Metal(loid)s in surface sediments of the middle and lower basins of the Usumacinta River (southeastern Mexico): variability and selenium enrichment.

Large tropical rivers remain poorly characterized relative to the widely studied large temperate rivers. The Usumacinta River in southern Mexico is North America's most extensive tropical fluvial system. Increasing anthropogenic pressures in the Usumacinta basin (e.g., agriculture, cattle raising, forest fires, deforestation, and urban development) threaten ecosystem health. However, our understanding of the characteristics of the basins, especially on metal(loid) remains limited. To address this, we investigated metal(loid)s in sediments along the Usumacinta River. Thirty-four surface sediments from 17 stations within the Usumacinta's mainstream and principal tributaries were sampled for metal(loid) analysis during the rainy and dry seasons. The sediment grain size and organic matter were analyzed and the concentrations of aluminium (Al), arsenic (As), barium (Ba), beryllium (Be), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), magnesium (Mg), manganese (Mn), nickel (Ni), silver (Ag), selenium (Se), strontium (Sr), antimony (Sb) , thallium (Tl), tin (Sn), vanadium (V), and zinc (Zn) were quantified by Inductively Coupled Plasma Mass Spectrometry. The indices enrichment factor (EF), geo-accumulation index (Igeo), contamination factor (CF), and pollution load index (PLI) were used to assess the health status of metal(loid)s in the surface sediments of the middle and lower basins of the Usumacinta River. Metal(loid) concentrations in Usumacinta River sediments fall within or below the range reported for tropical rivers with comparable geological settings, suggesting limited anthropogenic metal contributions. While the EFs of Ag, Ba, Be, Cu, Sr, Tl, and V was <1.5, suggests that they are linked to weathering, those of As, Co, Mn, Pb, Sb, Sn, and Zn were 1.0 < EF < 10 in various cases, Se was consistently >10 in all stations, seasons, and in the middle and lower basins, evidencing that sediments are Se-enriched. Using four pollution indexes, we found that the sediments of the Usumacinta River are either uncontaminated or moderately contaminated (enriched) with As, Cd, Cu, Cr, Ni, Pb, Sb, V, and Zn. They are consistently enriched in Se from natural sources across both basins and seasons, but this enrichment may be modulated by an anthropogenic signal. In general, results showed that concentrations were higher during the dry than during the rainy season, and that concentrations in the lower basin were higher than in the middle basin. EFs of most elements and stations were <1.5, likely indicating that these metal(loid)s are derived from natural weathering; only the EFs of Se were >10 in all stations and seasons across both basins, suggesting sediment enrichment. This enrichment can be linked specifically to the Cretaceous marine sedimentary rocks or the occurrence of seleniferous in the Sierra de los Cuchumatanes. However, an additional contribution related to phosphate-based agricultural inputs may be added. The patterns thus indicate a superposition of a regional geogenic baseline and a seasonally modulated anthropogenic signal.