Studies of arsenic, copper and lead in the soils of the Tamar Valley

Abstract

A more detailed soil survey than had previously been carried out in the Tamar Valley area of South West England confirmed higher than normal background levels of arsenic, copper and lead. These elevated background levels are attributed to the weathering of mineralized bed rock. Anomalous values were found directly above mineralized veins and in areas of past mining activity. Effects of aerial pollution are indicated by the combination of sodium dithionite and nitric acid extraction techniques, from which elemental association with secondary iron and sulphide minerals may be determined. Enhanced levels resulting from aerial pollution were found in a zone a few hundred metres around mine sites. This work has investigated elemental associations in soils. Using water, acetic acid, EDTA and sodium dithionite extraction methods, soluble copper and lead were found to be associated with exchangeable weak inorganic complexes, whilst arsenic is associated with secondary iron. When selecting a single extraction technique for the quantification of potentially soluble copper and lead EDTA is recommended, whereas for arsenic sodium dithionite should be used. The three elements follow the order arsenic > copper > lea d for complexation with inorganics and secondary iron, whilst lead > copper > arsenic for organic associations. An HPLC/UV/GFAAS interface was constructed and used to separate and detect organo-copper and -lead species in soil pore waters. Up t o 90% of the soluble copper and lead was found associated with low molecular weight polar dissolved organic compounds, especially citric and malic acids. Arsenic species were separated and detected on a HPLC/FAAS hydride generation interface. In aerobic soils arsenate was the predominant soluble species in the pore water. Where mineralization and mining activities have elevated arsenic levels arsenite and monomethylarsonic acid were found. In anaerobic soils the arsenite species predominates.