521.1 This section applies only to the portion of a major regulated project that consists entirely of bridge, roadway, streetscape, or railway work:

1. (a) In the existing Public Right-of-Way (PROW); or
2. (b) In the existing PROW and in the public space associated with the PROW.

521.2 For a project in the existing PROW, the property owner may comply with a requirement in this chapter to retain a Stormwater Retention Volume (SWRv) by:

1. (a) Retaining fifty percent (50%) of the SWRv requirement on-site and using off-site retention for the remaining volume pursuant to § 527, as applicable;
2. (b) Retaining one hundred percent (100%) the SWRv requirement on-site; or
3. (c) Retaining the SWRv requirement on-site to the maximum extent practicable (MEP), after proving that each opportunity to install retention capacity has been exhausted, as described in the Department's Stormwater Management Guidebook (SWMG).

521.3 A project in the existing PROW shall:

1. (a) Prioritize, to the MEP, the management of stormwater from the roadway, including stormwater draining from roadway beyond the area of land-disturbing activity; and
2. (b) Not be required to install a Best Management Practice (BMP) or landcover:
3. (1) That provides retention capacity greater than that required to achieve the SWRv that is calculated for the area of land-disturbing activity; or
4. (2) That is outside the area of land-disturbing activity.

521.4 An existing PROW project on an Anacostia Waterfront Development Zone (AWDZ) site may comply with a requirement in this chapter to achieve a Water Quality Treatment Volume (WQTv) by:

1. (a) Achieving the WQTv; or

(b) Achieving the WQTv to the MEP, after proving that each opportunity for installing retention and treatment capacity has been exhausted in compliance with the MEP process for existing PROW detailed in the SWMG.

521.5 To elect to retain the SWRv requirement on-site to the MEP for a project in the existing PROW, a person shall provide the following information demonstrating technical infeasibility or environmental harm:

(a) Detailed explanation of each opportunity for on-site installation of a BMP that was considered and rejected, and the reasons for each rejection, including each opportunity that could be created by reducing roadway width to create an expanded area for retention of the SWRv requirement or treatment of the WQTv requirement between the curb line and private property; and

(b) Evidence of site conditions limiting each opportunity for a BMP, including, as applicable:

(1) Data on soil and groundwater contamination;

(2) Data from percolation testing;

(3) Documentation of the presence of utilities requiring impermeable protection or a setback;

(4) Documentation of structural requirements that would not be satisfied by a BMP;

(5) Evidence of the applicability of a statute, regulation, court order, pre-existing covenant, or other restriction having the force of law; and

(6) Evidence of a District-approved use for the safe and effective transport of goods or people.

521.6 For a major regulated project in the existing PROW, the property owner may comply with on-site retention requirements by retaining more than the one and two-tenths (1.2) inch SWRv requirement for an area of the site or for an area that drains to the site, subject to the following conditions:

(a) Unless a Site Drainage Area (SDA) drains into the Combined Sewer System (CSS) or the Department approves an application for relief from extraordinarily difficult site conditions, the property owner shall comply with at least fifty percent (50%) of the one and two-tenths (1.2) inch SWRv requirement from the SDA through:

(1) Retention; or

(2) Treatment to remove eighty percent (80%) of total suspended solids to the MEP;

(b) Unless an SDA drains into the CSS or the Department approves an application for relief from extraordinarily difficult site conditions, the entirety of an area intended for use or storage of motor vehicles shall drain to each necessary BMP to comply with at least fifty percent (50%) of the one and two-tenths (1.2) inch SWRv requirement flowing from that entire area through retention or treatment to the MEP;

(c) Retention in excess of a one and two-tenths (1.2) inch SWRv requirement for one (1) area of the site or an area that drains to the site may be applied to the volume required for another area of the site;

(d) On-site BMPs shall retain a minimum of fifty percent (50%) of the one and two-tenths (1.2) inch SWRv requirement for the entire site unless the project complies with the SWRv requirement to the MEP;

(e) For any site that retains less than fifty percent (50%) of the SWRv requirement on-site, the property owner shall use off-site retention pursuant to § 527, as applicable, unless the project complies with the SWRv requirement to the MEP; and

(f) Retention of volume greater than that from a one and seven-tenths (1.7) inch rainfall event, calculated using the SWRv equation with a P equal to one and seven-tenths (1.7) inches, shall not be counted toward on-site retention.

521.7 If a project in the existing PROW that is retaining the SWRv to the MEP is not able to achieve retention of fifty percent (50%) of the SWRv for the entirety of an area intended for use or storage of motor vehicles, the Department may waive a requirement to provide treatment for that volume if the Department:

(a) Determines that a treatment BMP would displace or reduce the size of retention capacity to be installed; and

(b) Concludes that the displaced or reduced retention capacity would be as protective or more protective for District waterbodies than the alternative treatment BMP.

521.8 An existing PROW project that is retaining the SWRv or the WQTv to the MEP shall not be required to use off-site retention for the difference between the

required volume and the achieved volume.

SOURCE: Final Rulemaking published at 35 DCR 21 (January 1, 1988); as amended by Final Rulemaking published at 60 DCR 10640 (July 19, 2013); as amended by Final Rulemaking published at 67 DCR 818 (January 31, 2020); as amended by Final Rulemaking published at 72 DCR 012101 (October 31, 2025).