Stormwater Infiltration

When precipitation falls to the ground it either: evaporates, percolates, or becomes stormwater. Evaporated water forms clouds. Percolation is when precipitation soaks into the soil filling the spaces between grains of sand and gravel saturating the ground. Precipitation that does not naturally evaporate or percolate into the soil is stormwater.

Stormwater can flow over land, in channels or pipes into a surface water channel, or to a facility designed to place the stormwater into the ground. Stormwater that is filtered into the ground is called stormwater infiltration.

Percolation in natural, undisturbed areas, along with stormwater infiltration, including that collected from man-made areas such as clean roof run-off, sustain the groundwater aquifer that Redmond uses to supply 40% of its drinking water. 

The shallow groundwater aquifer in the valley areas of Redmond is only a few feet below the surface and is highly susceptible to contamination, so infiltration of contaminated stormwater is a potential problem.

City staff reviewed record drawings across the City and notified most owner/operators regarding their stormwater infiltration systems. If you were not contacted by the City, but suspect you have an infiltration system, check to see if there is City stormwater conveyance in the street near your property. If there are no catch basins near your property, it is possible that stormwater on your property infiltrates.

  1. Infiltration System Regulation
  2. Assessment
  3. Incentives/Schedule
  4. Protect Groundwater

In Critical Aquifer Recharge Area I

The City of Redmond requirements originate from the 2003 Wellhead Protection Ordinance and recent updates to the ordinance in 2010 and 2013. The Washington State Department of Ecology (Ecology) also requires registration, assessment and modification of many of these same infiltration systems throughout the entire state. Owners who meet Redmond's requirements will also be in compliance with the Ecology's rules on infiltration. 

Owners or operators of stormwater infiltration systems are required to evaluate and, if necessary, upgrade their stormwater infiltration systems to ensure protection of the underground drinking water resource. 

Outside Critical Aquifer Recharge Area I

These stormwater infiltration systems are regulated by Ecology. Ecology refers to perforated pipes or dry walls used to infiltrate stormwater below the ground surface as Underground Injection Control Wells, or UICs. To register your UIC-type stormwater infiltration system, visit the Department of Ecology UIC page.