Water flow paths and hydrochemical controls in the Birkenes Catchment as inferred from a rainstorm high in seasalts

At Birkenes, a small forested catchment with acidic soils in southernmost Norway, acid rain has resulted in high stream water H+, Al, and SO4 concentrations. Recent studies have revealed the complexity of the Al chemistry in Birkenes stream water, as inorganic Al is not regulated by one single solubility control. It has been hypothesized that this is due to the dynamic nature of water flow paths and the different Al solubilities in surface soils and subsoils. In this study we tested the flow path hypothesis as well as cation solubility controls, using soil solution lysimetry, before and after a storm event, exceptionally high in sea salts. Results indicate that considerable lateral flow through the organic surface layers to the stream did occur during rainstorms on nearly water‐saturated soils. In the organic surface horizons, Al concentrations were reduced and controlled by cation exchange, whereas in the mineral B horizons, Al was more soluble and close to equilibrium with gibbsite (pKs = −9.0 at 10°C).