 5 years.
6. Average annual recharge for the entire catchment was calculated (see data sheet). Mean annual precipitation in catchment G21B is 420 mm/s. Recharge calculations excluded the area of the sea water geohydrological regions. A low maintenance baseflow of 0.5 x 106 m3/a was set to ensure continued spring flow. The Buffels River is ephemeral and only flows for sort periods after heavy rainfall.
Until recently, Atlantis was entirely reliant on groundwater as a water source. It was hence assumed all basic human needs are to be met from the groundwater allocation. Groundwater is abstracted from Unit 1 for this purpose. Based on a population of 120 000 requiring at least 25 L/p/d, the BHN component of the Reserve was set at 1.09 x 106 m3/a.
In light of the response of the aquifer to abstraction during the last 20 years, the calculated groundwater allocation for Unit 1 appears reasonable. Those calculated for Unit 2 and Unit 3 therefore are also assumed reasonable. In assessing the validity of the calculated groundwater allocations, the following information was considered:
some 10 x 106 m3 of groundwater stored in Units 2 and 3 is brackish ( 1 500 mg/L) and is unlikely to be used for domestic or irrigation purposes without blending or treatment;
saline intrusion and the ingress of poor quality water from Unit 2 and Unit 3 poses a potential threat to Unit 1 and hence needs to be managed water quality monitoring is required;
approximately 5.8 x 106 m3/a is abstracted from the two wellfields in Unit 1. Abstraction is evenly distributed throughout the year. The gradual lowering of the water level by 5 m over a 20 year period in the vicinity of the wellfields has not resulted in any apparent negative ecological impacts.
abstraction will be distributed throughout the year, with summer abstraction being only marginally higher than abstraction during the winter months.
Based on quality of data available and duration of monitoring aquifer response to abstraction, the level of confidence of these estimates are ‘high’ to ‘very high’.
The following Resource Quality Objectives and limitations have been set to ensure integrity the aquifer system remains in tact:
no groundwater abstraction is allowed in the sea water geohydrological region
the dynamic water level may not drop below 10 mamsl at any time
static water level outside of the high volume abstraction geohydrological region may not decline over the long term
static water level in the high volume abstraction geohydrological region may not drop by more than 5 m below the ambient static groundwater level over the long term.
the dynamic water level may not drop by more than 5 m below static water level for a period longer than 7 days.
7. Example IRD Notice:
To ensure the ability of the groundwater component to satisfy the Reserve on a sustainable basis, the following groundwater allocations, resource quality objectives and drawdown limitations for the three homogeneous response units within Catchment G21B are set:
Unit 1: groundwater allocation is 9.09 x 106 m3/a
Resource quality objectives and drawdown limitations
static water levels outside the high volume abstraction geohydrological region may not decline over the long-term
static water levels in the high volume abstraction geohydrological region may not drop by more than 5 m below the ambient static groundwater level over the long term
dynamic water levels in the high volume abstraction geohydrological region may not drop by more than 5 m below static water level for periods longer than 7 days
Unit 2: Groundwater allocation is 3.45 x 106 m3/a
Resource quality objectives and drawdown limitations
static water levels may not decline over the long term
Unit 3: Groundwater allocation is 1.05 x 106 m3/a
Resource quality objectives and drawdown limitations
static water levels may not decline over the long-term
Other: - No groundwater abstraction is allowed in the sea water geohydrological region
Dynamic water levels may not drop below 10 mamsl
Groundwater levels and quality must be monitored at least quarterly and a review report submitted to the Catchment Management Agency annually
Should any of the above conditions not be met or the following changes detected within the catchment, then the IRD assessment must be reviewed by the Catchment Management Agency and the Minister of Water Affairs and Forestry informed thereof within 60 working days of the condition being detected:
deterioration of groundwater quality; and
impact on vegetation in the catchment.
The groundwater allocations of Catchment G21B must be reviewed within three years from the date of publication of this notice.
Should abstraction in the catchment exceed the groundwater allocation or any of the stipulated conditions not be met, the Minister may request a comprehensive assessment be conducted.
Conservative estimates were used. Some workers estimated recharge could be as high as 40% MAP in places. Fleisher and Eskes (1992) estimated recharge to be 16.3 x 106 m3/a in an area of 141 km2.
No IFR assessment was done, based on estimated flow over the last 20 years.
Bredenkamp, D.B. and Vandoolaeghe, M.A.C., 1992: Die ontginbare potensiaal van die Atlantisgebied; Gh report 3227, Directorate of Geohydrology, Department of Water Affairs and Forestry.
Department of Water Affairs and Forestry, 1995: General geohydrological map - Cape Town 3317; draft plot, Directorate of Geohydrology, Department of Water Affairs and Forestry.
Fleisher, J.N.E., 1990: The geohydrology of the Witzand wellfield; Report No. 2/90, Groundwater Programme, Division of Water Technology, CSIR.
Fleisher, J.N.E. and Eskes, S.J.T., 1992: Optimization and management of the Atlantis groundwater resource; Report 33/92, Groundwater Programme, CSIR, Stellenbosch.
Murray, E.C. and Tredoux, G., 1998: Enhancing water resources - factors controlling the viability of artificial groundwater recharge; Int.Conf.Proc. WISA Biennial Conference, Cape Town, May 1998, Paper 1C-5 pp1-8.
Muller, J.L. and Botha, J.F., 1986: A preliminary investigation of modelling the Atlantis Aquifer; IGS Bulletin 14, University of the Orange Free State, Bloemfontein.
Scott, D., 1999: Personal communications.
Tredoux, G., Hon, A.J. and Engelbrecht, J.F.P., 1989: A groundwater model for the Atlantis Aquifer; Project No. 670/2603/9, Programme, Division of Water Technology, CSIR.
Tredoux, G., 1999: Personal communications.
Vandoolaeghe, M.A.C. and Bertram, W.E., 1982: Atlantis grondwatersisteem - herevaluasie van versekerde lewering; Gh report 3222, Directorate of Geohydrology, Department of Water Affairs and Forestry.
Wright, A.H., 1991: The artificial recharge of urban stormwater runoff in the Atlantis Coastal Aquifer; unpubl. M.Sc. Thesis, Rhodes University, Grahamstown.