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 uni ts, between 1974 and 1997. The pH of Devoke Water, which was acidified only mildly, has returned to values estimated for the pre-industrial period (18 50 and earlier). Alkalinity in Devoke Water increased from ca 20 mu eq litr e(-1) in the early 1980s to ca 70 mu eq litre(-1) in the 1990s, and alkalin ities in three of the other waters have increased by ca 20 mu eq litre(-1) since the 1970s. For the two intensively monitored sites (Devoke Water and Levers Water), significant decreases in the concentration of non-marine sul phate are demonstrated, which have taken place concurrently with decreases in the atmospheric deposition of pollutant sulphur. Approximate calculation s 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. (C) 199 8 Elsevier Science Ltd. All rights reserved.