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Which Systems Need to Conduct Monthly Source Assessment Monitoring?

Monthly Monitoring

This page provides water system regulators and other interested parties with background information on how Drinking Water Services (DWS) determines whether or not water systems are required to conduct monthly source assessment monitoring.

Source Water Assessment Reports

DWS uses information found in source water assessment (SWA) reports to identify which water systems are required to conduct monthly source assessment monitoring. Water system regulators (that is DWS, county health departments, and Oregon Department of Agriculture) may not have a complete set of SWA reports in their files. The DWS Springfield Office can provide SWA reports to water system regulators or to the water system itself. Most of these reports are available as electronic files.

How to Obtain Additional Information

Contact the DWS Springfield office for assistance with the following:
  • Questions about the process identifying systems needing to do monthy assessment monitoring - contact the groundwater coordinator or refer to the susceptibility criteria, below
  • Questions regarding a particular water system, including why it is considered susceptible to viral contamination - contact the DWS regional hydrogeologist or refer to the water system's SWA report
  • Obtaining copies of SWA reports

Susceptibility Criteria

Some viruses are believed to survive as long as two years in groundwater. Since fecally-derived viruses are of most concern because they can cause acute illness, water systems identified as being susceptible to fecal contaminant sources located within their two-year water supply zone are required to conduct monthly source assessment monitoring.

A water system is identified as susceptible if both of the following conditions are true:
  1. DWS believes the aquifer and/or well/spring construction is highly sensitive to contamination, and
  2. A fecal contamination source has been identified within the 2-year Time-of-Travel Zone, Outreach Area, or Zone 1 (see Aquifer Zones below for definitions).
Only those groundwater sources that are identified as susceptible to fecal contamination and that treat with a disinfectant are required to conduct monthly source assessment monitoring. This is because while treating with a disinfectant may kill bacteria such as E. coli, it may not kill most viruses unless the treatment objective is to kill viruses and the appropriate contact time is achieved. Groundwater systems that do not treat with a disinfectent do not have to conduct monthly source assessment sampling since they are already sampling untreated water in the distribution system.

Groundwater Source Sensitivity
Groundwater sources can be sensitive to contamination due to two factors: aquifer sensitivity and well/spring construction. Aquifer sensitivity is evaluated using several factors (water depth, aquifer material, historical monitoring results) that are addressed in a water system's SWA report. Well and spring construction sensitivity (also addressed in the SWA report) is evaluated based on the Oregon Water Resources Department and PDF iconDWS construction standards. If the well or spring is inadequately constructed or there is no information on file regarding the construction, it is considered highly sensitive to contamination.

Aquifer Zones

  • 2-year Time-of-Travel Zone:  Refers exclusively to community (C) and non-transient non-community (NTNC) water systems that have wells. It identifies the 2-year water supply around the well(s) and is represented on the delineation map included in the SWA report for the water system (typically as Figure 1). On the delineation map for these water systems, there are multiple Time-of-Travel Zones identified that represent a 1-year, 2-year, 5-year, and a 10/15-year water supply. Contaminant inventory results for the 2-year Time-of-Travel Zone are given on the potential contaminant source (PCS) inventory map and in the appendix materials at the back of the report.  Note: that surface water is also considered a fecal contaminant source, but may not always show up in the potential contaminant source inventory, so you may have to review the map. Potential contaminant sources are number-coded on the map and in the report appendix.
  • Outreach Area:  Refers exclusively to transient non-community (TNC) water systems. It is an approximation of the 2-year water supply around the well(s)/spring(s) and is identified on the delineation map included in the SWA report for the water system. Unlike the 2-year Time-of-Travel Zone (see above), it is the only area identified on the delineation map. Multiple Time-of-Travel Zones are not identified for these water systems, as DWS is primarily concerned about acute illness threats which are typically associated with fecal contaminant sources. Until there is evidence to disprove our current understanding, we do not consider those same acute illness threats to be viable outside the outreach area.
  • Recharge Zones (Zone 1):  Refers to C and NTNC water systems that have springs. It identifies the portion of the watershed most likely to be contributing near-term groundwater to the spring (that is the travel time from the point of recharge, through the aquifer, to the spring is less than 2 years). It is our best estimate of what would likely be a 2-year Time-of-Travel Zone. Other zones (Zone 2, Zone 3, etc.) may also occur on the delineation map.