Source, release mechanism and migration of aqueous phase arsenic in the Fraser River Delta, Vancouver, {BC

Elevated arsenic was identified in soil and groundwater at a historic construction waste disposal site. Upon completion of a preliminary environmental assessment, roofing shingle waste was identified as the source of arsenic. Less certain was the mineralogic form of arsenic in the roofing shingle waste, the mechanism of release to the environment, and whether or not aqueous phase arsenic would reach the aquatic environment of a nearby major river. Others had postulated that vitrified arsenic within roofing shingles was not leachable, and site specific aggressive leachate testing confirmed this. However, elevated arsenic in groundwater (up to 26 mg/L) contradicted this theory and the results of the leachate testing. Our study demonstrated that vitrified arsenic in roofing shingles can be highly leachable, as based on the mineralogic form of arsenic within the shingles, the site specific geochemical conditions, and physical hydrogeology. The source of arsenic was determined using a combination of common petrographic and detailed microscopy methods. The release mechanism to the environment was identified according to the mineralogic form of arsenic in the source material and site-specific geochemical conditions. Surface complexation modeling based on site-specific measurements of natural absorbents, particularly hydrous ferric oxide in the soils allowed quantification of the arsenic sorption capacity within the aquifer. Aqueous speciation and coupled flow-solute transport modeling was carried out to predictively evaluate the fate and transport of aqueous phase arsenic towards the aquatic receiving environment. The results of the study confirmed the mineralogic form and arsenic release mechanism to the environment. Surface complexation modeling demonstrated that the aquifer possessed a substantial adsorption capacity for aqueous phase arsenic. Detailed assessment of the physical hydrogeology also indicated that dilution was an important natural attenuation mechanism. Coupled flow-solute transport modeling predicted that aqueous phase arsenic would not reach the aquatic receiving environment within a 1000 years.