GROUNDWATER RESOURCE ASSESSMENT FOR WATER SUPPLY TO THE HEUNINGVLEI SOCIO-ECONOMIC DEVELOPMENT AREA

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1 GROUNDWATER RESOURCE ASSESSMENT FOR WATER SUPPLY TO THE HEUNINGVLEI SOCIO-ECONOMIC DEVELOPMENT AREA C. Monokofala 1 and F.E. Wiegmans 2 1&2 Golder Associates Africa, P.O Box 13776, Hatfield, 0028, Ditsela Place, 1204 Park Street, Hatfield, Pretoria, Gauteng Province, South Africa; cmonokofala@golder.com ; fwiegmans@golder.co.za Abstract The Heuningvlei pipeline scheme was built in the 1980 s to supply water to rural communities in a low rainfall (<300 mm/annum) area - Northern Cape Province. In 2008, the Joe Morolong Local Municipality identified the need to refurbish and upgrade the pipeline scheme for socio-economic reasons. The safe yield and water quality information of existing sources supplying the scheme was unreliable. This was investigated by borehole test pumping and water quality sampling, which indicated reduced yields and deteriorating water quality since Water demands, which includes supply to communities for domestic use, schools, clinics and stock watering in the Heuningvlei area, was estimated at 2,380 m 3 / day or 868,700 m 3 /annum. The potable groundwater supply recommended from 11 existing boreholes is 316,937 m 3 / annum leaving a deficit of 551,763 m 3. The aquifers utilized for the existing water supply comprise fractured banded iron formations (BIF) and dolomite bedrock. Kalahari sedimentary and dolomite aquifers to the east of the pipeline scheme contain high saline water not suitable for domestic use. No surface water sources exist in the area and the feasibility of the socio-economic development project depends on establishing local groundwater resources that would not impact on existing sources. A target area was identified which is approximately 10 km south from the pipeline. This area is covered by the thick Kalahari sediments (up to 130m) underlain by dolomite bedrock with a potable groundwater balance of 2.3 million m 3 /a. Both the associated primary (Kalahari) and secondary (Dolomite) aquifers contain potable water. The target area was not investigated in the past due to perceived poor water quality (elevated salinity) conditions, very low (<10 %) borehole exploitability prospect and difficult drilling conditions. The paper will discuss the importance of recharge estimate and understanding of flow regime at sub catchment and local scale, use of an airborne magnetic survey in conjunction with ground geophysics, mapping of Kalahari sediment thickness, and successful drilling of exploration boreholes to exploit the deeper Kalahari sedimentary and dolomite bedrock aquifers. The successful development of localized potable water in a low rainfall area made it feasible to implement the Heuningvlei socio-economic development project. 1 INTRODUCTION The bulk of the groundwater resources for the Heuningvlei pipeline scheme were established during 1983 to 1989 (Partridge, 1989). A total of 17 boreholes were identified as production boreholes with a combined recommended yield of 24 l/s for 12 hours per day (1,037 m 3 / day). These investigations stressed the importance to monitor the aquifer under stressed conditions to enable evaluation of the long term sustainable yield potential of the groundwater resources. Monitoring data is however not available to assess the performance of these aquifer(s). The 1989 investigation cautioned against further local drilling activities prior to the assessment of aquifer performance under stressed conditions. In view of socio-economic planning and strategies for the Joe Morolong Local Municipality, feasibility studies were undertaken to refurbish and augment the water supply to the Heuningvlei area. Studies to establish sources outside the Heuningvlei pipeline area were only considered once the local potable resource potential was optimally developed. The sustainable potable yield information of existing sources used along the Heuningvlei pipeline was unreliable for water supply planning purposes. In 2006 the reported groundwater abstraction from existing pipeline boreholes operated by Sedibeng Water totaled 25,699 m 3 / month (856 m 3 / day), approximate 82 % of the 1989 recommended abstraction.

2 The projected water demand for the Heuningvlei socio economic development area is 2,380 m 3 / day. Water users include rural domestic, stock watering, schools and clinics. Water demand estimates and related statistics were obtained from Aurecon as presented in Table 1. Table 1: Estimated total water demand for the Heuningvlei area Type of water users Typical consumption l/c/d Population Theoretical demand m 3 /day Standpipe (200m) Schools Clinics Large stock ,402.6 Small stock Theoretical demand total 2,380.0 Theoretical excluding stock Theoretical stock only 2,027.6 Stock watering is the biggest user, with more than 28,000 large stock and 52,000 small stock populations, representing about 85% of the estimated total water demand. The apparent supply shortfall in relation to the water abstraction in 2006 is 1,512 m 3 / day. Identification or development of additional groundwater sources to meet the shortfall is required to make the planned Heuningvlei socio economic development project feasible. 2 STUDY OBJECTIVES AND APPROACH The main objectives of the study was to determine the safe yield and water quality of the Heuningvlei pipeline scheme existing boreholes, assess the resource potential within the Heuningvlei area and to establish potable groundwater sources aimed at meeting the project water demand of 2,380 m 3 /day. The study approach entailed three investigations phases which are outlined below: Phase 1: The first phase was to determine the safe yield and water quality of these existing boreholes and preliminary assess the resource potential within the Heuningvlei area to sustain a potable supply of at least 2,380 m 3 /day. Phase 2: The second phase was to identify target areas for further source development, and conduct preliminary hydrogeological assessments aimed at understanding the groundwater regime and water quality in target areas. Phase 3: This phase was to establish the shortfall identified from the verification results with the aid of further geophysical surveys, exploration drilling and test pumping of new sources. 3 STUDY AREA 3.1 Locality The Heuningvlei study area is located in the Northern Cape Province of South Africa in John Taolo Gaetsewe District Municipality. The Heuningvlei Pipeline Scheme is located in the north-westerly portion of the Moshaweng Local Municipality and also extends into the eastern portions of the Kagisano Local Municipality. It is approximately 120 km north of the Kuruman Town and 80 km north of Hotazel Manganese Mines, see Figure Geology The Heuningvlei well field is largely situated within the outcrop of Asbestos Hills Formation, Griqualand West sequence, which comprises largely of well jointed banded iron formation (BIF); dolomites of the Ghaap Plateau Formation in the Campbell Group and weathered lava. The bedrock geology of the area is largely concealed beneath the partially consolidated sediments of the Kalahari Group, which thickens to attain a maximum thickness of 180m. To the south the Kalahari cover becomes thin and patchy and large areas of bedrock are exposed in the mountainous areas. The deposition of these sediments was controlled by the pre-tertiary topography of the area. Intrusions of diabase sills

and dykes are present locally and their location could be mapped partially within the bedrock outcrop areas, as shown in Figure 1. 3.

3 and dykes are present locally and their location could be mapped partially within the bedrock outcrop areas, as shown in Figure Geohydrology Figure 1: Study área locality map Regional hydrogeological studies have been conducted in the past, with the most comprehensive one by Dr. P.J Smit (1977). From these studies, available borehole information and geohydrological information from published maps of the area the following can be concluded: The weathered lava aquifer is found in the Eiffel Madibeng area with borehole yields up to 2.0 l/s. Because of the shallow water levels the aquifer is vulnerable to groundwater pollution, containing excessive nitrate levels from stock watering. The weathered and fractured BIF bedrock aquifer yield potential is generally classified as poor to moderate borehole yield prospects ranging from 0.5 to 5 l/s. Karst aquifers associated with the Ghaap Plateau dolomite formations, are classified as good to moderate with expected borehole yields of more than 5 l/s. Most of the Heuningvlei project area is covered by Kalahari sediments which inhibit recharge via direct infiltration, which is generally associated with increased salinity in groundwater in the absence of lateral groundwater inflows. The intergranular aquifers are presented by the upper as well as basal sand and gravel beds. These aquifers are low productive with borehole yields generally less than 2 l/s, but with the ability to store large volumes of water. They are separated by red clays acting as a confining layer. The basal Kalahari sand and gravel formation and underlying bedrock is in hydraulic continuity. Drilling costs are generally high when establish production boreholes in this aquifer as well as underlying bedrock aquifers due to increased drilling depths and required telescopic borehole construction.

4 Several intrusive dykes, with low to impervious hydraulic conductivity, compartmentalize the aquifer. These dykes are present in the bedrock underlying the Kalahari sediments and at depth they become fresh rocks and acts as aquicludes, preventing lateral water flow. Generally the aquifer zone in the Heuningvlei Pipeline area has a high potential for long-term use, but the compartments sizes can limit the extent of the borehole performance. To optimally exploit the resource potential, abstraction boreholes will have to be located in most of these sub-compartments. 3.3 Groundwater Recharge The groundwater recharge is largely dependant on the quantity of annual rainfall and rainfall intensity (monthly rainfall). Monthly statistics for the Morokweng rainfall station (closest rainfall station) is presented in Table 2. Table 2 : Analysis of monthly rainfall data for the Morokweng/Severn rainfall station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Mean Median Percentile Percentile Percentile Percentile No. of years The significance of monthly rainfall on groundwater recharge in a semi-arid climate is generally as follows: 0 to 50 mm almost no recharge 50 to 100 mm low recharge, pending type and depth of bedrock cover 100 to 150 mm moderate recharge > 150 mm significant recharge To estimate aquifer recharge via direct infiltration from rainfall the Chloride Mass Balance (CMB) method was used. The concentrations of chloride in groundwater for boreholes in the Heuningvlei bedrock area with limited Kalahari sediment cover (< 15 m) were used to determine a representative value for recharge estimates. Elevated chloride values due to possible local contamination were excluded. The percentage recharge from rainfall varies from 7.5 to 1.3 with the average being 3.0 per cent. The mean annual recharge (which is 9 mm) represents the long-term maximum expressed as a mean, and includes recharge from super recharge events. This would overestimate the potential of the resource during extended dry periods. The median recharge (minimum that is available 5 out of 10 years) is therefore used for estimating the sustained groundwater recharge available for use. The relationship Y(median) = 0.68X(mean) 3.93 mm/a established by Wiegmans (2006) for the Kgalagadi DM area is used to estimate the sustainable resource potential. This equation yields a median recharge of 2.2 mm/a which is equivalent to Mm 3 /a over the 489 km 2 Heuningvlei bedrock outcrop area. 4 PHASE I: SOURCE VERIFICATION The phase 1 field investigation commenced in February 2009 involving a desk study, hydrocensus, and test pumping of existing boreholes for sustainable yield and groundwater quality assessments. A total of 18 pipeline boreholes. Boreholes with poor water quality (based on DWA 1998 domestic use guidelines) and low yielding were not subjected to test pumping, see Figure 2 for location of tested pipeline boreholes (blue dots).

Figure 2: Verified existing borehole locality map Step discharge rate and constant discharge rate tests were conducted on 34 existing boreholes with the respective recovery measurements.

5 Figure 2: Verified existing borehole locality map Step discharge rate and constant discharge rate tests were conducted on 34 existing boreholes with the respective recovery measurements. Excel based FC program developed by the Institute of Groundwater Studies (IGS) was used for analysing pump testing data. The recommended safe yields, proposed borehole use status, potability and test pumping results of the Heuningvlei pipeline scheme boreholes are summarized in Table 4. Water samples of the tested boreholes were collected at the end of the constant discharge pump test and submitted to UIS Analytical services in for physical parameters, micro and macro-chemical analysis. The groundwater quality in the Heuningvlei area is highly variable (Class 0 to Class 4 based on DWA, 1998 drinking water guidelines), with salinity and nitrate the main chemical elements which impact on water potability. Elevated Electrical Conductivities (EC), Total Dissolved Solids (TDS) and chloride (Cl) in boreholes are predominant found where Kalahari sediments thicknesses exceed 30 meters.

6 Table 3 : Summary of borehole yield testing and recommended abstraction Constant Discharge Test Recovery Test Safe Yield BH No Pump Intake mbgl Yield l/s Duration hours Draw down m Duration hours Residual Recovery m Aquifer m 2 /day l/s 12 hrs/day m³/a min , , , , , ,420 H , , , , , ,536 Total Daily Recommended Abstraction ,937 5 PHASE II: GROUNDWATER RESOURCE POTENTIAL AND TARGET AREAS FOR RESOURCE DEVELOPMENT The main recharge zone within the Heuningvlei pipeline has a footprint of 489 km 2, using the median recharge of 2.2 mm/a, a sustainable resource potential of 1,075,800 m 3 /a was calculated. The potable water (<class 2) supply recommended from 11 existing production boreholes is 316,937 m 3 /a. This leaves a water balance of 758,863 m 3 /a. The groundwater resources potential within the Heuningvlei pipeline is sufficient to meet the shortfall of 552,975 m 3 /a, however, the resource is not readily exploitable due to the inaccessibility of the mountainous area. The resource potential is also not sufficient to meet the water demand of rural communities obtaining water from own groundwater sources. Therefore target area 1 and 2 were identified for additional development of potable groundwater resources. Target areas 1 and 2 were also identified for verification of existing sources. In target area 1 three boreholes were pump tested Table 4. A combined yield of 3.0 l/s for 12 hrs (129.6 m3/day) was recommended from two boreholes. In target area 2, eight boreholes were subjected to pump tested - Table 4. Constant discharge rate tests were conducted on five of these boreholes (T9769, , , T9922 and G10439) and a combined abstraction rate of 7.8 l/s for 12 hours daily ( m 3 /day) was recommended. Stand alone community wáter supply sources in proximity to the pipeline route was also verified, including class 3 water quality sources. The combined recommended yield amounts to m3/day. Class 3 water quality should be blended at storage to attain suitable water quality. The combined potable groundwater resources from existing Heuningvlei pipeline scheme boreholes, target area 1 and 2 and stand alone sources is 1,730 m 3 /day. This leaves a shortfall of 650 m 3 / day to meet the projected wáter demand. The groundwater exploitation potential was assessed for both target áreas, of which results indicated that the potential in target area 2 was sufficient to meet the wáter supply shortfall. Regional groundwater contours indicated that groundwater flow from upstream catchments discharges through target area 2. The exploitable groundwater balance available at target area 2 has been reported by Wiegmans (2006) as 2,331,000 m 3 /a.

Borehole Number Table 4 : Summary of borehole yield testing and recommended abstraction in T1 and T2 Test Depthm bgl Constant Discharge Test Yield l/s Duration hr Draw Down m Recovery Test Duration

35 SDT -- -- -- -- -- -- Target Area 2 7.8 466.56 T9769 78.04 1.01 12 5.64 12 0.07 14 0.8 34.6 13-87006 87.04 2.03 12 7.24 9 0 29 1.0 43.2 13-87007 111.04 1.52 12 36.82 12 0.71 24 1.0 43.2 13-87008 99.

7 Borehole Number Table 4 : Summary of borehole yield testing and recommended abstraction in T1 and T2 Test Depthm bgl Constant Discharge Test Yield l/s Duration hr Draw Down m Recovery Test Duration hr Recovery m Aquifer T m 2 /day l/s Safe Yield 12 hrs/day Target Area min SDT Target Area T CAL G T T SDT SDT m³/d Total production boreholes PHASE III: GROUNDWATER SOURCE DEVELOPMENT Target area 2 (T2) was identified for groundwater source development aimed at establishing the shortfall. Both airborne and ground geophysical surveys were conducted. An airborne magnetic survey was conducted first followed by ground surveys at selected magnetic anomalies using: gravity, magnetic and 2D resistivity methods. 6.1 Airborne Geophysics An airborne magnetic survey was flown to delineate any possible intrusive dykes which may cut the underlying dolomite aquifer in several smaller compartments. The presence of several compartments requires spreading of production boreholes and limits the possibility to establish a closely spaced wellfield. A dual sensor air-borne magnetic survey was flown by Xcalibur Airborne Geophysics. Two wing-tip G823A 15m apart magnetic sensors were used. The line spacing was 100 metres and ground clearance 30 metres. The line direction was planned NW, approximate at right angle to the expected dyke direction. The dual magnetic sensors allow for improved interpolation of magnetic data and trends between flight lines. A total of 48 km 2 was flown over Target area 2 of which the residual and total magnetic field contour maps are presented in Figure 3. Figure 3:Magnetic field contour in T2

The air-borne data mapped the presence of several dykes, of which the interpreted positions are indicated (green lines) in Figure 3. 6.

8 The air-borne data mapped the presence of several dykes, of which the interpreted positions are indicated (green lines) in Figure Ground Geophysics Ground geophysical surveys conducted comprised magnetic, gravity and resistivity methods along selected traverses across major mapped air-borne magnetic anomalies. The initial survey was focused at using the gravity method to locate potential karts features in the dolomite bedrock. The second ground geophysical survey was focused at using the 2D resistivity imaging method to investigate the Kalahari sediment thickness (including layer conductivities) and underlying dolomite bedrock. The Abem SAS4000 terrameter and ES 464 switching unit with four multi core cables (150 metres long) and 1 metre stainless steel pegs were used. The Wenner-Schlumberger continuous profiling protocol was used. The measured pseudo sections were processed with a 2D inverse numerical modelling technique to give the estimated resistivities up to a depth of 116 m. The data was plotted as 2D resistivity sections and correlated with gravity and magnetic data to select drilling positions. An example is presented in Figure 4 and Figure 5. T3/1550 ( Q=4.3 L/s) Figure 4: 2D imaging profiling for Traverse 3 in target area 2 Heuningvlei T2 - Traverse Elevation (m) Station (m) Bouger Gravity (mgal) ELEVN. OBSG. Figure 5: Gravity and elevation profile for Traverse 3 in target area 2

9 6.3 Drilling Results in Target Area 2 The exploration drilling was done with the aid of air-percussion drilling technique whereby a telescoping approach was used to drill through thick Kalahari sand, budin clay and unconsolidated gravel in the transition zone between clay and bedrock. Total of 16 exploration boreholes were drilled. Drilling depths varied from m. The drilling exploration target was to develop both the upper Kalahari gravel and sand aquifer (above the Budin clay) as well as the basal Kalahari gravel and fractured/karst bedrock aquifers. Based on the drilling results the occurrence and distribution of groundwater in the Heuningvlei target area 2 is summarized as follows: Groundwater occurs mainly at depths ( metres) in weathered and fractured bedrock; Water interceptions in the weathered BIF and Kalahari gravels range from 102 to 120 metres, yielding 2.0 to 6.0 l/s (excluding dry boreholes); Water interceptions in underlying fractured dolomite aquifer range from metres, yielding 2 l/s. Casing was installed below major water strikes of >1 l/s in 9 exploration boreholes which were deemed successful. 6.4 Test Pumping of New Boreholes From the 16 boreholes drilled, 9 boreholes with airlift yields of 2 to 6 l/s were subjected to test pumping. The CDT s were conducted with the pumping times varying from 24 to 48 hours, and results of these are summarised in Table 5. Pump test analysis were done with the aid of FC based method which includes Cooper-Jacob, Theis and Basic FC developed by the University of the Free State (IGS). Table 5: Borehole management recommendations for newly drilled boreholes in Heuningvlei T2 Borehole Number Aquifer Pump Dynamic WL Critical WL Safe Yield Comments T (CJ) Intake 12 hrs/day m 2 mbgl mbgl /day mbgl l/s m³/d Production borehole Production borehole Production borehole Production borehole Production borehole Production borehole Production borehole Production borehole Production borehole Proposed Production Boreholes Total The newly established groundwater sources in target area 2 amounts to m 3 /day which meets the shortfall of 605 m 3 /day. The recommended existing and new production boreholes for the planned Heuningvlei socio-economic development project is shown in Figure 7.

Figure 6: Location of recommended production boreholes in Heuningvlei area 7 CONCLUSIONS A potable and sustainable wáter supply was verified and developed locally to meet the wáter demand of 2,380 m

10 Figure 6: Location of recommended production boreholes in Heuningvlei area 7 CONCLUSIONS A potable and sustainable wáter supply was verified and developed locally to meet the wáter demand of 2,380 m 3 /day for the Heuningvlei socio-economic development project. The groundwater exploitability in the area was previously mapped as very low (<10%). The understanding of the regional hydrogeological regime, use of appropriate geophysical techniques and drilling approach was fundamental in establishing a potable local groundwater resource. The median sustainable abstraction rate recommended for the 27 production boreholes is 2 l/s (12 hr/day). Groundwater levels in boreholes range from 60 to 90 mbgl. High yielding boreholes (>2 l/s) in the area were mainly found in the basal Kalahari gravel deposits below the Budin clay at a depth in excess of 90 mbgl. 8 REFERENCES DWAF, The South African Water Quality Guidelines, Domestic Use, Volume 1. Monokofala, C, Wiegmans, FE, Preliminary geohydrological investigation of Heuningvlei pipeline target area 1 and 2, Report , Golder Associates Africa. Monokofala, C, Wiegmans, FE (2008). Test Pumping of Heuningvlei pipeline boreholes and preliminary assessment of groundwater resource potential Report No /11521/1/E, VSA Geoconsultants Group

11 Monokofala, C., Sanitation protocol for rural communities in the Moshaweng local municipality - Heuningvlei area. Rep. no. VSAG/07/02/01. VSA Geoconsultants Group. Partridge, Robinson & Associates., (1989). Report on the results of a borehole drilling and aquifer testing programme for extensions to the Heuningvlei Well field, Bophuthatswana. SANS 241, South African National Standards, Drinking water standard, Edition 6, Smit, P.J Die Geohidrologie in die Opvangebied van die Moloporivier in die Noordelike Kalahari. Volume 1. Universiteit van Oranje-Vrystaat. Bloemfontein. Wiegmans FE., (2006). Water resources strategy for the Kgalagadi District Municipality. Groundwater situation assessment Volume 2 maps. VSA Geoconsultants Group WRC, Quality of Domestic Water Supplies. Volume 1: Assessment Guide. Water Research Commission Report No. TT 101/98

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