Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Arsenic
Arsenic is a metalloid with oxidation states of -3, 0, 3, and 5. It is widely distributed throughout the Earth's crust and is a major constituent of at least 245 mineral species. Natural sources of arsenic include volcanically derived sediment, sulphide minerals, and metal oxides. The most common arsenic mineral, globally, is arsenopyrite, which is commonly found in many vein gold deposits, such as those of Yellowknife. The most common source of arsenic in Canada is sulphide minerals. These minerals are typically composed of 0.02-0.5% arsenic; however, certain pyrite minerals may contain up to 5% arsenic (Hindmarsh and McCurdy, 1986; Pellerin, 2003).
Arsenicals are used commercially and industrially as alloying agents in the manufacture of transistors, lasers, and semi-conductors, as well as in the processing of glass, pigments, textiles, paper, metal adhesives, ceramics, wood preservatives, ammunition, and explosives. They are also used in the hide tanning process and, to a limited extent, as pesticides, feed additives, and pharmaceuticals, including veterinary drugs.
The principal sources of arsenic in ambient air are the burning of fossil fuels (especially coal), metal production, agricultural use, and waste incineration. Arsenic is introduced into water through the erosion and weathering of soil, minerals, and ores, from industrial effluents, and via atmospheric deposition (Hindmarsh and McCurdy, 1986; Hutton and Symon, 1986).
In surface water, arsenite (+3 valence) and arsenate (+5 valence) form insoluble salts with cations (usually iron) that are dissolved or suspended in the water. These particles generally settle out in sediments. This cleansing process occurs to a lesser extent in deep groundwater because of higher pH levels and lower iron concentrations (Hindmarsh and McCurdy, 1986).
Arsenic occurs in different forms (organic vs. inorganic) and valences depending upon the pH and oxidation potential of the water. In well-oxygenated surface waters, pentavalent arsenic (arsenate) is generally the most common species present (Irgolic, 1982; Cui and Liu, 1988); under reducing conditions, such as those often found in deep lake sediments or groundwaters, the trivalent species (arsenite) is the predominant form (Lemmo et al., 1983; Welch et al., 1988).