Taking It to the Limit

Since the National Pollutant Discharge Elimination System (NPDES) stormwater permit program came into being after the 1987 amendments to the Clean Water Act, discharges of stormwater from large and medium municipal separate storm sewer systems (MS4s), as well as stormwater related to industrial and construction activities, have been regulated by NPDES permits.

In California, permits have historically not included numeric effluent limits for municipal, construction, and industrial activities. Instead, municipal permits have required only that the discharge of pollutants be reduced to the maximum extent practicable, while discharges that resulted from industrial activities were required only to meet the standards of the best available technology economically achievable. However, due to the lack of enforceable effluent limits, some have argued that the permits in their current form do not adequately support water-quality efforts.

To settle the ongoing debate between environmental groups and the regulatory community in California, the State Water Resources Control Board assembled a blue-ribbon panel to address the feasibility of including numeric effluent limits as part of NPDES stormwater permits for municipal, industrial, and construction activities.

After much debate, the panel issued its report in June 2006 and made recommendations as to the technical feasibility of establishing numeric limits in stormwater permits, how such limits should be established, and what data should be required.

Municipal Permits
For area-wide municipal permits, the panel determined that it is not feasible to set numeric criteria for effluent levels at this time. Although the panel determined that it is possible to estimate acceptable mean effluent concentrations and variances for a number of constituents and recommended establishing action levels for catchments that require additional attention, it found that it is not technically feasible to establish legally enforceable effluent limits because of the variable and episodic nature of stormwater flows.

However, the panel did suggest that it is possible for municipalities to more diligently select, design, and monitor stormwater-quality best management practices (BMPs) to ensure that estimated mean effluent concentrations will be reasonably close to acceptable levels, and not exceed an agreed-upon value that is clearly above acceptability. As a result, expect to see greater scrutiny regarding the selection and maintenance of stormwater-quality BMPs in the future.

Construction General Permits
For construction general permits, the panel suggested that numeric limits are technically feasible—as long as background turbidity levels are taken into account—because enough is known about construction-site conditions and the various BMPs and technologies available to make effluent limits meaningful and practical.

The panel went so far as to recommend the use of active treatment technologies, such as those involving the use of polymers with relatively large storage systems, because they can provide consistently low discharge turbidity.

However, the panel did not address the possible toxic effects that some of these polymer-based treatments may have on water quality. The challenge going forward will be to consider the application and minimize any toxicity so treatment efforts don’t end up doing more harm than good.

Industrial General Permits
For industrial general permits, the panel basically took two approaches. First, it suggested that if there is a total maximum daily load (TMDL) in place, the numeric effluent limit should be set to conform to the TMDL waste load allocation based on appropriate flow rate assumptions. This will ensure that the allowable pollutant load does not cause the receiving water to exceed the water-quality standard.

If there is no TMDL present, the panel suggested taking the 1970s approach to wastewater treatment—where numeric limits are set according to the best currently available technology to treat the particular pollutants likely to be present in runoff based on the type of facility in question—and applying it to stormwater pollution prevention and treatment. However, this may prove challenging because a number of factors, including size, climate conditions, and constantly changing product demands, contribute to the variability of stormwater discharge from industrial sites. Treatment regimes will have to continually adapt to keep up.

While declining to provide prescriptive measures, the panel’s findings and recommendations provide guidance on how to move forward on the issue of numeric criteria for NPDES permits. And while each state will treat the situation differently—based on background conditions including soil type, climate, and rainfall—it is clear that numeric effluent limits are becoming a more common part of NPDES stormwater permits across the country.  
About the Author

John Moynier and Rosanna Lacarra

John Moynier is senior program manager for water resources working out of PBS&J’s Sacramento, CA, office and is an expert in water supply assessments, water resources planning and conservation, and water/wastewater operations. Rosanna Lacarra is a senior project manager/senior scientist working out of PBS&J’s Encinitas, CA, office. She specializes in water quality, particularly as it relates to stormwater programs and recycled water/wastewater.