Acid precipitation-related chemical trends in 18 rivers of atlantic Canada — 1983 to 1992

Using non-parametric techniques, we studied water chemistry changes from 1983 to 1992 in 18 rivers located at the extreme northeastern portion of North America. This period was marked by a decrease in sulfate (SO ₄ ^– ) production in eastern Canada from 1982 to 1986, followed by a levelling off from 1986 to 1992. Nitrate (NO ₃ ^– ) production and deposition generally increased over this whole period. We used two time windows, 1983 to 1989 and 1983 to 1992, to determine if changes in river acidification variables occurred over the ten year period. We found significant trends of increasing pH and acid neutralization capacity (ANC) concentrations at eight sites using both time windows, while SO ₄ ^–2 increased at five and seven sites during the same two time periods. Nitrate concentrations showed few consistent trends, while base cations showed increases in the earlier part of the data set and total organic carbon (TOC) showed long-term decreases. There were few significant trends in hydrogen (H⁺), and a slight decrease in SO ₄ ^–2 exports as opposed to increased concentrations over the same period. We attribute the discrepancy between SO ₄ ^–2 concentration and export trends to be due to evapotranspiration in the basins, causing a concentration of ions in water. Nitrate and calcium exports showed no trends, while TOC decreased at four or five sites, depending on the time window used. Longer data sets tended to produce more detectable trends. Overall, water chemistry in the region is showing the effects of reduced SO ₄ ^– loads and is not yet being affected by the increases in NO ₃ ^– deposition.