The Lite-Bala gold site is located between 3˚43'20'' - 3˚48'60'' north of the equator and 23˚6'00'' - 23˚9'60'' east of the meridian in the Nord-Ubangi province of the DRC ( Figure 1 ).

The site is composed of 8 gold deposits, including two inactive (Modenyo 1 and Mokpoloso) and 6 active (Mopodi1 alluvial deposits and Mopodi 1 vein deposits, Mopodi 2 alluvial deposits and Mopodi 3 alluvial deposits, Modenyo 2 alluvial and alluvial deposits, Mokpoloso alluvial deposits) and 6 camps (Mopodi 1, Mopodi 2, Mopodi 3, Bisengo, Mozaza, Modenyo). It is covered by a warm tropical climate with a rainy season from March to October and a dry season from November to February. The annual total rainfall reaches 800 - 1600 mm, and the vegetation is dense forest [11] [12] .

The relief of the site is that of the central basin of the DRC with low slopes and altitudes (350 - 700 m). The rocks of the gold site are metamorphic rocks, including schists and various sandstones. The soils have a red or yellow color characteristic of oxidation zones and belong to the Ferralsols group. The hydrographic network of the gold site is similar to that of the Congo Basin [11] - [13] . The population, estimated at around 6000 inhabitants, practices gold mining as its main activity. Cultivation (cassava and corn), livestock breeding and fishing are practiced around the site while small trade takes place there [4] [11] .

Simple random sampling was carried out at 0 - 25 cm for surface soils (SS) and 5 cm for sediments (RS) in zones 1 and 2, and stratified random sampling was carried out at 45 - 65 cm for deep soils (DS) in zone 2 using an auger during the rainy season (from 04 to 24/05/2023) and dry season (from 02 to 21/02/2024). Rock residues (RT) from mining deposits were collected in zone 1. The samples were dried and packed in plastic bags and a cooler [10] [14] [15] . The number (N) of soil samples is defined by the area of the area to be sampled (A) according to the formula N = $\sqrt{A}$ + 1 while that of sediments by the length of the river [2] [10] [16] . Figure 2 gives the mapping of the sampling points in zones 1 and 2 on the gold site.

The essential parameters of soils and sediments, determined for this study, are those which directly influence mobility, sorption and desorption phenomena, the valence of cations in solution as well as the infiltration rate of water containing trace elements. In the field, the determination of acidity (pH) and redox power (Eh) was carried out with a multiparameter probe Oyster Meter, Model 341,350 A are pH (NF T 90 008) and ORP (ISO 11271: 2002) by potentiometry with an Ag/AgCl electrode, saturated in KCl (E₀ = 0.199 mV) on a soil or sediment suspension in a solid-water ratio of 1:5 (v/v) while in the laboratory, organic carbon and particle size were determined respectively by the Walkley and Black method (not standardized) and by the sedimentation method (NF P 94-057) [10] [13] [17] .

The soil, sediment and rock residue samples (fraction < 2 mm) were mineralized with aqua regia after grinding, heated in a water bath at 100˚C for two hours, taken up in HCl diluted water (0.1 N) then filtered and diluted before analysis by high-frequency inductively coupled plasma atomic emission spectrometry (ICP-OES) at the Laboratory of Industrial Processes, Synthesis, Environment and New Energies (LAPISEN) of the Félix Houphouët-Boigny National Polytechnic Institute of Côte d’Ivoire [8] [10] [18] [16] .

The assessment of soil and sediment quality was carried out using grids and indices but also by determining the background pedogeochemical contents of soils and sediments by the “Median + 2* MAD” method (where MAD: median absolute deviation) (MM) and the “Tukey Inner Fence” (TIF) method. The anthropogenic inputs (%) due to exploitation were calculated from the EF (Equation (1)) while the pollution levels were determined using the geoaccumulation index (Igeo) (Equation (2)). The partition coefficient (kd) of ETM in soils and sediments was calculated by the ratio of total ETM (C_{ET total}) and soluble ETM (C_{ET soluble}) (Equation (3)) [8] [10] [14] [15] [19] . The different mathematical relations used are:

$\text{FE}=\frac{{\left[\frac{{\text{C}}_{\text{TE}}}{{\text{C}}_{\text{Normalizing element}}}\right]}_{\text{sample}}}{{\left[\frac{{\text{C}}_{\text{TE}}}{{\text{C}}_{\text{Normalizing element}}}\right]}_{\text{reference}}}$. (Equation (1))

With C_TE is concentration in trace element

C_{Normalizing element} is concentration of normalizing element

$\text{Igeo}={\log }_2\left[\frac{{\text{C}}_{\text{TE sample}}}{1.5\ast {\text{C}}_{\text{TE FPGN}}}\right]$ (Equation (2))

With FPGN the natural pedogeochemical background content

$\text{kd}=\frac{{\text{C}}_{\text{TE total}}}{{\text{C}}_{\text{TE soluble}}}$ (Equation (3))

With kd in [L/kg], and C in mg/kg (total ET) and mg/L (soluble TE)

The different classes of FE are: FE < 2 no enrichment, 2 < FE < 5 moderate enrichment, 5 < FE < 20 significant enrichment, 20 < FE < 40 very high enrichment and FE > 40 extremely high enrichment. Those of Igeo are: Igeo < 0 substrate or no pollution, 0 < Igeo < 1 unpolluted to slightly polluted, 1 < Igeo < 2 slightly to moderately polluted, 2 < Igeo < 3 moderately polluted, 3 < Igeo < 4 moderately to heavily polluted, 4 < Igeo < 5 heavily polluted, 5 ≤ Igeo extremely polluted [14] [19] - [21] .

Statistical processing of the data was carried out using R 4.3.2 and Excel 2013 software. It consisted of visualizing the data with barplots and calculating the means and medians [1] [17] .

Figure 3 gives the acidity (pH) of the soils and sediments of the gold site.

From Figure 3 , it is clear that the pH of soils and sediments is between 3.86 and 5.99 with low values in the dry season, respectively 4.14 ± 0.25 for deep soils (DS), 4.54 ± 0.60 for surface soils (SS) and 4.83 ± 0.52 for sediments (RS). Furthermore, in the rainy season, the pH values are 4.55 ± 0.35 for deep soils (DS), 4.83 ± 0.55 for surface soils (SS) and 5.53 ± 0.58 for sediments (RS). Figure 4 illustrates the redox power of soils and sediments at the Lite-Bala site.

This figure shows that the redox power varies, overall, between 216.36 and 576.63 mV with an average of 426.35 ± 41.64 and 387.58 ± 20.78 mV for deep soils (DS), 432.67 ± 54.93 and 372.78 ± 31.70 mV for surface soils (SS), and finally 390.00 ± 35.02 and 346.23 ± 39.58 mV for sediments (RS) respectively in the dry season and in the rainy season. The redox potentials in the dry season are higher than in the rainy season. Figure 5 shows the organic carbon contents of the soils and sediments of the Lite-Bala site.

The figure above reveals that the organic carbon contents of soils and sediments are between 0.11% and 0.80%. Deep soils display contents ranging from 0.22% ± 0.07% in the dry season and 0.26% ± 0.16% in the rainy season while surface soils (SS) have contents oscillating around 0.32% ± 0.07% in the dry season and 0.50% ± 0.26% in the rainy season. As for sediments, they have an average organic carbon (Corg) of 0.39% ± 0.07% in the dry season and 0.57% ± 0.10% in the rainy season. Corg contents are high in the rainy season than in the dry season and higher in sediments (RS). Figure 6 gives the texture of soils and sediments of the Lite-Bala site.

This figure indicates that the dominant texture for deep soils and surface soils is the silt-clay texture while for sediments, the texture is sand-silt and silt-sand.

Distribution of cadmium, lead, mercury and arsenic in soils, sediments and rock residues

Figure 7 shows the proportions of cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) in soils, sediments and rock residues.

This figure shows that arsenic (As) levels are high in surface soils (SS) and in mine tailings (MT), cadmium (Cd) levels in mine tailings (MT) and in sediments (RS), mercury (Hg) levels are high in surface soils (SS) and finally, lead (Pb) levels in deep soils (DS).

Cadmium, lead, mercury and arsenic contents in the different compartments studied

Figure 8 gives the cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) contents in the different mining residues of the Mopodi 3 deposit (GMP3) and the Mopodi 1 deposit (GMP1) of zone 1.

Figure 8 shows that the tailings from Mopodi 3 deposits (RGMP3) have elevated levels of arsenic (As) (0.069 mg/kg), cadmium (Cd) (0.272 mg/kg), mercury (Hg) (0.032 mg/kg) and lead (Pb) (0.008 mg/kg) compared to the tailings from Mopodi 1 deposits (RGMP1). Cadmium is the most abundant element in the tailings with an average of 0.1463 mg/kg. Figure 9 shows the levels of cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) in soils, sediments and rock tailings from zone 1 and zone 2 of the Lite-Bala gold site.

It should be noted that the high levels in zone 1 are those of cadmium (Cd) with respectively 0.1463 ± 0.1190 mg/kg in mining residues (MT), 0.0894 ± 0.0011 mg/kg in sediments (RS) and 0.0808 ± 0.0197 mg/kg in surface soils (SS) while in zone 2, the cadmium (Cd) levels are notable for deep soils (DS) and sediments with respectively 0.0885 ± 0.0304 mg/kg and 0.0782 ± 0.0610 mg/kg. Surface soils in zone 2 have high mercury levels (0.1380 ± 0.1093 mg/kg).

Table 1 reveals the background pedogeochemical content of the soils at the Lite-Bala site.

This table shows that the basic pedogeochemical contents of Lite-Bala soils are within the range of mercury (Hg) and cadmium (Cd) contents of the upper continental crust (UCC) reference material but have lower values for lead (Pb) and arsenic (As). Table 2 shows the linear regression of iron (Fe) and aluminum (Al) with cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As).

The p-value in this table shows that there is an acceptable correlation between cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) with iron (Fe) as well as with aluminum (Al). Therefore, iron (Fe) was used as the normalizing element.

Table 3 gives the enrichment factors for cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) in soils and sediments from the gold site.

This table shows that 79% of FE values are below 2, 13% between 2 and 5, 7% between 5 and 20 and finally less than 1% above 40. Sediments S1YL and S3MK are local control samples taken respectively at 1048 m and 1154 m from zone 2. The ETM enrichment studied is natural (substrate), with the exception of Pb in sediments (S2YL and S3YL) and As in soils of the Mopodi 3 deposit (SS2GMP3). Table 4 presents the contributions of cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) related to artisanal gold mining compared to the local pedogeochemical background of Lite-Bala.

This table shows that the contributions of lead (Pb) are significant for most of the soil samples (Mopodi 1 and Mozaza camps noted respectively MP1 and MZ, Mopodi 3 deposits noted MP3) as well as in the river sediments (Yalu and Mokpoloso whose codes are YL and MK). Arsenic shows a slight increase in the surface soils of the deposits (SSGMP1-SSGMP3) of zone 1 while Hg shows an increase in the surface soils of Mopodi 1 of zone 2 (SSMP1) compared to the pedogeochemical background contents of Lite-Bala.

Table 5 illustrates the level of pollution (Igeo) of soils and sediments in cadmium (Cd), lead (Pb), mercury (Hg) and arsenic (As) of the gold site.

Table 5 shows that 72% of the Igeo index values are below zero, 15% are between 0 and 1 and 6% are between 1 and 2. These values indicate that there is virtually no anthropogenic pollution on the site.

Table 6 gives the decimal logarithm of the partition coefficient of each trace element between the solid phase and the dissolved phase of each compartment.

This table shows overall that cadmium (Cd) with respective log coefficients of 7.56, 7.92 and 7.86 in the surface soils of the camps and deposits and in the sediments, is the most immobile of the four trace elements studied while lead (Pb) is the most mobile with respective log coefficients of 2.32, 2.59 and 3.83.

Figure 10 shows the level of correlation between pseudo-total cadmium (Cd), lead (Pb), mercury (Hg), arsenic (As) and gold (Au) contents and the similarities between surface soil (SS) and river sediment (RS) samples.

This figure shows a strong correlation between cadmium (Cd) and gold (Au) levels. Surface soil samples have high levels of cadmium, while river sediments have high levels of both cadmium and gold. The total inertia rate of 76.1% indicates that this information is reliable.