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Stormwater Infiltration Letter NGA Project No. 1040718 Zillah Street Residential Development Infiltration June 20,2018 Everett,Washington Page 4 Infiltration Pit 1 measured 4.0-feet long by 3.0-feet wide by 4.5-feet deep. Infiltration Pit 2 measured 4.0- feet long by 3.0-feet wide by 4.7-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 holes were monitored with a Great Plains Industries (GPI) TM 075 water flow meter for one hour for the steady-state portion of the test. The water was then shutoff and the water level was monitored within the pits every 15-minutes for one hour for the falling head portion of the test. Infiltration Pit 1 (Southern Parcel): After the 6-hour soaking period was completed,the water level was maintained at approximately 12-inches for one hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 0.02 gallons per minute (1.0 gallon per hour), which equates to an approximate infiltration rate of 0.134 inches per hour. The water was shut off after the steady-state period and monitored every 15 minutes for one hour. After one hour, the water level within the pit had dropped 0.125 inches,resulting in an infiltration rate of 0.125 inches per hour. Infiltration Pit 2 (Western Parcel): After the 6-hour soaking period was completed,the water level was maintained at approximately 12-inches for one hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 0.232 gallons per minute (13.92 gallons per hour), which equates to an approximate infiltration rate of 1.86 inches per hour. The water was shut off after the steady-state period and monitored every 15 minutes for one hour. After one hour,the water level within the pit had dropped 2.0 inches,resulting in an infiltration rate of 2.0 inches per hour. CONCLUSIONS It is our opinion that traditional stormwater infiltration systems within the northern property is feasible. An infiltration rate of greater than 0.3 inches per hour was achieved from our infiltration PIT tests within the northern lot. However, due to the very slow infiltration rate obtained from our on-site infiltration testing within the southern property, and the relatively silty nature of the site soils, it is our opinion that the native glacial till soils encountered within the southern property are not conducive to traditional infiltration systems. However, shallow low-impact design infiltration systems in the form of pervious pavements, bio swales, or rain gardens may be feasible at specific areas within the site depending on the final layout. This should be further discussed with the civil engineer. We recommend that any shallow infiltration systems installed on the site incorporate overflow components directed to an approved system. In accordance with the Table 3.5 of the Department of Ecology 2014 Stormwater Management Manual for Western Washington, correction factors of 0.7, 0.5, and 0.9 for CF,,, CFt, and the long-term conductivity loss factor,respectively were applied to the field measured infiltration rates of 0.125 and 2.0 inches per hour, obtained from the falling-head portion of the testing within Infiltration PIT 1 and 2, NELSON GEOTECHNICAL ASSOCIATES, INC.