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FAQs About Triggered Source Water Monitoring
Question 1: What is triggered monitoring?
Also called triggered source sampling, this requirement is to test the untreated source water (from a well or spring) for coliform when "triggered." The trigger is a positive result from a routine coliform sample in the distribution system. Groundwater systems are subject to the triggered monitoring requirement unless they are implementing the 4-log disinfection monitoring option.

Question 2: Our groundwater system does not disinfect the water (no chlorine, ultraviolet light, etc.). What do the triggered monitoring requirements mean for us?
If a routine distribution sample is coliform positive, you will need to collect a sample directly from each groundwater source (well or spring) in use at the time of the positive sample, in addition to collecting three repeat samples in the distribution system.

Question 3: Our groundwater system disinfects and is not conducting 4-log disinfection monitoring. What do the triggered monitoring requirements mean for us?
If a routine coliform test comes back positive, your system needs to collect:

An untreated triggered source sample within 24 hours from each groundwater source in use at the time the routine positive sample was collected, in addition to three repeat samples from the distribution system. Be careful filling out the lab forms (pdf) so it will be clear that your system has submitted two types of follow-up samples.

Your coliform sampling plan needs to include the triggered source samples, so whoever receives the positive result from a routine coliform sample will know what additional samples they need to take right away. See the revised coliform sampling plan template for systems serving up to 1,000 customers.

Question 4: We've never had a positive coliform sample in the distribution, so I doubt we'll be taking triggered samples. Are we off the hook from this requirement?

  • Even if no source samples are triggered, systems that disinfect the water using chlorine or ultraviolet light must submit at least one "raw" untreated water sample from each source per year to assess the water quality at the source. This type of source sampling (that is not tied to another test result) is called source assessment monitoring.

Question 5: Can a triggered source sample count as the required annual or monthly source sample for assessment monitoring?
Yes. Check with your regulator to make sure all source water sampling requirements are met.

Question 6: What happens if a triggered source sample from the well or spring comes back positive for total coliform?
As in the past, the operator may want to disinfect the well or spring (shock chlorination), utilizing professional judgment when applying the procedure. If there is a consistent problem with total coliform, Drinking Water Services (DWS) may ask the water system to begin 4-log disinfection monitoring to inactivate potential viruses in the source water.

Question 7: What happens if a triggered source sample comes back positive for E. coli?
The water system must collect five additional or "confirmation" source samples within 24-hours. In certain circumstances, a water system may skip confirmation sampling and move directly to corrective action if the regulator approves.

If any one of these five source samples is E. coli positive, the system must issue a “boil water” notice within 24-hours and take corrective action (pdf). Whether or not to require a boil water notice depends on individual circumstances and will be made in consultation with the regulating agency on a case by case basis. In some cases, a boil water notice could be lifted in a short time frame. For example, a system could increase the chlorine dosage so that it meets the contact time for 4-log treatment and show that the increased chlorine residual is detected throughout the distribution system. This could take less than a day (or even an hour) for a system with little distribution piping (such as for a transient system). Such a system would also need to start measuring chlorine residual daily before serving water to make sure 4-log disinfection is achieved. However, if the well or spring does not meet the current construction standards, corrective action could require reconstructing the source, or switching to a new source. For details regarding this issue, please consult with DWS hydrogeology staff in the Springfield office at 541-726-2587 (8am-5pm PT, Mon-Fri).

If additional samples confirm the presence of E. coli at the source, the water system may stop collecting triggered source samples during the period that they are implementing corrective action. At that point, the sampling has already confirmed fecal contamination in the untreated source water, so there is no need to continue checking its microbial content. The system must continue collecting routine coliform samples in the distribution system.

If any of the above five additional samples test positive for total coliform and not E. coli, see the answer to Question 6 above. The system would continue its routine coliform sampling in the distribution system and take a source sample again if triggered by another positive distribution sample.


Question 8: I took several coliform samples in the distribution system and more than one came back positive. How many triggered source samples do I need to take?
You need to take one triggered sample from each source in use at the time the routine samples tested positive. If more than one positive routine sample was taken on the same day, you still only need to collect one triggered sample per active source.


Question 9: I took a source sample at the same time as a routine distribution sample that tested positive for total coliform. Can that source sample count as the triggered source sample?
Yes, as long as the source sample was taken no earlier than the same day as the positive sample.

Question 10: Is representative sampling allowed to meet the triggered monitoring requirements?
Representative sampling may be allowed for larger systems with multiple groundwater sources and distribution system pressure zones if DWS has already approved the system's sampling plan. See more information about representative monitoring under triggered source water monitoring.

Question 11: We purchase water from another water system. If our routine coliform sample(s) test positive, who takes the triggered sample at the source(s)?
The wholesale water system must collect a triggered sample from each groundwater source that supplies water to the purchasing system (unless the wholesale system is conducting 4-log disinfection monitoring. Specific communications between the purchaser and wholesaler need to occur so that the source water requirements are met (see related Summer 2010 Pipeline article - pdf). These triggered sampling requirements should be spelled out in both systems' coliform sampling plans.