Hydrogeochemistry and rare earth element behavior in a volcanically acidified watershed in Patagonia, Argentina

The concentrations of REE in the Rio Agrio decreased by several orders of magnitude through the study area, as a result of dilution and chemical attenuation. A subtle shift in the slope of shale-normalized profiles of dissolved REE concentration was observed, from being weakly positive near the source of the Rio Agrio, to showing a weak middle REE enrichment trend in Lake Caviahue, to being weakly negative in the lower reaches of the river. The trend to a negative slope across the lanthanide series in the lower river is explained by selective partitioning of the heavier REE to hydrous oxides of Fe and Al suspended in the water column, and accumulating on the riverbed. Most of the decrease in REE load occurred immediately downstream of the confluence with a tributary that increased the pH of the Rio Agrio from 4.3 to 6.1. Although the mixed water was supersaturated with REE phosphate compounds, precipitation of LnPO4 is not believed to have been a dominant process because the predicted pattern of inter-element fractionation from phosphate deposition is inconsistent with the observed trends. Instead, REE attenuation most likely occurred from adsorption onto freshly precipitated hydrous aluminum oxide. The behavior of REE in the Rio Agrio watershed is broadly similar to what has been observed in watersheds that owe their acidity to oxidation of sulfide minerals.