| Arsenical herbicides were used extensively in sugar cane cultivation across the eastern portion of the Island of Hawaii. As a result, surface soil arsenic concentrations are ubiquitously elevated, averaging 260 mg kg -1 in former plantation soils. Arsenic (As) has migrated downward in the soil profile to depths of 50-80 cm, however, to date no As has been detected in groundwater wells within the study area (or elsewhere in Hawaii). Pedogenic solid phases in these Hydrudands, consisting of iron oxyhydroxides, allophane-like aluminosilicates, and metal-humus compounds, strongly adsorb As. In vitro As bioaccessibility ranges from <1 to 52 percent of total As. Risks to human health by direct contact (soil ingestion) are significantly reduced by low As bioaccessibility in Hydrudands; however some of the former sugar cane plantation soils likely have bioaccessible As (AsBAC) exceeding Hawaii Department of Health action levels, and will require mitigating actions.;Total As and key soil properties including pH, total organic carbon, total Fe, and reactive (citrate-dithionite (CD) and hydroxylamine hydrochloride extractable) Fe, Al, Si and P concentrations, were evaluated by multivariate linear regression modeling to predict AsBAC. Two predictor variable models, with total As as the first predictor and either total Fe, CD-extractable Fe, or CD-extractable Al, as the second predictor, were able to explain 85-86% of the variability (R2) in AsBAC.;Ferric chloride plus lime and ferrous sulfate plus lime were applied to As-contaminated soils in a field plot setting to determine the potential for reducing AsBAC by formation of additional Fe oxyhydroxide substrate. The two Fe sources performed similarly in reducing AsBAC over the 2 year observation period, with 30-41% reduction in AsBAC for 0.25 wt % Fe dosing (dry soil basis) and 59-63% reduction for 0.5 wt % Fe dosing. The cost of in-situ treatment of As-contaminated soil with ferrous sulfate plus lime to lower AsBAC is estimated to be an order of magnitude less than excavation and landfill disposal on the Island of Hawaii, making the technology a viable remedial alternative when remedial action objectives are based on AsBAC levels. |
