One well serving two residences is considered a Group B public water supply, but is placed in a separate classification as it has a different set of
regulations than those with 3 or more connections. Two-party water systems in Thurston County are regulated by the County Public Health and Social Services
Department, Environmental Health Division (Health Dept).
Find out the steps to getting a 2-party water system approved by the County.
Rules and Regulations
- The pump should be capable of producing a minimum of 17 gpm. A minimum 30 psi at maximum demand should be supplied to each property line or residence.
- 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.)
- Prior to use, the newly constructed system 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).
- On-going water quality monitoring: One bacteriological analysis per year and one nitrate analysis every three years is recommended.
- It is recommended that an operation and maintenance agreement be established between parties. Examples are available upon request.
NOTE: Expansion of a two-party system to provide additional connections will require that the system meet all public water system requirements of the Thurston County
Sanitary Code, Article III [PDF]. This includes a design of the proposed system
expansion by a qualified water system designer or engineer and subsequent approval by the Health Dept. For additional
Group B or 2-Party Water System Design Supplemental Application [PDF].
It is recommended that 2-party systems test for coliform bacteria every 12 months and nitrate every 36 months. 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].
Applications and Forms (all applications, forms and guidelines are in PDF format)
The following are frequently used Group B water system forms: