USING GEOPHYSICAL TECHNIQUES TO DELINEATE GROUNDWATER BEARING STRUCTURES AT THE SAGOLE HOT SPRING, LIMPOPO PROVINCE, SOUTH AFRICA

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1 USING GEOPHYSICAL TECHNIQUES TO DELINEATE GROUNDWATER BEARING STRUCTURES AT THE SAGOLE HOT SPRING, LIMPOPO PROVINCE, SOUTH AFRICA P.K. Nyabeze 1, A. Shabalala 2 A. and O. Gwavava 3 1 Geophysics Unit, Council for Geoscience, Pretoria, South Africa; pnyabeze@geoscience.org.za 2 Water Geoscience Unit, Council for Geoscience, Pretoria, South Africa; ashabalala@geoscience.org.za 3 Department of Geology, University of Fort Hare, Alice, South Africa; ogwavava@ufh.ac.za Abstract The Sagole hot spring is located in the northern Limpopo Province of South Africa. Investigations were carried out in order to investigate the groundwater aquifer and water chemistry. Results were envisaged to the understanding of the geothermal potential of the area. Regional scale airborne magnetic data and geology were used for identifying structures and lithological boundaries that are associated with thermal groundwater aquifers. Detailed ground follow-up and verification surveys were carried out across the target using magnetic, electrical resistivity tomography (ERT) and frequency-domain electromagnetic (FDEM) and radiometric methods. Water samples were collected from the spring eye and archival groundwater data was analyzed. The interpretation of the airborne magnetic data revealed the presence of west to east, northwest and intersecting lineaments at the hot spring. From magnetic data, the groundwater aquifer was found to be capped by basalt with hot rising to the surface along possible geological contacts, faults or fractures. The FDEM profile data across the aquifer zone had peak values above 100 ms/m. The inversion of ERT data defined a highly electrical conductive, low resistivity with thickness of about 60 m. Chemical analysis of the ground water revealed that the water does not have any indication of pollution. The thermal water was found to be of meteoric origin. The drilling of artesian thermal boreholes through the capping basalt should be explored. The hot water boreholes will be utilized by the community for domestic, irrigation and possible development of micro-geothermal systems.

2 1. INTRODUCTION The Sagole hot spring is situated in the northern Limpopo Province of South Africa (Figure 1). The main objective of the study was to assess the characteristics of the groundwater aquifer that is associated with the hot spring. The recorded water temperature at Sagole is 45 C (Olivier et al. 2011). The inferred heat source is at a depth of 4 to 6 km (Nyabeze et al. 2013). The results of the survey would be useful for understanding the groundwater aquifer. In addition the research will provide additional knowledge that would assist in the exploration for geothermal resource potential. Figure 1. Geology map of the Sagole hot spring area in the northern Limpopo Province of South Africa. 2. GEOLOGY Figure 1 shows the geological map of the study area. The Sagole spring is associated with the Tshipise fault that transects the contact between the Karoo Supergroup (Madzaringwe and Mikambeni Formations) to the north and Soutpansberg Group (Nzhelele and Musekwa Formations) to the south (Messina Map Sheet, Geological Survey, 1981). The Madzirangwe Formation consists of sandstone, siltstone and shales, and the Mikambeni Formation comprises mudstone, shale, sandstone with occasional thin coal seams (Johnson et al., 2006). The occurrence of basaltic lava in the area was reported by Brandl (1981). The Musekwa Formation is a 400 m thick volcanic assemblage that consists of; the Nzhelele Formation with a thickness of about 600 m constitutes the uppermost unit of the group, it consists of red argillaceous and arenaceous sediments together with layers pyroclastic rocks of which one is copper-bearing (Barker et al 2009).

3 3. METHODOLOGY 3.1 Regional Tectonics There is regional aeromagnetic data for the area that was collected at a line separation of 1000 m (Ledwaba et al., 2009). The regional aeromagnetic data was extracted for the Sagole area. The data was processed and filtered to generate a reduced to the pole dataset and the then a vertical derivative image. The vertical derivative image was interpreted for geological lineaments. Mapped geological faults were plotted on the aeromagnetic map. 3.2 Ground Geophysics Ground geophysical surveys were carried out in Sagole Village in the vicinity of the hot spring (Figure 2). In order to achieve the objectives geophysical surveys investigations were carried out along selected and accessible lines. The techniques that were applied included magnetics, electrical resistivity tomography, frequency domain electromagnetic profiling and radiometric methods. Electrical resistivity tomography (ERT) and frequency domain electromagnetic profiling (FDEM) were carried out during the dry season in order to reduce the effect of meteoric water on the subsurface characteristics. Electromagnetic surveys were carried out in on June 2009 along accessible traverses. Resistivity surveys were carried out in May 2011 and May 2013 respectively. Figure 2. A. Location of the ground geophysics survey locations and elevation contours. The terrain conductivity technique involves generating an electromagnetic field which induces currents in the earth; the resultant magnitude and phase of induced electromagnetic currents are related to the subsurface electrical conductivity. Surveys were carried out using a Geonics model EM34-3 frequency domain system which comprises separate receiver and transmitter coils that couple inductively with the ground. The technical and theoretical aspects of the EM34-3 technique are explained in a Geonics technical note (McNeill, 1980). The conductivity data was used to produce a gridded map and selected data for a west to east profile L1 was presented as profiles.

4 Resistivity surveys are useful for estimating depth of bedrock structures because different lithological units have differing resistivity values (ASCE, 1997), the method involves introducing current into the ground and measuring potentials to determine the effective or apparent resistivity of the subsurface Data was recorded using an Iris Syscal Pro 72 channel multi-electrode system. Resistivity data was modeled to produce a depth sections images using Res2DINV software (Loke, 2000). Radiometric surveys were carried out over selected areas to map variations in the concentration of radioactive elements potassium (K), uranium U) and thorium (Th). Useful information on geology and alteration can be deduced from analysing ratios of the concentration or the recorded radioactive counts of the three naturally occurring elements that is K, U and Th. Data was recorded using a RS-125 Gamma- Ray spectrometer, serial number The sensor recorded data at a height of about 0.5m. A station spacing of about 15m was achieved. Heat production depends on concentrations of U, Th and K which can be converted to heat generated per unit volume of rock per unit time (Jones, 1992), near surface weathering and alteration may cause redistribution of U and Th resulting in underestimates of the uppercrustal component of heat flow (Jones, 1992). 3.3 Water Chemistry Water samples were collected in May 2013, from the Sagole hot water spring for physico-chemical studies. The samples were collected using previously acid-washed high density polyethylene (HDPE) bottles, chilled to between 3 and 5 C and dispatched in a cooler box to an accredited analytical laboratory chemical for analysis. The following water quality parameters were measured in situ using portable, multi-sensor meters: Temperature, ph, Electrical Conductivity (EC), Total Dissolved Solids (TDS) and Dissolved Oxygen (DO). Cations were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) and anions by ion chromatography (IC). The sampling and presenting of water analyses in Piper diagrams is explained in detail in USGS field manual, the diagrams help to classify groundwater in terms of mineral composition (USGS, 2013). The evaluation of hydrochemical data for the salinity adsorption ration was used by Agarwal et al (2013). Water chemistry data from the national groundwater data base and historical information wre analysed. The ratio between halogens Br/Cl was computed to verify whether the source of the water had a sea water origin. The I/Cl ratio was used to infer the residence time of the groundwater. The Br/Cl and I/Cl ratios for sea water as 3.5x10-3 and x10-3 respectively, the source of the three halogens is assumed to be dissolved silicate rocks (Steinbruch and Merkel. 2008). 4. RESULTS 4.1 Airborne Geophysics The area has dolerite dyke swarms oriented roughly east-west, northeast and north west. Evidence of faulting and fracturing can be deduced from the discontinuous nature of the magnetic lineaments East to west trending lineaments can be seen on the vertical derivative magnetic map (Figure 3). The centrally located high intensity east to west trending magnetic feature could be a basaltic unit within the Soutpansberg Basin. An intersection of two lineaments was inferred at the Sagole hot spring. The high intensity east to west magnetic anomalies occurring to the north of the Sagole spring are probably dolerite dykes and sills that intruded the Karoo sediments.

5 Figure. 3. Vertical derivative magnetic image with mapped and interpreted lineaments. 4.2 Ground Magnetic Survey Follow up ground geophysical surveys confirmed the presence of the underlying basaltic occurrence to the south of the hot spring with sediments intruded by basalt to the north of the spring. East to west and northwest trending lineaments were interpreted (Figure 4A). Intersecting northwest and east to west trending lineaments were interpreted at the hot spring. The basalt is probably an aquifer confining zone. 4.3 Electromagnetic Survey The electrical conductivity data that was recorded using a 20m horizontal dipole configuration was gridded (Figure 4B), groundwater flow paths and a possible fault was interpreted.. The maximum depth of investigation using this configuration was about 15 m. The obtained results indicated the presence of a conductive top layer with a resistive bottom layer. The thermal water at the spring eye is associated with conductivity values above 100 ms/m (Figure 5). A lithological contact zone was inferred to the south from the EC anomaly contrast. In order to investigate the electrical conductivity variation at depth of 15 m and 30 m respectively, results of profiling using both a 20 m horizontal and vertical dipole on profile L1 were presented as profiles (Figure 5) Electrical Resistivity Tomography Results of a 10 m dipole-dipole ERT array survey that was carried out on thewest to east profile L1, were modeled to produce a depth section (Figure 6). There is a low resistivity vadose zone with a thickness of about 30 m. The low resistivity values below 25 Ohm.m could be attributed to water or salinity. The resistivity increases with depth from surface to about 70 m, having values above 500 Ohm.m. The maximum depth of the investigation of the ERT configuration was about 70 m. The higher bottom layer resistivity values could be associated with the mapped basalt.

6 Figure. 4. A. Ground magnetic survey data showing a survey stations, interpreted geology and lineaments. B.FDEM electrical conductivity data of the survey area. Figure 5. FDEM electrical conductivity data for profile L1across the spring for 20HD and 20VD survey configurations showing a decrease in EC with depth (increasing resistivity). Figure. 6. Electrical resistivity tomography data for profile along an EW oriented profile L1 showing a low resistivity top layer with a thickness of about 30 and increasing resistivity with depth.

7 4.5. Radiometric Data The lithology that is associated with the hot spring has relatively higher K and Th composition. The lower U composition can be attributed to its mobility (Figure 8). The average K, Th and U composition for the 20 recorded stations at Sagole were 0.39% for K, 1.19 ppm for U and 2.66 ppm for Th. Geological contacts or possible faults\lineaments were inferred from the radiometric data. Groundwater probably exploits inferred lineament. Figure 8. Profiles of K, U, Th recorded along line L1 oriented west to east Water Chemistry The physical and chemical characteristics of water at Sagole hot water springs are shown in Table 1. Water quality standards for potable water (SANS, 2006) are also given. The spring is alkaline with a ph of 9.3, the Total dissolved solids was 157 mg/l, the electrical conductivity was 24.3 ms/m and the total alkalinity (CaCO 3 ) was 79.3 mg/l. The values fell within the recommended South African National Standard for drinking water (SANS, 2006) indication that the war suitable for human consumption. Table 1. Physical and chemical characteristics of the Sagole hot water spring Parameter Value Recommended limit (SANS, 2006) Temp ( o C) ph ( ph units) EC (ms/m) 24.3 <150 TDS (mg/l) 157 < 1000 DO (mg/l) 1.98 Calcium (mg/l) 2.02 <150 Potassium (mg/l) 1.32 <50 Magnesium (mg/l) <0.05 <70 Sodium (mg/l) 64.5 <200 Chloride (mg/l) 36.2 <200 Nitrate (mg/l) <0.3 <10 Sulphate (mg/l) 16.9 <400 Total alkalinity (mg/l CaC0 3 ) 79.3 The high sodium and Chloride content is an indication of saline water. The pie charts in Figure 9 show the main cationic and anionic composition of the Sagole hot water spring for The evaluation of chemistry data from 1997 to 2013 showed that there was no significant variation in the composition of the Sagole hot water spring as shown in the Piper diagram (Figure 10A). The water is portable and does not have any indication of pollution. The Sagole thermal spring can be classified as sodium bicarbonate

8 chlorinated water. The water is also hard carbonate water because of the high concentration of CaCO 3. The other dominant elements found in the spring water are sodium (Na), sulphate (SO 4 ) and Chloride (Cl- ). The high Na+ concentrations probably originate from sodium-rich plagioclase feldspars from the sandstone and shale. Sagole (2013) : Anions (mg/l) Sagole (2013): Cations (mg/l) CaC03 60% Cl- 27% Na 95% Ca 3% K 2% S04 13% Figure 9. The main cationic and anionic composition of the Sagole thermal spring water for The Sodium-Adsorption Ratio (SAR) diagram (Figure 10B) indicates that the majority of water samples fall within C2 (EC<250 ms/cm, low salty water) and S2 (medium sodium content). C2 water is suitable for all plants. C2S2 water is suitable for irrigation but with a caution. The low Br/Cl ratio below 3.5x10-3 shows that the groundwater did not have a sea water origin, the I/Cl ratio is higher than x10-3, an indication of relatively lower residence time of the groundwater, see Table 2. The water may therefore be of meteoric origin. Figure 10. A. Piper diagram of composition of the Sagole thermal spring water: 1997 and 2013, B. Sodium-adsorption ratio (SAR) diagram of the Sagole thermal spring water.

9 Table 2. Analysis of halogens for source and residence time Halogen Values Ratios Br/Cl 2.31x x x x10-3 I/Cl x x10-3 Source of water chemistry (Olivier et al. 2011) 5. CONCLUSION AND RECOMMENDATIONS Regional geological structures were interpreted from the ragional aeromagnetic and geological data. Resistivity and electrical conductivity data indicated that the unsaturated, vadose zone has a thickness of approximately 30 m. Resistivity increases with depth from surface to about 70 m. The thermal aquifer is deep seated; occurring below the inferred basalt. The research confirmed the occurrence of the Sagole spring within the contact zone of two geological formations namely the Karoo Supergroup and the Soutpansberg Group rocks. Groundwater flows in a north to south direction was deduced from zones with high terrain conductivity values above 100 ms/m. The geology of the area also has an effect on the water quality. The weathering of this rock may result in the transportation of carbonates, which can increase the alkalinity of the water and result in high ph values that were observed. Chemical analysis of the ground water revealed that the water does not have any indication of pollution. The thermal water was found to be of meteoric origin. The drilling of artesian thermal boreholes through the capping basalt should be explored. The hot water boreholes will be utilized by the community for domestic, irrigation and possible development of micro-geothermal systems. The groundwater aquifer should be investigated below the 70 m depth, below the aquifer capping basalt. The generation of a 3-Dimensional model of the geology will assist in the understanding of the thermal aquifer. 6. ACKNOWLEDGEMENTS The authors would like to express gratitude to the Water Research Commission (Project K5-1959) and the Council for Geoscience for funding the project. The Council for Geoscience staff namely Mr. Matome Sekiba, Ms. Phathu Mulaudzi and Mr. Abongile Mbongonya are thanked for assistance with data acquisition. Mrs. Lindy. Heath from the Council for Geoscience is thanked for cartography work. The cooperation rendered by the community in Sagole Village is greatly appreciated. 7. REFERENCE Agarwal, M., Jain, S., and Shandilya, A.K. (2013) Hydro-Chemical Evaluation of Groundwater of Area Around Raisar, Bikaner District, Rajasthan, India. International Research Journal of Environment Sciences, Vol. 2(5), Rajasthan, India, ASCE, (1997) Geophysical Exploration for Engineering and Environmental Investigations. Technical Engineering and Design Guides as adapted from the U.S. Army Corps of Engineers: No. 23, 41, 75, 169, Barker, O. B., Brandl, G., Callaghan, C.C., Eriksson, P. G. and van Der Neut,M The Soutpansberg and Waterberg Groups and the Blouberg Formation. In The Geology of South Africa(edited by Johnson, M.R., Anhaeusser, C.R. and Thomas, R.J.),p Geological Society of South Africa, Johannesburg and Council for Geoscience, Pretoria, South Africa. Brandl, G : Geological Series, Sheet 2230 Messina and Explanation: The geology of the Messina area. Geol. Surv. S. Afr, Pretoria, South Africa. Johnson, M. R., Van Vuuren, C. J., Visser, J. N. J., Cole, D. I., and De V Wickens, H., Christie

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