Trace metal partitioning and potential mobility in the naturally acidic sediment of Lake Caviahue, Neuquén, Argentina

Lake Caviahue, in Patagonia (Argentina), is a very acid water body (Patagonia, Argentina) due to the influx of volcanic fluids. Over the past 18 years, the lake has been progressively alkalinizing and pH is close to the ferric iron precipitation threshold (pH>3,0). Should iron precipitate, water and sediment composition will be altered. To set a baseline, trace metal partitioning (Cu, Cd, Cr, Pb, Zn and Mn) was studied for three sediment cores (0 to 16 cm depth) at three sampling stations using a sequential extraction procedure (SEP) and the acid volatile sulfide/simultaneously extracted metals (AVS/SEM) protocol. The total metals content of the sediments ranged between 0 to 408 μg g-1 of dry sediment, with Pb>Cr>Mn>Cu>Zn>Cd. No Cd was measured above the limit of detection. The sediment was richest in Mn and Cr, two non-toxic metals at the pH / Eh combination of the lake basin. Total Pb was at a moderate contamination level although it is associated with the most recalcitrant fraction and therefore has low mobility. The sediment fractions with higher metal content were the oxidizable and residual, the former commonly associated with labile organic matter and pyrite while the latter are related to recalcitrant organic matter and rock forming minerals. In addition, a high correlation was found between Cr, Pb and Mn,thus these metals may be subject to analogous precipitation processes and possibly to co-variation in the volcano effluents. Furthermore, no metals were detected in the exchangeable/carbonates fraction, which is the most labile of all the sediment fractions. The metal content in (SEM) was likewise below the toxicity thresholds of two international sediment quality guidelines and the ratio AVS/SEM was over one, indicative of non-toxicity. Both results indicate that metal mobility in the acidic sediments of lake Caviahue is very low.