Integration of hydrogeophysical and geological investigations in enhancing groundwater potential assessment in Houtriver gneiss crystalline basement formation of South Africa

https://doi.org/10.1016/j.pce.2021.103009Get rights and content

Highlights

  • Multiple-method approach to groundwater exploration.

  • Houtriver gneiss crystalline aquifer studied.

  • Hydrogeophysical and geological tools used.

  • Multiple fractured zone from 30m to 72 m.

  • Test boreholes drilled to validate results and success.

Abstract

Groundwater exploration in crystalline basement aquifers such as the Houtriver gneiss formation in the Limpopo province of South Africa is often described as complex. This is because groundwater availability in such aquifers is largely a consequence of the interaction of several processes related to recharge, underlying geological features and fracture connectivity of the aquifer rock matrix. In this study, an integration of geophysical and geological investigations is applied in inferring potential drill targets within the Houtriver gneiss crystalline basement aquifer system. Results from the magnetic and frequency domain electromagnetic surveys were combined with geological investigations to identify sites where vertical electrical resistivity sounding was applied to infer the thickness and layering of weathered and fractured zones, as well as to identify potential targets where test boreholes were drilled. Constructed geo-resistivity pseudo-sections suggested that groundwater occurrence within this formation is described by a heterogeneous multiple-layered and fractured aquifer system with the main groundwater bearing zones ranging from a depth of 30–72m in most cases. Ten potential drill sites were identified from which three test boreholes were drilled and used to validate the results through a lithostratigraphic conceptual model developed from the correlation of the geophysical results with drill logs. The integration of hydro-geophysical and geological methods thus provided a comprehensive approach for resource assessment in the Houtriver gneiss formation.

Introduction

Crystalline basement aquifers form a fundamental component of water supply in sub-Saharan Africa with the notable emphasis being its contribution to agricultural and rural domestic water supply, (e.g. in Adams (2009) and Witthüser et al. (2010), and Robins et al. (2013), Holland 2010). The aquifer set up in the crystalline hard rock and its characteristics have been described by various researchers such as Davis and Turk (1964); Tardy (1971); Eswaran and Bin (1978); Ledger and Rowe (1980); Chilton and Smith-Carington (1984); Acworth (1987); Pickens et al. (1987); Nahon (1991); Wright (1992); Chilton and Foster (1995) and Jolly (1986). These aquifers are usually developed within the weathered overburden and fractured bedrock of crystalline rocks of intrusive and/or metamorphic origin which are mainly of Precambrian age and are, despite more often being described as strategic water supply, classified as low yielding (Wright, 1992). As such, groundwater exploration in such aquifers is often described as a more complex process since groundwater availability in such system is largely a consequence of the interaction of several processes related to recharge, underlying geological features and fracture connectivity of the aquifer rock matrix. In South Africa's Limpopo basin, for instance, has its groundwater resources largely contained within the Hout river gneiss crystalline basement formation (Holland, 2011). Like most basement aquifers, this formation is characterised by high variability in terms of the recharge mechanisms, groundwater resource potential and the approximate depth to which yielding boreholes need to be drilled. Historically, groundwater exploration within such crystalline basement aquifers has been centered on identification of structural features and geological contacts at depth within the rock formation. This is because groundwater movement and storage in this formation has been described by previous researchers to occur via fractures, faults, weathered zones and other secondary features that enhance the aquifer potential only locally with a varying aquifer thickness ranging from depth extending from 12m to over 120m (Jolly, 1986; du Toit, 2001). The assessment of the groundwater potential and borehole siting in typical crystalline formation aquifers has often presented to be challenging and more often drilling of numerous dry and seasonal boreholes has been reported for poorly sited wells across the formation. This is despite fact that a combination of hydrogeophysical and geological methods has over the years been successfully applied in some crystalline basement aquifers to increase borehole success rates through inferring the subsurface distribution of groundwater controlling structures such as the existence of any geological contacts, dykes and/or lineaments, approximate depth to groundwater table, thickness of the overlying weathered regolith, and the distribution fractured zones within the solid bedrock (e.g. in Zody 1969; Martinelli and Hubert, 1985; Metwaly, 2009, Chandra et al. (2010), Chuma et al., 2013). The current study demonstrates the effectiveness of applying integrated geophysical and geological investigations as appropriate exploration tools for inferring subsurface features useful in identifying potential drill targets in the Houtriver gneiss crystalline basement formation in the Limpopo Province of South Africa.

Section snippets

Study area and geological setting

The study was conducted in the Mamadila communal area, Polokwane municipality in the Limpopo province of South Africa (Fig. 1). In terms of water management area, the area lies within the Hout river catchment of the Limpopo basin. The Hout catchment has an area of 2478 km2 with an elevation ranging from 840 to 1739 m above mean sea level (mamsl), and an ephemeral river that flows intermittently following large and intense precipitation events into the Sand River. The catchment is characterised

Hydrogeophysical investigations

Several geophysical methodologies were systematically applied in three selected regions based on the occurrence of dykes and other geological features within the study area (Fig. 4). An understanding of each of the utilised methodologies is described in Telford et al. (1990).

The order of application for the geophysical methods (Fig. 5) was 1D ground magnetics using a proton magnetometer followed by frequency domain electromagnetic exploration methods using an EM34 unit as horizontal profiling

Results and interpretation

The interpretation of results from the field investigation was based on plots of both profiling (magnetic and EM) and VES field measurements (Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11). For each profile line in the respective regions, results of the lateral profiling using magnetic and electromagnetic methods were used to identify the anomalous sites (indicative of geological contacts or structures likely to control groundwater occurrence) are first presented. Anomalous zones from which

Discussion

The study results have shown that hydrogeophysical investigations and geological mapping have shown to be useful tools in the identification of drill targets for areas underlain by the geologically complex Houtriver gneiss crystalline basement formation. This was enhanced by the fact that groundwater storage and dynamics within such metamorphic crystalline formation is controlled is mainly influenced by the extend of zone of weathering and existence the fractures and fissures in the fresh rock,

Conclusions and recommendations

The integration of geophysical and geological investigations was successfully applied in inferring potential drill targets within the Houtriver gneiss crystalline basement aquifer system. Due to high heterogeneity in the crystalline basement formation, the aquifer characteristics varied from one place to another in terms of lateral and vertical distribution and hence groundwater availability was variable within the study area. The constructed geo-resistivity pseudo-sections suggested that

Author statement

The authors are grateful to all comments raised by the reviewers.

We have made all effort to address the necessary concerns and improve the quality of our diagrams.

Please find attached response to reviewers 4 concerns for the consideration. We very much hope that the revised manuscript is accepted for publication in the Journal. The response to reviewer 1 s comments are done on the review comment bullets on the reviewed word document that they attached and sent to us. We really appreciate their

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The Authors would like to appreciate and thank the sterling work done by the the support provided by the Danish Development Center through providing financial support of the project Enhancing Sustainable Groundwater Use in South Africa (ESGUSA), from which this work was part of. Further to this the authors also acknowledge Department of Water and Sanitation (Polokwane) ‘s resource assessment team for equipment and manpower assistance provided during field work.

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      Normally, geological lineaments represent groundwater controls such as fractures, dykes, faults and potential lithological contacts. Lithological cross sections constructed from drill logs samples collected during the development of monitoring wells under the ESGUSA project framework (Muchingami et al., 2021) confirms the heterogeneity and importance of the contribution of pegmatitic lineaments and dykes within the formation (Fig. 7). As such, any exploration work within the Hout river gneiss should endeavour to establish the exact location and orientation of such lineaments and other structures.

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