2.1 Study area
The Mutoshi aquifer is located approximately 350 meters from the Mutoshi open-pit mine (formerly Ruwe) northeast of the city of Kolwezi, about 8 km from the city center. The area is located between longitudes 25.50° and 25.54° and latitudes -10.66° and -10.70° (Figure 1). Access to the site is via the road that passes through the Mutoshi technical institute. The city of Kolwezi where our study area is located is in Lualaba Province, approximately 320 km from the city of Lubumbashi (Lukamba et al., 2007).
François (1973) identified several morphological zones in the Kolwezi mining area: the N'zilo promontory to the northwest, an area of rugged relief where relatively hard Kibarian rocks outcrop (quartzites and metamorphic schists). The Lualaba River (source of the Congo River) and some tributaries cross this massif at the bottom of deep gorges, in a series of waterfalls and rapids before emptying into the Upemba depression. The sandy plateaus of Manika and Biano are located southwest and northeast respectively, separated by the upper Lualaba Valley. Two gently sloping hills, interrupted by small reliefs, connect the Lualaba Valley to the plateaus.
According to Placet (1975), this structure serves as a hydrogeological reservoir in a region also characterized by a generally low slope, low surface runoff, and high infiltration. The average latitude of the region varies between 1000 and 1500 m, with depressions occupied by rivers. From a geomorphological perspective, the region exhibits karstic tendencies. The Lualaba is the most important river, originating on the high plateaus of Manika at 1420 m. The Kolwezi region falls entirely within the Lualaba watershed. The Lulua and Musonoie rivers are the two most important waterways in the Kolwezi region among the tributaries of the Lualaba (Geo Ouest, 2007)
2.2 Methodological approaches
2.2.1. Sampling and analysis methods
We collected samples from geological formations and groundwater in the study area. The rocks were analyzed to quantify concentrations of major and trace elements, while groundwater samples underwent detailed analysis of their physico-chemical parameters.
2.2.1.1 Sampling and analysis of geological formations
This study examined rock samples from the overburden layers of the Mutoshi deposit using lithogeochemical techniques such as X-ray diffraction to determine major and trace elements. First, representative lithological units overlying the Mutoshi aquifer were identified by referring to existing geological maps and conducting field verifications (Reedman, 1979). Rock samples were collected from fresh outcrops using a hammer and chisel. Approximately 2 kg of each lithotype were sealed in plastic bags to prevent contamination (Bowen, 1979). At least 5 samples per unit were collected. Sampling locations were recorded and marked on maps for reference. Samples were also collected from visible mineralized zones, such as faults, veins, or fractures, to study associated alteration (Levinson, 1974).
For soil sampling, small pits approximately 5 decimeters in diameter and 75 centimeters deep were dug at each grid point, following a predetermined sampling density of 50x50 meters (Govett, 1983). A total of 30 soil samples were collected. To ensure sample quality, certain key rules were followed, such as recording the location and time of sampling, noting field parameters, using airtight containers to prevent contamination, maintaining a depth of 50 to 75 cm to avoid root effects, and respecting the grid spacing (Reedman, 1979). The soil samples were placed in labeled plastic bags, with a basic description of the terrain noted in a sampling log for easy geological mapping of the encountered formations.
Laboratory analysis involved X-ray fluorescence (XRF) to determine the elemental composition of the lithological samples (Bowen, 1979). In the case of X-ray fluorescence, the sample is irradiated with X-rays, making it fluorescent and emitting secondary X-rays characteristic of its constituent elements. From the emitted wavelength spectra, the elements present are identified and quantified (Govett, 1983). This helps understand the enrichment and depletion of major and trace elements associated with mineralization in the overburden layers of the Mutoshi deposit.
2.2.1.2 Sampling and analysis of groundwater
Groundwater sampling at Mutoshi was conducted from existing tubular wells and boreholes after adequate purging to ensure collection of formation water (APHA, 1998). Field measurements included pH, electrical conductivity, temperature, and total dissolved solids, taken using calibrated portable meters according to standard methods (Hem, 1985). Samples intended for cation and anion analysis were filtered in the field through 0.45 µm membranes and appropriately preserved. Containers for cation analysis were acidified with ultrapure nitric acid to a pH below 2 (Reedman, 1979). Those not acidified were reserved for anion analysis. Samples were refrigerated and transported to accredited laboratories for analysis by inductively coupled plasma mass spectrometry (ICP-MS), ion chromatography, titration, and spectrophotometry, according to established protocols (Levinson, 1974).
2.2.2 Data processing methods
Processing data on Mutoshi's geological formations and groundwater involved hydrochemical and statistical methods.
2.2.2.1. Geochemical characterization of rocks
To characterize the studied rocks, geochemical data processing involved descriptive statistical analysis of the various chemical elements analyzed, including minimum, maximum, mean, standard deviation, and coefficients of variation (CV). This analysis also included comparing the mean values to Clark standards to assess the enrichment of chemical elements in the rock samples collected at Mutoshi. This statistical analysis was conducted on 30 samples and 10 elements (Cu, Co, Mn, Fe, S, Si, Ca, Mg, Al, and K).
2.2.2.2. Physico-chemical characterization of groundwater
Regarding the physico-chemical parameters of groundwater, their processing also involved descriptive statistical analysis, including the measurement of minimum, maximum, mean, standard deviation, and coefficients of variation (CV). The values of these parameters were compared to each other and also to WHO standards (Djahadi, et al., 2021). This statistical analysis was conducted on 20 samples and 21 physico-chemical variables (T°C, pH, Eh, CE, TDS, MES, MOS, Cu, Co, Mn, Fe, S, Al, Si, Ca, Mg, Na, K, HCO3, Cl, and SO4). To assess the relationship between these physico-chemical parameters of groundwater, a correlation analysis was conducted. This bivariate statistical analysis method allowed us to determine if variations in one parameter are associated with variations in the other. For this purpose, correlation coefficients (r), ranging from -1 to +1, were calculated to assess the strength and direction of the statistical link between the analyzed parameters (Mashauri, et al., 2023a).
The hydrochemical study of the waters required the use of tools such as the Piper diagram (Cidu et al., 2009; Bashir et al., 2017; Yao and Ahoussi, 2021; Hakim et al., 2022; Barhi et al., 2022) and Stiff diagrams to classify the waters and assess their ionic composition. Finally, using binary diagrams, we examined the origin of water chemistry and base exchange processes to better understand the interaction between groundwater and surrounding geological formations. These diagrams are frequently used in hydrochemical studies by several authors, including (Biemi, 1992; Ahoussi et al., 2012; Benjamin et al., 2023). All these analyses were conducted using Excel 2016, Tanagra 1.4, and Diagramme 6.77 software. The location of measurement points in the study area was represented using QGIS 3.43.5 software.