Acid Deposition

Terrain sensitivity and thresholds

Ecosystems have different sensitivities to acid depending upon their geology and soils. Thus the maximum level of acid deposition that terrain can withstand without harming ecological integrity, called the “critical load,” differs across ecosystems.13 Acid-sensitive terrain is generally underlain by slightly soluble bedrock and overlain by thin, glacially derived soils14 and has less buffering capacity.

Areas where the critical load has been exceeded in the Boreal Shield
Number of units above critical load, 2009
Map: areas where the critical load has been exceeded in the Boreal Shield. Click for graphic description (new window).
 
Source: adapted from Jeffries et al., 201018

Critical loads can be exceeded either when extremely sensitive terrain receives low levels of acid deposition or when less sensitive terrain receives high levels of acid deposition. The inset map shows where critical loads have been exceeded in the Boreal Shield Ecozone+. The potential for critical loads to be exceeded in northwest Saskatchewan is also a concern due to the high degree of acid sensitivity of many of the lakes in this area (68% of 259 lakes assessed in 2007/2008) and its location downwind of acidifying emissions from oil and gas developments.15 Similarly, transportation-related sulphur emissions in southwest B.C. are an emerging issue, with terrestrial critical loads exceeded in 32% of the Georgia Basin in 2005/2006.16

Sensitivity of terrain to acidity
Critical Load Index, 2008
Yellow through red categories are considered acid sensitive terrain
Map: sensitivity of terrain to acidity. Click for graphic description (new window).
Source: adapted from Jeffries et al., 201017