The Dawesley Creek–Bremer River drainage system, in South Australia, is affected by the potentially costly problem of acid drainage from the Brukunga mine site. This paper contrasts geochemical model predictions of water quality with detailed measurement of water chemistry in the field. This information is also used in the ecological risk assessment code, AQUARISK, to predict the degree of ecological detriment. These predictions are then assessed against independent, field-based biomeasures of algal and bacterial communities to test the performance of the code. The study demonstrates that the assessment of ecological risk obtained using AQUARISK, when coupled with geochemical modelling, accords quite well with the independent data from the biomeasures. Results from the study also suggest that only a single water-quality guideline value is required for aluminium, the major toxicant in the Dawesley Creek–Bremer River system.