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-150-10-8.38 - Pollutant Loading Analyses - Stormwater Practice Pollutant Removal Effectiveness

Current through June 14, 2024
A. Applicants will
				  frequently need to evaluate the potential pollutant removal effectiveness of
				  stormwater practices when conducting a pollutant loading analysis. To do this,
				  applicants can use the rated pollutant removal effectiveness as listed in the
				  tables in §§
										8.38(D) and
				  (E) of this Part as a basis of estimating
				  pollutant removal. These values have been derived from a variety of sources
				  based on actual monitoring data and modified, where appropriate, to reflect the
				  specific design and sizing criteria contained in §§
										8.19 through
				  8.35 of this
				  Part.
B. In some cases, the
				  pollutant removal rating values use median values from prior monitoring studies
				  when the studies included a significant number of facilities of similar design
				  criteria as those required in this Part. In other cases, the 75th quartile
				  values (or high end) are reported where it is recognized that the prior
				  monitoring was of insufficient sample size or was of practices with design
				  criteria not as robust as those required in this Part. Lastly, in many cases,
				  there is insufficient prior monitoring of practices for some or all of the
				  reported pollutants, but primary pollutant removal mechanisms are similar to
				  other practices; thus, a removal value is assigned, based on general literature
				  values and/or as a policy decision. In addition, most of the design criteria
				  for water quality BMPs incorporate pre-treatment requirements, such as the
				  requirement for a forebay or grass channel prior to infiltration. In these
				  cases, the rated removal efficiency of the practice is for the total
				  system.
C. In general, where
				  pollutant loading assessments are requested, applicants may use the rated
				  removal values as a basis for estimating pollutant load. However, other
				  pollutant removal estimates may be acceptable, provided the applicant submits
				  the source of these estimates and data used. All information supplied by the
				  applicant will be reviewed by the approving agency to determine the relevance
				  of the removal estimates to the situation.
D. Pollutant Removal Efficiency Rating Values
				  for Water Quality BMPs.
Water Quality BMPs
(Those Meeting Minimum Standard 3, §
																8.9 of this Part)
Median Pollutant Removal Efficiency (%)
TSS
TP
TN
Bacteria
WVTS
Shallow WVTS
85%2
48%3
30%2
60%2
Gravel WVTS
86%3
53%1
55%3
85%2
Infiltration Practices
Infiltration Basin
90%2
65%3
65%2
95%2
Infiltration Trench
90%2
65%3
65%2
95%2
Subsurface Chambers
90%2
55%2
40%2
90%2
Dry Well
90%2
55%2
40%2
90%2
Permeable Paving
90%1
40%1
40%2
95%2
Filters
Sand Filter
86%3
59%3
32%3
70%2
Organic Filter
90%2
65%2
50%2
70%2
Bioretention
90%1
30%2
55%2
70%2
Tree Filter
90%1
30%2
55%2
70%2
Green Roofs
Green Roofs
90%4
30%4
55%4
70%4
Open Channels
Dry Swale
90%1
30%2
55%2
70%2,6
Wet Swale
85%3
48%3
30%2
60%2
1 UNHSC, Roseen, R., T. Ballestero, and Houle, J. 2007b.
					 UNH Stormwater Center 2007 Annual Report. University of New Hampshire,
					 Cooperative Institute for Coastal and Estuarine Environmental Technology,
					 Durham, NH.
2 Center for Watershed Protection. 2007. Urban Stormwater
					 Retrofit Practices. Urban Subwatershed Restoration Manual Series - Manual 3.
					 Ellicott City, Maryland.
3 Fraley-McNeal, T. Schueler, R. Winer., 2007. National
					 Pollutant Removal Performance Database, v. 3. Center for Watershed Protection.
					 Ellicott City, MD.
4 Prescribed value based on general literature values
					 and/or policy decision.
5 50% of reported values of low end for extended detention
					 basins.
6 Presumed equivalent to bioretention; will require
					 diligent pollutant source control to manage pet wastes in residential
					 areas.
E. BMP Pollutant
				  Removal Rating Values for Other BMPs
Other BMPs
Median Pollutant Removal Efficiency (%)
TSS
TP
TN
Bacteria
Pretreatmen t BMPs
Grass Channel
70%1,2
24%3
40%2
NT
Sediment Forebay
25%4
8%5
3%5
12%5
Filter Strip
25%4
ND
ND
ND
Deep Sump Catch Basin
25%4
NT
NT
NT
Hydrodynami c Device
25%1
NT
NT
NT
Oil and Grit Separator
25%4
NT
NT
NT
Storage BMPs
Dry Extended Detention Basin
50%2
20%2
25%2
35%2
Wet Extended Detention Basin
80%3
52%3
31%3
70%3
Underground Storage Vault
20%2
15%2
5%2
25%2
"ND" means no data.
"NT" means no treatment.
1 UNHSC, Roseen, R., T. Ballestero, and Houle, J. 2007b.
					 UNH Stormwater Center 2007 Annual Report. University of New Hampshire,
					 Cooperative Institute for Coastal and Estuarine Environmental Technology,
					 Durham, NH.
2 Center for Watershed Protection. 2007. Urban Stormwater
					 Retrofit Practices. Urban Subwatershed Restoration Manual Series - Manual 3.
					 Ellicott City, Maryland.
3 Fraley-McNeal, T. Schueler, R. Winer., 2007. National
					 Pollutant Removal Performance Database, v. 3. Center for Watershed Protection.
					 Ellicott City, MD.
4 Prescribed value based on general literature values
					 and/or policy decision.
5 50% of reported values of low end for extended detention
					 basins.
6 Presumed equivalent to bioretention; will require
					 diligent pollutant source control to manage pet wastes in residential
					 areas.
F. Using the rated
				  efficiencies from the tables in §§
										8.38(D) and
				  (E) of this Part, applicants can reduce
				  post-development loadings to receiving waters when BMPs are designed,
				  installed, and maintained in accordance with the provisions of this
				  Part.
G. Estimating Pollutant
				  Removal of BMPs in Series
1. In some cases,
				  applicants may have one or more BMPs installed in a series as a so-called
				  "treatment train." In these cases, available research has shown that the
				  pollutant removal efficiency of specific BMPs, for specific pollutants, is
				  reduced for subsequent BMPs in the treatment train arrangement. As stormwater
				  migrates through the treatment train, coarser-grained particles are
				  preferentially removed by the prior BMP, leaving progressively finer particles
				  for practices down the line. The result is that for pollutants associated with
				  fine particulates, removal efficiency drops off significantly.
2. To account for this phenomenon, a widely
				  applied and generally accepted method has been to discount the rated removal
				  efficiency of the second BMP by a factor of between 75% and 50%, with
				  subsequent BMPs being reduced accordingly.
3. The Georgia Manual Method applies BMP
				  removals as below. This method does not apply to bacteria, where removal is
				  more a function of die-off than settling/attenuation; thus, the full efficiency
				  is applied to subsequent BMPs.
a. 100% of the
				  rated TSS removal efficiency to the first BMP
(1) If the TSS removal efficiency > 80%
				  for the first BMP; removal efficiency for the second BMP = 50% (regardless of
				  the pollutant constituent).
(2) If
				  the TSS removal efficiency <80% for the first BMP; removal efficiency for
				  the second BMP = 75% (regardless of the pollutant constituent).
b. For succeeding BMPs, removal
				  efficiency is applied at either 50% or 75% depending on the equivalent TSS
				  removal efficiency for the preceding BMPs (regardless of the pollutant
				  constituent).