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TNC Water Quality Monitoring

Water Quality Monitoring Requirements for Transient Non-Community Water Systems (TNCs) - TNCs must ensure that the system's drinking water quality is monitored according to federal and state requirements.  Monitoring is necessary to help demonstrate that the water provided to customers is safe to drink.  Monitoring requirements for TNCs can be divided into two general categories - testing for source permitting and routine monitoring. All samples must be analyzed by a certified laboratory and results submitted to the Drinking Water and Groundwater Protection Division (DWGWPD).

Additionally, based on local site conditions (e.g., geology and land use activities) and historical test results, the state may require additional monitoring to ensure the water is safe to drink. Such testing may include monitoring to determine if a source is under the direct influence of surface water (GWUDI), monitoring more frequently for contaminants routinely sampled by the system (e.g., nitrate or total coliform), or monitoring for other contaminants (e.g., petroleum products). The DWGWPD will notify the system if additional monitoring is required.  Here is a list of laboratories performing microscopic particulate analysis (MPA) testing.

Routine Monitoring
Once the source has been permitted and water is available to the public, the system must conduct ongoing monitoring for nitrate and total coliform to help demonstrate that the water remains safe to drink. Routine testing is important because water quality can change over time.  Click here for: Annual Monitoring Schedules for TNCs.

TNCs must sample once per year for nitrate. The sample should be taken as close to the source as possible and after any treatment. The maximum contaminant level (MCL) for nitrate is 10 milligrams per liter (mg/L). If a test result is 5 mg/L or more, but less than the MCL, the system will be required to monitor quarterly for nitrate and to try locate and remove the source of contamination. If a result exceeds the MCL, the system must take a confirmation sample within 24 hours and monitor monthly. If the average of the initial result above the MCL and the confirmation sample exceed the MCL, the system will be required to issue a “Do Not Drink” notice until the source of contamination is found and eliminated, treatment is installed to remove the contaminant (requires a Permit to Construct), or the source is replaced with a new permitted source of water.

Total Coliform
TNCs must routinely sample the distribution system for total coliform bacteria in accordance with a bacteriological sampling plan. Frequency of sampling depends on the source type (e.g., groundwater or surface water) and the number of people the system serves. Table 3 indicates the frequency and minimum number of samples to take.

If coliform is detected in a routine sample, the system must take three repeat samples and one source sample within 24 hours and five routine samples during the following calendar month that water is served to the public. Based on the results of the additional tests the system may have to notify their customers and/or issue a “Boil Water” notice.


Table 3.  Total Coliform Bacteria Monitoring Periods and Number of Routine Samples
for TNC Water Systems
Population* Groundwater Source Surface Water and
Groundwater Under the Direct Influence of Surface Water
Less than 1,001 1 per quarter 1 per month
1,001 to 2,500 2 per month 2 per month
2,501 to 3,300 3 per month 3 per month
3,301 to 4,100 4 per month 4 per month
More than 4,100 Contact the DWGWPD Contact the DWGWPD


*Based on the larger number of 1) the design capacity of the system’s Water/Wastewater permit, or 2) the sum of the residents and the average daily transient population (total number of transients served per month divided by the number of days per month).

In addition to the routine samples, systems that are not in service year-round (e.g., seasonal campgrounds) must collect a total coliform sample before, but not more than 10 days prior to, service startup (i.e., serving water to the public).



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VT DEC Drinking Water and Groundwater Protection Division 1 National Life Drive, Main Building, 2nd Floor  Montpelier, VT  05620-3521
Telephone 802-828-1535    Fax: 802-828-1541


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