EPISODIC ACIDIFICATION OF FRESH-WATER SYSTEMS IN CANADA - PHYSICAL AND GEOCHEMICAL PROCESSES

The occurrence of episodic acidification in Canadian streams, lake wat ers and shallow groundwaters has been reviewed, and the contolling mec hanisms identified. 'Episodes', which are periods of depressed alkalin ity during hydrological events, have been studied mainly in southeaste rn Canada, and occur at all sites where there is sufficient time resol ution of the observations, viz. Ontario, Quebec and Nova Scotia. An 'a lkaline episode', where acidity decreases during an event, has been re ported from one lake in the Canadian Artic. There is a bias towards th e examination of episodes stimulated by snowmelt or rain-on-snow, sinc e rainfall-stimulated episodes are poorly documented. Pre-event, rathe r than event, water dominates runoff during episodes. For this reason, biogeochemical reactions and the hydrological flowpaths in operation through the vadose and saturated zones are the principal controls on t he chemical characteristics of episodes. Most episodes are dominated b y base cation 'dilution' in circumneutral systems, and 'increase in st rong acid anions' (particularly sulphate) in acidic systems. Episodes dominated by nitrification or organic acids or stimulated by sea salt input are rare or have not been decumented. Direct input of event wate r may dominate only during particular circumstances at snowmelt. Then, direct chemical inputs from lake ice and lake snow cover may be of im portance in some systems.