1.4.6 Minimum Requirement 6 - Stormwater Treatment
Projects shall employ Stormwater Treatment BMPs in accordance with the following thresholds, standards, and requirements to remove pollutants from stormwater.
Each TDA within a project that triggers Minimum Requirement #6 must be reviewed to determine if Stormwater Treatment BMPs are required for the TDA.
It is possible for a project that triggers Minimum Requirement #6 to not need stormwater treatment BMPs in one or more individual TDA. If a TDA does not trigger the stormwater treatment thresholds below, the applicant must document why stormwater treatment BMPs are not needed within a given TDA.
When assessing a project against the following thresholds, only consider those surfaces that are subject to this Minimum Requirement as determined in Applicability of the Minimum Requirements and Additional Protective Measures.
The following require construction of stormwater treatment facilities:
TDAs in which the total of pollution-generating hard surface (PGHS) is 5,000 square feet or more in a threshold discharge area of the project, or
TDAs in which the total of pollution-generating pervious surfaces (PGPS) - not including permeable pavements - is three-quarters (¾) of an acre or more in a threshold discharge area, and from which there will be a surface discharge in a natural or artificial conveyance system from the TDA.
Projects that infiltrate stormwater in the South Tacoma Groundwater Protection District have additional and/or different thresholds and requirements for stormwater treatment. See the South Tacoma Groundwater Protection District Infiltration Policy available at https://www.tpchd.org/healthy-places/waste-management/business-pollution-prevention/south-tacoma-groundwater-protection-district for additional information.
1.4.6.2 Treatment Type Requirements
The type of stormwater treatment required is based upon the project scope and receiving waterbody. Below are the requirements for which treatment type applies to a project. Note, more than one treatment type may be required for a project. Stormwater treatment types determined to be applicable to the project TDA(s) must be provided. Projects required to provide oil control must also provide phosphorus, enhanced, and/or basic treatment (as applicable). Figure 1 - 5: Stormwater Treatment Flowchart below can be used as a tool to help determine which type of treatment is required for the project.
Figure 1 - 5: Stormwater Treatment Flowchart
Oil Control BMPs are required for areas within a project site that generate high concentrations of oil due to high traffic turnover or the frequent transfer of oil. Areas that require oil control include:
Areas of commercial or industrial sites subject to an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area, or 300 total trip ends per day.
Areas of commercial or industrial sites subject to petroleum storage and transfer in excess of 1,500 gallons per year, not including routinely delivered heating oil.
Areas of commercial or industrial sites subject to parking, storage, or maintenance of 25 or more vehicles that are over 10 tons gross weight (trucks, buses, trains, heavy equipment, etc.).
A road intersection with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use improvements.
Traffic count shall be based upon the prepared traffic study if one was required for the project. If a traffic study was not required, the traffic county can be estimated using information from the most recent Trip Generation Manual.
Phosphorus treatment is required for projects within watersheds that are sensitive to phosphorus and are being managed to control phosphorus. Direct and indirect discharges to Wapato Lake require phosphorus treatment when thresholds for Minimum Requirement #6 are met.
Enhanced treatment is required for industrial, commercial, multi-family, and/or high ADT roads that:
Discharge directly to fresh waters designated for aquatic life use or that have an existing aquatic life use; or
Discharge to conveyance systems that are tributary to fresh waters designated for aquatic life use or that have an existing aquatic life use; or
Infiltrate stormwater within ¼ mile of fresh waters designated for aquatic life use or that have an existing aquatic life use.
Even if all the thresholds above are met, enhanced treatment is not required for:
Direct or indirect discharges to the Puyallup River;
Landscaped areas of industrial, commercial, and/or multi-family project sites that do not involve any other pollution-generating sources (ex. industrial activities, customer parking, storage of erodible or leachable materials, wastes, or chemicals); or
Parking lots of industrial and commercial project sites dedicated solely to parking employee's private vehicles that do not involve any other pollution-generating sources (ex. industrial activities, customer parking, storage of erodible or leachable materials, wastes, or chemicals).
For threshold discharge areas within a project site with a mix of land use types, enhanced treatment is required when the runoff from the areas subject to enhanced treatment comprise 50% or more of the total runoff from the threshold discharge area. If the area subject to enhanced treatment is less than 50% of the total runoff from the threshold discharge area, only basic treatment is required.
For the purpose of determining when enhanced treatment is applied, ADT shall include:
Fully controlled and partially controlled limited access highways with Average Daily Traffic (ADT) counts of 15,000 or more;
All other roads with an ADT of 7,500 or greater
Areas that provide phosphorus treatment or enhanced treatment do not have to provide additional basic treatment. Phosphorus and enhanced treatment best management practices provide basic treatment.
Basic treatment is required for all areas within a project site that meet the stormwater treatment thresholds but are not required to provide enhanced treatment and/or phosphorus treatment.
Pretreatment best management practices are required:
When utilizing the following BMPs:
BMP 700 - Infiltration - Basins, Trenches, and Drywells
Where the basic, phosphorus, or enhanced stormwater treatment BMP or the receiving waterbody may be adversely impacted by non-targeted pollutants or may be overwhelmed by a heavy load of targeted pollutants.
1.4.6.3 Treatment Facility Selection, Design, and Maintenance
Stormwater treatment facilities shall be:
Selected in accordance with the Treatment Type requirements above;
Designed in accordance with criteria in Volume 4; and
Maintained in accordance with the maintenance standards in Volume 4.
1.4.6.4 Additional Requirements
The thresholds above apply to both a project’s onsite and offsite improvements. Once the project is required to meet this Minimum Requirement, all affected pollution generating surfaces are required to provide treatment. No net or average is permitted between non-pollution generating surfaces and pollution generating.
Direct and indirect discharge of untreated stormwater from pollution-generating hard surfaces to groundwater is prohibited, except as allowed by dispersion and infiltration through Onsite Stormwater Management BMPs or Infiltration Stormwater Treatment BMPs where the native soils meet the Soil Suitability Criteria. Certain projects in the South Tacoma Groundwater Protection District require stormwater treatment regardless of project size. See the South Tacoma Groundwater Protection District Infiltration Policy available at https://www.cityoftacoma.org/stormwatermanual
1.4.6.5 In-Lieu of Payment Option
In lieu of constructing stormwater treatment facilities, applicants may pay a fee to the City of Tacoma. The payment will offset construction of City owned and maintained regional stormwater facilities. In order to participate, the program must have capacity available and projects must meet certain eligibility requirements. The “Regional Stormwater Facilities Program” document available at www.cityoftacoma.org/stormwatermanual contains the program requirements, fees, and project feasibility criteria.
The purpose of stormwater treatment is to reduce pollutant loads and concentrations in stormwater using physical, biological, and chemical removal mechanisms so that beneficial uses of receiving waters are maintained and, where applicable, restored.