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FAQs about Monthly Source Assessment Monitoring

Question 1: What is monthly source assessment monitoring?
Drinking Water Services (DWS) determines which water systems that disinfect have groundwater sources susceptible to fecal contamination. If notified individually by DWS, these systems are required to collect monthly "raw" source water samples for twelve months (or only during the months of operation for seasonal systems) to determine whether any fecal contamination exists. Those not identified as susceptible will still need to collect at least one source sample per year if they disinfect, for example those systems treating with chlorine or ultraviolet light, to make sure the disinfection treatment has not been masking fecal contamination at the source. The only systems that do not need to submit at least one annual source assessment sample per source are those conducting 4-log disinfection monitoring.

Question 2: How do I mark my monthly source assessment sample for the laboratory?
Refer to the handout PDF icon"How to fill out a lab slip" (pdf) for instructions on how to label source samples for the lab. Always use the newest version of the coliform lab forms so that you will be properly credited for the samples submitted. You can obtain the current laboratory forms on the Laboratory Reporting Page or from your water testing lab.

Question 3: Can a triggered source sample be counted as a source assessment monitoring sample for the month or year?

Question 4: What happens if there is a total coliform positive result during the 12 months of source assessment monitoring?
No confirmation samples are required for total coliform detections at the source. We will track the results and make a determination at the end of the 12 month sample schedule. If there are two or more consecutive positive total coliform results, the regulator may recommend that the water system shock chlorinate the well (based on professional judgment and the system's history). If the 12 months of source data reveal a consistent problem with total coliform, DWS may ask the system to meet the standard of 4-log inactivation of viruses.

Question 5: What happens if there is a, E. coli positive result during the 12 months of source assessment monitoring?
After an E. coli-positive result from the source (unless corrective action must be taken immediately), the water system must collect five additional source samples within 24 hours. If any one of the five additional source samples is E. coli positive, the system must issue a boil water notice to consumers within 24 hours and take corrective action (pdf) to identify the source of contamination. 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 present 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 4-log disinfection monitoring (see FAQ Question 11 below) from this point forward as part of corrective actions. However, if the well or spring does not meet current construction standards, corrective action could require reconstructing the well or switching to a new source. The decision to reconstruct or switch to a new well will be made in consultation with the regulating agency and the DWS hydrogeology staff. If these additional samples confirm the presence of E. coli at the source, the water system may stop the 12 months of source assessment monitoring. At that point, the sampling has already confirmed fecal contamination in the untreated source water, so there is no need to continue the monthly sampling. The system should keep taking routine coliform samples in the distribution system, as always. However, if any (or all) of the above five additional samples test positive only for total coliform (not E. coli), the system should continue monthly source sampling, as well as their routine coliform sampling in the distribution system.

Question 6: Is representative or combined source sampling allowed during the 12 months of source assessment monitoring?
No. Samples must be taken from the sample tap (see question 7 below for more about sample taps) for the individual source(s) listed as susceptible to contamination in the notification letter you receive.

Question 7: What if there is no sample tap present prior to the point of treatment?
Not having a sample tap prior to treatment is considered a significant deficiency for a public water system. Once a properly located sample tap is installed, the water system would initiate the 12 months of source assessment monitoring. If you have questions on appropriate locations for a source sampling tap consult with your regulator.

Question 8: What if the source sample tap occurs after a pressure tank?
Regulatory staff would have to determine on a field visit whether the sample tap location is acceptable.

Question 9: What if there is a down-hole chlorinator installed in the well? How do I take a source sample in that instance?
The chlorinator should be turned off and water should be pumped to waste until there is no longer a chlorine residual measured at the source sample tap. A minimum of three well volumes of water should be pumped from the well, as calculated by the formula below:

Formula for determining well volume:

  • Length of water column (feet) = Depth of well (feet) - Static water level (feet) [Note: The well depth & static water level are listed on the well log from the time of well drilling.]
  • Volume of well (cubic feet) = Gallons per feet (from table below) x Length of water column (feet; from previous equation)

    Well Casing
    Diameter (inches)
    Gal. of Water per
    Feet of well Depth 

Question 10: Will the assessment monitoring schedules be visible on the Data Online website?
Yes. All source water sampling schedules will be entered into the drinking water database and will be visible on the water system's Coliform Sampling Schedule page in Data Online. Similar to routine coliform samples, we advise systems to collect the samples early in the week to allow follow-up samples and results to be available before the weekend.

Question 11: What is the process for changing from monthly source assessment monitoring to 4-log disinfection monitoring, that is ongoing verification of viral inactivation?
If a groundwater system can demonstrate that their source well or spring is constructed to required standards of both DWS and the Oregon Water Resources Department, the system can avoid the 12 months of source assessment monitoring by conducting disinfection (4-log) monitoring instead. Please consult with the hydrogeology staff at the Springfield DWS (541-726-2587) office concerning this question. Monitoring for 4-log disinfection requires the water system to measure the chlorine concentration at the entry point. It must be measured daily for those systems that serve 3,300 persons or fewer, and continuously for those that serve greater than 3,300 persons. The system will submit a monthly reporting form recording chlorine measurements.

  • To pursue the 4-log disinfection monitoring option, a public water system must demonstrate that it provides 4-log inactivation of viruses. This treatment can be achieved using disinfection that has been approved by DWS. Therefore, water systems must notify DWS if they wish to pursue this option. Be aware that the water system will be required to continue monthly source assessment monitoring until DWS confirms that 4-log treatment of viruses is achieved.
  • The PDF icon Disinfection Verification Form (pdf) is available to help water systems document if 4-log disinfection can be achieved. Water systems should complete and return one form for each groundwater source identified as needing to complete the 12 months of monitoring. The form explains what constitutes 4-log treatment of viruses and how to make that determination. Water systems that need help filling out the form should contact the agency representative responsible for their system (that is the local county Health Department, Oregon Department of Agriculture, or DWS staff). Submitted forms will be reviewed by the DWS staff person responsible for your county.
  • Once it has been determined that 4-log inactivation can be achieved, the water system will be notified of the minimum chlorine concentration required to be maintained at the entry point. The water system will submit completed monthly forms to DWS.?