A model of surface water acidification in Cumbria and its uses in long‐term research

Abstract

SUMMARY. 1. A description is given of a simple process‐based model of deposition‐catchment interactions in areas of Cumbria underlain by rocks of the Borrowdale Volcanic Series. The processes considered are evapotranspiration, nitrogen uptake by plants, dissolution and precipitation of A1(OH)3, basecation weathering, hydrolysis of Al3+, reactions of the carbonate system, and water flow routing. Model parametersare estimated from 1980s data for deposition and streamwater chemistry. It is shown that the‐ model is also applicable to tarns (small lakes). Analysis of 1970s streamwater data highlights uncertainties about deposition concentrations of NH4+ and catchment transformations of nitrogen.2. Using estimates of past deposition compositions, past compositions of tarn waters are estimated and indicate declines in pH I of up to 1 unit between c. 1850 and c, 1950, These changes are similar to those estimated independently from analysis of sediment diatoms. The model also shows that the historical pH trends are consistent with other paiaeolimnological evidence that Cumbrian soils have been acid for several thousand years. The model output is also consistent with direct evidence suggesting that the chemical compositions of Cumbrian tarns have altered little during the past 30 years.3. The model is used to estimate water compositions during high‐flow episodes at various times in the past, and shows that even a small increase in depositional acidity would have made high‐flow waters sufficiently acid and aluminum‐rich to limit faunal abundance. This is consistent with the limited amount of observational evidence that is available.4. The model can be used (a) for prediction, given scenarios for future deposition compositions or catchment behaviour, and (b) as a framework for planning future studies and interpreting data. Further work on the model should improve the reliability of its parameters, find better descriptions of processes dealt with in the current version, and introduce processes currently ignored. The present analysis indicates that fieldwork is needed to study nitrogen transformations, soil properties and biological responses to changes in streamwater chemistry.