GEOCHEMICAL FORMS AND BIOAVAILABILITY OF ARSENIC IN PESTICIDE-APPLIED COTTON SOILS OF TEXAS: AN INCUBATION STUDY

Injudicious application of arsenical pesticides in agricultural fields has rendered soils with elevated levels of arsenic. This is particularly true for the cotton soils of Texas where background concentrations of arsenic are significantly higher than normal. Rapid encroachment of suburban development on former agricultural lands has increased the probability of human contact with these soils with relatively high background arsenic concentrations. Human health risks from arsenic are associated with those forms of arsenic that are bioavailable for adsorption into human tissue, which is a function of geochemical speciation. In order to examine the relationship between bioavailability and geochemical-speciation of arsenic, an incubation study is in progress using an in-vitro method, which simulates conditions in the human gastrointestinal system. Two soil types have been chosen for this study: an organic vertisol of the Houston Black series (a typical cotton soil of Texas) and a sandy spodosol of the Immokalee series (minimal arsenic retention capacity, used for comparative purposes). The soils were amended with one inorganic and one organic arsenical pesticide at 3 rates, representing normal (45 mg/kg), high (225 mg/kg), and excessive (450 mg/kg) pesticide application. Concentrations of arsenic in various soil phases and the bioavailable fractions have been measured immediately after pesticide application (0 time), and after 6 months to study the effect of soil ageing on arsenic speciation and bioavailability. At 0 time, arsenic in Houston Black samples amended with high and excessive levels was predominately water-soluble (59% and 65%). However, after 6 months incubation, water-soluble forms of arsenic decreased to 27% and 16%, respectively. The presence of high levels of extractable Fe/Al in Houston Black soil resulted in high arsenic retention, leading to decreased arsenic bioavailability. In contrast, forms of arsenic recovered from Immokalee soils were predominately water-soluble at both 0 time and after 6 months incubation. A significant correlation was observed between soil speciation and in-vitro bioavailability of arsenic, indicating that soil chemical properties play a major role in arsenic bioavailability.