Prove It

After evaluating infiltration potential for hundreds of sites over my career, there are some sites I know will not infiltrate. It may be past experience in the area, or we may have a previous geotechnical engineering report with soil and groundwater information. A simple tech memo addressing the potential to infiltrate stormwater should check the box with the City review folks, right? Typically, NO! You have to “PROVE IT”, meaning we will be required to complete an infiltration test per whatever the applicable manual requires to prove we cannot infiltrate.

Many areas of the Puget Sound Region have glacial till soils as the surficial deposit. Glacial till, which “nature’s concrete”, is not a good infiltration receptor soil based on density and silt content. With estimates of recharge rates through till soils being 12 inches per YEAR, 2 x 10-5 inches/hour. Even the Washington State Department of Ecology recommends glacial till for use as an impermeable pond liner material.

I have written reports for till sites stating the above facts and citing the infeasibility criteria of infiltration rate is less than 0.3 inches/hour. Reviewers comment “You must conduct an infiltration test to Prove the rate is that low”. That usually means a Small Pilot Infiltration Test, which means I need a backhoe, I may need a water truck, I am going to have a staff person on-Site all day…..The cost to PROVE that the infiltration rate is less than 0.3 inches/hour in a material that both Ecology and I know is almost impermeable adds up quick, like thousands of dollars in subcontractor fees (backhoe/water truck) alone.

Here is a little tip, there are a number of things you can cite as “Infeasibility criteria” for the different stormwater infiltration elements (infiltration ponds/trenches) or Low Impact Development (LID) elements (permeable pavement/bioretention). They are all listed in the stormwater manuals. I have been told by the city review folks from numerous cities that they have been directed to require an infiltration test to confirm the rate if you cite an infiltration rate of less than 0.3 inches/hour as the infeasibility criteria for traditional or LID stormwater infiltration elements. So, if your site has till soils or say it is all silt, cite infeasibility criteria other than “infiltration rate below 0.3 inches/hour”. There are a bunch of other ones you can pick. For till sites “lack of separation distance where a minimum vertical separation of 3 feet to the seasonal high-water table, bedrock, or another impervious layer/restrictive layer would not be achieved below the bioretention cell”. This one has worked so far.

Interesting how the glacial till they wanted an infiltration test completed to Prove the rate was less than 0.3 inches per hour when citing that infeasibility criteria, magically becomes an “impervious surface/restrictive layer” under the lack of separation criteria.