Abstract.A 1-D, pelagic, reactive-transport model of a completely mixed, turbid, heterotrophic estuary -the Scheldt estuary -is presented. The model resolves major carbon and nitrogen species and oxygen, as well as pH. The model features two organic matter degradation pathways, oxic mineralisation and denitrification, and includes primary production as well as nitrification. Apart from advectivedispersive transport along the length axis, the model also describes O 2 , CO 2 , and N 2 air-water exchange. The aim of this study is to present a model which is as simple as possible but still fits the data well enough to determine the fate and turnover of nutrients entering the estuary and their spa-