Historical reconstruction and impact of the 2014 mining spill on El Molinito Dam sediments (Sonora River, northwestern Mexico)

Abstract

This study aimed to reconstruct the temporal variations of potentially toxic element (PTE) concentrations and enrichment levels in four ²¹⁰Pb-dated sediment cores, collected in Molinito Dam, NW Mexico, to assess the impacts of the 2014 mining spill on the Sonora River Basin. Sediment cores were collected with a gravity corer in February 2023; major and trace elements were analyzed through X-ray fluorescence spectrometry, except Hg and Cd, measured by atomic absorption spectrometry. Contamination degree was assessed via enrichment factor, and PTE sources were identified through factor analysis. The sediment cores spanned between 10 and 30 years; sediments were mostly composed of clayey silts, with organic matter and carbonate contents below 4 %. Element concentrations were similar in magnitude among cores, exhibiting minimal variations throughout their temporal profiles, except for distinct maxima of Mn, Fe, As, Cu, Pb, Ni, Cr, and Zn that reached different depths within each core. Most elements showed null to minor enrichment, except As, Cd, and Hg, which exhibited moderate to severe enrichment in some cores. PTE maxima were observed in sediment sections dated near 2014, which, considering the age model uncertainties, aligned with the timing of the 2014 mining spill. Additional maxima in older or younger sections were likely associated with hydrometeorological events or unreported spills. Although the enrichment factor for most PTEs was null or minor, the spill left a clear mark in the sediment record, which also showed that PTE concentrations returned to pre-event levels. These findings underscore the importance of reconstructing historical environmental conditions through sediment analysis to understand changes and evolution of aquatic ecosystems.