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)
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