Tracing of mercury sources in Nahuel Huapi National Park, Patagonia, Argentina, using lichens as atmospheric bioindicators

Full text: Usnea sp lichens were analysed in order to asses atmospheric sources of mercury in Nahuel Huapi National Park, Northern Patagonia, Argentina. Previous studies had shown that mercury contents in sediment and biota are high considering that within, or nearby, the region of study no relevant industrial or extensive agriculture activities exist. Pooled samples, made of at least 10 individuals, were analysed by Instrumental Neutron Activation Analysis. Sampling sites were defined following the steep precipitation gradient of the region, that ranges from 3000 mm.y-1 in the Western part of the park, to 500 mm.y-1 in the Eastern. Lithophile elements, associated with particulate matter entrapped by the lichen thalli, were identified, but mercury was not among these elements. Mercury contents range from (0.0558 ± 0.0083) to (1.38 ± 0.18) μg.g-1. The analysis of mercury contents determined in foliose lichens sampled from urban and periurban sites of Bariloche city, single relevant human settlement in the park, and from undisturbed regions, showed higher mercury contents in Bariloche air than in the surroundings. Transplanted thalli of fruticose Protousnea magellanica from undisturbed zones to urban sites confirmed this conclusion. 137Cs cumulative fluxes in the region were also determined from inventories of lake core sediments sampled nearby lichens sampling sites. Global transport followed by wet deposition is a potential source of mercury, and 137Cs is an anthropogenic isotope that can reach the Nahuel Huapi National Park only by atmospheric transport, stratospheric or tropospheric. Usnea sp mercury contents do not correlate with 137Cs cumulative fluxes along the precipitation gradient, leading to the conclusion that global transport and wet deposition is not the main mercury source in Nahuel Huapi National Park, and the analysis of the results suggests that more than one source are combined to impact the whole park area. (author)