LONG-TERM EFFECTIVENESS OF DRINKING-WATER TREATMENT RESIDUALS IN REDUCING SOIL ARSENIC BIOACCESSIBILITY: A GREENHOUSE STUDY

Human health risk from direct exposure to soil arsenic (As) via hand to mouth activities is restricted to a fraction of total soil As available to the gastrointestinal system. Evidence from previous incubation studies in our laboratory indicate that drinking-water treatment residuals (WTRs) are effective in reducing bioaccessible forms of As. However, no studies have been conducted so far to validate the long-term stability and effects of the WTRs on As bioaccessibility under real or simulated field conditions. In this context, we set out to assess the effect of two types of WTRs on As bioaccessibility in two soils with varying physico-chemical properties over a four year time period in a greenhouse setting. The two soil types under study, Immokalee (low pH sand) and Orelia (high pH loam) were selected on the basis of their potential difference in terms of As reactivity. Soils were spiked with sodium arsenate at the rate of 1500 mg kg-1 As and then amended with Al- WTR and Fe-WTR at two rates (5% and 10%). Rice was the test crop for 1st year of study. The first soil sampling was done immediately after spiking (time zero), and then after 6 and 12 months of equilibration time. Selected columns were retained for longer periods (without grass cover) to understand the effect of soil-aging on As speciation and bioaccessibility. After year 1, soils were sampled on an annual basis – currently the study is in its third year. In-vitro extraction schemes were employed to estimate As bioaccessibility in gastro-intestinal and stomach phases. Immokalee soils with very low As retention capacity showed a decrease in As bioaccessibility immediately after spiking with the WTRs. There was a 15% decrease in As bioaccessibility at 5% Al-WTR amendment rate, which increased to 25% at the higher application rate compared to the unamended controls. These numbers were significantly lower in case of Orelia which has higher As retention capacity. Results show that As bioaccessibility in the WTR-amended soils decreased significantly (p<0.05) with time compared to controls. However, there was no statistical difference in the effects of Al- and Fe-WTR on As bioaccessibility. Hence, As bioaccessibility in the soils is dependent on soil properties, WTRs rates, and soil aging.