TIME TREND OF WET DEPOSITION ACIDIFYING POTENTIAL AT 5 ECOLOGICAL MONITORING SITES IN EASTERN CANADA 1981 - 1993

The Acidifying Potential (AP) of wet deposition has been previously de fined by the expression AP = SO4- - [Ca++ + Mg++]. The time series ove r the period 1981 to 1993 for sulphate, calcium plus magnesium and AP concentrations are examined at five ecological monitoring research sit es in the eastern half of Canada. At all sites there was a reduction i n the concentrations of all 3 parameters over the whole period. As a r esult the reduction in AP was between 70 and 80% of the sulphate reduc tion at the 4 sites where precipitation acidity is highest. The annual wet deposition of AP showed a substantial decline in the first half o f the period, but has levelled off in the second half, in spite of con tinuing sulphate decline. In assessing the impact of acidic deposition on surface water systems it is, thus, important to account for the co unteraction of sulphate decreases by base cation decreases. This is pa rticularly important for simulation modelling, either diagnostic using past data, or predictive using future sulphur dioxide emission scenar ios, where in general cations have been assumed to remain constant.