Continental-scale variation in chloride/bromide ratios of wet deposition.

Chlorine and bromine play crucial roles in atmospheric element cycles and are important environmental tracers in catchment investigations, so understanding their distribution at the Earth's near-surface is imperative for deciphering their behaviour. This study presents the first continental-scale analysis of Cl- and Br- concentrations of wet deposition, based on six and half years of weekly samples collected across North America. A recently developed imputation algorithm was applied to estimate the high proportion of censored Br- values, as well as the other eight analytes, based on the multivariate relationships of nine analytes. The results are consistent with previous studies that have found that the concentrations of these two ions in wet deposition, and the Cl-/Br- ratios of wet deposition decrease with distance inland. Close to the coast, Cl-/Br- ratios of wet deposition are similar to the ratio found in seawater (~288 by mass), rapidly decrease to approximately a third of the seawater ratio at ~200km inland, and then decrease at a much lesser rate to reach mass-ratios of 20 to 10 at ~1500km inland, following a logarithmic regression. The Niagara Falls and the Great Salt Lake are identified as localised sources of atmospheric solutes based on elevated Cl-/Br- ratios of wet deposition at proximal sites. Our observations provide further confidence in the findings presented in previous studies that have shown that Cl-/Br- ratios systematically decrease with increasing distance from the coast, despite the potential confounding impact of other processes, such as weather patterns, chemical behaviour and anthropogenic activity. Our results provide improved estimates of Cl-/Br- ratios of wet deposition source water in the absence of site-specific data.

EDTA-assisted Pb phytoextraction.

Pb is one of the most widespread and metal pollutants in soil. It is generally concentrated in surface layers with only a minor portion of the total metal found in soil solution. Phytoextraction has been proposed as an inexpensive, sustainable, in situ plant-based technology that makes use of natural hyperaccumulators as well as high biomass producing crops to help rehabilitate soils contaminated with heavy metals without destructive effects on soil properties. The success of phytoextraction is determined by the amount of biomass, concentration of heavy metals in plant, and bioavailable fraction of heavy metals in the rooting medium. In general, metal hyperaccumulators are low biomass, slow growing plant species that are highly metal specific. For some metals such as Pb, there are no hyperaccumulator plant species known to date. Although high biomass-yielding non-hyperaccumulator plants lack an inherent ability to accumulate unusual concentrations of Pb, soil application of chelating agents such as EDTA has been proposed to enhance the metal concentration in above-ground harvestable plant parts through enhancing the metal solubility and translocation from roots to shoots. Leaching of metals due to enhanced mobility during EDTA-assisted phytoextraction has been demonstrated as one of the potential hazards associated with this technology. Due to environmental persistence of EDTA in combination with its strong chelating abilities, the scientific community is moving away from the use of EDTA in phytoextraction and is turning to less aggressive alternative strategies such as the use of organic acids or more degradable APCAs (aminopolycarboxylic acids). We have therefore arrived at a point in phytoremediation research history in which we need to distance ourselves from EDTA as a proposed soil amendment within the context of phytoextraction. However, valuable lessons are to be learned from over a decade of EDTA-assisted phytoremediation research when considering the implementation of more degradable alternatives in assisted phytoextraction practices.

Topsoil (0-5 cm) composition in eight arctic catchments in northern Europe (Finland, Norway and Russia).

Frozen topsoil samples (0-5 cm) were collected during March/April 1994 in eight Arctic catchments in northern Europe (four in Russia, three in Finland, one in Norway), at different distances and in different wind directions from the emissions of the Russian mining, roasting and smelting industry on the Kola Peninsula. Between 14 and 25 sites were sampled in each of the eight catchments ranging in size from 12 to 35 km2. Results show that close to the smelters in Monchegorsk and Nikel, topsoil is clearly enriched in Ag, As, Bi, Cd, Co, Cr, Cu, Fe, Ni, Pb, Sb, Se, Te and V. Cu and Ni median contents in topsoils collected close to Monchegorsk are about 600 times higher than in the Finnish catchments. The effect of open-cast mining and waste dumps of alkaline rocks from the nephelinite industry near Kirovsk can be seen in the elevated contents of Al, Ba, K, La, Mn, Na, Sr, Ti, Y and Zn in topsoil collected in a nearby catchment. For many elements, however, variations observed in single catchments are as great as the total regional variation. Several elements (e.g. Cd, Hg, Cu, Ni, Pb, S, Zn) show strong positive correlation between the organic content of the sample and the element content observed. Thus it is necessary to determine the organic content of the samples and correct the element levels when using depth-related soil samples (here, the 0-5 cm topsoil layer) for regional mapping.