Hydrogeochemistry and trace elements in groundwater of artisanal gold mining villages, Kédougou–Kéniéba Inlier, eastern Senegal

Artisanal and small-scale gold mining (ASGM), the largest anthropogenic source of mercury worldwide, has expanded in the Birimian Kédougou–Kéniéba Inlier of eastern Senegal, where communities rely on dug wells and boreholes tapping discontinuous basement aquifers. We characterized groundwater hydrogeochemistry and trace elements using three datasets: field parameters and major ions (ion chromatography) at 51 sites in November 2011 (end of the rainy season), trace elements by ICP-MS at 26 of these sites, and trace elements by ICP-OES at 22 sites in June 2014 (onset of the rainy season), eight of which provided valid pairs between campaigns. Data quality was assessed systematically; Hg, As and V in 2014 were treated as screening data, and Sb was discarded. Groundwater was predominantly Ca–Mg–HCO₃ type (84%), reflecting silicate weathering. Nitrate exceeded 50 mg L −1 at 27% of sites, including boreholes, indicating rapid transfer from the land surface. Element associations (Sr–Ca, Ba–K, Ni–Co–Mn–Zn) were reproduced in both campaigns. Sabodala well 3 showed a persistent, localized, acidic metal-rich signature (Ni up to 890 µg L −1 , Co 210 µg L −1 , Mn 643 µg L −1 ; within 15% between campaigns), compatible with sulfide oxidation; the nearby industrial mine is a plausible but unconfirmed source. Manganese exceeded 80 µg L −1 in 23–31% of samples. At the paired sites, conservative solutes were 17–31% higher in June 2014; because season and year are confounded, this is consistent with, but does not demonstrate, a first-flush effect. A screening-level assessment of the quantified elements identified Sabodala well 3 as unfit for drinking (hazard index 20.5 for adults). Arsenic screening signals ranked highest in northern villages, a preliminary pattern that requires quantitative confirmation. The 2014 method (Hg detection limit 13 µg L −1 ) could not determine compliance with the 6 µg L −1 mercury guideline, and antimony could not be assessed. Dedicated methods for Hg, As and Sb are required.