Fractionation and distribution of arsenic in soils contaminated by cattle dip

Abstract

Soils surrounding cattle dips in Australia are known to be highly contaminated with As and are potentially of major concern to the environment and human health. A study of 11 dip sites in northern New South Wales (NSW) has revealed considerable surface soil (0–10 cm) contamination with As (37–3542 mg As kg⁻¹ soil). In addition it was shown that considerable movement of As down through the soils had occurred with concentrations at 20 to 40 cm ranging from 57 to 2282 mg As kg⁻¹ soil. At one particular site, an As concentration of above 14 000 mg As kg⁻¹ soil was determined at a depth of 40 to 45 cm. A sequential fractionation scheme, based on a soil P fractionation, was developed to assess the chemical nature, and thus the potential bioavailability and mobility of As, at the sites. Soil As is separated into six fractions with (i) anion exchange resin, (ii) NaHCO₃, (iii) NaOH, (iv) NaOH following sonication, (v) HCl, and (vi) HCl/HNO₃. Although substantial differences between sites and soil depths were present, some general trends were apparent. Most sites contained substantial concentrations of As in the two most labile fractions, indicating high potential for phytotoxicity and leaching. The bulk of the contaminant As at the sites seemed to be associated with soil amorphous Fe and Al minerals.