In addition to evaluatingstormwater retention, the DC Department of the Environment (DDOE) considered how the program might create orexacerbate stormwater pollution hotspots. For the purposes of this analysis,DDOE considered stormwater pollution hotspots to be parts of waterbodies withdisproportionate stormwater pollution impacts, either in terms of erosivevolumes or the pollutants in that volume. Several important points supportDDOE’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-siteretention will result in the installation of more BMPs retaining stormwaterfrom developed areas that currently have little or no retention. In addition toproviding more overall retention, as discussed above, the volume retained bythese BMPs will be more heavily composed of first-flush volume. First-flushvolume is the volume that washes off a site during the beginning of arainstorm, and it tends to have higher concentrations of pollutants than thevolume washing off at later points in the storm. Second, with or without off-siteretention, all regulated development sites in the District will achievesignificantly more retention than is currently being achieved under the statusquo (DDOE’s existing regulations do not require retention).
Third, the location of off-siteretention BMPs is likely to provide more protection for the relativelyvulnerable non-tidal tributaries to the Anacostia and Potomac Rivers and RockCreek, as compared to strict on-site retention. DDOE assumes a typical off-siteretention scenario would shift retention from regulated sites with highretention costs in the densely developed downtown to retrofit sites outside ofthe downtown core, where the cost of retention is significantly lower. Thesesites outside of the downtown core typically drain into the relativelyvulnerable tributaries. By contrast, much of the District’s downtown coredrains into the tidal Anacostia and Potomac Rivers. Because of their size andtidal mixing, these waters are generally less sensitive to erosive flow andlocalized pollutant impacts than the tributaries. In short, off-site retentionis likely to result in a further increase in protection for the District’stributaries (its most vulnerable waters), compared to strict on-site retention.
DDOE also evaluated the potentialimpact of off-site retention in terms of Environmental Justice (EJ). DDOE doesnot expect a negative EJ impact and sees the potential for a positive EJimpact. For the reasons discussed above, DDOE expects that high-cost retentionsites in the densely developed and relatively affluent parts of the downtownbusiness district would be relatively likely to forego on-site retention infavor of purchasing SRCs from low-cost retrofit sites in less densely developedand less affluent areas. This could provide a net increase in the installationof aesthetically pleasing green infrastructure in less affluent parts of theDistrict. In addition to these aesthetic benefits, these retention BMPs wouldprovide more protection for the waterbodies in those communities, helping tomake them better resources for community members. (DDOE Proposed Regulations)
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