Groundwater/Wellhead Protection

Wellhead Protection is a pro-active approach to preventing contamination of groundwater we use for drinking water. Cleaning polluted groundwater can often cost 100 times more than preventing pollution in the first place. 

Wellhead Protection (WHP) is a requirement for local governments to protect the health of people using groundwater supplies for drinking water. The City of Redmond WHP Program is compliant with Chapter 246-290 of the Washington Administrative Code (WAC) and, therefore, the federal Safe Drinking Water Act. 

Redmond’s Wellhead Protection Program staff evaluate and reduce risks to groundwater by:

  • Identifying areas most vulnerable to impact Critical Aquifer Recharge Areas. 
  • Identifying existing impacts and activities that have the potential to degrade groundwater or reduce groundwater availability.
  • Developing strategies to prevent degradation or loss of groundwater resources from occurring.
  • To learn more about monitoring to make sure risks are identified before they can reach a supply well , click here to learn more (PDF). 
  • Managing existing impacts to ensure appropriate remediation and clean-up is done.
  • Per source water protection requirements, the city collects an inventory of potentially contaminated sites located in the CARA every other year. This inventory is available by request through the city’s public records request website. Click here to submit a request through our Support Home Page (

Wellhead Protection Program staff work closely with the Water Operations Division to protect our municipal supply wells.

Critical Aquifer Recharge Areas

Redmond recently updated groundwater/wellhead protection areas using robust computer modeling. The updated areas are called 

Critical Aquifer Recharge Areas (CARA) and were adopted on April 16, 2019.

As part of the update, Redmond convened a Sounding Board of people with diverse perspectives who learned about the groundwater model, participated in a series of meetings, and helped make decisions related to some model settings.

CARA I and CARA II are areas are where the aquifer is vulnerable to contamination, and where infiltration is important for replenishing, or recharging, the groundwater supply. These special protection areas are based on the time it takes for groundwater to travel to a municipal supply well.

  • CARA I represents the area where groundwater takes up to five years to travel to a municipal supply well.
    • This is the area where groundwater supply is most vulnerable to contamination from pollutants.
  • CARA II represents the area where groundwater takes up to 10 years to travel to a municipal supply well, plus additional sensitive areas.

CARA I and CARA II have requirements for development and businesses that help protect groundwater in CARA I and II. See Related Documents and Other Resources sections on this page for more groundwater information.

CARA I  II_City_Limit
(Click to enlarge image)

  1. Groundwater
  2. Protecting Groundwater
  3. Monitoring Program

Beneath the Downtown, Avondale Rd and southeast Redmond areas lie sand and gravel that was deposited long ago by glaciers and rivers. Within the tiny spaces of this geologic material flows groundwater that supplies 40% of Redmond’s drinking water. Municipal supply wells pump groundwater from this shallow water resource, called an aquifer. It’s then treated for safety and delivered as drinking water.

The aquifer is replenished by precipitation and surface water that infiltrates down through the soil. Since pollutants such as oil and chemicals can also infiltrate down to groundwater in the aquifer, special protection areas are established to keep municipal supply wells safe from contamination.


Septic Systems

Septic system postcard

It is critical to maintain your septic system if it is in a groundwater protection zone (Critical Aquifer Recharge Area). The CARA provides approximately 40% of Redmond’s drinking water. Proper maintenance helps keep Redmond’s drinking water clean. If septic systems are not properly maintained, they no longer receive their benefits. Instead, they can harm groundwater and pollute lakes, streams, and beaches. They can even cause sewage to surface in your yard or back up into your house. To prevent this, a septic system owner also needs to operate their system properly, such as throwing all food waste into the compost or trash can instead of tossing it down the drain (see more at Taking care of your septic system - King County).

For Additional Septic Information
Visit the King County Sewer System page, On-site Sewage System (OSS) Program - King County.

3 Steps to Lengthen the Life of Your Septic System

  1. Pump Your Septic Tank
    1. Septic Tanks are typically pumped every 3 to 5 years, depending on water use. Generally, septic tanks accommodating larger households will need to be pumped more frequently.
  2. Check for Septic System Failure
    1. Call your septic system professional if you experience any of these indicators of septic system failure:
      1. Odor
      2. Wet spots or standing water near the septic tank or drain field
      3. Backups in toilets, drains, or sinks
      4. Bathtubs, showers, and sinks drain very slowly
  3. Regularly Inspect Your System
    1. Contact a certified septic system professional to inspect and monitor your system with the following recommended frequency: 
      1. Gravity systems: Every three years
      2. Pressure distribution systems: Annually
      3. A proprietary system such as an aerobic treatment unit (ATU), membrane bioreactor
      4. (MBR), drip irrigation, and other products:
      5. Annually, or more often if required by the manufacturer
      6. Mound or sand filter systems: Annually

Find a Certified Septic System Professional

Visit the King County Site to search for certified septic system professionals: King County list of on-site sewage system professionals.