Environmental Health
Drinking Water & Wells
Public Water Systems
  Two Single-Family Residential Water Supply  

One well serving two residences is considered a 2 Single-Family Residential water supply. Three or more connections are regulated by Article III of the Thurston County Sanitary Code as a Group B public water system.

Rules and Regulations


  1. The pump should be capable of producing a minimum of 800 gallons per day. A minimum 20 psi at maximum demand should be supplied to each property line or residence.
  2. The pressure tank(s) should be sized so that the drawdown provides adequate pump protection and/or storage.
    Pump protection: 2.5 x pump rate = gallons. (This is the minimum drawdown capacity of the tank.)
  3. Prior to use, the newly constructed supply should be disinfected with a chlorine dosage of no less than 10 parts per million (ppm) as measured after a contact time of 24 hours (10 ppm is equivalent to approximately 1 quart chlorine bleach per 1,000 gallons water).
  4. Develop a Shared Well Agreement between the two parties.
  5. Evidence of well and waterline access agreement filed with the Thurston County Auditors' office if the two respective parcels sharing the well are separate legal lots of record.
  6. Test for coliform bacteria every year and nitrates every 3 years.

    For information on the Thurston County lab, see Water Quality Lab. To view a list of other certified testing facilities in Washington, see the Dept of Ecology Accredited Lab List [PDF].

NOTE: Expansion of a 2 Single-Family Residential water supply to provide additional connections will require the supply to meet all the requirements of Article III that apply to public water systems. This includes a design of the proposed system expansion by a qualified professional engineer and subsequent approval by the Health Department. For additional information, see Group B Public Water System Design Supplemental Application [PDF].

Applications, Forms, and Additional Resources (some documents are in PDF format, unless otherwise noted)

The following are frequently used forms:

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This page last updated: 05/18/22