Infiltration & Retrofits

What is stormwater infiltration?

When precipitation falls to the ground it generally does one of three things: it evaporates, it percolates, or it 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.

Stormwater is the portion of precipitation that does not naturally evaporate or percolate into the soil. This 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.

How does infiltration affect Redmond's groundwater?

Precipitation and stormwater infiltration sustain the groundwater aquifer that Redmond uses to supply 40% of its drinking water. Percolation in natural, undisturbed areas provides clean water to the groundwater aquifer. Stormwater collected from man-made areas such as clean roof run-off can also beneficially recharge the aquifer.

Stormwater from roads, parking lots, and industrial areas however, may pick up oils, metals, and other contaminants not beneficial to the aquifer. 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.

How do I know if I have a stormwater infiltration system?

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. For questions regarding infiltration systems contact Steve Hitch at 425-556-2891.

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What types of stormwater infiltration systems are regulated and by whom?

Stormwater systems that infiltrate untreated stormwater into dry walls, perforated pipes, trenches, bottomless vaults or infiltration ponds (Typical Stormwater Infiltration Systems) are regulated by the City’s ordinance. Owners or operators of stormwater infiltration systems located in Wellhead Protection Zones 1 and 2 are required to evaluate and, if necessary, upgrade their stormwater infiltration systems to ensure protection of the underground drinking water resource.

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.

Stormwater infiltration systems outside of Wellhead Protection Zones 1 and 2 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. This link will take you directly to the Department of Ecology website where you can register your UIC-type stormwater infiltration system.

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The City of Redmond Existing Stormwater Infiltration Assessment (Assessment) Criteria was developed to help owners and the City assess the site specific risks presented by stormwater draining into the ground. Owners were required to complete an assessment of their stormwater systems to ensure protection of the groundwater resource by February 3, 2011. Most affected owners have completed their assessments.

The criteria for completing assessments and modifications to existing stormwater infiltration systems were adopted by the City Council as Ordinance 2521 in 2010. Ordinance 2704 was adopted in August 2013 to extend the schedule and provide incentives to owners to complete their assessments and modifications to protect groundwater.

Click here to access the Assessment Form and Instructions for Completing the Assessment. The Assessment can be completed by answering 12 questions in four categories as follows:
  1. Land Use - information on specific actions, materials, and products being used or stored within areas that drain to the infiltration system.
  2. Location - proximity to public or private drinking water wells.
  3. Infiltration system description - size and treatment capacity of the system and depth to groundwater.
  4. Risk Reduction - list pollution prevention measures already in place at the site.

Points are assigned for answers in each category, and at the end of the form the points are totaled. Sites with scores less than seven (7) are identified as low risk sites. Low risk sites must implement Best Management Practices (BMPs) applicable to their site to help ensure long term protection of groundwater.

Sites with scores higher than seven (7) are identified as potential significant groundwater hazard sites. Higher risk sites must implement required BMPs as well as modifications to their stormwater system or operations.

The Assessment can be used as a guide to determine which types of modifications can be applied to a site to reduce its risk score.

Incentives are available to encourage timely implementation of stormwater system modification. Emphasis is being placed on the higher risk facilities, addressing them earlier in the schedule and providing larger incentives to encourage quicker risk reduction. Please read (Ordinance 2704) for complete details on the incentive program and elimination of permit fees for required modifications

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Assessment Review and Modifications

The goal of the assessment process is to determine the risks to groundwater at a site and determine what modifications can be made to operations, physical structures or to the stormwater system to reduce the risk. The goal is to reduce each site's risk score to less than eight (8) points by putting permanent measures in place to protect groundwater.

City staff will review the Assessment for completeness and accuracy; all questions must be answered on the assessment. City staff will be ready to help owners work through the assessment and find resources to help answer any questions.

The Flow Chart graphically describes the process and approximate schedule for completing the assessment, review, work authorization, design, permitting, installation of modifications and incentive process.

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What is the schedule? What incentives are available?  

Ordinance 2704 provides incentives to owners/operators that work with the City to promptly address the risks identified in the assessment and make the modifications required by the City on schedule. The schedule varies depending on the risk score for each site.

Assessment scores for sites located in Wellhead Protection Zone 1 and 2, have been tentatively ranked and divided into 3 primary groups based on their score. Sites with the highest risk to groundwater will start work on their facilities first:

Group One parcels with scores of 19 and higher will receive notification letters between October 2013 and October 2014. They will work with the City to design and install approved modifications after receiving notification. 

  • Incentive to complete work quickly: 
  • If all work is completed within 2.5 years, owner will be eligible for reimbursement of 75% of approved costs.
  • If all work is completed in over 2.5 years and less than 3.5 years, owner will be eligible for reimbursement of 50% of approved costs.
  • There will be no reimbursement for work completed after 3.5.

Group Two parcels with assessment scores of eight to 18 will receive notification letters for beginning work in January 2018.

  • Between 2013 and 2018, Group 2 owners will be required to implement stormwater best management practices to protect groundwater.
  • City approved modifications will be eligible for reimbursement of 60% of approved costs for work completed within 4.0 years of notification.
  • There will be no reimbursement for work completed after 4.0 years.

Group Three parcels with scores seven and below will not be required to modify their systems but will be required to meet appropriate stormwater best management practices.

Ordinance Update

The City of Redmond's Natural Resources staff collaborated with businesses and the Chamber of Commerce through a series of meetings in 2012 and 2013 to update the schedule for implementation, work with a group of pilot sites that are preparing to implement their changes, and proposed significant incentives to offset the cost of modifications necessary to protect the aquifer. The process has been endorsed by Ecology as meeting their assessment requirements and is supported by the businesses that participated in its development.

What do I need to do NOW to help protect the groundwater?

You can ensure that activities at your site are not creating a stormwater or groundwater hazard by ensuring chemicals are handled and stored safely, spills are avoided, and that chemical use is minimized where possible to help protect our drinking water resource.

All facilities are required to implement the Required Operational and Structural BMPs to protect groundwater from stormwater contamination. These BMPs include, but are not limited to, basic practices like:
  • forming a spill prevention team
  • good housekeeping
  • preventative maintenance
  • spill cleanup
  • training
  • regular inspections, and
  • record keeping
These measures are required by the City of Redmond Wellhead Protection and Stormwater Codes.

Click here for the complete version of Section IV of the Stormwater Manual that describes BMPs for specific business types such as:
  • fueling facilities
  • materials handling
  • vehicle maintenance
  • outside manufacturing activities
  • parking
  • landscape maintenance 
  • dust and sediment control 
  • storage of equipment and materials 
  • storage in tanks 
  • spill response 
  • many other best management practices
The complete Stormwater Management Manual for Western Washington Volume IV can help to provide information on design, operation and maintenance of all stormwater systems. City of Redmond code requires that "any facility, activity, or residence in the City in which hazardous materials or other deleterious substances are present shall be operated in a manner that prevents their release to the environment" and ensures that they do not "pose a significant groundwater hazard."

Please use chemicals and equipment carefully. If you would like to request a technical assistance visit from a City staff member or King County's Local Hazardous Waste Management Program to help you find different ways to reduce the risk of pollution you may contact us at

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Other Resources

You may download a shallowest depth to groundwater map that shows the approximate depth to groundwater in many areas of the City. You will need this information when completing your Assessment.

If you need other information related to your site you may contact the City. We will need your parcel number to quickly help find information for your location. You may contact us at

In addition, the following links will help you complete your registration and assessment:



United States Environmental Protection Agency

The U.S. Geological Survey

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