Bioaccessibility of arsenic in soils developed over Jurassic ironstones in eastern England

Jurassic ironstones outcropping over parts of eastern England give rise to soils with arsenic concentrations in excess of the UK soil guideline value of 20 mg kg(-1) for residential areas. Total arsenic concentrations were determined for 73 ironstone derived soils and bioaccessible arsenic determined using an in vitro physiologically based extraction test. The bioaccessible arsenic concentration for these soils was found to be well below the soil guideline value with a mean concentration of 4 mg kg(-1) and a range of 2-17 mg kg(-1). The bioaccessible fraction ranges from 1.2 to 33%. Data from a sequential extraction test based on the use of aqua regia as the main extractant is presented for a subset of 20 of the soils. Chemometric data reduction is used to demonstrate that the bioaccessible arsenic is mainly contained within calcium iron carbonate (sideritic) assemblages and only partially iron aluminosilicates, probably berthierine, and iron oxyhydroxide phases, probably goethite. It is suggested that the bulk of the non-bioaccessible arsenic is bound up with less reactive iron oxide phases.