Modeling Water Quality Impacts of

It is a common practice in assessing water quality impacts of stormwater runoff to use hydrology-based “water quality ” models to estimate total concentrations of chemical contaminants at a particular location in the runoff and/or receiving waters. Those estimates are then compared with US EPA worst-case-based water quality criteria and state water quality standards. One of the stated “Purposes ” listed for the California Water Environmental Modeling Forum (discussed in Newsletter 10-8) was assessment of “water quality impacts. ” However, the results of stormwater hydrology-runoff models, such as some of those discussed at the Forum workshop and in the literature, do not, in fact, properly assess water quality impacts. This is largely because they focus on total concentrations of chemicals rather than on those forms that are, or could become, available to affect water quality/beneficial uses of the receiving water. It has been known since the 1960s that the total concentration of a potential pollutant, such as a heavy metal, pesticide, or other organic, is not a reliable indicator of the pollutant’s water quality impact or of impairment of aquatic life-related beneficial uses of a water. This is because many chemical constituents of water quality concern exist in