Thesis Abstracts 2003

Arsenic Risk Assessments: The Importance of Bioavailability

By: Christopher Ollson (PhD.)

Supervisors: Dr. K.J. Reimer and Dr. I. Koch

Abstract

Arsenic is a contaminant of concern at some of the largest contaminated sites in North America. It has attracted much attention as a result of documented increases in both cancerous and non-cancerous diseases in human populations exposed to arsenic in drinking water at concentrations greater than 100 ppb. However, there has yet to be a reported incidence in the scientific or medical literature of arsenosis in populations exposed to elevated concentrations in soil.

The focus of this work was to assemble a variety of analytical and statistical approaches to conduct arsenic risk assessments in a new fashion, which may provide an explanation as to why exposures to arsenic contaminated soils do not manifest as risks in ecological or human populations. Yellowknife, NT was selected as the study site for this work, because elevated concentrations of arsenic in the area are of concern to local residents and regulatory authorities. It was the only contaminant of concern in Yellowknife, thus removing any potential confounding influences that may occur with multiple contaminants.

The multi-variant statistical technique, principal components analysis was shown to be a useful tool in segregating naturally impacted and anthropogenically influenced arsenic concentrations in soils. The natural background concentration of arsenic in Yellowknife was determined to range from 3 to 150 ppm, and in some cases as high as 300 ppm. Soils in the Yellowknife area were separated into four soil types based on sample location (likelihood of exposure), organic carbon content, and fractionation of how arsenic was bound in the samples; rock, tailings, mine organic and residential soils. Chemical fractionation of arsenic in soils by sequential selective extraction (SSE) revealed that arsenic was bound very differently in rock, tailings and organic soils; thus suggesting that each soil type had different potenital arsenic mobilities and toxicities. Arsenate (As(V)) was the dominant species of arsenic detected in all soil extractions.

Over 100 soil samples from Yellowknife were subjected to a simulated gastric fluid extraction to measure the bioaccesible fraction of arsenic from soils. Although tailings and rock samples had higher total arsenic concentrations in the samples, organic soils with lower total arsenic concentrations had a higher arsenic bioaccessibility and resulted in similar total arsenic concentrations in the GFE extracts. This suggests that the amount of arsenic that was bioaccessible in the human or mammal gastrointestinal tract is similar from all four soil types examined. It was shown that arsenate, a relatively toxic form of arsenic, was the dominant form (>90%) in all GFE extracts.

The influence of arsenic bioaccessibility in an ecological and human health risk assessments was presented. In both cases the inclusion of site specific soil arsenic bioaccessibilities served to reduce the estimated potenital risk, in some cases below dose levels that would be considered to pose a potential risk to receptors. Probabilistic risk assessments were conducted and confirmed the results of the deterministic approach, but also allowed for a distribution of potential risk to be calculated. The approach taken in these risk assessments may serve to explain why the ecosystem of Yellowknife appears to be in good health and why residents are not suffering from arsenosis.

Although Yellowknife was used as a case study, the incorporation of metal and metalloid bioaccessibilities into human health risk assessment will serve as a better predictor of actual risk at other contaminated sites.