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

• Factors that control groundwater quality are investigated.
• Arsenic mobility is limited in dug wells by their adsorption onto Fe-oxyhydroxides.
• Elevated AsT concentrations in borewells are attributed to water – rock interaction.

This study investigates the processes that control dug well and borewell water chemistry in crystalline basement aquifers in semi-arid environment. Changes in SO42 − and NO3− were observed in both dug well and borewell samples. Six dug wells and seven borewells had high NO3− concentrations relative to SO42 − suggesting prevailing oxidizing redox conditions in these samples. The highest NO3− and SO42 − 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.