QUEBEC LAKE SURVEY .2. ORIGIN AND EXTENT OF ACIDIFICATION

Over 95% of the province of Quebec is located on the Canadian Shield where the bedrock offers a very low acid neutralizing capacity. Some areas of Quebec are more vulnerable than others because of shallow soils or soils presenting very low carbonated content. Lakes in these areas tend to be more acidic and more sensitive than elsewhere on the Precambrian Shield. There are two major areas showing high sensitivity (ANC less-than-or-equal-to 100-mu-eq L-1): the southwestern portion of Quebec and the easternmost part of the province. In the former, lake acidity is mainly driven by SO4 deposition, while in the latter area, organic acidity appears to be the main controlling factor. The origin of acidification can be related to the pattern of SO4 deposition, the higher loadings being measured in the southwestern portion of Quebec. These depositions follow a west to east decreasing gradient as they are mostly related to sources located west and southwest of Quebec.