Arsenic Concentrations in Quaternary Drift and Quaternary-Tertiary Buried Channel Aquifers in the Athabasca Oil Sands (In Situ) Area, Alberta

Sampling of selected wells from Quaternary drift aquifers and Quaternary-tertiary buried channel aquifers, in the Athabasca Oil Sands (in situ) Area, was conducted between the summer of 1999 and the spring of 2001. The purpose of the sampling program was to obtain high-quality water samples that could be used to better define baseline groundwater chemistry in these geological units.

Twenty-one water samples were collected from Quaternary intertill-intratill aquifers. Three water samples were collected from Quaternary-Tertiary buried channel aquifers. For each of the samples collected, field determinations of pH, oxidation-reduction potential, temperature, conductivity, dissolved oxygen and total alkalinity were determined. Samples were collected by the Alberta Geological Survey and analyzed by various laboratories for concentrations of major, minor and trace elements, organic compounds, as well as stable and radiogenic isotopes. One of the elements included in the water-sample analysis is arsenic.

The majority of the Quaternary intertill-intratill aquifer water samples have arsenic concentrations below the analytical detection limit. Four of the water samples have arsenic concentrations that exceed the analytical detection limit of 0.01 mg/L. Arsenic conditions appear to be controlled by a combination of the following mechanisms: 1) adsorption to oxide and clay minerals; 2) competition between arsenic species and phosphorus ions for adsorption sites; 3) release of arsenic brought about by changes in pH; and 4) proximity to bedrock.

All three samples from the Quaternary-Tertiary buried channel aquifer contained arsenic concentrations above the analytical detection limit. Arsenic concentrations in the Quaternary-Tertiary buried channel aquifer appear to be controlled by a combination of the following mechanisms: 1) competition between arsenic species and phosphorus or bicarbonate ions for adsorption sites; 2) release of arsenic brought about by changes in Eh; 3) dissolution of carbonate or evaporite minerals releasing associated arsenic; and 4) proximity to bedrock.