Thesis Abstracts 2001

Characterization of Arsenic in a Short Terrestrial Food Chain Yellowknife, Northwest Territiories.

By: Hough, Christopher M.A. M.Sc.(Enviro. Sc.)

Supervisors: Dr. Kenneth J. Reimer and Dr. Iris Koch.

Abstract

The city of Yellowknife, Northwest Territories, Canada, is host to two active gold mining operations, the Miramar Con and Giant mines, and one inactive operation, the Negus mine. Arsenic, associated with gold bearing refractory and non-refractory ores as arsenopyrite (FeAsS), is generated in mine waste during the gold extraction and refining processes. As a result of over 60 years of mining activity, several large, well characterized waste (or tailings) ponds have been produced in the vicinity of the mines.

Previous studies have shown that arsenic is present at elevated levels in soil, tailings and biota collected from or near Yellowknife’s tailings ponds. In September 2000, deer mice (Peromyscus maniculatus), plant and soil/tailings samples were collected from these areas—collecting these samples constituted a terrestrial food chain that can be used to assess ecological health.

Arsenic concentration and form (species) in soils/tailings, plants and deer mouse tissues (including liver, skin, carcass, kidney and stomach contents) were determined by neutron activation analysis (NAA), hydride generation-atomic absorption spectrometry (HG-AAS), high performance liquid chromatography-hydride generation-atomic absorption spectrometry (HPLC-HG-AAS) and high performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS). Total arsenic concentrations in soils/tailings ranged from 54.1 to 14100 parts per million (ppm) with a mean of 1630 ppm. In biota samples, total arsenic concentrations ranged from 0.56 to 120 ppm (wet weight) for plants (median of 3.2 ppm), and from non-detectable to 72 ppm (wet weight) for tissues (median of 2.7 ppm). Furthermore, arsenic concentrations in all sample matrices from the Con and Giant mines were higher than those from the background area, which is most likely the result of greater arsenic exposure.

Relationships between arsenic concentrations in soils, plants and tissues were generally weak, owing much to the limited bioavailability of arsenic in soil and plants. Bioconcentration factors for plants (from soil) and tissues (from soil and plant) were also generally low (less than one), which indicates that arsenic is not bioconcentrating in this food chain. BCFs (from soil) for individual plant species were not found to be significantly different, therefore no difference was shown between plants with respect to the ability to take up arsenic from soil.

Speciation of selected biota samples showed that the predominant species in plants were arsenate (As V) and arsenite (As III), while arsenite and dimethylarsinic acid (DMA) were predominant in tissues. Arsenobetaine and trimethylarsine oxide were also identified in smaller quantities in several liver, kidney and skin samples, but were not found in any plant samples. The determination of arsenobetaine in certain tissues is thought to be the result of a previously unknown metabolic pathway or due to ingestion of arsenobetaine-containing forage. The presence of the predominately more toxic inorganic arsenic forms in plants and As(III) in tissues indicates that these plants and deer mice may be at risk.

A model was used to quantify the potential arsenic exposure to mice in this area. Comparisons were made between estimated body burdens and those actually determined in mice, as well as between daily doses of arsenic from arsenic-containing forage and known toxicological parameters. Estimated body burdens were much higher than the actual body burdens (mean of 560 times higher), and are most likely due to differences in diet, the metabolism of arsenic, or an adaptation to the arsenic-rich environment of Yellowknife. All daily doses, with the exception of that for the background deer mice, exceeded the lowest LD50 value for oral arsenic exposure if 100 % of the arsenic ingested was absorbed into the mice. No dose exceeded this value, though, when the doses were recalculated using bioavailability values suggested by the exposure model. These two findings indicate that deer mice collected in this study may not be affected greatly by the arsenic levels in soil and plants. Considering that the deer mice collected in this study could represent a “worst case” scenario for mammalian arsenic exposure, the low arsenic body burden of this bio-monitor implies that other mammals and higher trophic level consumers (including humans), which are exposed to much less arsenic, are at even less risk from the arsenic levels observed.

Ultimately, the data included in this thesis will form a part of a larger study to evaluate risk due to the presence of arsenic in the Yellowknife area. This new study will seek to determine risk to both the environment as well as to humans.