4920 24TH AVE W Geotech Report 2023-06-22

GeoTest Services, Inc. October 8, 2021 2405 Hillside Lane, Everett,WA Project No. 21-0845 Storm water Treatment The on-site stormwater facilities may require some form of pollutant pretreatment with an amended soil prior to on-site infiltration or off-site discharge.The reuse of on-site topsoil is often the most sustainable and cost-effective method for pollutant treatment purposes. Cation exchange capacities, organic contents, and pH of site subsurface soils were also tested to determine possible pollutant treatment suitability. Cation exchange capacity, organic content, and pH tests were performed by Northwest Agricultural Consultants on three soil samples collected from the explorations shown in Table 1. A summary of the laboratory test results is presented in Table 1 below: Table 1 Cation Exchange Capacity,Organic Content,and pH Laboratory Test Results Sample Approximate Cation Exchange Organic Test Pit Geologic Depth Elevation Capacity Content pH ID Unit (ft) (ft ASL) (meq/100 grams) (%) TP-1 3.0 215 Relict Topsoil 21.6 4.60 5.9 TP-2 0.25 225 Topsoil 18.2 5.74 5.9 TP-3 4.0 205 Transitional Beds 21.6 4.68 5.4 Note:ASL—Above Sea Level Suitability for on-site pollutant treatment is determined in accordance with SSC-6 of the SWMMWW. Soils with an organic content greater than or equal to 1 percent and a cation exchange capacity of greater than or equal to 5 meq/100 grams are characterized as suitable soils for stormwater treatment. Based on the results shown in Table 1, the topsoil, underlying relict topsoil, and native soils found on site are suitable for stormwater treatment purposes. On-site soils can be amended by mixing higher silt content soils or adding mulch (or other admixtures) to elevate the cation exchange capacity and organic contents. On-site amended soil requires additional testing to confirm compliance with ecological regulations. GeoTest is available to perform additional laboratory testing as part of an expanded scope of services if the soil is to be amended. Alternatively, the owner may elect to import amended soils with the desired properties for planned treatment facilities. Note that the near-surface native soils can be expected to have low infiltration rates. Geotechnical Consultation and Construction Monitoring GeoTest recommends that we be involved in the project design review process. The purpose of the review is to verify that the recommendations presented in this report are understood and incorporated in the design and specifications. 17