The 2014 Western Washington Stormwater Manual (Ecology, 2014) recommends use of the Pilot Infiltration Test (PIT) for estimating infiltration rates to be used in design of Green Stormwater Infrastructure (GSI) facilities. Other states across the country have adopted the PIT as the preferred method for estimating infiltration rates. Unfortunately, the PIT approach ignores important physical processes and overestimates porous medium hydraulic conductivity by 20 to 60 percent, depending on test geometry (area and water depth) and porous medium hydraulic properties. Although the non-conservative nature of this approach is partially mitigated as size of the test excavation increases, the cost and logistical challenges (e.g., sufficient water supply) can become prohibitive.
This paper provides an alternative approach, the borehole permeameter (BP) method, that better represents the three components (pressure, gravity, and capillarity) of infiltrative flow or recharge from an excavation or borehole completed within the unsaturated zone. The method is well suited for estimating infiltration rates for use in design of green stormwater infrastructure (GSI). The BP approach has been used in open excavations (similar to the PIT approach), in temporary wells installed using a vactor (vacuum extraction) truck, and in deeper drilled wells completed within the unsaturated zone.
The BP procedure involves pumping water into the borehole/excavation at a steady rate and monitoring head rise until the head becomes constant. The test results provide estimates of bulk “field saturated” hydraulic conductivity (K) using approximate analytical solutions that were originally developed in the 1950’s and have been refined in recent decades. Bulk K is a composite parameter that incorporates the effects of groundwater mounding, as well as fine-scale porous medium layering and larger-scale porous medium variability within the tested interval. The term “field saturated” refers to the fact that ponded infiltration into unsaturated porous media may entrap some amount of air, causing the effective porous medium K to be a much as a factor of two lower than if the porous media were completely saturated (i.e., no entrapped air).
The BP approach has been tested and refined during numerous stormwater infiltration assessments, and examples of the approach are provided in this paper.