Hydrogeologic Assessment Report for Deep UIC Wells Venema Natural Drainage System

The City of Seattle (Seattle Public Utilities [SPU]) has implemented a Natural Drainage System (NDS) program to utilize Green Stormwater Infrastructure (GSI) to filter, attenuate, divert, and/or infiltrate stormwater runoff in multiple watersheds within the City. This program focuses on returning affected watersheds to their pre-development hydrology. The Venema NDS Project proposes to utilize GSI within selected street right-of-ways to reduce runoff into the natural stormwater conveyance system via localized deep infiltration along selected blocks. Bioretention swales with an underdrain and deep infiltration systems (Class V Underground Injection Control [UIC] wells) make up the basic components of the NDS system in order to provide water quality treatment and flow control for the Basin. Low-permeability sediments cover most of the project area limiting the potential for shallow stormwater infiltration. Subsurface explorations encountered unsaturated Vashon advance outwash, a permeable geologic unit, at depth beneath the surficial low-permeability deposits. Therefore, the NDS approach proposes to infiltrate treated stormwater runoff into Vashon advance outwash via UIC wells connected at the downstream end of the proposed NDS system. Groundwater modeling analyses combined with seepage and slope stability simulations demonstrate deep infiltration will avoid adverse impacts to offsite steep slopes. The deep UIC well system is expected to decrease surface erosion by removing a portion of the surface water runoff from the existing stormwater conveyance system while improving instream flows to Venema Creek.