Reversal of acidification in upland waters of the English Lake District

Water chemistry data are reported for five high-altitude standing waters in the English Lake District, with current average pH values in the range 5–7. The waters show long-term increases in pH, ranging from 0.3 to 0.8 pH units, between 1974 and 1997. The pH of Devoke Water, which was acidified only mildly, has returned to values estimated for the pre-industrial period (1850 and earlier). Alkalinity in Devoke Water increased from ca 20 μeq litre−1 in the early 1980s to ca 70 μeq litre−1 in the 1990s, and alkalinities in three of the other waters have increased by ca 20 μeq litre−1 since the 1970s. For the two intensively monitored sites (Devoke Water and Levers Water), significant decreases in the concentration of non-marine sulphate are demonstrated, which have taken place concurrently with decreases in the atmospheric deposition of pollutant sulphur. Approximate calculations suggest that the catchment of Levers Water was a sink for sulphur in the 1970s, and that the catchment of Devoke Water may currently be a source of sulphate. For neither Devoke Water nor Levers Water is there evidence of a long-term decrease in the concentration of non-marine base cations.