The Tongon industrial mine is located in northern Côte d’Ivoire (N9˚57'-5˚76˚ W5˚42˚-13˚68˚), approximately 628 km northeast of Abidjan, the economic capital.

Soil samples were collected using a hand dredge in and around the Tongon industrial mine area to assess the lateral dispersion of mining-related contaminants (Figure 1). The location of the respective sampling sites is shown in Figure 2.

The samples collected were packaged in plastic bags, labelled according to the sampling site. The samples were kept in a cool box for transport to the laboratory, where they were placed in a freezer at (−4˚C) [7] .

After drying in an oven (60˚C) for 24 hours, the sample was pre-sieved on a

sieve to remove coarse elements such as pieces of shell, branches and leaves before being sieved using a sieve. Fractions smaller than 2 mm were kept in hermetically sealed plastic bottles to protect them from moisture, then stored in a dark, cool (20˚C) cupboard for analysis.

The analyses to determine the total arsenic concentration in the soil samples required the synthesis of aqua regia. This aqua regia was synthesized from 65% pure concentrated nitric acid HNO₃ and 37% pure concentrated hydrochloric acid HCl at a ratio of 1/3 (v/v). A solution of hydrofluoric acid HF concentrated to 48% pure and a solution of boric acid H₃BO₃ prepared from distilled water were also used to determine the total arsenic concentration. The sediments were mineralized using a reference method for marine pollution studies [7] . The principle of this method is based on the total dissolution of sediments by hydrofluoric acid (HF) and aqua regia (HNO₃: HCl; 1:3, v/v) at high temperature. The use of HF is essential because it is the only acid that completely dissolves silicates and all metals [8] . 0.2 g of dry, homogeneous sediment or soil sample placed in a Teflon tube previously washed with acid underwent hot mineralization using 1 mL of aqua regia (HNO3: HCl; 1:3, v/v) and 6 mL of concentrated HF (48%). Heating was carried out in a water bath at 120˚C for 2 hours 30 minutes. After cooling in ambient air, the residue was taken up in a boric acid solution (2.70 g in 20 mL of distilled water). The final volume was reduced to 50 mL and left to stand for 6 hours. After the samples had been concentrated and digested, arsenic concentrations were determined by inductively coupled plasma mass spectroscopy (NexION 2000 ICP-MS, USA). The minimum detection limit (MDL) for arsenic was 0.0005 mg/L.

The quality of the analyses was checked using blanks and duplicates. To prevent uncertain contamination, all the laboratory equipment used was washed with phosphate-free soap, rinsed twice with distilled water and soaked in 10% nitric acid for 24 hours. They were then rinsed twice with distilled water and dried at room temperature. The limits of detection (LOD) for the elements were calculated as follows:

LOD = 3 × s m (5)

where “s” is the standard deviation of 10 blank measurements and “m” is the slope of the calibration curve obtained. The prepared blanks are analyzed at the same time as each batch of 10 samples.

Analysis of variance (one-way ANOVA) was used to assess the difference between sampling sites. The difference was considered significant at P < 0.05. Whenever the difference was significant, Tukey’s test was performed to identify groups that were particularly different from each other. Person correlation analysis was used to establish a relationship between the heavy metal content of soils, suspended particles, sediments and water. The correlation coefficient is between −1 and 1. The higher its absolute value (towards 1), the more the variables are correlated. On the other hand, the closer it is to 0, the weaker the correlation. Statistical analyses (mean value, minimum, maximum, standard deviation, and analysis of variance and Pearson correlation) were carried out using STATISTICA 2005 software (version 7.1). Spatial distribution maps were produced using ArcGIS-10.2 software. 2.

Figure 3 shows the spatial distribution of arsenic concentrations in soils in the Tongon mine area.

Average arsenic concentrations in soils range from 137.917 ± 0.046 mg/kg to 3808.987 ± 0.030 mg/kg (Figure 3(a)). These average concentrations are very high and well above the world average for arsenic-free soils, which is 6 mg/kg [9] . The reference values for arsenic-contaminated soils are set at 20 mg/kg and 29 mg/kg respectively by the environmental protection legislation of certain

European countries such as France, Italy and Holland [9] . The concentrations obtained are all above these standards. The overall average for the site was 534.929 ± 1166.267 mg/kg. They are higher than the concentrations determined in other studies on soil samples from gold mines in Côte d’Ivoire. In their studies of soil samples from certain gold mines in Côte d’Ivoire, the authors found arsenic concentrations of up to 278.6 mg/kg [2] . They also found concentrations of 294.38 ± 195.40 mg/kg at Afema and 7.38 ± 4.39 mg/kg at Bonikro in other industrial mines [3] .

The high concentrations of arsenic in soils are thought to be due not only to naturally occurring levels in soils but also to their distribution, which often originates from mine tailings. Their wide distribution can be explained by the remobilization of arsenic from waste rock. Rainwater infiltrates the waste rock, leading to arsenic being distributed over large areas and accumulating in the soil [2] .

Figure 4 shows the health risks associated with exposure to arsenic-contaminated soil for children, adult men and adult women, respectively. For children, the mean value is 205.18 ± 52.47. For adult men, the mean value is 16.16 ± 4.13, and

for adult women, the mean value is 15.16 ± 3.88. Human health risk assessment has shown that children are at the greatest disadvantage when it comes to exposure risks such as direct inhalation of dust [5] . The risks of exposure are well above the critical limit of 1 and are, in descending order, children > adult women > adult men. The risks are higher for children. Contaminated soil could be used as a passageway and playground for children, who could inhale, ingest and eat contaminated soil [8] . Soil ingestion by children is a primary exposure route of concern for contaminated soils [5] . Similarly, arsenic hazard quotients have been reported in other gold mining areas in Ghana [8] . These studies identified trends similar to those found in our study. Similarly, they found that higher levels of arsenic in the topsoil presented a potential health risk for children [8] .