An approach to modelling contaminant behaviour in aquatic systems is presented based on the principle of superposition of linear systems. The approach separates the modelling into two parts: part one, is the description of the contaminant behaviour and generation of system responses, part two, is the analysis of a given situation through the addition of system responses. It is argued that separating the modeling into two parts may facilitate model use in remedial action planning and facilitate model evolution. The technique is illustrated with PCBs in Lake Ontario.