SOIL AND STREAM WATER CHEMISTRY VARIATIONS ON ACIDIC SOILS - APPLICATION OF A CATION-EXCHANGE AND MIXING MODEL AT THE CATCHMENT LEVEL

A wide database on soil matrix, soil water and stream water from the B irkenes catchment in southern Norway is used in evaluating a model bas ed on cationic exchange reactions and mixing of end-members. It is ass umed that the end-member composition is established during flow throug h small sized pores, and that this water is transported and mixed in r egions of the soil with larger pore sizes without new equilibration wi th the solid phase. Synthetic data based on reasonable mixing regimes reveal patterns rising from the underlying mechanisms. It is shown tha t the average composition of mixed end-members displays ionic equilibr ium relationships similar to those in the end-members themselves if th ree conditions hold: (1) the end-member composition is randomly distri buted, (2) there are numerous end-members, and (3) there is some regul arity governing their mixing. Field soil water data display a large va riation in ionic composition for all the ions considered (Ca2+ + Mg2+, Al3+ and H+) with the largest range seen for the E horizon, while the solid-soil data indicate that the ion exchange surfaces are most vari able in the 0 horizon. The results show that the simple mechanisms use d in the model are a possible explanation to the observed data. The he terogenous 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 effec ts of acidic deposition: assessment of a lumped-parameter model of soi l water and stream water chemistry. Water Resour. Res., 21: 51-63) and the Birkenes model (N. Christophersen, H.M. Seip and R.F. Wright, 198 2. A model for stream water chemistry at Birkenes, Norway. Water Resou r, Res., 18: 977-996). The more integrated in space a signal is, the m ore confidence may be put into such lumping of charateristics. Then th e lumped ionic exchange reactions are best described by functions like the theoretical relationships for the end-members.