Hydrogeochemical processes controlling groundwater quality around Bomboré gold mineralized zone, Central Burkina Faso

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Résumé

This study investigates the processes that control dug well and borewell water chemistry in crystalline basement aquifers in semi-arid environment. Changes in SO4 2− and NO3 −were observed in both dugwell and borewell samples. Six dug wells and seven borewells had high NO3− concentrations relative to SO4 2− suggesting prevailing oxidizing redox conditions in these samples. The highest NO3− and SO4 2− concentrations were found in few borewells. These high concentrations could be attributed to low recharge rates and chemical weathering of sulfide minerals. The acidic conditions may have promoted AsT adsorption in the majority of dug wells. The saturation indices and correlation coefficients showed that most dug wells and some borewells were supersaturated with respect to Fe-bearing minerals, implying that trace elements such as AsT, Cu, Cr, Ni and Zn were likely to coprecipitate with residual Fe minerals. The low mobility of AsT in dug wells could be also explained by the limited carbonate mineral abundance in the weathered layer. By contrast, the elevated AsT concentrations observed in the majority of borewells reflected an extended water–rock interaction that had led to deportonation of surface charges of the aquifer minerals, thereby inhibiting AsT adsorption. The average concentration of FeT and total coliform content of dug well samples exceeded the World Health Organization (WHO) limits for drinking waters. Likewise, the average concentrations of FeT, AsT (~55% of samples) and Pb in borewells were higher than the WHO permissible limits. The study showed that the local groundwater resources are exposed to both anthropogenic and geogenic sources of pollution.

Mots-clés

Borewells, Dug wells, Arsenic, Trace metals, Groundwater quality