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Summary Band 38

Wessel-Bothe, Stefan: Simultaneous transport of ions with different matrix affinity in loess soils under field conditions - measurement and simulation. Bonner Bodenkundl. Abh. 38 (2002), 218 S.


For the purpose of measuring leaching processes in field soils and developing a related simulation model the anions Br, NO3, SO4, H2PO4 and MoO4 and the cations Na, K, Mg, and Zn were applied on the surface of a cambisol. Leaching processes were observed over a period of 22 months by analyzing suction cup solutions and saturation extracts.

A transport influenced by preferential flow was detected after a period of 30 days. During this short period the applied ions had already migrated into different soil depths due to their heterogeneous sorption properties. After 22 months the following leaching depths were observed: Bromide and nitrate >= 135 cm, magnesium approximately 100 cm, sulfate approximately 90 cm, sodium approximately 60 cm. Zinc, potassium, phosphate, and molybdate had only been transported over very short distances.

In most cases, the concentrations in the suction cup solutions for bromide, nitrate, sulfate, sodium, and magnesium were much higher than in saturation extracts that were taken simultaneously. Phosphate, molybdate, potassium and zinc, however, showed high concentrations in suction cup solutions only at the beginning of the transport experiment. At the end of it, their concentrations were in most cases much lower than those of saturation extracts, as they had been decreasing during the experiment. The different matrix affinity of the applied ions and the flow conditions in soil were identified as the main reasons for those deviations.

Calculations of metal species in the soil solution supported the presumption of co-transport of magnesium and calcium together with sulfate and thus the unexpected high transport velocity of magnesium in the field experiment could be explained.

Alterations of the pH value by applying elemental sulfure and lime on two plots considerably affected the concentration of some of the solutes. Nevertheless, no effects on the leaching velocity of those solutes could be observed.

On the basis of field and simultaneously conducted laboratory studies (Spang 2000) a model for simulation of solute transport on field scale was developed (Klennert & Helfrich 1997, Niemeyer 2000). In this model, the water transport is calculated based on measurements of the pressure head in different soil depths and spontaneous adsorption is computed by different adsorption isotherms. An important innovation of the model is the estimation of solid state diffusion using different equations for computing the diffusion of soluble ions into rod-shaped, spherical, and cylindrical particles.

After calibration of the model with the observed tracer transport a satisfying simulation of water transport could be realized. Transport simulations of adsorbed solutes showed that some isotherm parameters determined by laboratory studies can be transferred to field conditions with satisfying results, while others may be transferred only with restrictions.

Transport simulations of phosphate, potassium and zinc were only possible by considering the solid state diffusion. For transport processes under natural conditions diffusion coefficients were observed, that were one to four dimensions smaller than those of laboratory studies. Reasons for these deviations might be the varying temperature and water content in the soil and the influence of various chemical parameters which affect the solubility of ions, i.e. pH value and DOC concentration. Only adsorption and diffusion coefficients which were determined within a cinetic batch experiment for zinc (Kuhl 2002) lead to simulations that were in accordance with field results. All in all the consideration of solid state diffusion for modeling transport processes in the field seems to be a useful alternative to estimating adsorption cinetics by equations of any order and therefore an improvement.