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As part of a study to determine the magnitude of atmospheric inputs of materials into the lakes of central Ontario, a four-station network of bulk deposition and wet-only precipitation samplers was established in the Muskoka-Haliburton and Sudbury regions to determine the deposition (rag m -2 yr-l) and volume weighted concentration (gg 1-1) of Pb, Cu, Ni, Zn, A1, Mn and Fe in precipitation. Large temporal variations in the monthly deposition of all metals were observed. The variations for Cu and Ni exhibited seasonal patterns which could be attributed to a combination of source and wind direction factors. Concentration and deposition of all metals at Muskoka-Haliburton were generally as low or lower than median North American values from the literature. At Sudbury, the large local smelting industry contributed to the elevated Cu, Ni, Zn, and Fe deposition measured in the region (up to two orders of magnitude larger than Muskoka-Haliburton); A1 and Mn values were not elevated. Deposition ofCu, Ni and Fe was inversely related to distance from the largest point source at Sudbury. The importance of dry deposition is greatest at Sudbury where dry inputs of Cu, Zn, A1, and Fe generally exceed wet inputs. In contrast, wet deposition of metals at Muskoka-Haliburton predominates over dry. Calculation of an enrichment factor (normalized against Mn) showed that the levels of Pb, Cu, Ni and Zn observed in the precipitation of central Ontario require an additional non-crustal source (either natural or anthropogenic) for explanation.* For the purposes of this paper, 'heavy metals' will be functionally defined to include all metallic elements not present as major cationic species in surface waters.Water, Air, and Soil Pollution 15 (1981) 127-152. 0049-6979/81/0152-0127503.90.

Precipitation in south-central Ontario, with mean pH between 3.95 and 4.38 and frequent event pH's of < 4.0, is more acidic than that in the Sudbury, Ontario, region and in Scandinavia, and as acidic as that in the northeastern USA. As in New England, a major component of the total acidity is strong mineral acid. Because most lakes in south-central Ontario have very low buffering capacities (10–200 μeq∙L−1), high acid loadings are likely to lead to acidification of many lakes and streams in the forseeable future. Although it is not yet possible to estimate the time required for this to happen, there is already some evidence of declining buffering capacity in a few lakes. Key words: acidic rain, buffering capacity, acidic lakes, precipitation, alkalinity

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