Natural Sources
Sulphur is a non-metallic element that occurs naturally in numerous minerals, including barite (BaSO4), epsomite (MgSO4·7H2O), and gypsum (CaSO4·2 H2O). Hexavalent sulphur combines with oxygen to form the divalent sulphate ion (SO42-). The reversible reaction between sulphide and sulphate in the natural environment is often referred to as the "sulphur cycle." Natural sources of sulphur include volcanoes, decomposition and combustion of organic matter, and from sea salt over the oceans. Particles of sea salt formed by the breaking of myriads of bubbles are an important source of atmospheric sulphate. The atmosphere is the main vehicle for transport of sulphur from various sources (Kellogg et al. 1972).
Anthropogenic Sources
Sulphates are discharged into the aquatic environment in wastes from industries that use sulphates and sulphuric acid, such as mining and smelting operations, kraft pulp and paper mills, textile mills and tanneries. Iron pyrite (FeS) may be leached from abandoned coal mines and the sulphide ions converted in surface waters to sulphates. Sulphates are also released during blasting and the deposition of waste rock in dumps at metal mines. This is a significant source of sulphate generation in British Columbia (Jarman, personal communication). The burning of fossil fuels is also a major source of sulphur to the atmosphere. Most of man's emissions of sulphur to the atmosphere (about 95%) are in the form of SO2. Sulphate fertilizers are identified as a major source of sulphate to ambient waters (Kellogg et al. 1972).
Sulphate Concentrations in Receiving Waters
Freshwater
Sulphate concentrations in Canadian Lakes typically range from 3 to 30 mg/L according to Katz (1977). In a survey of river waters in western Canada, excluding the province of BC, sulphate concentrations ranged from 1 to 3040 mg/L; most concentrations were below 580 mg/L (Environment Canada, 1984). Dissolved sulphate data for BC freshwaters, collected for acid rain assessment (Phippen et al. 1996), and for the Canada-BC Water Quality Monitoring Agreement (BC MELP and EC, 1996a-ac) are compiled in Table 1. Mean concentrations range between about 2 and 30 mg/L for most lakes and rivers throughout the province. However, some lakes in the Cariboo Region and in Richter Pass near Osoyoos have unusually high natural sulphate levels in the thousands of mg/L (Warrington, personal communication). Seasonal fluctuations in dissolved sulphate concentrations are obvious in most rivers, with low concentrations during freshet and elevated concentrations during the low winter flow periods as shown for the Bear River at Stewart BC (Figure 1). Also, in a study of rainfall-induced changes in chemistry of a British Columbia coastal creek, Whitfield et al. (1993) noted decreases in sulphate concentrations (1.7 to 1.0 mg/L and 1.9 to 1.5 mg/L) during two rainfall events. Concentrations returned to pre-storm conditions over a period of several days.
Seawater
Seawater contains about 2700 mg/L sulphate (Hitchcock, 1975) and it has been estimated that about 1.7 million tonnes of sulphate are added annually to the Canadian atmosphere from sea spray (Katz, 1977).