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Stormwater Infiltration Letter NGA Project No. 1035918 Aikins Residence Development Stormwater Infiltration May 16,2018 Everett,Washington Page 4 We monitored the water level during the steady state portion of the test within both infiltration pits. Checking the water level every 15-minutes, we observed the water level had remained at 12-inches for the steady state portion of the test with no inflow of water in both infiltration pits. After the steady state portion of the test,the tests were concluded due to the water level remaining at 12-inches. CONCLUSIONS Based on the results of the small-PITs, the relatively silty nature of the native soils that underlie the site, presence of shallow perched groundwater within areas of the site, and the presence of surficial undocumented fill, it is our opinion that the onsite native glacial till soils encountered at depth are not conducive for stormwater infiltration systems. However, low-impact design infiltration systems such pervious pavements and rain gardens could be feasible within the site. Any proposed low-impact design infiltration systems should be determined by the civil engineer during the final site design. If pervious pavement or rain gardens are utilized within the site,we recommend that an overflow component directed to an approved point of discharge be incorporated into the design. Additionally, a minimum separation of 1- foot from the base of the pervious pavement subbase course and any groundwater must be maintained. We did observe perched groundwater within our most northern exploration at a depth of 5.5 feet below the existing ground surface. Any low-impact design infiltration systems should be design in accordance within the City of Everett standards. We recommend that the pervious pavement surfacing be underlain by a minimum of 12 inches of clean crushed rock. We recommend that any proposed low-impact infiltration systems such as pervious pavements or rain gardens be placed as to not negatively impact any proposed or existing nearby structures and also meet all required setbacks from existing property lines, structures, and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be located within proposed fill areas within the site associated with site grading or retaining wall backfill as such condition could lead to failures of the placed fills and/or retaining structures.We should be retained to review pavement design and oversee installation. USE OF THIS LETTER This letter was prepared for Mr. and Mrs. Aikins and their agents, for use in planning and budgeting the above-referenced project only. Our services included an evaluation of the infiltration capability of the site soils at specific locations, and should not be considered as an in-depth geotechnical study of the site or an evaluation of the overall site stability. This letter may be used for bidding and estimating purposes,but our letter, conclusions, and interpretations should not be construed as a warranty of the subsurface conditions. The subsurface conditions between explorations may vary. A contingency for varying conditions should be incorporated into the project plans. NELSON GEOTECHNICAL ASSOCIATES, INC.