Are ion exchange processes important in controlling the cation chemistry of soil- and runoff waters ?

Soil- and stream water elemental concentrations from a subcatchment in the Lake Gårdsjön area have been used to evaluate the importance of ion exchange processes on the transport of cations to aquatic ecosystems. The importance of cation exchange processes in the upper organic and upper B soil horizons was demonstrated using lysimeter water data from a recharge area and soil water flow simulated with SOIL model during winter rain events with high sea-salt concentration. The importance of the hydrological conditions, such as water flow and water pathway, silicate weathering and the ion exchange of Al with H⁺ on the streambed materials in controlling cation concentrations in soil and stream waters are also discussed. With the SAFE model, the contribution of cations from ion exchange by depletion of base cations from the exchange matrixes compared to from weathering was also assessed. SAFE calculations indicate that the release rate of base cation by ion exchange to runoff water has decreased since 1945 and is very low, approx. 0.1 keq/ha per year, at present time as a result of soil acidification due to S and N inputs.