May 28, 2010
Riding the Wave of Impending Stormwater Regulations: Water Quality
By: Aislinn Creel
A paradigm shift is occurring in the way in which we control pollutants generated by stormwater runoff. This shift is evident by examining the existing and proposed stormwater regulations and design standards. Stormwater runoff (non-point source) pollution is one of the most difficult to quantify and manage, yet critical when it comes to protecting our receiving waterways from harm.
Current standards for water quality criteria in the Commonwealth of Virginia (applicable to all state agency projects and every land-disturbing activity regulated under §10.1-603.8 of the Code of Virginia) target a reduction of total phosphorus and are based on a comparison of existing and post-developed pollutant loading or pollutant removal efficiencies. Percent impervious cover is used to determine pollutant loading in both the performance-based and the technology-based methods and various recognized BMPs are then selected based on assigned pollutant removal efficiencies. To date, BMPs are sized to treat peak discharge rates and rely heavily on the principals of detention time and sedimentation for treatment.
By contrast, the 2007 Energy Independence and Security Act mandates for all federally-owned project sites (>5,000 ft2) that design strategies to maintain or restore the pre-development hydrology of the property with regard to temperature, rate, volume and duration of flow be employed. This aforementioned maintenance or restoration of pre-development hydrology can be accomplished by either controlling the 95th percentile rainfall event, or by analyzing pre- and post-development hydrographs. Either way, the federal mandate requires that 100% of the excess runoff determined be either infiltrated, evapotranspired or captured and used.
Similarly, LEED® standards (Leadership in Energy and Environmental Design) for water quality treatment of stormwater runoff require that 90% of the average annual rainfall is treated using acceptable BMPs and that the BMPs used are capable of removing 80% of the average annual post-developed TSS loads. To obtain LEED® credit for stormwater design: quality control, a stormwater management plan that reduces impervious cover, promotes infiltration or captures and treats the runoff must be implemented.
The focus has shifted from management of discharge rates to management practices focused on volume that better mimic natural systems, i.e., infiltration, evapotranspiration and capture and use. Rather than assuming that retention and sedimentation will primarily treat harmful pollutants liberated by stormwater runoff, regulators are now focusing on mandates to treat entirely on-site the “first flush”; that is, zero discharge of runoff from small (1”-2”) storm events is allowed. EPA is currently in development of a new stormwater rule which may address performance standards for new and re-development; expanding coverage of regulated stormwater discharges to areas with rapid development; provisions for existing discharges (retrofits); permitting and non-permitting alternatives; and special provisions for Chesapeake Bay and maybe other sensitive waters. Based on the already mandated standards for federal sites and similar philosophies used in LEED criteria, it seems likely that the EPA will use a similar philosophy in development of the new rule. We’ll be tracking the progress of this new rule and you can, too, at www.epa.gov/npdes/stormwater/rulemaking.