Bioaccessible arsenic in soils of former sugar cane plantations, Island of Hawaii

Arsenical herbicides were used extensively for emergent weed control in Hawaiian sugar cane cultivation from 1913 to about 1950. As a result, surface soil arsenic concentrations average 280mgkg^-1 across more than 60km² of former sugar plantation land in the eastern portion of the Island of Hawaii. This study was conducted to elucidate the relationship between soil properties and arsenic bioaccessibility in the iron-rich volcanic soils. Soils are predominantly Andisols, formed by weathering of basaltic lava and tephra, with pedogenic solid phases consisting of short-range order iron oxyhydroxides, allophane-like aluminosilicates, and metal-humus compounds. These reactive solid phases strongly adsorb oxyanions, such as phosphate and arsenite/arsenate. High arsenic sorption capacity limits desorption and vertical migration within the soil column and prevents contamination of the underlying groundwater aquifer, despite high arsenic loading and precipitation rates. In vitro arsenic bioaccessibility, as measured by the SBRC gastric-phase test, ranges from 2% to 35% and averages 9% of total arsenic. Bioaccessible arsenic is higher in less weathered soils (Udifolists, Typic and Lithic Hydrudands) and lower in more weathered ash-dominant soils (Acrudoxic Hydrudands). Soil weathering indicators, such as reactive iron content, are strong predictors of arsenic bioaccessibility. Based on evidence from soil mineralogy, geochemistry and arsenic speciation, as well as limited soil arsenic bioavailability/bioaccessibility comparisons, risks to human health from direct contact (soil ingestion) are significantly reduced by low arsenic bioaccessibility. Nonetheless, some soils within former sugar cane cultivation areas contain bioaccessible arsenic concentrations exceeding Hawaii Department of Health risk-based action levels, and will require mitigating actions. Even higher levels of soil arsenic contamination have been identified at former pesticide storage and mixing areas, but are generally of localized extent.