A tool for modelling chemical and multidimensional transport processes in soil. ORCHESTRA (Object Representation of Chemical Speciation and Transport models) represents an entirely new approach to the implementation of chemical and physical models within computer-based models. ORCHESTRA does not represent a specific model but is a generic toolbox that allows users to create their own computer models using conceptual models which they have either developed themselves or have been developed by others. In ORCHESTRA, models are composed of objects, which are stored in an object database and are fully user-definable. Such objects represent parts of the model at various levels of abstraction, e.g. simple numerical expressions, chemical reactions and even complete physico-chemical model systems. These objects can subsequently used as building blocks themselves.