Abstract

Background: Large tropical rivers remain largely undiscovered compared to the widely studied large temperate rivers. The Usumacinta River in southern Mexico is North America's most extensive tropical fluvial system. The increasing anthropogenic activities in the Usumacinta basin (e.g., agriculture, cattle raising, forest fires, deforestation, and urban development) threaten the ecosystem's health. However, our understanding of the characteristics of the basins, especially on metal(loid) concentrations, remains limited. To address this, we investigated metal(loid)s in the sediments along the Usumacinta River.

Methods: 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 by conventional techniques, and the concentrations of Al, As, Ba, Be, Cd, Cr, Co, Cu, Fe, Pb, Li, Mg, Mn, Ni, Ag, Se, Sr, Sb, Tl, Sn, V, and Zn were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). 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 (M) and lower (L) basins of the Usumacinta River.

Results: Metal(loid) concentrations (µg g -1 ) ranges were Ag 0.07-0.40, As 1-6, Ba 8-103, Be 0.1-0.3, Cd 0.1-0.8, Co 4-10, Cr 21-150, Cu 1-6, Li 3-10, Mn 105-400, Ni 18-198, Pb 3-13, Sb 0.01-0.50, Se 0.1-2.0, Sn 0.2-1.7, Sr 22-180, Tl 0.04-0.19, V 7-22, and Zn 19-79. The enrichment factor (EF) of Ag, Ba, Be, Cu, Sr, Tl, and V was <1.5, indicating that these were not enriched and can be associated with natural weathering processes. Conversely, the average EFs of As, Co, Mn, Pb, Sb, Sn, and Zn were 1.0 10 in various cases; Se was consistently >10 in all stations, seasons, and in the M and L basins, evidencing that sediments are Se-enriched. Using four pollution indexes, we concluded 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 with Se through natural sources in both basins and seasons.

Conclusions: In general, results showed that metal(loid) concentrations were higher during the dry than in the rainy season and that such concentrations in the L basin were higher than in the M. Generally, the enrichment factors (EFs) of most elements and stations were <1.5, indicating that such metal(loid)s are derived from natural weathering; only the EFs of Se were >10 in all stations, seasons, and in both basins, evidencing that sediments are enriched. The Igeo values confirmed this, as Se exhibited Igeo > 5 in twelve sites in the rainy season, indicating anomalous conditions.