Use of Paleoflood Deposits to Determine the Contribution of Anthropogenic Trace Metals to Alluvial Sediments in the Hyperarid Rio Loa Basin, Chile

Abstract

Toxic trace metals are a common and significant contaminant in riverine ecosystems, and are derived from both natural and anthropogenic sources. Determining the contributions of metals from these sources has proven difficult, in part, because physical and biogeochemical processes alter the nature (e.g., grain size, mineral composition, organic matter content) of the source materials as they are transported through the drainage network. This study examined the use of paleoflood deposits located along the hyperarid Rio Salado, a tributary to the Rio Loa of northern Chile, to construct local background functions and enrichment factors (LEFs) to differentiate between natural and anthropogenic metal sources. Significant variations in metal content occurred between river reaches and flood deposits of a given reach; these variations were primarily related to changes in sediment source that may reflect differences in El Niño and La Niña precipitation patterns. Three conservative elements (Al, Fe, Co) were examined to construct background functions for seven trace metals. Cobalt yielded the most effective background functions for As, Cd, Ni, Pb, and Zn; Fe was selected for Cr, and Al for Cu. The resulting LEFs approximated 1, illustrating that paleoflood deposits produced effective background functions, and could be applied to downstream sites contaminated by mining activity.

Fluvial palaeohydrology in the 21st century and beyond

“…These large rivers exhibit a diversity of channel patterns (though dominantly anabranching), relatively long‐term channel–floodplain evolutionary histories, and complex floodplain mosaics and connectivity processes, all of which pose difficulties for palaeodischarge estimation. Greater success for palaeodischarge reconstructions has been achieved in the Atacama Desert, where Late Glacial–Holocene palaeoflood reconstructions of flood magnitude have been related to paleo‐El Niño events by interpreting flood stratigraphy, water geochemistry, trace metals, and fluvial sediment inputs to the coastal zone (Magilligan et al, 2008; Miller et al, 2019; Ortlieb, 2000; Rogers et al, 2004; Wells, 1990, among others). In Patagonia, the southernmost region of South America, palaeodischarge and frequency reconstructions have been achieved for Holocene GLOFs (Benito & Thorndycraft, 2020; Benito et al, 2021; Vandekerkhove et al, 2020).…”

Methods and Advances in the Forensic Analysis of Contaminated Rivers

“…For example, within the Rio Loa basin of northern Chile, Cu/Sb ratios were effective at differentiating between mining and geogenic trace metal sources. High Cu concentrations were associated with waste materials from local Cu mines, whereas elevated Sb concentrations were associated with natural mineralized rocks, particularly sediments derived from the El Tatio Geyser Basin [39]. In other instances, it may be possible to utilize the unique element or mineral associated with a specific source.…”