| Summary: | The occurrence of episodic acidification in Canadian streams, lake waters and shallow groundwaters has been reviewed, and the controlling mechanisms identified "Episodes", which are periods of depressed alkalinity during hydrological events, have been studies mainly in southeastern Canada, and occur at all sites where there is sufficient time resolution of the observations, viz Ontario, Quebec and Nova Scotia. n "Alkaline episode", where acidity decreases during an event, has been reported for one lake in the Canadian Arctic. here is a bias towards the examination of episodes stimulated by snowmelt or rain-on-snow, since rainfall-stimulated episodes are poorly documented. re-event, rather than event, water dominates runoff during episodes. or this reason, biogeochemical reactions and the hydrological flowpaths in operation through the vadose and saturated zones are the principal controls on chemical characteristics of episodes. ost episodes are dominated by base cation "dilution" in circumneutral systems, and "increase in strong acid anions" in acidic systems. pisodes dominated by nitrification or organic acids or stimulated by sea salt input are rare or have not been documented. irect input of event water may dominate only during particular circumstances at snowmelt. hen, direct chemical inputs from lake ice and lake snow cover may be of importance in some systems.
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