Governments and managers have generally conducted inadequate cost-benefit analysis (CBA) by failing to incorporate environmental impact costs. This is not a straightforward task, owing to the intangibility, in monetary terms, of most environmental goods and services. By means of a theoretical example, we present a practical approach to correct cost-benefit analysis for environmental externalities that do not need natural resource valuations. The theoretical example is based on ventures that potentially pollute river waters. However, we think that the methodological approach presented here may be used to internalize (in monetary terms) the costs of negative external impacts on the environment, which may result from any type of venture or project.