Actual environmental hazards, such as heavy metal contamination of soil led to a growing need of computer tools that can assist the work in the fields of risk assessment and remediation. At present there are a number of different approaches for modelling transport processes in the rhizosphere. Although there are some more widely applied models of nutrient transport, which have given very good predictions in a number of cases, the need of understanding the physicochemical and chemical processes in the rhizosphere can only be satisfied with coupled speciation-transport models. An initial study on the suitability of different available geochemical codes (MIN3P, ORCHESTRA, PHREEQC) for modelling chemical speciation, transport and uptake in the rhizosphere has been conducted by a European network formed from the COST 631 Action (Nowack et al. 2004). However the wider application of such codes is hindered by their special needs and restrictions. The calibration of these codes to experimental data is not conveniently solved until now. No comparison of the model outputs with experimental data has been conducted. The development of the model presented here is conducted with using a dataset from a compartment system in which the temporal changes in chemical composition of soil solution have been measured with increasing distance from the root surface (Vetterlein et al. 2005). The chosen case study is the transfer of toxic arsenic from the soil to the plant. An artificial substrate based on quartz fertilized with different macro- and micronutrients, mixed with goethite in different amounts and spiked with arsenate was packed into compartment systems. Soil solution samples were collected weekly by micro suction cups installed horizontally with a spatial resolution of 6 mm. The solution samples were analysed for the major elements and As species. The computer model being developed consists of two main modules: (i) the initialization module performs the calibration (inverse modelling) of the model and determines the initial parameters for the transport module, (ii) the transport module (forward modelling) copes with the three main problems in the rhizosphere: plant uptake, transport and speciation. The chemical speciation is calculated by the widely applied and accepted geochemical code PHREEQC (Parkhurst and Appelo 1999). The development is carried out in MATLAB, which has the possibility of calling external programs, and offers robust numerical methods and the possibility of creating a graphical user interface, which could be essential for a wider applicability of the model. The optimization of the initial parameters is implemented by applying the simplex method to minimize the cumulative quadratic difference between measured and modelled concentrations in the soil solution samples gained from the compartments before planting. The differential equations of the transport module are to be solved in separate steps from the speciation by applying the split operator technique. Some demonstrative results are going to be presented to illustrate the applicability and the predictive capability of the model. References: (1) Nowack et al. in: Rhizosphere 2004 - Perspectives and Challenges GSF - Bericht 05/05, 241. Parkhurst and Appelo (1999) USGS Water-Resources Investigations Report 99-4259. (2) Vetterlein et al. in: C. J. Li et al. (Eds.) Plant nutrition for food security, human health and environmental protection, Tschinghua Univ. Press 2005. 636-637.