Fly Ash-Borne Arsenic in the Soil-Plant System

Abstract

Arsenic could be threat to the health of children who might ingest contaminated soil and to the food chain through uptake by plants. The effects of As from fly ash and fly ash/biosolids mixtures on the element concentrations in corn plants were evaluated and the potential for reducing As availability to plants by mixing fly ash with biosolids was also evaluated. A risk assessment for soil As was carried out to evaluate permissible concentrations of soil As and safe As loading rates. The results from large-scale field experiments with soil treated with fly ash/biosolids blends were used as input data to analyze pathways of the most intense transfer of As to a target organism. The direct soil ingestion by children USEPA: the soil-human pathway) was considered the most important exposure route for soil As. Even when the permissible annual loading rate (2 kg ha^-1 As) is exceeded several fold, plant As uptake was not significant. Only young plants or plants grown under greenhouse conditions show significant increases in As uptake; however, no crop yield reductions or any symptoms of toxicity were observed. High loads (6–9 kg ha^-1year^-1) of ash-borne As did not significantly affect its concentrations in corn grain nor enhance As leaching from the soil profile. Mixing fly ash with biosolids appeared to be an effective method of reducing As availability in soils, especially for fly ashes with high As concentrations applied at the high rates. A conservative risk analysis (based on child ingestion of contaminated soil as the most likely path of environmental exposure) shows that As concentrations in soil can reach 4014 g^-1without a hazard to exposed organisms.