Resource Directed Measures for Protection of Water Resources: Groundwater Component
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- Case Study - Catchment N13A
GRAAFF -REINET0. The Moordenaars groundwater unit, located some 20 km west of Graaff-Reinet has been used to supplement the town’s water supply for the last 10 years. Exploration work, well field development and ongoing monitoring and re-evaluation has resulted in the response of the aquifer to abstraction being reasonably well understood. A good conceptual model of the site was developed by Parsons (1987) while Steffen Robertson and Kirsten (1988) installed a series of production boreholes and established the groundwater production scheme. Routine monitoring of the response of the aquifer to monitoring was also carried out by SRK (1992). Due to declining groundwater levels, the yield of production boreholes were re-assessed (Woodford, pers.comm., 1999). 1. The total area of Catchment N13A is 430 km2. However, the catchment was divided into three homogeneous response units (Unit 1, Unit 2, Unit 3). Division was based principally on topographical, geomorphological and geohydrological characteristics. 2. Three geohydrological region types were also recognised, namely a high volume abstraction geohydrological region (only Unit 2), a riparian vegetation geohydrological region and a terrestrial vegetation geohydrological region. Division was based on the hydrological function of each geohydrological region type in the catchment. 3. An extensive hydrocensus was carried out by Parsons (1987) and static piezometric levels in the catchment were defined. These conditions essentially represent ambient groundwater levels (reference conditions) as groundwater abstraction in the catchment was limited at that stage. Groundwater flow mimics surface water drainage patterns. Parsons (1987) referred to a groundwater flow convergence zone which follows a path similar to that of the Moordenaars River. Groundwater levels fluctuate by about 3 m in the central low-lying parts of the aquifer system. Spring flow at Corndale was estimated at 50 000 m3/a. Borehole yields are highly variable, but generally are ‘low’ to ‘moderate’ (92% of boreholes have yields less than 5 L/s). High yields (+20 L/s) are associates with specific geological features. Hydraulic conductivity ranges between 0.5 m/d and 10 m/d while storativity of the rock is in the order of 0.001. The hydraulic gradient at Corndale was measured at 0.0043. Groundwater quality in the catchment is good. The 25 th, 50 th and 75 th percentile of electrical conductivity are 80 mS/m, 120 mS/m and 180 mS/m respectively. The water has a NaCl character and is thus classified as a ‘B’ type water. No groundwater contamination has been identified and groundwater quality is considered representative of pristine conditions. Groundwater abstraction for agricultural purposes (domestic, stock watering) amounts to about 400 000 m3/a while abstraction from the Mimmosadale wellfield amounts to some 240 000 m3/a. 4. The wellfield used to supplement Graaff-Reinet’s water supply has been over-pumped (Woodford, pers.comm., 1999). This resulted in a steady decline of groundwater levels. However, the impact remain localised (ie. within 100 m of the production boreholes). Management actions were implemented and rate of abstraction was reduced from 665 m3/d to 180 m3/d. This resulted in a rise in groundwater levels. The current rate of abstraction appears to be sustainable over the long term. Because of over-abstraction, present status category of the high volume abstraction area in Unit 2 was set at ‘E’. Outside of this region limited groundwater abstraction takes place. The groundwater is used for farm domestic supply, stock watering and small scale irrigation. Current conditions therefore are similar to the reference conditions described above. 5. As a result of the over-abstraction in the high volume abstraction geohydrological region, a ‘c’ management class was set. This requires that a comprehensive Reserve determination be initiated. Until such time as the more detailed determination can be completed, only 75% of the groundwater allocation determined by the IRD method should be abstracted. 6. Using the formulae presented by Kirchner et al. (1991), the average annual recharge in each geohydrological region type was determined (see data sheet). A MAP of 380 mm/a was used in Unit 1 while the MAP of Units 2 and 3 was taken as 295 mm/a. In setting the groundwater allocation, no corrections were made for baseflow as the Moordenaars River is ephemeral. Further, the population in the catchment is limited (less than 200) and all basic human needs are met from groundwater resources. In assessing the validity of the groundwater allocations presented in the data sheet, the following information was considered: some 0.4 x 106 m3/a is abstracted throughout the catchment for agricultural purposes while approximately 0.24 x 106 m3/a is abstracted from the Mimmosadale well field for town supply. a localised lowering of the water level in the vicinity of the well field has occurred, but no detrimental ecological impacts have been reported. Based on the quantity and quality of data available for the IRD assessment, the level of confidence of the assessment was set as ‘medium’. As the calculated long-term average annual recharge of Unit 2 and the volume of groundwater abstracted (mostly from the high volume abstraction geohydrological region) are similar, the IRD assessment is considered inappropriate and a more detailed comprehensive assessment is required. The IRD assessment can be used for issuing preliminary water usage authorisation permits, but must be reviewed within one year. Local groundwater management and monitoring is to be implemented immediately. 7. Example IRD Notice: To ensure the ability of the groundwater component to satisfy the Reserve, the following preliminary groundwater allocation, resource quality objectives and drawdown limitations for three homogeneous response units within Catchment N13A are set: Unit 1: Groundwater allocation is 3.82 x 106 m3/a Resource quality objectives and drawdown limitations: The static water level and groundwater quality may not decline over the long-term The dynamic water level in the riparian geohydrological region may not drop by more than 5 m for a period of longer than 7 days Unit 2: Groundwater allocation is 0.92 x 106 m3/a Resource quality objectives and drawdown limitations The static water level in the high volume abstraction geohydrological region may not drop by more than 10 m below the ambient static groundwater level (710 mamsl) over the long-term. The dynamic water level in the high volume abstraction geohydrological region may not drop by more than 20 m below static water level for a period longer than 7 days. The static water level and water quality outside the high volume abstraction geohydrological region may not decline over the long-term The dynamic water level in the riparian geohydrological region may not drop by more than 5 m for a period of longer than 7 days Unit 3: Groundwater allocation is 0.12 x 106 m3/a Resource quality objectives and drawdown limitations: The static water level and groundwater quality may not decline over the long-term The dynamic water level in the riparian geohydrological region may not drop by more than 5 m for a period of longer than 7 days Other:
A comprehensive Reserve determination for Unit 2 must be initiated forthwith Groundwater abstraction, water levels and quality in the high volume abstraction geohydrological region must be monitored at least quarterly and a review report submitted to the Catchment Management Agency annually Annual monitoring of water levels in the rest of the catchment is recommended. Should any of the conditions not be met or the following changes be detected within the catchment, then 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 the vegetation in the catchment. The groundwater allocations of Units 1 and 3 of Catchment N13A must be reviewed within five years from the date of publication of this notice. ReferencesKirchner, J., van Tonder, G.J. and Lucas, E., 1991: Exploitation potential of Karoo aquifers; WRC Report 170/1/91, Water Research Commission, Pretoria. Parsons, R.P. (1986) The exploration and evaluation of groundwater units south and west of Graaff-Reinet, Cape Province, South Africa; unpubl. M.Sc. Thesis, Rhodes University, Grahamstown. Steffen Robertson and Kirsten (1988) Wellfield development at Graaff-Reinet - Phase II; Report CE5892/2 prepared for the Graaff-Reinet Municipality, Steffen Robertson and Kirsten, Cape Town. Steffen Robertson and Kirsten (1992) Wellfield monitoring - Graaff-Reinet - Third quarterly report July to September 1992; Report 164555/M5 prepared for the Graaff-Reinet Municipality, Steffen Robertson and Kirsten, Cape Town. Woodford, A.C. (1999) Personal communications. DATA SHEET Case Study - Catchment N13AGraaff-Reinet
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