Environmental geochemistry of soils and stream sediments from Anka and Birnin-Gwari artisanal gold mining areas, NW Nigeria

• Main Author: Waziri, Nuhu Musa
• Published: University of East Anglia 2012
• Subjects: 577
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590698

The geochemistry of surface soils and stream sediments from two areas in the north-western Nigeria Schist Belt was studied in order to assess the environmental impact of artisanal mining of quartz-gold-sulfide mineralization. XRF determination of total elemental concentration was carried out, along with sequential extraction procedures (SEPs) and in vitro bioaccessibility tests using ICP-AES. The results show that the soils in both the Anka and Birnin-Gwari area are highly enriched in silica, zirconium and markedly depleted in base cations due to intense tropical weathering. The results further show that artisanal mining has only caused severe contamination, especially with respect to Pb and Cu, in the Anka area, highlighting the importance of mineralogical differences in the ore deposits. Most trace elements partition strongly into the carbonate and Fe/Mn oxides fractions in samples from the Anka area, indicating possible risk of mobilization under reducing, slightly acidic conditions; the exchangeable phase being the least significant in both areas. Very high bioaccessibility values, which correlate strongly with the sums of the SEP steps, were obtained for Pb, Cu and to a lesser extent, As and Mn in soils of the Anka area and the minimal risk levels and tolerable daily intakes are greatly exceeded. Low pH was found to enhance the bioaccessibility of Pb, Cr and Cu, while rising pH appears to favour the release of As. Only the bioaccessibility of Cr was found to change with the length of extraction time and the ratio of the mass of sample to extraction fluid volume only affects the extraction of As. The results show significant human health risks, mostly in the Anka area due to processing of ores and improper disposal of tailings. Ore processing at the mine sites and an end to using the tailings as construction materials may reduce the exposure of the local population to potentially toxic elements.