Wednesday, November 15, 2006

Geochemical Speciation and Bioavailability of Arsenic in Chemically Variant Soils Amended with Sodium Arsenite: A Greenhouse Study.

Saurabh Sharma1, Rupali Datta2, Dibyendu Sarkar1, and Rachana Nagar1. (1) Univ of Texas at San Antonio, 6900 N. Loop 1604 West, San Antonio, TX 78249, (2) Univ of Texas at San Antonio, 6900 N. Loop 1604 West, San Antonio, TX 78249

High residual level of arsenic at former agricultural lands is a critical environmental issue as soil arsenic may pose serious health risk through direct contact, ingestion or inhalation. A major factor governing the toxicity of arsenic in soil is its bioaccessibility that depends on its interaction with soil components. Little attention has been paid to investigate reactivity, distribution and stability patterns of arsenical pesticides in different soil types. To address the issue of soil variability on arsenic bioaccessibility, a greenhouse column study is being performed allowing for dynamic interactions between soils, water, plants and  pesticide. Three agricultural soils (i.e. Eufaula, Millhopper and Orelia series) differing in their physicochemical properties were selected for these investigations. Selected soils were amended with sodium arsenite (a common pesticide prior to the nineties) at three rates (0, 45, and 450 mg/kg As) and were analyzed immediately after spiking (time 0) and after 4-month soil pesticide equilibration. Vetiver grass (Vetiveria zizanioides) was used as the test crop. Noticeable vetiver growth was observed in Orelia soils amended with 450 mg/Kg of As. However, no vetiver growth was observed in the other soils at the higher amendment rate. The majority of the arsenic was leached out from the Eufaula and Millhopper soils after the first leaching event. A sequential extraction scheme was employed to identify geochemical forms of arsenic 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” bioaccessible arsenic fractions. Results from this study verified those obtained from the static incubation experiment, and demonstrated that As bioaccessibility is a function of soil speciation of As and that soil-As forms are a function of soil chemical properties. As the soil aged, a greater percentage of total soil-As was rendered unavailable to the simulated human gastrointestinal system.