Assessment of the impact on sediment quality from abandoned artisanal mine runoffs in a semi-arid environment (the Sonora River basin—Northwestern Mexico)

Impact to streams that receive runoffs from the abundant abandoned artisan-scale mines disseminated throughout the middle and upper Sonora river basin in central Sonora state, Northwestern Mexico, was evaluated by determining total concentrations, enrichment factor (EF), geoaccumulation index (Igeo) and geochemical and granulometric fractionations of heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in sediment of those streams. Total concentrations of As were also determined. Excluding Fe, the metal found in highest concentrations in all sites was Mn. Findings indicate that sediment enrichment is moderate only for Pb (3.76) and Zn (3.68) in 1 point, and for Cu (3.61) in a different site, out of the 11 points under study. Enrichment is absent or minimal for the remaining elements and sites. Maximum contamination levels as established by Igeo were moderate for Cu (1.14) and lower than moderate for Cu (0.77) and Pb (0.32) in one site each; and absent for all other elements and sites. The granulometric analyses found a close correlation between the percent mass composition of sediment in gravel, sand, clay and silt, and the distributions of metal content in those compartments, with the exception for Cu and, to a lesser extent, Pb. The geochemical fractionation indicates the metal found in the exchangeable fraction in a highest proportion was Mn, while the highest percent in the reducible fraction corresponds to Pb and Mn. At present, sediments of streams in the middle and upper Sonora River basin show a low or null impact on their quality by the sporadic runoffs from abandoned artisan mines they receive.