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Groundwater Source Monitoring Requirements
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Triggered Source Water Monitoring
Also called triggered source sampling, triggered source water monitoring tests the untreated groundwater source (from a well or spring) for coliform bacteria when "triggered." The trigger is a positive (present) result from a routine coliform sample in the distribution system. Groundwater systems that disinfect are subject to the triggered monitoring requirement unless they are implementing the 4-log disinfection monitoring option. The triggered samples must be incorporated into the water system's coliform sampling plan.
Representative Triggered Source Water Monitoring Plans: Systems with large distributions or multiple pressure zones may request to take triggered source samples only at those sources contributing to the location where the routine coliform distribution sample was positive. Prior approval of a representative triggered source water monitoring plan is required.

Source Assessment Monitoring
Source assessment monitoring refers to ongoing, periodic sampling of the source water that is not triggered by another test result. There are two types of source assessment monitoring:
  • Annual Source Assessment Monitoring: Requires at least one "raw" untreated water sample from each source per year from all water systems that disinfect the water in any way - either with oxidizing agents such as chlorine or ultraviolet light. A triggered source sample can meet this requirement for that calendar year.
  • Monthly Source Assessment Monitoring: Groundwater sources that are determined to be at higher risk of fecal contamination are required to collect monthly source samples for twelve months (or only during the months of operation for seasonal systems or sources) to determine whether any fecal contamination exists. Drinking Water Services (DWS) will notify water systems individually if monthly source assessment monitoring is required.

Additional or Confirmation Monitoring
After one E. coli-positive result from the source, a system must take five additional source samples within 24 hours to confirm fecal contamination, unless the system pursues corrective action immediately with regulatory approval.

4-log Disinfection Monitoring
In lieu of triggered monitoring, a groundwater system that provides 4-log (99.99%) inactivation or removal of viruses may pursue disinfection monitoring with regulatory approval. This option is only available to systems that: a) DWS hydrogeologists determine do not need source reconstruction; and b) demonstrate that they achieve 4-log virus treatment before the first user. If approved, the water system will be assigned a minimum chlorine residual that must be maintained and monitored at the entry point.