Trends in surface water acidification in Europe and North America (1989-1998)

During the last 20 years, emission reductions in Europe and North America have resulted in decreased atmospheric S-deposition of up to 50%, while N-deposition has stayed almost constant. Data from 98 ICP Waters sites were tested for trends in concentrations of major chemical components for the 10-year period 1989-1998 using the nonparametric seasonal Kendall test. The sites were grouped into regions and types for meta-analysis. All of the regions had highly significant downward trends in SO₄ ^2-* concentrations. Nitrate concentrations, on the other hand, show no regional patterns of change. Concentrations of base cations declined in most regions. All regions showed tendencies of increasing DOC. The low ANC sites showed the largest rates of recovery: Neither the high NO₃ ^- or low NO₃ ^- groups of sites exhibited significant trends in NO₃ ^- concentrations. Alpine (non-forested) sites show clear and consistent signals of recovery in ANC and pH, and appropriate (relative to SO₄ ^2-* trends) rates of base cation decline.