Heavy metals inputs in northern Patagonia lakes from short sediment cores analysis

Full text: Short sediment cores were sampled from lakes located in Nahuel Huapi National Park, Northern Patagonia, Argentina, in order to determine heavy metal inputs to these water bodies from the analysis of elemental concentration profiles. Elemental contents were determined by Instrumental Neutron Activation Analysis, with special attention on mercury determinations, and measuring also potential pollutants Sb, Ag, As, Cr, Ti, V. In order to identify different processes that may happen within the sediments the concentration profiles of major and minor elements Fe and Mn, associated with diffusive diagenetic processes, Al, Ca, and Na, geochemical tracers (Rare Earth elements, Sc, Hf, Ta, Th, and U), trace elements Ba, Br, Co, Cs, Sr, Rb, and Zn, and organic matter contents by LOI, were determined. Core dating was performed by measuring natural 210Pb and anthropogenic 137Cs, and by tephra identification. Total 210Pb, 226Ra, associated with supported 210Pb, and 137Cs specific activity profiles were measured by low background gamma-ray spectrometry. Sedimentation processes were found to be affected by volcano ashes deposition, and by diffusive diagenetic processes with heavy metal mobilization after sediment deposition. No evidence of input increase to lake environments of trace elements Sb, Ba, Cs, Zn, Co, Hf, Ni, Se, Sr, Ti, U, and V was observed. High As concentrations (up to 250 μg.g-1) were found associated with Mn and Fe oxides diffusion processes. Strong association between Br concentrations and organic matter contents was observed in sediment cores, but no evidence of relevant increase of Br inputs to the aquatic systems studied was observed in recent years. Ag concentrations were found to be enriched in upper core layers over base line values determined from deep layers measurements, which range from 0.1 to 0.2 μg.g-1. Hg concentration profiles of sediment cores sampled from lakes, or lake branches, with higher productivity (organic matter contents of upper most layer ranging from 17 to 24 %) showed a peak at 1 to 2 cm below lake bottom, with concentration values ranging from 0.6 to 2.8 μg.g-1. The water body with lower productivity (6 % organic matter contents) does not show this peak in the concentration profiles, and Hg contents range from 0.2 to 0.3 μg.g-1 in upper core layers. Two background levels were determined from deep layers measurements; the lower associated with pre industrial time according to core dating, ranging from less than 0.07 to 0.2 μg.g-1, and the other associated with modern times, ranging from 0.1 to 0.3 μg.g-1. (author)