ARSENIC GEOCHEMISTRY IN SOILS CONTAMINATED WITH ORGANO-ARSENICAL PESTICIDES

In addition to natural sources, large quantities of arsenic (As) can enter into the environment as organo-arsenical compounds through agricultural activity. Monosodium methanearsonic acid, disodium methane-arsonic acid and cacodylic acid are utilized as herbicides in cotton farming, whereas roxarsone (3-nitro-4-hydroxyphenylarsonate) is utilized as feed additive/antibiotic agent in poultry farming. The potential for human exposure to As has increased tremendously due to the encroachment of suburban areas into former agricultural lands. Recent studies have focused on the health risk posed by long-term human exposure to low-level As-contaminated systems, particularly due to soil ingestion from incidental hand-to-mouth activity by children. Organic arsenicals are considered non-carcinogenic, and hence assumed safe to be used. However, insufficient data is available on the stability of organic arsenicals in soils - if they remain in organic form or are converted over time to inorganic forms. A laboratory incubation study was conducted to estimate geochemical speciation of As and its in-vitro bioavailability as a function of soil properties. Five different soil types were chosen based on their potential differences with respect to As reactivity: an acid sand with minimal As retention capacity, a sandy loam with relatively high concentration of Fe/Al-oxides, a low-pH clay soil, an organic (muck) soil, and a high pH calcareous soil. The soils were amended with sodium dimethylarsonate (DMA) at three rates: 45, 225, and 450 mg/kg. A sequential extraction scheme was employed to identify geochemical forms of As in soils (soluble, exchangeable, organic, Fe/Al-bound, Ca/Mg-bound, residual). Concentrations of these operationally defined soil As forms were correlated with the “in-vitro” bioavailable fractions of As to identify the As species that are most likely to be bioavailable. Arsenic bioavailability and speciation studies were done at 0 time (immediately after spiking the soils with pesticides) and after four-months and one-year incubation periods. Our results show that soil properties greatly impact geochemical speciation and bioavailability of DMA, and indicate that usage of organic arsenicals as pesticides in mineral soils may not be a safe practice from the viewpoint of human health risk.