LAUSR

The Sonora River and its tributary streams (Tinajas, Bacanuchi) were impacted in 2014 by an acid solution spill (approximately 40,000 m3). This study aims to presents a clear and supported overview to determining the spill's consequences on the environment and the people inhabiting the area. The elements quantified were those found in the spilled solution: Al, As, Cu, Fe, Mn, Pb, and Zn. Potential Toxic Element (PTE) concentration means from 187 sediment samples were, in mg.kg-1: Al = 7,307, As = 16.6, Ba = 128 Cu = 106 Fe = 15,764, Mn = 566, Pb = 46 and Zn = 99. Differences between PTE concentrations in the most impacted sediments and those of the local baseline, sampled in streams not affected by the spill and regional baseline values, were not statistically significant. The similarity of PTE concentrations among sediments may be explained by natural geological enrichment, historical mining impacts, and a low increase of PTE in sediments after the acid spill because of natural and anthropogenic attenuation. Mainly heavy rains, natural pedogenic carbonates, and remedial work done by the mining company (retaining dam, adding lime; precipitation, collecting formed solids, and transport to the mine). The Contamination Factor (C.F.), Enrichment Factor (E.F.), and Geo-accumulation Index (Igeo) were determined. The C.F. indicated low and moderate contamination in all elements. Cu exhibited the highest E.F., from moderate to significant enrichment. The Igeo generally ranged from -0.02 to 0.15. Cu and Zn were classified as moderately to heavily contaminated. In local baseline sediments, the Cu C.F. varied from moderate to very high contamination, the Cu E.F. from moderate to significant enrichment, while the As, and Pb Igeo ranged from uncontaminated to moderately contaminated. In general, normalization demonstrated a high degree of Cu enrichment at sites 1-14. Sequential extractions indicated that only Cu was found in all fractions, including a significant exchangeable fraction in the very impacted sediments (1-14). The other PTEs were distributed between the Fe/Mn oxide fraction and the residual phase. Principal Components Analysis for PTE concentrations indicated three different groups with similar geochemical patterns and allowing to identify the PTE potentially sources: the first sediments from sites 1-14 were the impacted sediments in accordance with pH and electrical conductivity results, the second group from sites 15-20 showed characteristics of the mineralized environment, and the third from sites 21-30 were unrelated to the spillage. The area impacted by the acid solution spill reached approximately 30 km downstream, just roughly 15% of the initially considered area.