Le site d’orpaillage de Tchaga est situé à 65 Km à la direction Sud-Est d’Ati. Il se trouve entre la latitude 12˚71' et 13˚33' Nord et la longitude 17˚60'et 18˚40' Est ( Figure 1 ). Tchaga couvre une superficie d’environ 5 Km². Il est implanté sur la rive gauche du fleuve Batha. L’orpaillage s’y est développé depuis 2016, date marquant le début de la ruée vers l’or au Nord du Tchad.

This attraction to the yellow metal has encouraged the installation of several homes in the area on an almost permanent basis as well as the development of several other economic activities indirectly linked to the exploitation of gold.

The Pan-African Range of Central Africa (CPAC) is part of a vast portion of a whole that includes the Pan-African tectonic belts of Central Africa, the Brasiliano belts of NE Brazil and the Trans-Saharan Pan-African Range ( Toteu et al., 2001 ; Van Schmus et al., 2008 ). This chain extends across the African continent, in Nigeria, Cameroon, Chad, the Central African Republic (CAR) and continues eastward to Sudan, Uganda and Tanzania. From current knowledge, this domain is formed by convergence between the West African craton, the Congo-São Francisco cratons and the Latea and Saharan meta cratons ( Castaing et al., 1994 ; Liégeois et al., 2000 ; Toteu et al., 2001 ; Abdelsalam et al., 2002 ). On the northern edge of the Congo craton, this chain shows the characteristics of a collision chain ( Toteu et al., 2001 ), with external layers of regional extension, high pressure granulitic metamorphism, intense migmatization, regional scale strikes and the presence of molassic deposits ( Toteu et al., 2001 ). Chad is located in a mobile Pan-African zone between the Congolese craton to the south, the West African craton to the west and the Nile craton to the northeast (rather poorly defined). The crystallophyllian rocks and ancient granitoids of Tibesti in the north, Ouaddaï in the east, the central massif in the center, Mayo Kebbi and the Baibokoum region in the south represent the base of these cratons. The Precambrian formations constituting these massifs were affected by the Pan-African orogeny (700 to 520 Ma).

A large part of Chad, Western Sudan, Northern Cameroon and the CAR would constitute “the Central African Belt” (Central African zone) of the Pan-African mobile zone. The southern limit with the Congolese craton is marked by the lineaments of Sanaga and Adamaoua (Cameroon), but that with the Nile craton is less obvious. For Vail (1978) , most of eastern Chad, Sudan, eastern Tibesti up to Mayo Kebbi presents a Cratonic zone (Uweinat Craton, Archean).

The continental terminal is essentially composed of tertiary deposits covered by quaternary formations not exceeding a thickness of 50 m. We find in places slightly consolidated sandstone and clayey sand;

The hydrogeological plan of the Batha area is divided into two types of aquifers of unequal importance and distinct by their lithological nature, their depth and their power ( Kusnir & Schneider, 1993 ). Continuous aquifers are made up of reservoirs whose formations are sedimentary types with predominance of alluvium and alterites and discontinuous aquifers made up of reservoirs of fissured or fractured rocks are smaller.

Deep water (wells and boreholes) and surface water (washes and rivers) were sampled ( Figure 1 ) during this phase. During this phase, physical parameters of the water were carried out such as hydrogen potential (pH) and electrical conductivity using a multiparameter instrument. To find normal samples for laboratory analysis, the following steps are taken:

The 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: (i) research focused on chemical elements major (cations, anions, alkalinity and heavy metals) ( Table 1 ) and (ii) 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). The analyzes were carried out to meet the expectations of research focused on the major chemical elements: cations (Ca²⁺; Mg²⁺; Fe²⁺; K⁺; Na⁺; ${\text{NH}}_{\text{4}}^{\text{+}}$); anions ( ${\text{Cl}}^{-}$, ${\text{NO}}_3^{-}$, ${\text{HCO}}_3^{-}$, ${\text{SO}}_4^{2-}$), alkalinity and heavy metals (CN⁻; Zn; Pb and Hg).

Volumetric dosing spectrometry is carried out by:

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

The analysis of physical parameters revealed that the pH obtained during the work varies at a maximum of 7.89 and a minimum of 6.54 for an average of 7.45. The waters studied in the area are neutral ( Figure 3(a) ). The pH values measured in the field are within the WHO standard set in 2011. The waters analyzed are basic to slightly basic (6.54 × pH × 7.89). It is a favorable environment for chemical and biochemical reactions. It is also a determining factor for the growth of microorganisms living in water, in particular bacteria since most of them prefer neutral or slightly alkaline pH values between 6.5 and 8.5 ( Mara, 2004 ).

The conductivity of water is an indicator of changes in the composition of materials and their overall concentration. It is proportional to the quality of ionizable salts. It provides information on the degree of overall mineralization of surface waters. High temperatures act on electrical conductivity by acting on the mobility of salts. Natural waters serve as solvents for a considerable number of solutes, which in aqueous solutions are either completely associated into ions or partially ionized. High conductivity reflects either normal pH, or most often high salinity ( Elmorhit, 2009 ). The value of electrical conductivity oscillates between 62.8 µs/cm and 4000 µs/cm. It is 4000 µs/cm) in the waters of Abdebe, therefore higher than the WHO 2011 standard. This explains that the water of Abdebe has a high mineralization compared to the others. The waters of the other sites have a very low conductivity value around 62.8 µs/cm, which could explain the low mineralization, i.e. lightly charged.

The heavy metal analysis results are recorded in Table 2 .

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 which subsequently pass through protected areas. In traditional gold panning techniques, the risks and dangers for the physical environment generally result in deforestation, the destruction of plant cover and soil, and the pollution of water resources often resulting from the use of gold mining products chemicals in treatments. 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. 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. This results in land degradation which is then made unsuitable for agriculture. Gold panning is a generating activity that causes enormous disruption to the natural environment as well as to the health of gold miners and local residents. Indeed, the environmental impacts of mining activity are as numerous as they are visible in pollution and chemical contamination: soil, water, air, etc.

The water used to clean the ore is often released directly into the natural environment without treatment ( Figure 7(a) , Figure 7(b) ). The use of settling basins is not systematized in Chad, more precisely in Tchaga, and if these settling basins exist, they are generally undersized ( Figure 7(a) , Figure 7(b) ). The water thus released is then heavily loaded with suspended matter and increases the turbidity of waterways. When sinking, the gold miners reach the water table which is at a depth of 20 to 25 m on average. They use motor pumps to release significant quantities of water. The use of motor pumps on the site leads to the evacuation of several liters of water per day, thus contributing to the reduction in the level of the groundwater table. Even if water consumption is negligible at this level, it should be noted the discharge of used oils and hydrocarbons which can pollute water resources. Washing and gold extraction using mercury are activities that consume a lot of water. Metals in acidic mining effluents are generally metabolic poisons, particularly heavy metals. The daily life of gold miners requiring a daily need for water (nutrition, laundry, shower, etc.) adds up to all these activities. The pumped water is used by residents of the site for daily needs (toilet, cooking food and drinking by some) which can constitute a source of poisoning for local residents. In short, the impacts on water are the depletion of water resources (massive use of water, water discharge during sinking) ( Figure 7(a) , Figure 7(b) ). Mining waste (oil, hydrocarbons, human faeces, plastic, wastewater, waste rock residues and dumps, etc.) is also another important cause of pollution of water resources by leaching or infiltration.

Likewise, used batteries left inside wells can pollute groundwater resources. Chemicals used in mining, such as flotation reagents and solvents, can leach into nearby water bodies. This leads to water pollution which affects water quality and aquatic life. Indeed, the topography of the study site and of the Batha region in general decreases towards Lake Fitri. The holes are left open and the surroundings are littered with spoil and treatment residues ( Figure 7(c) , Figure 7(d) ).

Prospecting and ore extraction activities degrade soil quality. In artisanal mines, concern for the environment does not exist in the minds of gold miners. Gold panning operations (excavation, use of explosives, gradual increase in mining depth) may be likely to affect the pre-existing state of equilibrium within the virgin rock mass ( Poulard et al., 2017 ). When the Tchaga site was discovered, large pits were made ( Figure 7(c) , Figure 7(d) ); which contributes to soil degradation through erosion. Man also contributes to irreversible soil degradation, associated with the abandonment of mining shafts and galleries not rehabilitated after exploitation, piles of abandoned mining waste rock and lithological inversions giving the depth elements towards the surface and vice versa. This situation greatly modifies the landscape ( Affessi et al., 2016 ; Digbo et al., 2021 ), distorts the composition of the soils which become poorer over time ( Keita, 2017 ). It impacts the vegetation regeneration process, endangers gold miners with the risk of accidents ( Abdou Amadou, 2020 ) and triggers the phenomena of landslides and soil erosion ( Minkilabe et al., 2018 ; Soma et al., 2021 ). The same is true during sinking during which shafts are dug in order to extract the ore. This irreversibly degrades the soil because the soil is not replenished after abandoning the sites, this leads to modification of the landscape. The sinking results in the degradation of the soil which is left to gully, soil erosion, soil pollution by the piles left in the well. Soil pollution results in the presence, at abnormal concentrations (different from those naturally present), on the surface or in the first meters of soil, of undesirable elements in the form of solid particles ( Poulard et al., 2017 ). Ground pollution by mining waste (oil, hydrocarbons, human faeces, plastic, wastewater, etc.) produced on the site must be reported. Above all, there is the contamination of the soil by chemicals (mercury) used during the extraction of gold, with the disadvantages of erosion, infertility, pollution by mining waste, contamination by harmful substances. Indeed, soil degradation seems to be a factor in the reduction of arable land and also a factor in the displacement of the population in search of more fertile arable land. This is the cause of the reduction in agricultural productivity. Furthermore, waste (plastic bags, wastewater, used batteries, clothing) from extraction is spread all over the site. Even more, the soils are polluted by waste oil spills and by hydrocarbons from pumps.

It has been shown that for each gram of gold obtained by amalgamation, approximately two (2) grams of mercury escape into the ambient environment, directly polluting soil and water, not to mention the inhalation of gas by the user and their neighborhood. During the operation, approximately 40% of the mercury would have escaped into nature in the form of metal balls or vapors (two grams of mercury “evaporate” per gram of gold recovered). The released mercury can then be drained into waterways and deposited in sediments where it is transformed by bacterial action into methyl mercury, an organic compound with high bioavailability which can easily enter the food chain. “Mercury vapors produced when amalgam is heated to temperatures above 350˚C for gold recovery. Mercury vapor can be carried quite far by winds. It is deposited on unprotected soil, water and food or even precipitated in the form of acid rain, etc. These vapors are partly absorbed by the gold miners and people on the site. Thus, chronic exposure to the product does not only affect direct users who inhale the vapors, but also more indirectly the entire population who live and find their food in the contaminated zone. Direct releases of mercury in liquid form during gold concentrate amalgamation operations in the soil, the leaching of which by runoff water causes the mobilization and dispersion of heavy metals in the environment, particularly in surface water (rivers, rivers, Lake Fitri and water reservoirs) and in groundwater by infiltration. The methylation of mercury is favored by the physicochemical conditions of the aqueous environment, thus leading to the most toxic and dangerous form of mercury for public health, etc.

When burning sponge gold in the open air, gold miners and anyone nearby can inhale large quantities of up to 80% of mercury vapors. Mercury vapors can remain on clothes and shoes and can therefore be breathed in for a long time after the operation. Also, the wind can transport a large quantity of vapor far from the place of emission where it can be deposited on homes, plants and the ground. The waters studied do not contain mercury because its content in all the samples analyzed is 0 mg/l ( Table 2 ).

Arsenic is a toxic element which can cause dermatological problems after 5 years of exposure ( Meek et al., 2011 ). Ingestion of arsenic through drinking water can cause peripheral neuropathy, skin, bladder and lung cancers. Cardiovascular system problems can also be observed in children consuming water contaminated with ars.

The chemicals used for gold processing (mercury and cyanide) will pollute waterways and aquifers ( Fofana et al., 2009 ). Cyanide is a very toxic element. Exposure to hydrogen cyanide can cause neurological disorders including dizziness, confusion, headaches and can quickly lead to death in the event of acute poisoning. Dermal contact may cause skin irritation. Hydrogen cyanide vapors can cause eye irritation. Although some studies suggest lethality from 180 mg/l; INRS (2018) considers that absorption of a total dose of 50 to 100 mg of hydrogen cyanide is sufficient to cause death in an adult. SML effluents can contain more than 37 mg/l. As gold miners’ effluent is unprotected, accidental ingestion can easily cause serious health problems. Although the discharge of cyanidation water has not shown contamination of surface water (ponds), this could happen in the future if the discharge is indeed true and continuous.

Lead is a chemical element harmful to human health. According to a WHO study, exposure to lead can cause cardiovascular disease, kidney problems, and reduced human fertility ( Meek et al., 2011 ). Exposure to lead can also cause stunted growth in children. The case of Zamfara in Nigeria is very instructive on this subject since many children in several villages had died following exposure to lead from dust from artisanal gold mining activities. 400 children died from lead poisoning and more than 3000 children suffered lead poisoning. There is therefore reason to be seriously concerned about this given that the gold miners are in permanent contact with this water and that in addition there is the presence of children who work on the Tchaga site. It should also be noted that in this area, surface water is at a certain time of the year the only source of water for livestock but also for household chores.

During the crushing and sieving phases of the gold ore, enormous quantities of dust are released and spread into the ambient atmosphere. Inhalation of silica dust can cause numerous pathologies such as rhinitis, sinusitis, or broncho-pleuro-pulmonary manifestations with cardiac complications. Silicosis is the most well-known pathology linked to the inhalation of silica dust. The silicotic risk can occur several years after exposure, hence the need to take protective measures to protect workers as well as children who are most of the time also present on gold panning sites. Although dust measurements were not carried out on the Tchaga site, it is necessary to look into raising awareness among gold miners of the silicotic risk in view of the enormous quantities of dust released during the grinding phases and sieving of the ore and that the Chadian legislator must set the quantity of admissible dust according to the silica content.