Stormwater Atlas

water-quality

Water Quality Volume and Treatment

Sizing basis for water-quality capture/treatment and pollutant-removal expectations.

ReviewedPrevalence 48%~286 manuals tagged14 citationsModel: heuristic+editorialRegional: arid_west, cold_climate
Provisional practice synthesis for discussion — not a design manual or adopted regulation. Controlling state, regional, and MS4 criteria apply when more stringent. Verify citations against the linked jurisdiction manuals.

Practice

Water-quality volume or treatment criteria appear in a large share of atlas manuals (field populated in roughly half overall and a majority of Tier A). Common framings include a fixed rainfall or runoff depth, a percentile storm, pollutant-load reduction targets, and retention-first hierarchies before manufactured treatment. Numeric depths and methods vary by climate and permit program.

Water-quality sizing basis options

Draft national guidance grounded in atlas practice — not a design manual or adopted regulation.

Sizing basisSuggested national guidanceAtlas note
Fixed depth (e.g. first inch / capture depth)State an explicit rainfall or runoff depth for WQv; differentiate new vs redevelopmentWQ method field present in 95% of manuals (88% Tier A)[1,2,3,4,5]
Percentile / statistical stormAllow a documented percentile storm (e.g. 90th) where climate data support itCommon Tier A framing alongside depth standards[1,2,3,4,5]
Load-based / performanceAllow pollutant-load or performance targets when the controlling permit uses them; couple to verified BMP ratesLess universal than depth/percentile; verify locally[1,2,3,4,5]
Retention hierarchyPrefer retention / runoff reduction before treatment-only BMPs where soils and groundwater allowHierarchy language widespread in GI/LID manuals

Tier A water-quality method exemplars

Truncated atlas field text from Tier A manuals — verify against source PDFs before design use.

SlugStateMethod summary
alabama-handbookALFirst-flush capture: the first 1 inch of runoff carries 90% of the pollution load from a storm (USGS, 1984); detentio…
ak-stateAKTreat runoff from 90% of average-year storms (goal: 80% average annual post-development TSS reduction). WQv = (Rv) × …
albuquerque-nm-gsiNMBernalillo County Ordinance 2107-8 requires storm water treatment volumes of 90% storm capture for new development an…
boston-maMAOn-site retention/infiltration mandatory for all development or redevelopment: infiltrate a volume equal to 1 inch of…
baltimore-mdMDQualitative control uses water quality volume and recharge volume per the 2000 Maryland Stormwater Design Manual (Rev…
bellevue-waWAImplements Ecology DOE Manual-aligned Minimum Requirements #1–#9 for new/redevelopment (site plans, construction SWPP…

Guidance

  1. 7.1Field-verified

    Define water-quality volume (or equivalent treatment volume) with an explicit rainfall or runoff basis and clear new versus redevelopment rules.[1,2,3,4]

  2. 7.2Editorial

    Prefer retention and runoff reduction before treatment-only BMPs where soils and groundwater allow.[1,2,3,4]

  3. 7.3Corpus pattern

    Publish presumed pollutant-removal rates only as guidance with jurisdiction footnotes.[1,2,3,4]

  4. 7.4Editorial

    Apply the stricter of this guidance or the controlling MS4 or state manual.[1,2,3,4]

Regional notes

Arid West often uses larger capture depths or retention standards. Cold climates adjust for frozen soils and snowmelt. Coastal and karst settings may limit infiltration credits.

Open questions

Single national WQ depth versus climate bands. Whether nutrient criteria belong in the MOP. Relationship to CGP and MS4 minimum measures.

References

Numbered footnotes for guidance clauses and atlas notes. Field-verified entries come from structured atlas evidence; corpus-pattern entries are keyword-matched excerpts.

  1. [1]AlaskaAlaskaField-verifiedTable 3-4; Section 3.3.1
    “Treat the runoff from 90% of the storms that occur in an average year. For Alaska, this equates to providing water quality treatment for the runoff resulting from a rainfall depth of 1.25 inches or less. The goal is to reduce average annual post-development TSS loadings by 80%. WQv = (Rv) × (A) × (P) / 12”
  2. [2]AlabamaAlabamaField-verifiedPage 9, Chapter 2
    “the first flush equates to the first 1" of runoff which carries 90% of the pollution load from a storm (USGS, 1984)”
  3. [3]MontanaMontanaField-verifiedSection 1.3.1, Pages 1-2 to 1-3
    “infiltrate, evapotranspire, and/or capture for reuse the post-construction runoff generated from the first 0.5 inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation... remainder... remove 80 percent total suspended solids (TSS)”
  4. [4]ConnecticutConnecticutField-verifiedPage 46, Water Quality Volume Calculation
    “WQV = (P)(R)(A)/12 where: WQV = water quality volume (cubic feet) P = 1.3 inches (90th percentile rainfall event) R = volumetric runoff coefficient = 0.05+0.009(I) I = post- development impervious area (percent) A = post-development total drainage area of site or design point (square feet)”
  5. [5]FloridaFloridaField-verifiedChapter 3, Page 103
    “temporary (or permanent) sediment or wet detention basin providing 3,600 cubic feet of storage per acre drained must be provided”
  6. [6]Bernalillo CountyNew MexicoField-verifiedRecap of BernCo Stormwater Quality Ordinance, Page 9
    “storm water treatment volumes (90% storm for new development, 80% storm for redevelopment) must be addressed within the boundaries of a proposed site development/redevelopment plan”
  7. [7]City of BaltimoreMarylandField-verifiedTable 1, Page 5
    “New development Treatment of 1.0 inch... Redevelopment Treatment of 0.5 inch; or 50% removal of impervious surface area; or Combination of both.”
  8. [8]City of BellevueWashingtonField-verifiedD1-04.2
    “MINIMUM REQUIREMENT #6 - RUNOFF TREATMENT / #5 - ON-SITE STORMWATER MANAGEMENT”
  9. [9]Boston Water and Sewer CommissionMassachusettsField-verifiedSection 3.5 On-site Retention of Stormwater, Pages 3-7 to 3-8
    “A volume of runoff equal to one inch of rainfall multiplied by the total impervious area on site must be infiltrated prior to discharge to a storm drain or a combined sewer system for projects less than 100,000 square feet of floor area. For all projects which are at or above 100,000 square feet of floor area, the proj”
  10. [10]VermontVermontCorpus patternpages 16-16
    “ture and treat the portion of runoff containing the majority of pollutants. The water quality volume (WQv) is the volume of runoff resulting from the 90th percentile rainfall event, which is equivalent to the first inch of rainfall. Achievement of the WQTS wit”
  11. [11]IdahoIdahoCorpus patternpages 69-69
    “. Nationally, many jurisdictions specify a treatment volume, referred to as the water quality volume, designed to capture the first flush component of the storm water runoff. This is achieved by specifying a rainfall amount (e.g., first 1/2-inch, 1-inch, or ot”
  12. [12]OhioOhioCorpus patternpages 59-59
    “ng Treatment Volume In most cases flows into the sand filter are limited to the water quality volume (WQv). Therefore other measures may be necessary to meet flood control detention requirements either () by diverting all runoff exceeding the water quality vo”
  13. [13]Rhode IslandRhode IslandCorpus patternpages 55-55
    “AND PERFORMANCE CRITERIA 3-14 surface (i.e., 1 inch of runoff) (CWP, 2007). The water quality volume requirement may be waived or reduced by applying the LID Stormwater Credit outlined in Section 4.6 of this document. The WQv is calculated using the following”
  14. [14]State of Hawaii Department of Transportation, Highways DivisionHawaiiCorpus patternpages 56-56
    “ention facilities. Sizing of volume-based facilities is based upon the required Water Quality Volume (WQV) which is calculated using the following formula. The WQV volume is based on a 1-inch Design Storm Runoff Depth (P) representing the 85th percentile, 24-h”
  15. [15]MarylandMarylandCorpus patternpages 42-42
    “Chapter 2. Unified Stormwater Sizing Criteria ...............................Water Quality Volume 2.4 Basis for Determining Water Quality Treatment Volume As a basis for design, the following assumptions may be made: " Measuring Impervious Cover: the measu”
  16. [16]New York StateNew YorkCorpus patternpages 41-41
    “e defined, and sub-catchments are delineated, the designer should calculate the water quality volume. This initial calculation of WQv may have to be revised after runoff reduction techniques are applied. Step 3: Apply Runoff Reduction Techniques and Standard S”
  17. [17]GeorgiaGeorgiaCorpus patternpages 120-120
    “xpressed in terms of engineering design criteria through specifica- tion of the water quality volume (WQv), which is equal to the runoff generated on a site from 1.2 inches of rainfall. The WQV must be treated to the 80% TSS removal performance goal. This stan”
  18. [18]West VirginiaWest VirginiaCorpus patternpages 328-328
    “st day modeled in 1977) n = End day of modeling (Last day modeled in 2007) Tv = Treatment Volume of the cistern, assuming the Tv applies to a drainage area that is only the rooftop draining to the system. The spreadsheet calculations should always be inclu”
  19. [19]MinnesotaMinnesotaCorpus patternpages 395-395
    “eotechnical professional. Step 4: Compute runoff control volumes. Calculate the Water Quality Volume (Vwq), Channel Protection Volume (Vcp), Overbank Flood Protection Volume (Vp10), and the Extreme Flood Volume (Vp100). Details on the Uni- fied Stormwater Sizi”
  20. [20]AlaskaAlaskaCorpus patternpages 33-33
    “Alaska Storm Water Guide December 2011 1-20 Chapter 1: Overview of Storm Water Regulations 1.2.4 Contained Water Discharge Permits A waste disposal general permit (Permit Number”

Supporting jurisdictions

11 jurisdictions cited in this section (outline tags ~286 manuals; full atlas has ~299).

vt-stateidaho-bmp-catalogohio-rainwaterri-statehawaii-dot-pcbmpmd-stateny-statega-statewest-virginia-manualmn-state-2008ak-state