Effect of Soil Ageing from Arsenic Bioaccessibility and Geochemical Speciation: A Greenhouse Study

Shahida Quazi, Dibyendu Sarkar, and Rupali Datta
Environmental Geochemistry Laboratory, University of Texas, San Antonio, San Antonio, TX

Soil ingestion is regarded as an important human exposure pathway of contaminants. For agricultural farmland turned residential or recreational land use scenarios, the ingestion of arsenic (As)-contaminated soil by children is a serious point of concern. Mobility and availability of As is controlled by reaction with soil solids. Degree and strength of retention of As by soil constituents may vary greatly and change temporally. The binding mechanism influences the extent to which contaminant transport is attenuated. Studies have shown that upon ageing, intra-particle surface diffusion of contaminants within micropores may result in reduced mobility, leading to differences in the potential for As release and hence, bioaccessibility. This study aimed at evaluating the effect of time on soil As fractionation and bioaccessibility in a temperature/humidity controlled greenhouse column setting involving dynamic interactions between soil, pesticide, water and plants. The study used four soil series (Immokalee, Millhopper, Pahokee Muck, and Orelia) based on their potential difference in regards to As reactivity. Soils were amended with sodium arsenate at two rates representing concentrations typically found at Superfund sites. Soil samples collected at different time periods (0, 6, 12 months, and 3 years) were extracted for soil-As forms by a sequential extraction technique and for bioaccessible As by in vitro gastrointestinal method. Immediately after spiking, total extractable As was almost entirely in the soluble form, resulting in high bioaccessibility. However, As bioaccessibility decreased significantly after 1 year as the soils aged resulting in increased As retention along with diffusion to surface micropores. This trend of decreased bioaccessibility with increasing time continued after 3 years demonstrating the impact of ageing on the geochemical fate of as in soils.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas