If geoenvironmental models are going to be descriptive of mineral deposit type and effect on the environment, it is necessary to develop tools that link geologic control of water quality to effects observed in aquatic ecosystems. The Biotic Ligand Model is a geochemical speciation model used to produce Water Quality Criteria for trace-metals protective of aquatic ecosystems and is proposed for use in the National Mineral Environmental Assessment to predict geologic effects on aquatic organisms. Here I present the first use and validation of the Biotic Ligand Model as a field-based assessment tool of the influence of rock type on aquatic organisms. I will demonstrate that the model is capable of discriminating between lithologies that may impair water quality and that the model is quantitatively more predictive of biological responses to trace-metals than other toxicity models. However, data collected as part of the Central Colorado Assessment Project suggest that in streams contaminated by multiple trace-metals (i.e., common reality), the model fails to set criteria protective of aquatic invertebrates. I will discuss the use of Water Quality Criteria as targets for remedial actions and implication on fisheries.