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Initial Distribution System Evaluation (IDSE)

The goal of the initial distribution system evaluation (IDSE) is to identify the sites in the distribution system where customers may be exposed to high levels of DBPs. These sites will then be used for Stage 2 compliance monitoring. NTNC water systems serving less than 10,000 population are exempted from IDSE requirements but will be required to do Stage 2 compliance monitoring.

Systems had a choice of four IDSE options when the rule was originally implemented:

  • Standard Monitoring: Monitoring takes place over one year.
  • Systems Specific Study: Uses existing monitoring data or hydraulic modeling.
  • 40/30 Certification: Uses existing data if all results are < 1/2 the MCL. (Note: not available to newly chlorinating or new systems that chlorinate)
  • Very Small System Waiver: Available to water systems with less than 500 population. (Note: not available to newly chlorinating or new systems that chlorinate)

New water systems that add a disinfectant other than UV and existing systems that begin adding a disinfectant must conduct Standard Monitoring or do a System Specific Study using hydraulic modeling in order to comply with the IDSE requirements. 
Systems that were able to utilize either the 40/30 certification or very small system waiver when the rule was originally implemented were required to complete a Compliance Monitoring Plan (CMP) before compliance monitoring began, but otherwise had no other requirements under the IDSE portion of the rule.