Resource Directed Measures for Protection of Water Resources: Groundwater Component
CASE STUDY - CATCHMENT X2H NELSPRUIT
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CASE STUDY - CATCHMENT X2HNELSPRUIT0. The section of the Crocodile River between Kwena Dam and Nelspruit was selected as the first field application of the IRD method. Two significant water resources were identified. This example only presents the determination from Kwena Dam to just upstream of Montrose Falls. Previous investigations and long term monitoring of the Crocodile River resulted in development of a good surface water quantity and quality database. However, little data were available for the groundwater component of the IRD assessment. Data available on the NGDB mostly fell outside the area of interest and were of low accuracy. The groundwater study which formed part of the water quality situation assessment of the Crocodile River catchment presented similar findings (DWAF, 1995). Available data, however, did show borehole yields to be low (40 % of boreholes ‘dry’ and a further 40 % had yields less than 0.3 L/s). From this it was concluded that groundwater is not utilised to any great extent. National scale groundwater maps (Vegter, 1995) and harvest potential maps (Seymour, 1995) and the incomplete regional scale geohydrological map of the area provided a broad perspective of prevailing geohydrological conditions. Lack of groundwater data resulted in the level of confidence of this assessment being ‘very low’. 1. Geographic boundaries of the study area were selected on the basis of surface water criteria. Two ecoregions were identified (Great Escarpment Mountains, Lowveld). Though geology clearly played a major controlling factor, it was proposed the area underlain by dolomite be identified as a separate homogeneous response unit as it probably has a major influence on the hydrology (both surface and groundwater) of the area. 2. Five geohydrological region types were identified (see map). The area in the immediate vicinity of the rivers could be classified as either spring flow, river flow or riparian vegetation geohydrological regions. Further, the entire dolomitic outcrop could be classified as a land stability geohydrological region. The major portion of the catchment, however, was classified as a terrestrial vegetation geohydrological region. 3. Lack of data precluded reference conditions being defined during the field test. This is regarded as a major limitation and results in the assessment being of low confidence. 4. Based on limited data and a brief visit to the area, it was proposed little groundwater abstraction takes place. Excluding Nelspruit, no significant potential sources of contamination were identified in the study area. Present status category was hence set at ‘A’. 5. In light of limited groundwater usage, a management class of ‘a’ was accepted. This requires the groundwater component of the IRD be set, appropriate monitoring implemented and the IRD assessment be reviewed within 5 years. 6. Based on river flow data from gauging stations X2H013Q01 and X2H075Q01, recharge was estimated to be approximately 15% of MAP. About 60% of MAR is baseflow. Because of the lack of data, the significant water resource was treated as a single homogeneous response unit. It was nonetheless recognised the dolomitic aquifer system probably contributes the bulk of groundwater to baseflow in the river. Recharge was calculated at 194 x 106 m3/a. Annual low maintenance baseflow required to protected aquatic ecosystems in the significant water resource was set by the IRD project team at 116 x 106 m3/a. It was assumed the total volume would be derived from groundwater inflow into the river. It was agreed by the IRD team that most basic human needs were being met from surface water resources with only limited supplies being obtained from groundwater. The groundwater allocation for the significant water resources was thus set at 78 x 106 m3/a. The allocation is set on the basis that: groundwater levels do not decline and groundwater quality does not deteriorate groundwater abstraction does not impact flow in the river In assessing the validity of the calculated groundwater allocations, the following information was considered: mean annual baseflow is approximately 135 x 106 m3/a - as not all recharge enters the river the calculated recharge of 194 x 106 m3/a was considered reasonable. excluding the dolomitic aquifer system, groundwater abstraction is unlikely to impact the hydrological regime as borehole yields are low (less than 0.3 L/s) and geohydrological conditions unfavourable for large scale abstraction. over-exploitation of the dolomitic aquifer system could have a major impact on river flow, appropriate management of the aquifer is hence required. 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 the significant water resource are set: Groundwater allocation is 78 x 106 m3/a Resource quality objectives and drawdown limitations the regional static water level and groundwater quality may not decline over the long-term. abstraction should not significantly alter the groundwater hydraulic gradient in the vicinity of the river. all groundwater abstraction of greater than 1 L/s within 500 m of the river must be licenced all boreholes in the dolomite area must be registered and abstraction licenced. appropriate groundwater level and quality monitoring is to be implemented. this IRD assessment must be reviewed within 5 years. Key learning points to emerge from the field test were: a hydrocensus is essential to collect basic field data and develop a conceptual model of an area. the process of checking quantification against the conceptual model is critical. in areas of little or no data, drilling and pumping tests may be required to erify assumptions made in the IRD assessment. geohydrological input into defining geographical boundaries and/or ecoregions must be reassessed. the relationship between catchments and significant water resources must be clearly defined as must the relationship between IRD assessments and individual licence applications. as IFR’s are set at monthly time steps, the groundwater allocation must use a similar scale, at present the groundwater allocation is considered to be evenly distributed throughout the year. the issue of time lag between recharge, contribution to stream flow and impact of abstraction was found to be an area that needs to be understood. the need to understand WR90 data and undertake basic data interpretations as part of the groundwater assessment was apparent - this is an area where closer links are required between the surface and groundwater disciplines. groundwater utilised by plantations and/or large tracts of alien vegetation could have a major impact on the groundwater allocation - inclusion of this aspect needs to be addressed. the need for monitoring and aquifer management was apparent, but the question who was going to ensure that it was actually being done was less clear. ReferencesDepartment of Water Affairs and Forestry, 1995: Water quality situation assessment of the Crocodile River catchment, Eastern Transvaal; Volumes 1 - 10, Water Quality Management Series, Department of Water Affairs and Forestry, Pretoria. Seymour, A., 1995: Explanation report for the groundwater harvest potential map of South Africa 1995, with notes on secondary aquifer system classification; Unpubl. Provisional report, Department of Water Affairs and Forestry, Pretoria. Vegter, J.R., 1995: An explanation of a set of national groundwater maps; Report TT 74/95, Water Research Commission, Pretoria. DATA SHEET Case Study - Catchment X2H Nelspruit
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