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Licentiate thesis, 1999

Control of runoff chemistry by processes in the near-stream zone

Fölster, Jens


The near-stream zone has been identified as an important control of runoff chemistry. In northern forested areas, near-stream zones have been found to be both natural sources and sinks for acidity. The near-stream zone can be a buffer against acidity by cation exchange and sulphate reduction. Oxidation of sulphide and leaching of organic acids are natural sources of acidity from the near-stream zone. A relatively high pH and base saturation in the near-stream zone favours nitrogen cycling processes. Hence, the near-stream zone has a potential to control nitrogen leaching from the catchment. Nitrogen can be retained by denitrification, microbial immobilisation and plant uptake. On the other hand, mineralisation, nitrification and leaching of organic matter can make the near-stream zone into a source for nitrogen to the stream water. The geomorphology and the hydrological flow paths are likely to be crucial for the function of the near-stream zone. This thesis reports the results from an investigation on a small forested catchment in central south Sweden, designed to clarify the effect of processes in the nearstream zone on acidity and nitrogen chemistry in runoff. Both hydrological and water chemistry data suggested that runoff was dominated by superficial ground water, which agrees with what is commonly found in Swedish till soils. The Kindla catchment is heavily affected by acidification with a pH in the stream water between 4.4 and 4.6. Despite the severe acidification, there were no indications of forest decline at the Kindla catchment that retains nitrogen in deposition efficiently. The leaching of inorganic nitrogen was only 3% of bulk deposition. There were no indications that the nearstream zone acted as a buffer against acidity or nitrogen leaching from upiand soils. The main function of the near-stream zone at Kindla with respect to runoff chemistry was rather to serve as a source for organic acids and organic nitrogen and, to a small extent, nitrate and ammonium. The contribution of organic anions to acidity was however overshadowed by a high sulphate concentration, except during a spring flood episode when organic matter was flushed out. Indications of sulphate reduction in the ground water were found but this probably had no influence on stream water chemistry. Ammonium was almost only found during the spring flood episode, when 72% of the annual transport of ammonium in the stream took place. Nitrate was found more frequently, but mainly at concentrations below 100 µg L⁻¹. This nitrate probably originated in the near-stream zone. The results contrasted to what was found by other authors with respect to the intluence of the near-stream zone on stream water chemistry. This would imply that since the function of the near-stream zone appears to vary greatly between catchments, there is a need for further catchment studies that also account for within catchment variation in order to achieve a more general picture of the impact of near-stream zones on runoff chemistry.


watersheds; runoff; forest soils; acidification; eutrophication; air pollution; sulphur; nitrogen; sweden

Published in

Rapport / Sveriges lantbruksuniversitet, Miljöanalys
1999, number: 1999:5
Publisher: Department of Environmental Assessment, Swedish University of Agricultural Sciences

Authors' information

Swedish University of Agricultural Sciences, Department of Environmental Assessment

UKÄ Subject classification

Oceanography, Hydrology, Water Resources
Environmental Sciences

URI (permanent link to this page)