Integrating variations of soil-adsorbed cations into a cation exchange model

Abstract

A computer simulation of cation exchange reactions in heterogeneous soils is provided. It is used to examine the relationships between theoretical cation exchange equations and lumped cation concentrations for acid organic horizons of podzolic, peat and shallow organic soils. The study is based around field data for exchangeable cations at the Birkenes catchment in southern Norway. Field measurements of the relative proportions of soil exchangeable cations show substantial variation and correspond to a wide range of micropore water chemistries. Spatial and temporal heterogeneity is introduced in the simulations to represent natural variation in both hydrological conditions and exchangeable cation proportions. Modelled results show that heterogeneity in soil-adsorbed cation ratios results in highly scattered relationships between cations in the soil-micropore water, with contrasting behaviours at low and high anion concentrations. At low anion concentrations, hydrogen-ion and sodium concentrations are negatively correlated, whilst poor correlations are observed for the more highly charged cations. At high anion concentrations, no pair of cations dominates the total cation charge and relationships are much more scattered. Despite this, any alteration of anion concentration results in a systematic change in average cation concentrations. Similar patterns are observed when further heterogeneity is introduced by varying the local micropore equilibrium constants for the exchange reactions. The averaged or lumped data approximately follow laws analogous to the micropore cation exchange formulations. However, as the degree of heterogeneity increases, so too does the extent of the deviation away from the behaviour of a purely homogeneous system. At a gross level, the inter-relationships of cations of different charge become more linear with increasing heterogeneity. The results, in conjunction with two earlier companion papers, provide: (1) a method allowing the effects of heterogeneity to be investigated for cation exchange reactions; and (2) strong justification for the continued practice of lumping techniques in catchment scale hydrochemical models.