Water pollution control agencies are implementing control programs for chemical contaminants in urban stormwater runoff because concentrations of total forms of some contaminants in receiving water exceed numeric water quality standards. While some assert that stormwater-associated contaminants are causing water quality problems (impairment of beneficial uses), there are significant reasons to question the reliability of that claim. While urban stormwater runoff frequently contains many chemicals in sufficient concentrations to cause exceedance of numeric US EPA water quality criteria in receiving waters, exceedance of a water quality criterion/standard applied to total concentrations is not a demonstration of water quality impairment The US EPA water quality criteria were developed for worst-case or near-worst-case exposure to available forms of the contaminants. Such exposure conditions would not be expected with short-term, episodic runoff events. Substantial portions of many of the chemical contaminants in stormwater runoff are associated with particulates and would hence be expected to be largely unavailable to affect aquatic life-related beneficial uses of receiving waters. Furthermore, evidence of beneficial use impairment caused by urban stormwater runoff has not been forthcoming to document the claims. It is concluded that many of the contaminants associated with urban stormwater runoff from residential and commercial areas do not impair beneficial uses of receiving waters. The current US EPA water quality criteria have limited applicability to assessing potential water quality concerns for stormwater runoff. Guidance is presented on how urban stormwater runoff-associated contaminants should be evaluated and regulated to control use impairment without significant unnecessary expenditures for contaminant control.

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