BIOAVAILABILITY OF ARSENIC IN CATTLE DIPPING VAT SITE SOILS AS A FUNCTION OF SOIL CHEMISTRY

Widespread application of high levels of arsenic in cattle dipping vats used for treating cattle ticks has resulted in severe contamination of soil and ground water with this Group-A human carcinogen. Because of the absence of a universally applicable soil model, many baseline risk assessment studies involving these sites have used an extremely conservative estimate that 100% of the arsenic present in soil is bioavailable, by equating arsenic bioavailability in water with that in soils. However, several in-vivo animal studies have shown that bioavailability of arsenic in soils is significantly lower than in water. This is because arsenic exists in many geochemical and mineralogical forms, many of which are insoluble in human digestive juices and are not likely to be bioavailable. Therefore, the assumption of 100% bioavailability significantly overestimates the calculated risk, proportionately elevating site cleanup expenses.

The present study evaluated arsenic bioavailability in cattle dip vat site soils as a function of soil properties. Twelve soil samples were selected from former cattle dip vat sites in Australia and Florida, which varied widely in terms of soil physico-chemical properties that affect arsenic retention. The soils were subjected to an in-vitro gastrointestinal (IVG) procedure to determine arsenic bioavailability. The IVG protocol consisted of a low pH stomach phase (IVGS) and a high pH intestinal phase. Absorption through the intestinal membrane was simulated using iron-oxide coated filter paper strips. Arsenic extracted by the intestinal-absorbed phases (IVGIA) and the IVGS phase was correlated with geochemical forms of arsenic. In addition, the extracted fractions (Total As, IVGS, IVGIA) were evaluated against the properties of the soil samples. Multiple regression analyses indicated that 85% of the variability in the arsenic bioavailability using the IVGS method, and 88% using the IVGIA method could be explained on the basis of variability in the soil properties considered (pH, salinity, clay content, total P, total Ca + Mg, SOM, oxalate Fe + Al). Results obtained in this study clearly demonstrate the influence of soil properties on arsenic speciation and bioavailability in contaminated soils.