**2. Arsenic as a health issue and its presence in soil**

The World Health Organization has established the inorganic arsenic maximum tolerable daily intake at 2 μg kg−1 body weight [1]. Inorganic arsenic intake may lead to gastrointestinal, cardiovascular, central nervous system diseases, as well as bone marrow depression and selective cancers (kidney, lung, bladder) [1, 2]. The World Health Organization and the United States Environmental Protection Agency have established drinking water standards at 10 μg As L−1. Compounding the arsenic water and food threshold levels is problematic because arsenic speciation influences arsenic toxicity, with arsenite (As(III)) being perceived as appreciably more toxic than arsenate (As(V)) [3].

Arsenic soil surface horizon concentrations vary from 0.1 to 67 mg kg−1, with a geometric mean of 5.8 mg kg−1 [4]. Among sedimentary deposits, argillaceous sediments generally have greater arsenic concentrations (trace to 13 mg kg−1) [4]. In Missouri, Aide et al. [5] measured soil arsenic concentrations in 22 pristine soil profiles and reported that the epipedons exhibited arsenic concentrations from 2 to 12 mg kg−1, whereas the argillic and cambic horizons exhibited greater arsenic concentrations, ranging from 10 to 30 mg kg−1. The source of the observed arsenic was speculated to be simply inherited in the parent material. Naturally occurring As-bearing minerals include: arsenopyrite (FeAsS), cobaltite ((Co,Fe)AsS), enargite (Cu3AsS4), erythrite (Co3(AsO4)2 8H2O), orpiment (As2S3), proustite (Ag3AsS3), realgar (AsS), and tennantite (Cu12As4S13) [3, 4].
