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Stormwater Infiltration Letter NGA Project No. 1040718 Zillah Street Residential Development Infiltration June 20,2018 Everett,Washington Page 3 geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 4. The logs of test pits are attached to this letter and are presented as Figures 5 and 6. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions,the test pit logs should be reviewed. 32At the surface of all of Infiltration Pit 1 and 2, and Test Pit 2, we encountered approximately 1.0 to 1.2 feet of surficial grass,and topsoil. Underlying the topsoil in Infiltration Pit 1 and 2, and Test Pit 2 and the ground surface in Test Pit 1,we encountered approximately 1.0 to 3.0 feet of loose to medium dense,dark brown to brown silty fine to medium sand with varying amounts of gravel, organics and debris that we interpreted as undocumented fill soils. Below the fill in all of our explorations, we encountered mdium dese to dense, orange-brown to gray silty fine to medium sand with gravel that we interpreted as native glacial till soils. The native glacial till soils encountered within the explorations within the northern property were noticeably sandier that the glacial till soils encountered within our explorations within the southern property. All of our test pits were terminated within the native glacial till soils at depths in the range of 4.0 to 6.0 feet below the existing ground surface. Hydrogeologic Conditions We did not encounter groundwater seepage within our explorations. If groundwater is encountered during construction we would interpret this water 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 Department of Ecology 2014 Stormwater Management Manual for Western Washington was utilized to determine the long term design infiltration rate of the site soils within each property. According to this manual, on-site infiltration testing consisting of the Small-Scale Pilot Infiltration Test (PIT) was used to determine the long-term design infiltration rates. The subsurface soils generally consisted of silty sand with varying amounts of gravel and cobbles that we interpreted as native glacial till soils. We conducted one small-scale PIT within each property for a total of two tests as shown on the attached Site Plan in Figure 2. NELSON GEOTECHNICAL ASSOCIATES, INC.