Water Flow Paths and the Spatial Distribution of Soils and Exchangeable Cations in an Acid Rain‐Impacted and a Pristine Catchment in Norway

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Abstract
The dynamic pattern of soil water transport is a major factor in determining the chemistry of streamwater. In the acidified Birkenes catchment (southernmost Norway) the streamwater chemistry is, to a first approximation, explained by mixing solutions from the forest floor, the B horizon and the deep peat, in various proportions depending on the hydrological conditions. Paradoxically, a direct physical contact between the forest floor and the B horizon on the one hand and the stream on the other is lacking, as the stream banks largely consist of peats. To investigate this paradox, soils and their levels of exchangeable cations were studied in a 100 m × 100 m grid. Results indicate that the exchange sites of the surface peat along the stream are significantly enriched in Al, probably due to return flow of Al‐rich B horizon water. This view is supported by the similarity of the solution chemistry in surface peats and B horizons. Exchangeable base cations dominate in the forest floor upslope. Forest floor solutions, an important component of streamwater during intensive storms, are depleted in Al and may bypass the A‐enriched surface peats via ephemeral flow channels. A parallel study in a pristine catchment in mid‐Norway shows a similar accumulation of Al in return flow areas. This indicates that acid deposition is not a prerequisite for elevated levels of exchangeable Al in the surface organic layers of return flow areas.

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