The intent of LEED Sustainable Sites Credit 6.2 is “to limit disruption and pollution of natural water flows by managing stormwater runoff,” and the requirements necessary for meeting this intent are clearly spelled out: capture 90% of the average annual rainfall and treat it by removing 80% of the total suspended solids (TSS). While the rating system does not define TSS it does recommend reliance on field monitoring data that is compliant with the Technology Acceptance Reciprocity Partnership (TARP) protocol or the Technology Acceptance Protocol-Ecology (TAPE) when screening stormwater best management practices (BMPs).
Although seemingly straightforward, experience has raised some concerns. There is a caveat in the LEED criteria that BMPs “designed in accordance with state or local program” are acceptable. But because most state and local programs do not require robust TARP or TAPE studies to qualify for 80% TSS credit, this is contrary to the original LEED intent. In addition, the technologies listed as preferred by LEED (alternate surfaces and nonstructural techniques) are rarely tested utilizing these protocols.
While many states require 80% TSS as a common removal requirement, very few define TSS. And failing to define a representative particle size distribution can have major implications for BMP performance and approval criteria. This is especially true for proprietary BMPs that are sized based on assumptions about the TSS load. If the actual TSS load is less coarse than assumed, the BMP will be undersized and can fail to protect receiving waters. To ensure consistent design and performance across BMP types, standards should be expanded to include conservative particle size assumptions.
The TARP and TAPE protocols set criteria for reviewing stormwater treatment practices. Although a growing number of local stormwater programs require data collected in accordance with one of these protocols, the majority of programs leave the decision up to the site engineer. All too often the engineer is left to sort through incomparable data collected under outdated or unrecognized protocols and potentially cherry-picked by the BMP manufacturer. This can lead to BMPs that do not meet the TSS removal objectives being specified on LEED projects.
If the appropriate review were to take place, oil / grit separators, hydrodynamic settling devices, and screening systems would rarely meet the intent of this LEED credit on their own. A growing body of research has shown that proprietary BMPs employing unit processes such as filtration and infiltration provide superior performance to flow through gravity systems and should be relied on to earn credit for SS 6.2.
It is important to note that the majority of studies conducted under the TARP and TAPE protocols involve proprietary treatment devices. Non-structural and many land-based practices have yet to be evaluated in accordance with TARP or TAPE, but are listed as preferred for LEED credits. While not suggesting these practices are inferior to proprietary systems, it is important that the same level of scrutiny should be applied to all types of stormwater treatment practices. Requiring TARP or TAPE compliant data is a sound approach, but in order for the process to work, alternate means of approval should be eliminated.
Implementing uniform criteria for BMP use and approval on LEED projects is essential if we truly hope to “limit disruption and pollution of natural water flows by managing stormwater runoff.” As the industry standard for “green building” LEED typically sets the bar high. Unfortunately the “local standard” component of the stormwater criteria section creates a loophole that currently allows for the approval of inferior BMPs in some jurisdictions.