Resource Directed Measures for Protection of Water Resources: Groundwater Component
CASE STUDY - CATCHMENT A23+ PIENAARS RIVER
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- Table 1: Distribution of water quality parameters
- References
- Acknowledgements
CASE STUDY - CATCHMENT A23+PIENAARS RIVER0. The study area is located north of Pretoria along the Pienaars River between Roodeplaat Dam and Klipvoor Dam. Groundwater resources in the area have been extensively developed as part of former homelands and self-governing states water supply programmes (Hobbs, 1996). The north-eastern part of the study area includes part of the Springbok Flats aquifer which has been studied extensively because of elevated nitrate levels (Tredoux, 1993). The study area is covered by regional scale hydrogeological maps prepared by Haupt (1995) and Barnard (1996) and national scale maps of Vegter (1995) and Baron et al. (1998). As a result of a lack of specific crucial data (eg. groundwater abstraction) and because much of the data used in the assessment was derived from regional scale maps, the level of confidence of this IRD assessment is set as ‘medium’. Further, interaction between surface water, groundwater and the wetland is not well understood. This results in the conceptual model of the system being of ‘low’ confidence. The confidence of this assessment is therefore set as ‘medium-low’. 1. The total area of the geohydrological assessment area is 4 821 km2 (see map) and includes six quaternary catchments. Geology, topography and eco-regions were considered before defining homogeneous response units. The area is dominated by minor intergranular and fractured aquifers with only the basalts in the north-east being classified as a major aquifer system. Geology plays a major role in defining topographical characteristics and changes in the study area. In turn, topography controls the position of surface water catchment boundaries. It was found characteristics in each quaternary catchment were sufficiently similar to use quaternary catchment boundaries to delineate geohydrological response units. However, geohydrological catchments do not coincide with those of surface water bodies (see map). This further hinders development of a sound conceptual model upon which to base this IRD assessment. 2. Because of the great depth to groundwater (average depth to groundwater is in the order of 18 m) it was concluded the river and wetland are largely influent in nature (ie. discharge to the groundwater system). This was supported by surface water chemistry data (fluoride) and natural baseflow data which showed little or no natural baseflow occurs during the dry winter months. Sewage effluent discharge in the upper reaches of the Pienaars and Apies Rivers and irrigation return flows have resulted in substantially elevated baseflow in the Pienaars River. It is interesting to note that effluent discharge from the sewage works (145 ML/d) is of a similar order of magnitude to river flow up-gradient of the Klipvoor Dam during winter (2.1 m3/s). This further supports the assumption of lack of groundwater discharge during dry months. Based on existing knowledge of the catchments groundwater does not play a major role in sustaining river flow, riparian vegetation or sensitive eco-systems. Excessive lowering of the water table, however, could induce greater losses from the river and wetlands. Sustainable management of the groundwater system is thus required to maintain the integrity of the Reserve. No substantial groundwater abstraction schemes were identified in the catchment, but hand pumps and other low-yielding pumps are used to supply a population of some 800 000 with their basic water requirements (based on data supplied by VSA GeoConsultants). Groundwater resources thus probably have a much higher social importance than ecological importance. It is questionable whether the groundwater system sustains terrestrial vegetation in the study area. As depth to groundwater is generally greater than 18 m, most of the mixed bushveld vegetation is probably sustained by water stored in the relatively thick sandy soil horizon and vadose zone. Nonetheless, the entire area is classified as a terrestrial vegetation geohydrological region. 3. It is beyond the scope of this report to present a detailed description of prevailing geological and geohydrological conditions (see Haupt, 1995; Vegter, 1995; Hobbs, 1996 and Barnard, 1999). Available information was used to develop a conceptual model of the study area, and to define reference conditions and the response of the aquifer system to anthropogenic influences. A limitation of this assessment is the lack of quantified groundwater abstraction data. Because of the availability of reticulated surface water, most irrigation is presumed to be based on surface water. An exception to this is groundwater-based irrigation in the Springbok Flats area. Hobbs (1996) reported annual abstraction from the southern Springbok Flats area for irrigation purposes amounted to some 9 000 000 m3/a. The Springbok Flats aquifer is classified as a major aquifer system and has characteristics different from the predominantly Karoo aquifer systems adjacent to the Pienaars River. Groundwater is widely used to supply water for domestic consumption to the rural, semi-rural and semi-urban communities and for stock watering. Assuming groundwater abstracted for domestic supply is similar to that used for irrigation, total groundwater abstraction in the study area could be in the order of 20 000 000 m3/a. Because of the dependancy of the population on groundwater, most of the aquifer systems in the study area are sole source aquifers which require a high level of protection to ensure sustainability of the water supply. The regional groundwater flow direction mimics that of rivers. A number of data sets were used to assess depth to groundwater. The average depth to groundwater measured in the study area after 1980 and excluding the Springbok Flats aquifer system is 19.2mbc while the 25 th, 50 th and 75 th percentiles were calculated to be 10.3m, 16.7m and 25.0m respectively. When only considering depth to water in the vicinity of the lower reaches of the Pienaars Rivers, depth to groundwater is slightly deeper (25 th, 50 th and 75 th percentiles determined to be 11.2m, 18.9m and 27.8m). No evidence exists which indicates that depth to groundwater in close proximity to the river is significantly shallower than elsewhere in the catchment. Implications of this are: the river is influent in nature and not sustained by groundwater discharge baseflow is largely supplied from water held in the vadose zone after significant summer rainfall little groundwater is evapotranspired Six sets of monitored groundwater level data were obtained from the NGDB. Groundwater levels fluctuate by approximately 5 m in the vicinity of rivers. Further away, levels fluctuate by some 8 m. Groundwater quality in the study area is generally good, but variation can be significant. Extremely high nitrate levels have been recorded in the basalts of the Springbok Flats (Tredoux, 1995). Nitrate concentrations regularly exceed 50 mg/L (N0x-N). High fluoride concentrations have also been recorded (up to 14 mg/L) while TDS in excess of 8 000 mg/L have be measured. However, no anthropogenic contamination is evident. Data from Hobbs (1996) was used to determine the distribution of some key groundwater quality parameters (Table 1). Typically, TDS ranges between about 200 mg/L and 1000 mg/L and has a Ca-Mg-HC03 character (Type B, as defined by Vegter, 1995). Table 1: Distribution of water quality parameters
4. Three mechanisms were used to define the present status of groundwater in the six quaternary catchments. As no reliable data were available regarding groundwater abstraction, the description presented in the groundwater usage present status assessment was used to set the status as B. This classification is supported by: comparing estimated abstraction to determined recharge, and the fact that available water level monitoring data does not indicate declining groundwater levels. No sign of groundwater contamination was observed. However, it must be stressed available data is not suitable for proper contamination status assessment. High usage of pit latrines in the study area requires that at least a ‘B’ status category be assigned. Because of the nature of available information, an assessment of present status using aquifer vulnerability and land use is probably most appropriate. Aquifer vulnerability is generally low with medium vulnerability observed in the central parts of catchments A23H and A23J towards Klipvoor dam and including the area of the wetland. Land use in the study area is expected to have low to moderate impact on prevailing aquifers. Irrigation, low density pit latrines and stock farming occur through the catchment. Localised activities such as mining, high density pit latrines, small waste water treatment works and waste disposal may result in varying levels of localised contamination. Present status category using vulnerability and land use information is set at ‘B’. Operational management of the catchment must address localised contamination issues. Based on the three mechanisms for setting present status of the geohydrological regime, an overall present status category of ‘B’ is appropriate. 5. The management class of the entire study area is set as ‘a’. This requires the Reserve be reviewed every 5 years. Further the management class does not require reduction of the groundwater allocation determined in Step 6. 6. Annual recharge in the study area was determined to be 78.10 x 106 m3/a (see attached table). This estimate is considered somewhat conservative but is in keeping with the approach for preliminary Reserve determinations. Basic human needs in the study area were estimated to be some 7.31 x 106 m3/a. The IRD team set the required maintenance baseflow at 19.8 x 106 m3/a. Though it is expected that groundwater contributes little to natural baseflow, it was conservatively assumed that at least half natural baseflow is derived from groundwater. The Groundwater Allocation, therefore, was conservatively set at 60.88 x 106 m3/a. In spite of the low confidence conceptual model, results of previous work suggest the Groundwater Allocation is reasonable. As borehole yields are generally low (less than 2 L/s) it is unlikely widespread impacts resulting from over-abstraction will be felt. Better information regarding population and groundwater abstraction is required while appropriate on-going monitoring is needed to ensure aquifer integrity over the long term. 7. Appropriate monitoring of abstraction, groundwater levels and quality is required to evaluate the validity of this assessment of the groundwater component of the Reserve. Based on the management class, this Reserve determination is to be reviewed every 5 years. Monitoring adjacent to and further away from surface water bodies as well as near areas of significant groundwater abstraction is needed. 8. Example IRD Notice: To ensure the ability of the groundwater component to satisfy the Ecological Reserve and Basic Human Needs on a sustainable basis, the following groundwater allocation and resource quality objectives within Catchment A23+ are set: Groundwater allocations: Catchment A23B 15.52 x 106 m3/a Catchment A23C 5.30 x 106 m3/a Catchment A23F 6.85 x 106 m3/a Catchment A23G 10.91 x 106 m3/a Catchment A23H 11.73 x 106 m3/a Catchment A23J 10.57 x 106 m3/a
declining groundwater levels; or deterioration of groundwater quality; or impact on vegetation in the catchment. The Groundwater Allocations of Catchment A23+ must be reviewed within five 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 Reserve determination. Key learning points to emerge from the field test were: conceptual uncertainties with surface - groundwater interaction were identified as a major limitation whenever setting the Reserve. quantification of recharge will be critical for all IRD’s, therefore improvement of skills in this arena is required. inclusion of recharge insert maps on the 1 : 500 000 hydrogeological maps could be very useful in this regard. depth to water plays a critical role in determining whether surface and groundwater interact, thus depth to groundwater insert maps should also be included on the regional hydrogeological maps. groundwater monitoring is vastly different to surface water monitoring with the biggest difficulty being the vast areas to be covered by a single borehole - monitoring requirements for Reserve determinations should be incorporated into the national groundwater monitoring program. groundwater system boundaries do not coincide with surface water catchment boundaries - a project should be initiated to identify regional aquifer system boundaries for the entire country. stressed catchments need to be identified for prioritising future IRD assessments - groundwater abstraction in relation to groundwater allocation / harvest potential / exploitation potential could be a way of doing this a need exists to quantify population and groundwater abstraction in each quaternary catchment. Notes on Pienaars River data sheet 1. Recharge estimates were based on a catchment water balance model and estimates presented by Taylor (1980), Nel (1992), van Tonder et al. (1993), Fayazi (1994), Haupt (1995),Vegter (1995), Hobbs (1996) and van Tonder (1999). 2. Generally, the recharge estimate used is about half of that of other workers. Most workers estimate recharge to range between 25 and 40 mm/a (3% to 5% MAP) while the average depth computed in this study amounts to 16 mm/a (2.5% MAP). The recharge estimate used, therefore, is considered conservative. 3. A higher percentage MAP (3.5%) was used for catchment A23B because of higher rainfall in the catchment and higher recharge levels presented by Vegter (1995). 4. Though interaction between the influent stream and groundwater is not well understood, it was assumed half the maintenance baseflow of 19.8 x 106 m3/a is derived from groundwater and half from the unsaturated zone. Baseflow from groundwater was then determined proportionally for each quaternary catchment. 5. Basic human needs were calculated by assuming a population of some 800 000 people requiring 25 L/p/d and proportionally assigning requirements based on quaternary catchment size. ReferencesBarnard, H.C., 1999: An explanation of the 1 : 500 000 general hydrogeological report; Draft report, Directorate of Geohydrology, Department of Water Affairs and Forestry, Pretoria. Fayazi, M., 1994: Regional groundwater investigation of the northern Springbok Flats; Technical Report Gh 3684, Directorate of Geohydrology, Department of Water Affairs and Forestry. Haupt, C.J., 1995: Explanation of the 1 : 500 000 hydrogeological map 2326 Pietersburg; WRC Report TT 75/95, Water Research Commission and the Department of Water Affairs and Forestry, Pretoria. Hobbs, P.J., 1996: JICA study - Situation report on the geology, soils and hydrogeology of the Maglies Water extended supply area; Report prepared for EVN Consulting Engineers by VSA GeoConsultants, Pretoria. Nel, G.P., 1992: Regionale grondwater-ondersoek van die Springbokvlakte-suid met spesiale verwysing na die grondwater-ontrekking and aanvulling; Technical Report Gh 3787, Directorate of Geohydrology, Department of Water Affairs and Forestry. Taylor, C.J., 1980: Regional groundwater investigation around Warmbaths, northern Transvaal; Technical Report Gh 3174, Directorate of Geohydrology, Department of Water Affairs and Forestry. Van Tonder, G., 1999: Personal communications. Vegter, J.R., 1995: An explanation of a set of national groundwater maps; Report TT 74/95, Water Research Commission, Pretoria. Vegter, J.R. and Pitman, W.V., 1996: Recharge and streams; Paper presented at workshop on groundwater-surface water issues in arid and semi-arid areas, pp 1- 20. AcknowledgementsA number of individuals and organisations provided data and assisted in undertaking this IRD assessment. The following are specifically thanked for their input and assistance: Ms Eva Baron DWAF Mr Julian Conrad CSIR Mr Phil Hobbs VSA GeoConsultants Dr Mannie Levin Africon Mr Bill Orpen DWAF Prof. Gerrit van Tonder IGS
DATA SHEET Case Study - Pienaars River Roodeplaat Dam to Klipvoor Dam
 Case Study 4: The Pienaars River between Roodeplaat and Klipvoor Dams.  Case Study 4: Pienaars River: Comparison of groundwater elevation and depth to groundwater (after van Tonder, 1999) 1 Editor’s note: Author to re-draw figure following poor file format conversion. GW1/ Department of Water Affairs and Forestry, South Africa Version 1.0: 24 September 1999 Directory: docs -> Water%20Resource%20Protection%20Policy docs -> Application for acem docs -> Observational Assessment of Teaching Practices Teaching Assessment Initiative Proposal Submitted to The Teachers for a New Era project docs -> Traditional British values docs -> From Warfighters to Crimefighters: The Origins of Domestic Police Militarization docs -> Borzoi Club of America Register of Merit Program I. What is a Register Of Merit docs -> Protecting the rights of the child in the context of migration docs -> United Nations E/C. 12/Esp/5 docs -> 9th May 1950 the schuman declaration docs -> Getting To Outcomes® in Services for Homeless Veterans 10 Steps for Achieving Accountability Water%20Resource%20Protection%20Policy -> Resource Directed Measures for Protection of Water Resources: River Ecosystems Download 1.54 Mb. Share with your friends:
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