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City of Stockton and County of San Joaquin Page 53 NPDES Permit CAS0083470 Response to Comments document entitled, "Interim Permitting Approach for Water Quality-Based Effluent Limitations in Storm Water Permits" (EPA 833-D-96-001, September 1996) in Finding 35. This document states that, "[a]lthough [NPDES] permits must contain conditions to ensure that water quality standards are met, this does not require the use of numeric water quality-based effluent limitations." USEPA therefore acknowledges that these limits are optional for MS4 permits. On the subject of the feasibility of numeric effluent limits, the USEPA guidance document states that, "EPA has found that numeric limitations for storm water permits can be very difficult to develop at this time because of the existing state of knowledge about the intermittent and variable nature of these types of discharges and their effects on receiving waters." Indeed, it is important to note that, to date, neither USEPA nor the State Board has provided guidance on a methodology for deriving numeric water quality-based effluent limitations for MS4 permits. Because numeric effluent limitations require strict enforcement, the establishment of these limits in the Tentative Order would compel the Permittees to treat their MS4 discharges. This is the only way they could assure compliance with numeric effluent limitations. The process for developing these limits is: (1) characterize MS4 discharges; (2) evaluate treatment technologies for effectiveness and compliance with the MEP standard; and(3) identify the achievable treatment standards that would become numeric effluent limitations. Accomplishing the first task is technically infeasible at this time because, as stated in the USEPA guidance document, "[s]torm water discharges are highly variable both in terms of flow and pollutant concentrations, and the relationships between discharges and water quality can be complex." This statement alludes to the fact that storm water quantity and quality are functions of an area's geology, topography, land use, and individual storm characteristics (i.e., duration, intensity, and number of dry days prior to the storm). The application of ill-conceived numeric effluent limits can have serious consequences. As stated in the USEPA guidance document, "[d]eriving numeric water quality-based effluent limitations for any NPDES permit without an adequate effluent characterization, or an adequate receiving water exposure assessment...may result in the imposition of inappropriate numeric limitations on a discharge...This could lead to overly stringent permit requirements, and excessive and expensive controls on storm water discharges,not necessary to provide for attainment of WQS. Conversely, an inadequate effluent characterization could lead to water quality-based effluent limitations that are not stringent enough to provide for attainment of WQS." If numeric effluent limitations are deemed infeasible, USEPA recommends the same approach that was used to develop the Tentative Order: the iterative BMP approach. As stated in the USEPA guidance document, "[t]he interim permitting approach uses best management practices (BMPs)in first-round storm water permits, and expanded or better-tailored BMPs in subsequent permits." The USEPA guidance document was also cited by the State Board in its Order WQ 2000-11 as support for the State Board's endorsement of the iterative BMP approach. Thus, the use of this approach comports with federal and State law with regards to renewal of MS4 permits. The Tentative Order does notcontain compliance schedules for achieving compliance with water quality standards. Rather, as Finding 26 indicates, immediate compliance with Discharge