Soil and stream water chemistry variations on acidic soils. Application of a cation exchange and mixing model at the catchment level

Abstract

A wide database on soil matrix, soil water and stream water from the Birkenes catchment in southern Norway is used in evaluating a model based on cationic exchange reactions and mixing of end-members. It is assumed that the end-member composition is established during flow through small sized pores, and that this water is transported and mixed in regions of the soil with larger pore sizes without new equilibration with the solid phase. Synthetic data based on reasonable mixing regimes reveal patterns rising from the underlying mechanisms. It is shown that the average composition of mixed end-members displays ionic equilibrium relationships similar to those in the end-members themselves if three conditions hold: (1) the end-member composition is randomly distributed, (2) there are numerous end-members, and (3) there is some regularity governing their mixing. Field soil water data display a large variation in ionic composition for all the ions considered (Ca 2+ + Mg 2+, Al 3+ and H +) with the largest range seen for the E horizon, while the solid-soil data indicate that the ion exchange surfaces are most variable in the O horizon. The results show that the simple mechanisms used in the model are a possible explanation to the observed data. The heterogenous nature of a catchment like Birkenes, and the structure seen in the soil water data, support the application of lumping techniques within comprehensive catchment models, e.g. in MAGIC (B.J. Cosby, R.F. Wright, G.M. Hornberger and J.N. Galloway, 1985. Modelling the effects of acidic deposition: assessment of a lumped-parameter model of soil water and stream water chemistry. Water Resour. Res., 21: 51–63) and the Birkenes model (N. Christophersen, H.M. Seip and R.F. Wright, 1982. A model for stream water chemistry at Birkenes, Norway. Water Resour, Res., 18: 977–996). The more integrated in space a signal is, the more confidence may be put into such lumping of characteristics. Then the lumped ionic exchange reactions are best described by functions like the theoretical relationships for the end-members.