Acid-sensitive waters of the English Lake District: a steady-state model of streamwater chemistry in the upper Duddon catchment

Data on deposition and streamwater chemistry, obtained for the upper catchment of the River Duddon in the 1970s and 1980s, are reviewed. These data, together with soil chemical data, are used to deduce key processes in the deposition-catchment interaction, the analysis being based on current concepts of acidification. The processes are incorporated into a steady-state model that allows streamwater compositions to be calculated. The large baseflow pH range (5-7) of Duddon streams is accounted for in the model by a range of base cation weathering rates. Other processes invoked are evapotranspiration, the uptake of nitrogen by plants, dissolution of Al(OH)(3) in the mineral soil, precipitation of Al(OH)(3) in the baserock zone and in streamwater, Al(3+) hydrolysis, and reactions of the carbonate system. Both cation exchange and sulphate adsorption are ignored, because they are assumed to influence rates-of-change between steady states, but not steady-state water compositions per se. The model can be used to estimate variations in streamwater composition with flow. Model calculations suggest that a 50% decrease in depositional acidifying components (sulphur oxides and NH(4)(+)) would result in increases of up to 1 pH unit in streamwaters with present-day baseflow pH values of 5 or less. It appears that water quality in the upper Duddon is currently more sensitive to inputs of NH(4)(+) than of H(2)SO(4). To improve the reliability of model predictions, more information is required on (a) the pH dependence of base cation weathering, (b) transformations involving nitrogen, (c) aluminium chemistry and (d) partial pressures of CO(2) in soil and baserock.