In addition to evaluating stormwater retention, the DC Department of the Environment (DDOE) considered how the program might create or exacerbate stormwater pollution hotspots. For the purposes of this analysis, DDOE considered stormwater pollution hotspots to be parts of waterbodies with disproportionate stormwater pollution impacts, either in terms of erosive volumes or the pollutants in that volume. Several important points support DDOE’s conclusion that stormwater retention credits (SRC) trading is not likely to have a net negative impact, and may have a net positive impact, in terms of hotspots.
First, off-site retention will result in the installation of more BMPs retaining stormwater from developed areas that currently have little or no retention. In addition to providing more overall retention, as discussed above, the volume retained by these BMPs will be more heavily composed of first-flush volume. First-flush volume is the volume that washes off a site during the beginning of a rainstorm, and it tends to have higher concentrations of pollutants than the volume washing off at later points in the storm.
Second, with or without off-site retention, all regulated development sites in the District will achieve significantly more retention than is currently being achieved under the status quo (DDOE’s existing regulations do not require retention).
Third, the location of off-site retention BMPs is likely to provide more protection for the relatively vulnerable non-tidal tributaries to the Anacostia and Potomac Rivers and Rock Creek, as compared to strict on-site retention. DDOE assumes a typical off-site retention scenario would shift retention from regulated sites with high retention costs in the densely developed downtown to retrofit sites outside of the downtown core, where the cost of retention is significantly lower. These sites outside of the downtown core typically drain into the relatively vulnerable tributaries. By contrast, much of the District’s downtown core drains into the tidal Anacostia and Potomac Rivers. Because of their size and tidal mixing, these waters are generally less sensitive to erosive flow and localized pollutant impacts than the tributaries. In short, off-site retention is likely to result in a further increase in protection for the District’s tributaries (its most vulnerable waters), compared to strict on-site retention.
DDOE also evaluated the potential impact of off-site retention in terms of Environmental Justice (EJ). DDOE does not expect a negative EJ impact and sees the potential for a positive EJ impact. For the reasons discussed above, DDOE expects that high-cost retention sites in the densely developed and relatively affluent parts of the downtown business district would be relatively likely to forego on-site retention in favor of purchasing SRCs from low-cost retrofit sites in less densely developed and less affluent areas. This could provide a net increase in the installation of aesthetically pleasing green infrastructure in less affluent parts of the District. In addition to these aesthetic benefits, these retention BMPs would provide more protection for the waterbodies in those communities, helping to make them better resources for community members. (DDOE Proposed Regulations)