Gamboké and MBibou are the two gold panning sites studied which are respectively between 09˚32'37'' and 09˚54'38'' north latitude and between 14˚82'06'' and 14˚98'71'' East longitude. Covering an area of 1103.58 km², gold mining began on these sites a few decades ago (1930). The hydrographic network is almost invariable in one direction and seems to unify in most cases at NNW-SSE to NNE-SSW. This network is determined by a tropical regime during the maximum rise in rainwater (September-October) and strong interannual fluctuations in flow rates. The relief of the study area is in the form of a plain, which is why the altitudes vary very little, ranging from 342 to 553 m. The area presents a gently undulating landscape due to the phenomenon of erosion, which is alternated by very flat valleys. The soils of this area are favorable for almost all kinds of plantations. The main types of soils are: vertisols, hydromorphic soils, sandy-clayey soils or sandy soils and ferruginous, red soils.

The Mayo Kebbi massif, located in the southwest of Chad between the Congo craton to the south, the West African craton to the west and the Sahara Metacraton to the east. This was formed during the Pan-African orogeny, between 800 and 570 Ma ( Figure 1 ). The Pan-African orogenic belts constitute the main segments of a vast Neoproterozoic orogenic belt, associated with the formation of the supercontinent Gondwana between 725 and 550 - 500 Ma [9] . This massif is made up of two series of green rocks which are the Zalbi Series (SZ) to the West of Léré and the Goueigoudoum Series (SGG) to the North of Pala, in addition to the Intermediate Mafic Complex (CMI) ( Figure 1 ). These two series and the CMI represent greenstone belts. Everything is covered by Phanerozoic sedimentary deposits [9] .

Deep water (wells and boreholes) and surface water (washes and rivers) were sampled during the field campaigns. During this phase, physical parameters of the water were measured, such as hydrogen potential (pH) and electrical conductivity using a multi-parameter instrument. To find normal samples for laboratory analysis, the following steps are taken:

Physical parameters were measured in situ to correlate with the concentration of heavy metals. Physical parameters such as hydrogen potential (pH) and electrical conductivity (µs/cm) were measured in the laboratory using a branded multimeter.

Analyzes of groundwater samples were carried out at the Hydro-Geosciences and Reservoirs Laboratory (LHGR) of the University of Farcha and at the National Water Laboratory (LNE) for metals: 1) research focused on chemical elements major (cations, anions, alkalinity and heavy metals) and 2) spectrometry by volumetric dosage, molecular absorption spectrophotometer and flame spectrometer ( Table 1 ).

The samples taken during the field campaign were analyzed at the National Water Laboratory (LNE). Soil analyzes consist of weighing 10 g of the prepared soil sample and putting it in an Erlenmeyer flask. 100 ml of demineralized water was added to this measured sample for dissolution. Everything is placed on a stirrer under tension for homogeneity and then filtered. For this purpose, measuring devices are used to determine the concentration of the elements sought in the samples.

The analyzes were carried out to meet the expectations of research focused on the major chemical elements: cations (Ca²⁺; Mg²⁺ Fe²⁺; K⁺; Na⁺; ${\text{NH}}_4^{+}$); anions (Cl⁻, ${\text{NO}}_3^{-}$, ${\text{HCO}}_3^{-}$, ${\text{SO}}_4^{2-}$), alkalinity and heavy metals (CN⁻; Zn; Pb and Hg) ( Table 1 ).

Volumetric dosing spectrometry is carried out by:

Table 1 methods are used to determine the concentration of cations and anions.

The concentrations of physicochemical parameters determine the quality of a soil Table 2 .

Hydrogen potential (pH) of soils

The pH of the soils ranges from 4.52 to 7.18 with a mean of 6.44 and a standard deviation of 0.88. Its average, slightly below the WHO standard, attests that the soils in the study area are acidic ( Figure 2 ). Soil acidity is believed to be due to the precipitation of hydroxides, increasing the acidity of the environment and consuming metals.

The bicarbonate ( ${\text{HCO}}_3^{-}$) content of soils is between 0.00 mg/l and 507 mg/l, with an average of 150.91 mg/l and 234.77 standard deviation. The Gamboké soil samples analyzed show an absence of bicarbonate (0.00 mg/l), but on the other hand those from MBibou show a very high concentration (503.7 mg/l), ( Figure 4(a) ).

Soil pH values indicate high pollution in our study areas, because lowering pH increases the solubility and mobility of heavy metals in the soil. The pH value of the samples is acidic, so there could be a source of pollution that could extend from the sites through the flow of water and production of dust. This justifies the Cu, Cd and CN contents below in waters which are beyond the WHO standard.

The quality of water is based on the concentration of physicochemical parameters. The result of the elementary statistics of the physical parameters (pH, conductivity and turbidity) made it possible to determine these different values (minimum and maximum), averages and standard deviations in the table below. ( Table 3 )

EC: electrical conductivity; STD: total dissolved solids.

1) Hydrogen potential (pH) of water

Hydrogen potential (pH) indicates the acidity or basicity of water. The pH of the water analyzed varies from 7.40 to 8.00 with an average value of 7.71 and a standard deviation of 0.18. Its average value shows that these waters have an alkaline pH and comply with the WHO standard.

In a gold panning area, pH is one of the most important factors controlling the mobility of ETMs at the water-sediment interface and plays an important role in the adsorption of metals on the soil surface. An acidic pH promotes the high solubility of heavy metals and their mobility in the soil, particularly through the dissolution of metal salts or the destruction of the retention phase. Conversely, an alkaline pH limits or immobilizes the transition of metals from the solid phase to the aqueous phase [14] ( Figure 5 ).

2) Electrical conductivity (EC)

The electrical conductivity of water is the expression of its mineralization and is expressed in microsiemens per centimeter (µs/cm). It is a measure of ability to conduct an electric current. It varies from 300 to 830 μs/cm on the two gold sites, for an average of 491 μs/cm. Compared to the WHO standard (≤2500 μs/cm), the waters are weakly mineralized. This low mineralization could be explained by the fact that the waters of the sites studied do not contain enough dissolved mineral salts, such as chlorides, sulfates, etc. High conductivity reflects either normal pH or high salinity [15] [16] .

Water turbidity is the determination of water clarity. Turbidity has a minimum value of 10.1 NTU and a maximum of 116,000 NTU, with an average of 18664.0 NTU and a standard deviation of 35523.2 NTU. The average turbidity is above the 2017 WHO standard (≤5 NTU), and the water quality is very poor. This abnormal variability in surface water turbidity is due to ore processing activities. According to Vigouroux et al. (2005 and 2006), high turbidity contributes to the disruption of photosynthesis and reduces the dissolved oxygen content of waters caused by particles suspended in the water. Likewise, high turbidity reduces water transparency and pollutes the environment by affecting aquatic species.

3) Chemical characteristics of water with heavy metals

In order to determine pollution, the levels of heavy metals in the water are compared to current standards (WHO standards).

The heavy metals present in waters, on the one hand, come from the heritage of the geochemical background and on the other hand from the accumulation of anthropogenic contributions. The heavy metal contents of water, the values of which are recorded in the table below. ( Table 4 )

After observing the analysis results of 10 water samples including 5 heavy metals (Cu, Cd, CN, Pb and As), we note that the concentration of Cu, Cd and CN in surface water is high compared to (Pb and As). Half of the samples have a high cyanide content, Cu and Cd are observed in all samples, Pb is obtained in 2 samples and As is present at very low content in all samples. Taking into account the results of the water analyzed, the Pb and As present a low content which is below the limit values set by the WHO for drinking water; on the other hand, the contents of Cu, Cd and CN exceed this standard.

Cadmium, copper and Cyanide, very toxic, are discovered in surface and underground water (mining wells) at concentrations higher than the limit values set by the WHO. They are a source of concern since gold miners and animals consume water. Their accumulation in the body leads to kidney (Pb, Cd), neurological (Pb, As, Cd), hepatic (Cd) problems and causes cancer [20] .

Mining activities pose a threat to natural ecosystems, particularly in protected areas and areas of high conservation value. Artisanal mining sites produce mining waste which can pollute the soil and waterways that subsequently pass through protected areas. Finally, like any human settlement, we observe on artisanal mining sites, environmental pollution by waste and organic matter and a significant accumulation of rubbish and batteries from the torches used by gold miners in the mines.

Mining activity impacts the soil at all stages. In Gamboké, as in Mbibou, the sites are dotted with holes, often shallow or very deep, creating small reliefs on the surface and modifying the natural landscape. Digging effectively contributes to the degradation of the soil which leads to infertility and intensive erosion processes. This imbalance is at the origin of alluvial deposits in the valleys. These uncontrolled activities can cause disruptions in the natural drainage of watercourses.

Indeed, gold panning is the cause of soil degradation which is a factor in the reduction of cultivable areas and also a factor in the search for more fertile land. It is the result of a reduction in food production and the emergence of conflicts following the occupation of land. This artisanal activity makes it unsuitable not only for agriculture but also for grazing. In addition, soil pollution is caused by the discharge of solid and liquid waste on sites. According to [21] , these extraction procedures are not reversible in most cases and can be catastrophic on a generational scale.

The movement of “waste rock by runoff water, the erosion of hillsides, and ore washing residues lead to the silting of rivers during the rainy season. During low water periods, the watercourses are mostly dry. The abnormally high content of suspended matter in these watercourses is manifested by the muddy and turbid appearance of the watercourses” [22] ( Figure 7(a) , Figure 7(b) ). Water is used throughout the chain of gold panning activities. During sinking, the free water table is reached, and the gold miners use motor pumps to evacuate large quantities of water, contributing to the reduction of the level of the groundwater table.

Gold processing activities with mercury consume a lot of water. For example, it takes around 200 liters of water to process a 50 kg bag of gold ore. The chemical products, solid waste (used batteries) and liquids (used oils) used contaminate surface and groundwater by leaching or infiltration. Furthermore, the use of mercury in the purification of gold and certain chemical particles contained in the subsoil leads to sedimentary deposits that pollute aquatic and atmospheric environments ( Figure 7(c) , Figure 7(d) ). In Guyana, research carried out by [23] on the biochemical cycle of mercury highlighted the aggravating role of gold panning activity, through additional releases of metallic mercury, through certain soil erosion which promotes the mobilization and transport of metallic mercury to the lowest points (shallows, watercourses). The results of these authors are confirmed in the present research.

In Gamboké, as in Mbibou, these mining activities have led to the escape or death of animals on the sites. This is due to excessive tree cutting and sound production. Gold miners use a lot of wood during digging, which quite destroys ecological niches and reduces the number of certain animals. On the gold panning site, the digging and installation of miners are activities that have a very negative impact on wildlife.

To reach gold horizons, excavations often require clearing which results in the significant loss of plant species, notably tree savannahs and grassy savannahs [23] . Indeed, the clearing operations carried out by gold miners on the activity sites gradually contribute to the destruction of the plant cover. Deforestation exposes the soil, facilitates the infiltration of rainwater [23] and erodes the soil. In traditional gold panning techniques, the risks and dangers for the physical environment generally result in deforestation, destruction of plant cover and soil. The destruction of plants is also reinforced by the search for nuggets which are known to be found especially between the roots of certain plants. The installation of mining shafts also requires clearing and deforestation to reinforce the walls of the shafts and prevent landslides. As no monitoring is put in place, the savannah is almost destroyed. The destruction of trees contributes to the destruction of wildlife habitat (migration or loss of certain animals) and a loss of tree species useful to humans. The progressive loss of flora (Figures 8(a)-(c)) gradually impacts the climate of the site: an increase in wind speed, high evaporation and thermal variabilities. A reduction in vegetation leads to drought and a reduction in agricultural yield. High flow turbulence leads to the transport and deposition of sediments in streams, which reduces stream beds and their flow [24] . ( Figure 8 )