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Coliform Resources

The information on this page is designed for and intended for use by Drinking Water Services County and Department of Agriculture partners who have specialized training and are registered as environmental health specialists. If you have questions regarding this material please contact Drinking Water Services at (971) 673-0405.

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The Groundwater Rule (GWR), which took effect December 1st, 2009, applies to all public water systems that use groundwater sources or purchase groundwater. The primary purpose of the rule is to protect the public from fecal-related bacterial and viral pathogens in public groundwater systems. E. coli is used as the indicator of fecal contamination. If a groundwater source (well or spring) is found to be fecally contaminated, or a significant deficiency or rule violation is identified during a water system survey, the public water system must take corrective action to assure that their consumers are adequately protected. See the following resources for more information on implementing the rule.


Coliform Monitoring Resources

As of April 1, 2016 a detailed investigation is required after the MCL for E. coli is exceeded or a second level 1 coliform investigation is triggered in a 12 month period.  The individual responsible for conducting sanitary surveys at the water system where the investigation was triggered must complete the investigation within 30 days and submit the completed investigation form to DWS.

Groundwater Rule procedures following a positive routine coliform sample

Under the Groundwater Rule, triggered source water monitoring is required after (or "triggered by") a total coliform positive routine sample.

Resources for addressing confirmed E. coli-positive sources under the Groundwater Rule


Representative and Combined Source Monitoring

Systems with large distribution or multiple pressure zones may request to take triggered source samples only at those sources contributing to or representative of the location where the routine coliform distribution sample was positive. Systems with difficulty sampling at each individual source may request to take triggered or assessment source samples at the combined source location prior to any treatment. Prior approval of a representative or combined source monitoring plan is required. Contact your DWS Technical Services staff for further information.

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