Routine Coliform Monitoring Requirements

Group A public water systems are required to perform routine coliform monitoring. The minimum number of samples required each calendar month depends on the type of water system, how many connections are active, how many people are served, and the source of the water. A month's population is the total number of residential and non-residential users. The population and connections for your water system and the assigned coliform monitoring schedule can be found on the Water Facilities Inventory form (WFI).

A reduced monitoring schedule is not possible for a system that is served by a surface water source or a groundwater source influenced by surface water (GWI).

The table below lists the minimum number of routine samples required for community (COMM), non-transient non-community (NTNC), and transient non-community (TNC) water systems. Refer to the table in WAC 246-290-300 if your water system is larger than listed below.

Population Served
During the Month

Monthly Routine Samples

0 0*
1-1,000 1**
1,001-2,500 2
2,501-3,300 3
3,301-4,100 4
4,101-4,900 5
4,901-5,800 6
5,801-6,700 7
6,701-7,600 8
7,601-8,500 9

*If your water system does not serve water to any consumers during a calendar month then monitoring is not required for that month. If this is due to unusual circumstances, contact your DOH regional office. If your system remains open during a month it is normally closed, sampling is required according to the above table.

**A non-community water system that uses only protected groundwater sources and that serves less than 25 people in any month may have a reduced monitoring schedule. Consult the system's WFI. This does not apply to systems using a surface water source or a groundwater or spring source influenced by surface water (GWI).

For more information:

Northwest Regional Office - Kent
Coliform Program: 253-395-6775
Main Office: 253-395-6750

Southwest Regional Office - Olympia
Coliform Program: 360-236-3045
Main Office: 360-236-3030

Eastern Regional Office - Spokane
Coliform Program: 509-329-2134
Main Office: 509-329-2100