Rhode Island Code of Regulations › Title 250 - Department of Environmental Management › Chapter 150 - Water Resources › Subchapter 10 - Wastewater And Storm Water › Part 8 - Stormwater Management, Design and Installation Rules › Section 250-RICR-150-10-8.9 - Minimum Standard 3: Water Quality

Current through June 14, 2024
A. Stormwater
				  runoff must be treated before discharge. The amount that must be treated from
				  each rainfall event is known as the required water quality volume (WQv). The
				  required WQv is calculated as described in §§
										8.9(E) through
				  (J) of this Part and excludes LID credits
				  allowed under §
										8.18 of this
				  Part.
B. The WQv must be treated by
				  at least one of the structural BMPs listed in §§
										8.19 through
				  8.25 of this Part at each
				  location where a discharge of stormwater will occur. Structural BMPs are
				  generally required to achieve the following minimum average pollutant removal
				  efficiencies: 85% removal of total suspended solids, 60% removal of pathogens,
				  30% removal of total phosphorus for discharges to freshwater systems, and 30%
				  removal of total nitrogen for discharges to saltwater or tidal systems. Based
				  upon results published in the scientific literature, the structural BMPs listed
				  in §§
										8.19 through
				  8.25 of this Part will meet
				  these standards when properly designed, constructed, and maintained.
				  Pretreatment is required for water quality treatment practices where specified
				  in §§
										8.19 through
				  8.25 of this
				  Part.
C. BMPs targeted to remove
				  other pollutant(s) of concern and/or to achieve higher pollutant removal
				  efficiencies may be required for impaired receiving waters, drinking water
				  reservoirs, bathing beaches, shellfishing grounds, Outstanding National
				  Resource Waters, Special Resource Protection Waters, tributaries thereto, and
				  for those areas where watershed plans, including Special Area Management Plans
				  or Total Maximum Daily Loads, have been completed. In some cases, the
				  permitting agencies may require that an applicant prepare and submit a
				  pollutant loading analysis developed in accordance with the provisions of
				  §§
										8.36 through
				  8.38 of this
				  Part.
D. Applicants or other
				  interested parties may petition DEM and CRMC to add one or more BMPs to the
				  list of acceptable structural stormwater controls described in §§
										8.19 through
				  8.25 of this Part by
				  submitting monitoring results and supporting information developed in
				  accordance with the provisions of the Technology Assessment Protocol in
				  §§
										8.39 and
				  8.40 of this
				  Part.
E. The required WQv, which
				  results in the capture and treatment of the entire runoff volume for 90% of the
				  average annual storm events, is equivalent to the runoff associated with the
				  first 1.2 inches of rainfall over the impervious surface (i.e., 1 inch of
				  runoff). The water quality volume requirement may be waived or reduced by
				  applying the LID Stormwater Credit outlined in §
										8.18 of this Part. The WQv
				  is calculated using the following equation:
WQv = (1") (I) / 12
Where:
WQv = water quality volume (in acre-feet)
I = impervious area (acres)
F. A minimum WQv value of 0.2 watershed
				  inches (0.2 inches over the entire disturbed area) is required, which requires
				  the calculation of the total site disturbance. This minimum treatment volume is
				  necessary to fully treat the runoff from pervious surfaces on sites with low
				  impervious cover, i.e., less than 20% of the disturbed area. However, this
				  requirement does not imply that every pervious subarea of disturbance must be
				  treated with a structural water quality BMP.
G. For facility sizing criteria, the basis
				  for hydrologic and hydraulic evaluation of development sites should be as
				  follows:
1. Impervious cover is measured from
				  the site plan and includes all impermeable surfaces; and
2. Off-site areas shall be assessed based on
				  their "pre-development condition" for computing the water quality volume (i.e.,
				  treatment of only on-site areas is required). However, if an off-site area
				  drains to a proposed BMP, flow from that area must be accounted for in the
				  sizing of a specific practice.