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Doctoral thesis, 2001

Critical loads of acid deposition for surface water

Rapp, Lars


Cutting down emissions of sulphur and nitrogen compounds is the most obvious long-term solution to the acidification problem. The critical load concept has proved a very useful basis for the European negotiations about how much the emissions have to be reduced in order to protect the environment. Great reductions have been achieved and in the near future we are facing a situation where we have to decide to what extent further emission reductions are needed. To be able to make such decisions, we have to take into account the uncertainties of the critical load concept. Otherwise, incorrect decisions may result in further acidification or, on the contrary, economical investments without due cause. The main objectives of this thesis are to validate and elucidate the model used in Sweden to determine the critical load of acidity for surface waters i.e. the steady state water chemistry model (SSWC) as well as to explore a potential alternative (PROFILE). As a way of testing the models, diatom reconstructions of pre-industrial lake chemistry are used. The acidification status of Swedish surface waters was also assessed with particular attention to uncertainties. The SSWC model’s prediction of pre-industrial pH did not correspond well with the diatom reconstructions for pH values below 6.5. The discrepancies can be related to short-term fluctuations in modern lake chemistry. Applying the F-factor (a component of SSWC) to time series of runoff chemistry generated by the SAFE model suggests that the F-factor worked best during the acidification phase when soil processes buffer incoming acidity. However, the empirical functions for estimating F from contemporary lake chemistry are not well adapted to the recovery phase when the F-factor turns negative due to recovery processes in the soil. Exploring the PROFILE model as an alternative to SSWC revealed that calculating critical loads for individual lakes is not feasible when using the PROFILE model due to lack of catchment specific input data. However, the use of soil parameter distributions is possible since the critical loads used in international negotiations are not for individual sites but for distributions of ecosystems in large areas. While direct consideration of soil properties instead of lake chemistry is promising, there are problems of hydrology, data acquisition and spatial integration which will have to be overcome if PROFILE is to be aviable alternative for calculating surface water critical loads.


critical loads; surface waters; lakes; acidification; steady state water chemistry; SSWC; F-factor; palaeolimnology; PROFILE; SAFE; Sweden

Published in

Acta Universitatis Agriculturae Sueciae. Silvestria
2001, number: 207
ISBN: 91-576-6091-3
Publisher: Department of Environmental Assessment, Swedish University of Agricultural Sciences

Authors' information

Rapp, Lars
Swedish University of Agricultural Sciences, Department of Environmental Assessment

UKÄ Subject classification

Environmental Sciences

URI (permanent link to this page)