Ambient concentrations of metals in surface waters have become an important consideration when establishing water quality criteria and conducting risk assessments. This study sought to estimate amounts of copper that may be released into fresh and estuarine waters considering ambient concentrations, toxicity thresholds, and bioavailability. Cumulative distribution functions of ambient copper concentrations were compared statistically for individual sites within 14 surface waters of North America and Europe to identify differences among mean distribution variables (e.g., slopes, intercepts, and inflection points). Results illustrated that the majority of distributions among sites differed significantly. These differences illustrate the variability in ambient copper concentrations in surface waters due to geographic location, regional geology, and anthropogenic influence. Additionally, surface water quality data were used for streams and lakes in Chile, Europe, and North America (including 1 saltwater estuary) to estimate bioavailable copper concentrations in ambient surface waters (based on predictions using biotic ligand models). The amount of dissolved metal that could be added to surface waters without exceeding toxicity thresholds was calculated by subtracting ambient surface water concentrations from chronic (reproduction) no-observable-effect concentrations (NOEC) for Daphnia magna using the freshwater data and 48-h median-effect (normal shell development) concentrations (EC50) for Mytilus edulis using that for saltwater. Because ambient dissolved copper concentrations were, on average, only a small fraction (18%) of predicted effects threshold, an average of 14 ± 17 μg/L (±SD) of copper could be added before exceeding the D. magna chronic NOEC or the M. edulis EC50. However, several sites were identified as having ambient copper concentrations in excess of these toxicity thresholds. The risks posed by copper to sensitive indicator species in surface waters can now be readily computed to facilitate site-specific consideration of point and nonpoint sources of metal.