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Stormwater Infiltration Letter NGA Project No. 1035918 Aikins Residence Development Stormwater Infiltration May 16,2018 Everett,Washington Page 3 were visually classified in general accordance with the Unified Soil Classification System, presented as Figure 3. The logs of the explorations are presented as Figure 4. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed description of the subsurface conditions,the exploration logs should be reviewed. At the surface of Infiltration Pits 1 and 2, and Test Pit 1, we encountered approximately 0.8 to 2.0 feet of surficial topsoil. Underlying the surficial topsoil in each of our explorations, we encountered medium dense or better, light brown to gray, silty fine to medium sand with gravel, and trace iron-oxide staining,which we interpreted as native glacial till deposits. Infiltration Pit 1 and 2, and Test Pit 1 were terminated within the native glacial till soils at depths in the range of 4.5 to 6.5 feet below the existing ground surface. Hydrogeologic Conditions We encountered groundwater seepage within Test Pit 1 at a depth of 5.5 feet below the existing ground surface. We interpret the groundwater observed within our exploration as perched groundwater. Perched water occurs when surface water infiltrates through less dense,more permeable soils, such as topsoil and the weathered horizon, and accumulates on top of a less permeable soil. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of groundwater to decrease during drier times of the year and increase during wetter periods. INFILTRATION TESTING The subsurface soils generally consisted of compact silty sand with varying amounts of gravel that we interpreted as native glacial till deposits to the depths explored. We conducted on-site infiltration testing in accordance with the 2014 SWMMWW to determine the long term design infiltration rate of the site soils. On-site testing consisted of a Small Pilot Infiltration Test (Small PIT) within each parcel to determine the long-term design infiltration rates. We conducted a Small PIT within Infiltration Pit 1 and Infiltration Pit 2, as shown on the attached Site Plan in Figure 2. Infiltration Pit 1 measured 4.0-feet long by 3.0-feet wide by 4.5-feet deep while Infiltration Pit 2 measured 4.0-feet long by 3.0-feet wide by 5.0-feet deep. The holes were filled with approximately 12- inches of water and this level was maintained for six hours for the pre-soak period. At the end of the pre- soak period, the water flow rate into the hole was monitored with a Great Plains Industries (GPI) TM 075 water flow meter for one hour. NELSON GEOTECHNICAL ASSOCIATES, INC.