Evaluation of the use of soil ion exchange properties for predicting streamwater chemistry in upland catchments

The potential of soil ion exchange chemistry for predicting streamwater quality is evaluated using soil and streamwater chemical data from ten upland catchments in NE Scotland. The study catchments vary from those dominated by acid hill peats, alpine soils and podzols to those dominated by more base-rich soils, including cambisols and gleysols. Soil and streamwater chemical data combined with precipitation and parent material chemistry are also used to investigate sources and pathways of water movement. In all soils studied, Ca and Mg are the dominant exchangeable base cations in the surface soil horizons. In most soils, Na becomes increasingly important on the exchange complex with depth down the soil profile. Plots of the relative proportions of Na:Ca:Mg in streamwater show that, during periods of high discharge, streamwater chemistry tends to become relatively more Na-rich compared with Ca and Mg. Using triangular diagrams, streamwater chemistry can be described as a mixture of geochemically distinct packages of water derived from precipitation inputs, specific parent materials and key soil horizons, although spatially important soils within catchments may be relatively unimportant in controlling streamwater chemistry. Changes in streamwater chemistry at high flow can be explained by dilution of water derived from groundwater sources or the B/C horizon, either with laterally flowing water from the upper soil horizons, or with precipitation. In conclusion, changes in the relative proportions of Na:Ca:Mg in streamwater during storms suggest that precipitation chemistry may play a greater role than hitherto suggested in modifying solute chemistry during periods of high how.