Figure 1. Process for calculating pollutant control credits
In late 2015, a new Municipal Stormwater Permit was issued for San Diego, Riverside and Orange counties in California. It set forth more stringent performance standards for onsite retention and pollutant control, and it provided a broader classification of Priority Development Projects (PDPs). It also included new language permitting participation in an alternative compliance program, allowing developers to satisfy onsite performance standards through implementation of an offsite project demonstrating a greater overall water quality benefit.
The Water Quality Equivalency (WQE) document was prepared to provide standards to determine whether an alternative compliance project (ACP) will achieve greater water quality benefits than retention or biofiltration on a PDP site.
Two categories of ACPs were defined: structural practices and natural system management practices (NSMPs).
Structural practices detain, retain, filter, remove or prevent the release of pollutants to surface waters. Retrofits add or modify BMPs at existing developments, regional practices treat storm water from more than one development, and water supply practices capture storm water and replenish water supplies.
NSMPs restore or preserve predevelopment watershed functions. Land restoration practices restore developed land to predevelopment conditions, land preservation practices preserve undeveloped land and stream rehabilitation practices restore streams to a natural stabilized condition.
The co-permittees identified technical areas where available science appeared unable to verify greater improvement to water quality. They selected volume as the metric for equivalency. The volume credited to an ACP is adjusted based on pollutant load and land-use factors, amount of the 85th percentile storm captured and best management practice (BMP) implemented. A greater water quality benefit was defined as the earned storm water pollutant control volume (VE) from the ACP, which is equal to or greater than the volume of storm water not retained or biofiltered on a PDP.
Three-Step Process
Step 1: PDP Storm Water Pollutant Control Impacts. Calculate the storm water volume not retained or biofiltered at a PDP. Calculate the required volume and calculate the volume retained/biofiltered on a PDP. The difference is the volume deficit. This step is not required for applicants constructing independent ACPs because they are constructed without knowledge of specific PDPs.
Step 2: ACP Storm Water Pollutant Control Benefits. Calculate the storm water pollutant control benefits of an ACP. This consists of four variables: design capture volume, land-use factor, BMP efficacy and VE. VE is the volume of water treated by the ACP considering site-specific factors presented in the equation: VE = L (ΔV + V2B2 - V1B1), where:
VE = Earned storm water pollutant control volume (cubic ft)
L = Land-use factor
ΔV = Change in design capture volume (V1 - V2)
V1 = Impacted condition design capture volume for ACP (before ACP)
V2 = Mitigated condition design capture volume for ACP (after ACP)
B1 = Impacted condition BMP efficacy factor
B2 = Mitigated condition BMP efficacy factor
Design capture volume (DCV) tributary to the ACP is determined through the same methodology outlined for PDPs in the San Diego County BMP Design Manual (2015). ACP applicants determine DCV values for impacted and mitigated ACP conditions and then calculate the difference between the two.
L is the ratio of pollutant concentrations generated by an ACP tributary compared to the pollutant concentrations generated by a reference PDP tributary. The guidance provides concentration values for key pollutants across several land use categories to facilitate this comparison. The reference tributary typically is characterized by the drainage area of a specific PDP, and also may be represented by the land use composition at the watershed scale. Land use factors are determined for each pollutant for which the receiving water is impaired, and the lowest overall result is applied in the equation.
B quantifies the ability of an ACP to remove pollutants in runoff from the tributary drainage area. This factor ranges from 0.00 to 1.00. The BMP efficacy factor includes pollutant removal efficiency and provided capture. Pollutant removal efficiency specifies the difference between BMP types to remove pollutants, and provided capture quantifies the water quality benefits when an ACP manages less than the 85th percentile storm volume.
Step 3: Storm Water Pollutant Control Credits. Greater water quality benefit is demonstrated when VE from an ACP is greater than or equal to the deficit of the storm water pollutant control volume from a PDP.
Case Studies
Two property development projects were evaluated in Orange County, where water quality trading credits would be generated by installing storm water retention BMPs: a 23-acre property redevelopment using subsurface infiltration within a 241-acre catchment, and a 4-acre surface area flood control basin retrofit (infiltration) within a 1,000-acre catchment. Earned storm water pollutant control volume calculations were performed for each project and tributary area. A triple-bottom-line economic benefit cost analysis was conducted for two scenarios for each project.
In Scenario 1, alternative compliance provided incentives for a project proponent to build retention for the entire catchment in 10 years and sell credits thereafter. In Scenario 2, with no alternative compliance allowance, water quality benefits occurred as the catchment redeveloped over a 200-year horizon. This analysis concluded that, in general, overall economic benefits improved two-fold with alternative compliance due to the more rapid improvement in water quality that occurred when alternative compliance incentivized more rapid construction of storm water retention BMPs.
