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 280 mg kg−1 across more
than 60 km2 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.
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