10.1 BMP 700 - Infiltration - Basins, Trenches, and Drywells
This Chapter provides site suitability, design, and maintenance criteria for infiltration basins, trenches and drywells designed to provide stormwater treament.
The infiltration BMPs described in this chapter include:
BMP 720 - Infiltration Trenches
Depending on how the infiltration facility is sized, infiltration basins, trenches, and drywells can be used to ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management, Minimum Requirement 6 - Stormwater Treatment, Minimum Requirement 7 - Flow Control, and Minimum Requirement 8 - Wetlands Protection.
Infiltration basins, trenches, and drywells can provide phosphorus treatment, enhanced treatment, and basic treatment.
Infiltration basins, trenches, and drywells can only be used where subsurface conditions are appropriate. The stormwater treatment function is achieved as stormwater filters through the native soils or through the engineered soils below the facility.
Pretreatment is required before infiltration basins, infiltration trenches, and drywells.
10.1.3 Design Criteria for Infiltration Basins, Infiltration Trenches, and Drywells
10.1.3.1 Site Suitability for Infiltration Basins, Infiltration Trenches, and Drywells
10.1.3.1.1 Infiltration Rates: Measured (initial) and Design (long-term)
Refer to Volume 4 - Appendix A - Appendix A Determining the Design Infiltration Rate for the allowable methods for determining infiltration rates.
The design (long-term) infiltration rate should be used for drawdown time and routing calculations.
For treatment purposes, the measured (initial) soil infiltration rate shall be 9 inches/hour or less. Design (long-term) infiltration rates up to 3 inches/hour can be considered if the infiltration receptor is not a sole-source aquifer, and the treatment soils meet the physical and chemical. suitability criteria.
10.1.3.1.2 Drawdown Time
Refer to Appendix A - Appendix A Determining the Design Infiltration Rate for the allowable methods for determining infiltration rates. Document that the water quality design storm volume (indicated by an Ecology approved continuous simulation model, assuming a 15-minute timestep) can infiltrate through the infiltration basin surface within 48 hours - see Section 10.1.3.2 Sizing Criteria for Infiltration Basins, Infiltration Trenches, and Drywells.
This drawdown restriction is intended to meet the following objectives:
Aerate vegetation and soil to keep the vegetation healthy, and
Enhance the biodegradation of pollutants and organics in the soil.
10.1.3.1.3 Native Soil Physical and Chemical Suitability for Treatment
The soil texture and design infiltration rates shall be considered along with the physical and chemical characteristics specified below to determine if the native soil is adequate for removing the target pollutants. The following soil properties must be carefully considered in making such a determination:
Cation exchange capacity (CEC) of the treatment soil must be ³5 milliequivalents CEC/100 g dry soil (USEPA Method 9081). Consider empirical testing of soil sorption capacity, if practicable. Ensure that soil CEC is sufficient for expected pollutant loadings, particularly heavy metals. CEC values of >5 meq/100g are expected in loamy sands, according to Rawls, et al.
Depth of soil used for infiltration treatment must be a minimum of 18 inches.
Organic content shall be a minimum of 1%. Organic content shall be measured on a dry weight basis using ASTM D2974.
Waste fill materials should not be used as infiltration soil media nor should such media be placed over uncontrolled or non-engineered fill soils.
Engineered soils may be used to meet the design criteria .
10.1.3.1.4 Depth to Bedrock, Water Table, or Impermeable Layer
The base of BMP 710 - Infiltration Basins, BMP 720 - Infiltration Trenches, and BMP 730 - Drywells shall be ³ 5 feet above the seasonal high-water mark, bedrock (or hardpan) or other low permeability layer. A separation down to 3 feet may be considered if the groundwater mounding analysis, volumetric receptor capacity, and the design of the overflow and/or bypass structures are judged by the site professional to be adequate to prevent overtopping and meet the site suitability criteria specified in this section.
10.1.3.1.5 Subsurface Characterization
A soils report is required to design an infiltration trench, infiltration basin, or drywell. See Appendix B - Appendix B Soils Reports requirements.
10.1.3.2 Sizing Criteria for Infiltration Basins, Infiltration Trenches, and Drywells
The size of the infiltration facility can be determined by routing the influent runoff file generated by an Ecology approved continuous simulation model, assuming a 15-minute timestep, through it. The primary mode of discharge from an infiltration facility is infiltration into the ground. However, when the infiltration capacity of the facility is reached, additional runoff to the facility will cause the facility to overflow.
Infiltration facilities can be sized using an Ecology approved continuous simulation model, assuming a 15-minute timestep:
In the Ecology approved continuous simulation model, assuming a 15-minute timestep, use:
The pond element to represent infiltration basins for BMP 710 sizing.
The trench element to represent infiltration trenches for BMP 720 sizing.
The trench element or SSD Table element to represent drywells for BMP 730 sizing.
The maximum ponded depth should be between 2 and 6 feet with at least 1 foot of freeboard.
Facilities sized for 100% infiltration meet the stormwater treatment sizing requirements, flow control sizing requirements, and LID Performance Standard Requirements.
Input the dimensions of the proposed facility.
Input the infiltration rate and reduction factor (safety factor)
When the infiltration rate was calculated using the Simplified Approach (see Appendix A - Appendix A Determining the Design Infiltration Rate), either
i. Enter the measured (initial) saturated hydraulic conductivity (Ksat) as the infiltration rate and the Total Correction Factor as the reduction factor, OR
ii. Enter the estimated design infiltration rate as the infiltration rate and use a reduction factor of 1.
b. When the infiltration rate was calculated using the Detailed Approach, enter the preliminary design infiltration rate as the infiltration rate and the aspect ratio (Ar) as the reduction factor.
Input a riser height and diameter.
Run the model for the Developed Mitigated Scenario
Check to see if the Percent Infiltrated is 100%. If the Percent Infiltrated is less than 100%, increase the facility size.
(B) For 91% infiltration (stormwater treatment)
Use the same steps as used when determining the facility size for 100% infiltration except the target Percent Infiltrated is 91%.
Ensure that the water quality design flowrate can infiltrate through the BMP within 48 hours using the equation below.
V - Facility Volume Below Overflow - Not Including Freeboard (in3)
Ksat - Estimated Design Infiltration Rates (in/hr)
SA - Surface Area at Mid-Depth Dimension (in2)
(C) Partial Infiltration to Meet Flow Control or LID Performance Standard and Stormwater Treatment Requirements
Use the same steps as used when determining the facility size for 91% Infiltration and also ensure the design meets the required flow control standard or LID Performance Standard. This may require the Percent Infiltrated to be greater than 91%.
(D) To meet the LID performance standard
Using the output files from an Ecology approved continuous simulation model, assuming a 15-minute timestep, ensure the total of any bypass and overflow from the facility meets the applicable LID performance standard.
10.1.3.3 Construction Criteria for Infiltration Basins, Infiltration Trenches, and Drywells
Initial basin excavation should be conducted to within 1-foot of the final elevation of the basin floor. Excavate infiltration trenches and basins to final grade only after all disturbed areas in the upgradient project contributing area have been permanently stabilized. The final phase of excavation should remove all accumulation of silt in the infiltration facility before putting it in service.
Infiltration facilities should generally not be used as temporary sediment traps during construction. If an infiltration facility is to be used as a sediment trap, it must not be excavated to final grade until after the upgradient contributing area has been stabilized. Any accumulation of silt in the basin must be removed before putting it in service.
Relatively light-tracked equipment is recommended for use within infiltration areas for excavation and cleaning to avoid compaction of the basin floor. The use of draglines and trackhoes should be considered for constructing infiltration basins. The infiltration area should be flagged or marked to keep heavy equipment away.
10.1.3.4 Verification Testing of the Completed Infiltration Basin, Infiltration Trench, and/or Drywell
Verification testing of the completed facility is recommended to demonstrate that the facility performs as designed. Use the same method for saturated hydraulic conductivity as used in the planning stages to ensure comparable results. Perform the testing after stabilizing the construction site. If the rates are lower than the design saturated hydraulic conductivity, the applicant shall implement measures to improve infiltration capability of the facility and retest. Replacement of the top foot of soil or more may be needed to ensure the facility performs as designed. Longer-term monitoring of the facility may be needed for some facilities. Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/Site Development Group may require verification testing on a case-by-case basis.