Development of Site Specific Metals Criteria for CSO Discharges to Headwater Streams

The combined sewer service area in Atlanta is unique in that the roughly 12,100 acre combined sewer area is along a ridgeline through the City and discharges comprise the headwaters of several small streams. The combined sewer overflow (CSO) treatment facilities discharge to headwater streams in the Chattahoochee (West watersheds) and Upper Ocmulgee (East watersheds) River Basins. In November 1997, a Federal District judge ruled that the City of Atlanta's CSO discharges were causing violations of water quality standards for metals and fecal coliform. In addition, the Georgia Department of Environmental Protection (EPD) required compliance with water quality standards at the point of discharge of the CSO facilities with no allowance for dilution.In developing a plan to characterize water quality issues related to metals toxicity associated with CSO discharges in headwater streams, several types of data were collected. First, metals levels in CSO and separate stormwater were characterized using “Clean Sampling and Analytical Techniques” to the maximum extent practical. Second, both total recoverable and dissolved metals samples were collected to allow calculation of chemical translators for evaluation of compliance with dissolved metals criteria. Third, toxicity testing was conducted with CSO discharges (chlorinated and dechlorinated samples) including standard 48-hour acute tests and time-variable toxicity tests. Finally, testing to develop site specific water quality criteria for metals using water effect ratio (WER) testing was initiated after the first three steps of the characterization were completed.Significant progress was made towards development of site specific criteria for CSO discharges. Toxicity testing demonstrated that the facilities were eligible for site specific criteria because no toxicity was present if chlorine is removed. Chemical characterization using Clean Methods significantly narrowed the list of the metals of concern and focussed efforts on copper and zinc. Chemical translators were developed that provide significantly more relief from EPA criteria than default translators in EPA guidance. Tests evaluating WER of CSO discharges is underway and indicates that additional adjustment of criteria may be achieved through these tests.