For the simulation of spatial concentration patterns of 'down-the-drain' chemicals mathematical models were coupled with a Geographic Information System (GIS) to predict concentrations in the receiving surface waters, using the detergent chemicals Linear Alkylbenzenesulfonate (LAS) and Boron and the Itter stream, a small tributary to the Rhine, as an example. In order to estimate the release of the aforementioned chemicals into the Itter two different model concepts have been implemented and evaluated. The first concept shows that mainly the connectivity to the STPs and the elimination processes taking place in them influence the predicted concentrations. In the second concept, concentrations in various stretches of the Itter are calculated based on the release via point sources (STPs). The second concept, accompanied by statistical analyses, shows a high correlation between the half-life of LAS in the riverine water and the water quality parameters TOC and ammonium, This study is closely linked to the ongoing project GREAT-ER. (C) 1999 Elsevier Science Ltd. All rights reserved.