Delineation of preferential flow pathways in a tropical crystalline rock aquifer in Tarkwa, Ghana using integrated hydrogeophysical methods

The desire to develop hydrogeological base maps and to protect groundwater resources to augment sustainable water resources management in Ghana features prominently in Ghana's Water Sector Strategic Development Plan, 2012–2025 (Ministry of Water Resources, Works and Housing 2012) and Medium-Term National Development Policy Framework (MTNDPF), 2022–2025 (Ghana National Planning Development Commission 2021) documents, which are hinged on and in consonance with Goal 6: Clean Water and Sanitation of the Sustainable Development Goals, 2015–2030 (United Nations General Assembly 2015). Globally, crystalline rock aquifers are important for the provision of potable water for supporting livelihoods in tropical and humid climates (e.g. Gustafson & Krásný 1994; Aristide et al. 2021). In Ghana, crystalline rock aquifers, referred to in Ghana as Basement Complex Aquifers, cover more than half of the land area being locally important for the provision of groundwater, especially in the rural areas (Gill 1969; Dapaah-Siakwan & Gyau-Boakye 2000). Groundwater occurrence in crystalline aquifers is linked with the development of secondary porosity and hydraulic conductivity resulting from jointing, shearing, fracturing, and weathering (Lloyd 1999; Singhal & Gupta 1999). These secondary hydraulic features create preferential flow pathways and vertical head gradients that produce fast groundwater and contaminant travel times (Cook 2003; Ewusi et al. 2017a).

Despite the importance of these crystalline aquifers in Ghana, they are plagued by contamination from anthropogenic activities like indiscriminate refuse dumping and contaminated land leaching (Agbotui et al. 2021), mining (Ewusi et al. 2017b), agricultural inputs (Okofo et al. 2022), and discharge from septic tanks (Anornu et al. 2017). To effectively manage the groundwater resource in the crystalline aquifers and also protect them against contamination, requires that the preferential flow pathways are well characterised and delineated (Singhal & Gupta 1999; Cook 2003) so as to predict contaminant fate and transport, interpret groundwater levels in boreholes, delineate well head-protection zones, design supply boreholes, estimate groundwater resource amounts, target borehole sampling depths, and develop conceptual and numerical groundwater models. The methods used to delineate preferential flow pathways include field geological mapping (Medici et al. 2021; Medici & West 2022), single borehole dilution testing (BDT) (Tsang & Doughty 2003; Agbotui et al. 2020), borehole inspection camera (Zemanek et al. 1970; Paillet & Pedler 1996), core logging (Shuter & Teasdale 1989), packer testing (Quinn et al. 2012), flow logging (Paillet 2001; Day-Lewis et al. 2011), slug testing (Ren et al. 2018; Medici et al. 2019), and well-to-well tracer testing (Singhal & Gupta 1999; Cook 2003). Paillet & Pedler (1996), provide details about the strengths and weaknesses of these methods and the hydraulic parameters they can delineate in fractured aquifers. Paillet & Pedler (1996) espouse the integrated use of the methods as a complement to each other for better elucidation of preferential flow pathways.

In this study, borehole dilution tests, slug testing, and field geological mapping are used in an integrated manner to characterise preferential flow pathways and hydraulic parameters. The combined methods provide complementary data and are relatively cheap, and logistically simple to set up and interpret. A review of Ghanaian hydrogeological literature indicates that published hydrogeological investigations have focused on groundwater modelling (Yidana et al. 2015), groundwater quality, and resource quantity (Yidana et al. 2014; Ewusi & Seidu 2018) assessments that focus on catchment groundwater flow dynamics. These precedent approaches did not incorporate preferential flow pathways, whose characterisation and delineation are very important for protecting groundwater resources in crystalline rock aquifers.

This study will therefore be important for (i) characterising flow variation with depth for the development of conceptual and numerical groundwater models; (ii) demonstrating the utility, novelty, and promise of the integrated hydrogeophysical tests (single borehole dilution and slug tests, and field geological mapping) in characterising tropical crystalline aquifers in Ghana and the West African sub-region. This study aims to delineate preferential flow pathways in crystalline rock aquifers in Tarkwa, Ghana by the combined use of BDT, slug testing, and geological field mapping. The specific objectives of this research are using: (i) the BDT to map flow horizons, quantify the specific groundwater velocities and develop borehole flow models; (ii) slug testing to determine open borehole interval hydraulic conductivity, transmissivity, and storage coefficient and (iii) field geological mapping to characterise geological discontinuities, and fracture porosity to understand the flow geometry. We then use the above datasets to develop borehole scale conceptual models for feeding into future numerical models.

The Tarkwa area forms part of the Ankobra basin, with two major rivers, the Huni River and Bonsa River draining the north and south, respectively. The climate is considered to be sub-humid tropical with eight wet months and four dry months (Kuma 2007). Data from the University of Mines and Technology (UMaT) Meteorological Station between 1970 and 2020 show that the minimum, mean and maximum rainfall values are 1,381, 1,798, and 2,174 mm, respectively. Similarly, the annual pan evaporation for the Tarkwa area ranges from 905 to 1,065 mm, with a mean value of 971 mm.

Data for this research were gathered from two monitoring boreholes located in UmaT named Hilly and Valley. The Hilly borehole is located close to the staff residence, whereas the Valley borehole is close to the Bediabewu stream that drains the area. Data that were used to prepare the geological map were obtained from the Ghana Geological Survey and Federal Institute for Geosciences and Natural Resources (2009).

The Tarkwa area is underlain by rocks of the Tarkwaian Group (Figure 1), which consists of a distinctive sequence of sedimentary rocks that occurs within a broad band along the interior of the Ashanti belt. It is a synclinorium that runs from Axim in the south to Konongo in the north, a distance of 240 km. The lowermost units (Kawere conglomerates) of the Tarkwaian are dominated by polymitic conglomerates with interbedded quartzites, sandstones, and Phyllites; the total thickness varies from 200 to 700 m (Kesse 1985). These are succeeded by a relatively thin but economically very important Banket series which is only about 120–160 m thick in the Tarkwa district and includes the gold-bearing quartz conglomerates which are interbedded with sandstones, quartzites, and grade upwards into Phyllites (Tarkwa Phyllites). These Phyllites are also quite variable in thickness (120–400 m) but are widespread and grade upwards into thick sequences (approximately 1,400 m) of sandstones with interbedded quartzites and Phyllites (Griffis et al. 2002). Faulting and jointing have been observed in rocks in the area, and the prominent joints have an ESE-WNW trend. However, joints with NW–SE and N–S trends have also been observed (Hirdes & Nunoo 1994; Kuma & Younger 2004). Typically, faulting in the area is characterised as strike-parallel faults and dip-parallel faults (Hirdes & Nunoo 1994; Kuma & Younger 2004). The Tarkwaian has been subjected to moderate folding, and at least five episodes of deformation have been observed. The initial deposition took place in a basin setting with low to steep-angle normal faulting. Subsequent compression and folding led to the development of thrust faults and reversing of previous normal faults. The final stages involved further thrusting in a southwest direction.

Rocks in the Tarkwa area inherently lack primary porosity and permeability but have developed secondary porosity through fissuring and weathering in the shallower depths. Consequently, aquifers in the area are semi-confined and unconfined, and composed of silt, clay, and sand which are weathering products of the basement rocks (Kuma 2004). Kortatsi (2004) reported that borehole depth in the Tarkwa area is between 18 and 75 m with an average of 35.4 m. Additionally, borehole yields vary between 0.4 and 18 m³/h with an average of 2.4 m³/h.

BDT is a single borehole method for characterising flow variation with depth in boreholes through the sequential monitoring of fluid conductivity changes from the injection of a tracer into the borehole water column (Ward et al. 1998a). Aside from the single borehole characterisation, the BDT is also very useful in verifying groundwater velocities from borehole-to-borehole tracer tests (Novakowski et al. 2006) and establishing flow horizon and borehole connectivity (West & Odling 2007). The BDT can be conducted for either ambient or pumped using either a uniform or point tracer injection. Details of the BDT strengths and weaknesses, analytical principles, and field methodology are presented in the precedent references.

For the BDT implemented in this study, water columns in two boreholes under ambient conditions were uniformly injected with sodium chloride (NaCl). The BDT was used for this study because, in comparison with flow metre logging and packer testing, the BDT is relatively cheaper to set up, can detect crossflows, and is sensitive to detecting very low ambient flows (Maurice et al. 2010). NaCl tracer was injected because it is cheap, readily available, and will not cause adverse environmental effects at the injection concentrations (Ward et al. 1998a, 1998b). NaCl also provides enough specific electrical conductivity (SEC) contrast between background and post-injection profiles.

For the field operation, first, background SEC at 25 °C in μS of the borehole water was taken with a Heron temperature, level, and conductivity (TLC) metre logger. Metal weighted hose pipe injectors were inserted into the 100- and 50-m deep wells. After this NaCl was dissolved in a calculated volume of water, mixed thoroughly, and injected uniformly along the water columns in the wells (see injection parameters in Table 1) via the injector.

Slug testing is a single borehole test method used to determine the in situ hydraulic conductivity in the vicinity of a borehole by measuring the recovery of water level within it after an instantaneous change in the head via the introduction of a slug (Schwartz & Zhang 2003). There are two types of slug tests: the falling and rising head tests (Butler 1997). The slug test is often used because it is relatively cheap and easy to set up (Butler 1997; Weight 2008). In addition, the test is more suited to contaminated boreholes, where pumping of contaminated water from the borehole is prohibited by regulation although aquifer hydraulic parameters need to be determined (Butler 1997). The limitations of the slug test are that it can only evaluate K in the vicinity of the tested borehole, which is not representative of the entire aquifer near-instantaneous slug introduction is difficult to achieve when using water as a slug (Butler 1997). The reader is directed to Butler (1997), Weight (2008), and Campbell et al. (1990) for the details on the principles, field setup, and analytical procedures of the slug test.

The falling-head slug test method was used on both wells, in accordance with the design guidelines set out by Butler (1997). The slug was introduced into the boreholes by pouring a known volume of water into the boreholes, after which the head drop was monitored with groundwater transducers and dippers (see set-up parameters in Table 2).

The monitoring continued until water level recovery was about 90% of the initial water level before slug introduction. Initially, response data were converted from pressure (kPa) to metres of the head. Noisy early-time data were ignored and a new test initiation point was assumed where an exponentially decreasing recovery pattern is present. After this, the methods of Hvorslev (1951), Bouwer & Rice (1976) and Cooper et al. (1967) were used to estimate the screened interval hydraulic conductivity for both boreholes. In addition, the method by Cooper et al. (1967) also yielded transmissivity and storage coefficient values for the two boreholes.

Mapping and characterisation of fractures, joints, bedding planes and other rock discontinuities in crystalline aquifers are important for developing a conceptual understanding of flow in aquifers (Cook 2003) as these discontinuities are important for the storage and transmission of water and flow geometry mapping (National Research Council 1996; Singhal & Gupta 1999; Cook 2003; Singhal 2008).

Horizontal and vertical scanline surveys to map the discontinuities and fracture network were undertaken along slope cuts that were representative of the geology of the two boreholes. Dip orientation, dip inclination, fracture spacing, aperture size and aperture form, and fracture trace persistence were measured. Rock discontinuities were plotted in stereographic projections with Stereonets software v.11 (Allmendinger et al. 2012) and analysed for the main discontinuity trends and orientation. Fracture connectivity, infill material, and persistence were qualitatively analysed. In addition, scaled cliff face and aperture sizes from fieldwork photography were used to estimate the rock porosity and hydraulic conductivity (Singhal & Gupta 1999).

For the Valley borehole (Figure 3(a)), the initial profile concentration ranges between 0.4 and 1.4 g/L, with the highest concentrations on the upper part of the borehole, reducing to the borehole bottom. The profiles show uniform spacing, without any persistent kinks, indicative of horizontal diffuse flow. The concentration ratios (Figure 3(b)) for the normalised concentrations show that the dilutions increase towards the bottom of the borehole, indicative of higher horizontal flow flows at the borehole bottom. Although the profile concentrations do not show persistent kinks, the ratios show flow features with higher flows at depths 19, 35, 43, and 48 mbgl. The profile dilution slopes and correlation coefficients for the Valley borehole are shown in Table S1 in the supplementary section. The R² ranges between 0.7510 and 0.9836, showing the dominance of horizontal flow in the borehole. The specific discharge computed from Equation (2) for the entire depth interval of the open section of the borehole is shown in Figure 3(c). The discharge ranges between 1.4 × 10⁻³ and 6.3 × 10⁻²m/d, increasing from below the casing to the bottom of the well between depths 45 and 50 mbgl.

The flow model (Figure 3(d)) shows diffuse horizontal flow signatures distributed throughout the borehole depth produced by foliated and thinly laminated Phyllites.

Around the Valley borehole area, the heavily foliated and thinly laminated Phyllites dominate the area as shown by the cliff cutting face (Figure 8(a)). Compared to the Phyllites in the Hilly area, however, the Phyllites in the Valley area have mineral concretions on the beds (Figure 8(b)) which have the potential to restrict groundwater flow. The area is also faulted. Overall, the heavily foliated and laminated nature of the Phyllites account for the diffusive groundwater flow from the borehole dilution testing results for the Valley borehole.

For both boreholes, the faults identified may have the potential to either serve as preferential flow pathways for enhanced groundwater flow or seal flow pathways.

The aperture sizes for the fractures range from 2 to 10 mm for T4 and T1, respectively. On the rock face, fracture type T4 (2 mm) occurs more frequently thereby dominating the porosity of the rock face, followed by fracture type T3 (5 mm). The total estimated porosity from the scaled mapping of the rock face is 23%, which is several orders of magnitude higher than the storage coefficient estimated from the slug tests. In terms of hydraulic conductivity, fracture type T1 (10 mm) dominates flow, followed by T3 (5 mm).

The dilution profiles from the BDTs in both boreholes reflect the geology and the borehole location. The Hilly borehole dilution profiles are characterised by a combination of horizontal and downward flows. The upper portions of the borehole may be located in the weathered regolith and the sparsely fractured sandstones, which is/are responsible for the vertical flow and no flow intervals, respectively, in the upper part of the borehole. The uniform dilution/horizontal flow at the bottom of the Hilly borehole is indicative of flow on the thinly laminated beds in the Phyllites. The downward flow in the borehole is the result of head differences between flow features because the borehole is located at a possible recharge area.

In the Valley borehole, the rapid tracer dilution in its open section and the dominance of the horizontal flow may be due, respectively, to flow concentration on foliated and laminated beds in the Phyllites and equal hydraulic heads between flow features in transition area boreholes (Tóth 1963, 2009; Kuma 2004). Flow concentration in the borehole may have developed flow features over time, thereby making the Valley borehole more transmissive than the Hilly borehole.

In this study, the BDTs were able to discriminate and delineate preferential flow pathways and intervals in the boreholes. These findings have several implications. The first is that the aquifer is stratified and fractured with preferential flowpaths that will result in very fast contaminant transport times. The second is that comparing the BDT method to whole well depth interval methods like the slug and pumping tests for delineating, detecting, and characterising flow features in boreholes in a fractured crystalline terrain, the BDT method provides a better way of ‘seeing’ the flow features because the BDT depends on specific depth interval analyses of dilution effects of the flow features. In times of aquifer contamination, it is the hydraulic parameters of these flow features that dictate flow contaminant fate and transport behaviour and not the underestimated entire well depth hydraulic parameters deduced from slug and pumping tests. The third implication is that in planning, designing, costing, and implementing more expensive borehole geophysical logging and packer testing programmes in crystalline rocks, the relatively cheaper BDT method could be used as a preliminary tool to provide a priori flow horizons for targeted testing. Finally, in the design of monitoring boreholes in the Hilly area, depth-specific piezometers must be installed to specifically tap different flow horizons and stratifications.

Despite the usefulness of the BDT results in this study, flow horizons could be better delineated via the use of multiple hydrogeophysical approaches (Paillet & Pedler 1996; Datel et al. 2009) and continuous or finer resolution SEC logging.

The slug tests enabled the estimation of hydraulic parameters (hydraulic conductivity, storage coefficient and transmissivity). In the knowledge of the authors, this study is the first that attempts to estimate storage parameters in boreholes in the study area, which is a very important parameter in modelling contaminant fate and transport, and the estimation of available groundwater reserves. The authors reiterate that the estimated hydraulic conductivities will be several orders of magnitude lower than that estimated by packer tests for example.

The similarity of the order of magnitude difference between the estimated hydraulic conductivity for the two boreholes reinforces the difference in the time of pressure dissipation and tracer dilution signals in the boreholes. The preponderance of diffuse flow features in the Valley borehole over that of the Hilly borehole accounts for the increased hydraulic conductivity and storage parameter.

At the borehole scale, the estimated hydraulic conductivities (5.09 × 10⁻⁴–7.66 × 10⁻² m/d) in this study fall within the values stated by Cook (2003) (10⁻⁷–10⁻²m/d) but several orders of magnitude lower than pumping test results (0.51 m/d) from Asante-Annor & Ewusi (2016) in the Tarkwain Phyllites of the Adansi area. The transmissivity value estimated from the slug test for the Valley borehole (3.18 m²/d) falls within the range and is also of the same order of magnitude as the transmissivity average value (6.1 m²/d) obtained by Asante-Annor & Ewusi (2016). However, the Hilly borehole value (4 × 10⁻² m²/d) is two orders of magnitude lower than that of Asante-Annor & Ewusi (2016). These hydraulic parameter dissimilarities could be attributable to scale effects, as the slug tests data capture flow features in the vicinity of the boreholes, whereas pumping tests measure far field hydraulic features. Despite the scale effect weakness, this study presents a relatively cheap method of estimating the hydraulic parameters for the two boreholes.

Geologic mapping has provided information on flow geometry in the boreholes. The dips and dip directions measured in this study are in consonance with the structural features noted by Kuma (2007) and Bhattacharya et al. (2012). Groundwater flow in the study area is on preferential pathways like fractures and foliated beds in the direction of dips, with little cross-communication between flow features due to the paucity of non-strata bound joints, but for a few mapped normal faults.

The estimated storage coefficients computed from the geologic scaling from the cliff face are two orders of magnitude bigger than those from the slug test results. This difference in values could be attributable to the opening up of cliff face fractures and flow features due to stress relief from anthropogenic slope face cutting compared to the overburden closing up of fractures in the boreholes (National Research Council 1996; Singhal & Gupta 1999).

In this study, we combined borehole dilution tests, slug tests, and geological mapping to characterise preferential flow pathways and hydraulic parameters in two boreholes in the Tarkwain Phyllites.

The borehole dilution tests were able to delineate flow horizons and depth intervals to estimate specific discharge and to develop flow models for the boreholes. Slug tests yielded borehole scale hydraulic conductivity, transmissivity, and storage coefficients. From the hydraulic conductivity values, the Valley Borehole is two orders of magnitude more transmissive than the Hilly Borehole, which is in agreement with borehole dilution profile analyses and geology of the boreholes. The geological mapping produced the surface expression of flow features and estimation of storage coefficients, albeit with the caveat that there is likely to be a marked difference between flow features on the surface and in the boreholes due to overburden pressure relieve effects.

Overall, this study has demonstrated a cost-effective but integrated way of characterising flow in boreholes in the Tarkwain Phyllites, which is a very important aquifer for the study area. The methods used in this study are encouraged for undertaking preliminary and explorative borehole hydrogeophysical investigations before relatively expensive methods are deployed. We recommend the approach used in this study, not only for Ghana, but for the West African sub-region because of the preponderance of fractured crystalline aquifers in the region and the cheapness and analytical simplicity of the borehole dilution test. To improve this study, continuous or finer SEC resolution profiling and other hydrogeophysical methods like packer testing, caliper, borehole CCTV, and flow logging will be useful to delineate and quantify flows in the boreholes. In addition, conduction of pumping tests to constrain borehole transmissivities and storage coefficients will be useful to fine tune the findings from our study.

We thank Ebenezer Ansah, Albert Kafui Klu, Eric Ameamu, and Charles Agbotui for helping with the fieldwork.

The fieldwork for this research was made possible by funding from the Accra Technical University (ATU) Research Fund (ARIF) Grant 2020. The ARIF Grant did not have any hand in our Journal choice.

All authors participated in funding acquisition. P.Y.A., J.S., and M.B.-A. did project administration, conceptualized the study, performed methodology, investigated the study, did data curation, visualized, validated, wrote the original draft, wrote, reviewed and edited the article. A.E. did project administration, collected resources, conceptualized the study, performed methodology, supervised the study, wrote, reviewed, and edited the article. A.W. and B.A. conceptualized the study, performed methodology, supervised the study, wrote, reviewed, and edited the article.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.