Recovery is defined as the return to pre-industrial levels of acidity and other components that counteract acidity. Great Britain was the only region that did not show a decrease in sulfate concentrations. Reduced emissions don't automatically translate to immediate improvement in streams, lakes and rivers, though. In Maine, for example, some waters continued to acidify even though sulfuric acid concentrations in rain decreased. Recovery in alkalinity, expected in regions where sulfate concentrations declined significantly, was observed in all three regions of Europe, especially in the 1990s.
Alkalinity is a marker for the ability a body of water had to resist damage from acid rain.
The scientists speculate that it is possible that the supply of acid neutralizing chemicals in rocks and soils in the other Northern Hemisphere regions has been depleted by decades of acid rain. The research was conducted under the auspices of the United States Environmental Protection Agency.