Recovery from acidification in the Tillingbourne catchment, southern England: catchment description and preliminary results

Measurements of acid deposition and streamwater chemistry made in 1979 1982 and 1999 2000 are compared for a small, acid-sensitive catchment in Southeast England. The location, geology, soils, vegetation and hydrology of the catchment are described. The catchment is located on an acidic cretaceous sandstone with a low permeability clay sub-stratum. Soils are predominantly podzol and gley, with some mesotrophic peat. The catchment is forested. Mean volume-weighted concentrations in precipitation have changed approximately in proportion to emission changes. SO42 has declined by 61%, H by 75%, both NO 3 and NH 4 by 37% and Cl by 26%. Changes in wet deposition are greater, sulfate deposition declined by 69%, non-marine SO2 by 73%, H deposition by 75%, NO and NH 4 3 4 by 50% and Cl by 41%. Sulfate deposition in throughfall, a surrogate for total deposition measurement, has declined by 82% and non-marine SO42 by 86%. Some of these changes are due to alterations in the tree cover and location of the collectors. In 1979 1982, the flux of NO 3 and NH 4 in throughfall was less than in rainfall, 7.5 compared with 11.3 kg N ha 1 year 1, showing that N uptake by the canopy was greater than dry deposition of these species. However, in 1999 2000, the throughfall flux of N was greater than rainfall, 19.6 compared to 5.7 kg N ha 1 year 1, indicating that canopy uptake is not occurring to the same extent. Surface water was sampled at the same locations in the catchment during the two periods. At the catchment exit, mean pH increased, from 3.93 to 4.21 mg l 1, and SO2 declined from 20.2 to 16.7 mg l 1 Ž18%.. The decrease in SO2 is much less than the reduction in 4 4 deposition, suggesting that the predicted recovery is being delayed by release of sulfur from the soil. In contrast, NO concentrations in the catchment waters increased from 0.22 to 0.52 mg N l 1 Ž133%. despite the reduction in 3 N deposition. NH 4 concentrations were low during both study periods. It is concluded that recovery from acidification is probably occurring, but is possibly being delayed by desorption of soil S. The catchment is also showing signs of increasing N saturation, despite a reduction in N inputs.