Underground Injection Control - Infiltration Trenches with Perforated Pipe Information

Washington State Department of Ecology

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Infiltration Trenches with Perforated Pipe

Infiltration trenches with perforated pipe must be registered with Ecology. They must be designed, constructed, operated, and maintained according to either the current Stormwater Management Manual for Western (2005) {SMMWW} or Eastern (2004) Washington or an equivalent manual approved by Ecology (www.ecy.wa.gov/programs/wq/stormwater/tech.html).

Below is a summary of the treatment requirements in Western WA for designing an infiltration trench for rule authorization:

Infiltration trenches with soils NOT considered a treatment best management practice (BMP): solids removal/basic treatment is required.
Infiltration trenches with soils considered a treatment BMP: pretreatment BMP is required.

For soils to be adequate for treatment they must meet the design standards in the SMMWW (SSC-6-9, vol. III, p.3-84, 85) www.ecy.wa.gov/biblio/0510031.html:

Cation exchange capacity.
Minimum of 18 inch depth.
Short term and long term infiltration rates.
Separation between ground water table and the base of the trench must be at least 5 feet (down to 3 feet is considered if overtopping does not occur - mounding analysis is required).

Last updated January 2008