The formation of disinfection byproducts (DBPs) is one of the most important issues faced by water suppliers today. Based on epidemiological evidence and the large number of people exposed to DBPs, the USEPA has proposed the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR) as a DBP control measure beyond those already required for public water systems.

Under the Stage 2 DBPR, the USEPA requires utilities to revisit the selection of distribution system sampling sites and include locations with high DBP concentrations in their compliance monitoring program. This is accomplished by performing an Initial Distribution System Evaluation (IDSE).
The schedule for complying with Stage 2 DBPR, and particularly with the IDSE, is unusually rapid as shown in the timeline above. Change may be expected as the Proposed Stage 2 DBPR Rule is currently under review.
•  Community Water Systems (CWSs) that add a primary or distribution system residual disinfectant other than ultraviolet (UV) light.

•  Consecutive CWSs that add a primary distribution system residual disinfectant other than UV.

•  Nontransient Noncommunity Water Systems serving at least 10,000 people.

Systems that meet certain requirements may not have to perform an IDSE. These systems include those with very low trihalomethane/haloacetic acid (THM/HAA) levels (< 40/30 µg/L) and small systems that receive a waiver.

Requirements for IDSE differ by system size; type (surface water vs. ground water systems, consecutive vs. wholesale vs. combined systems, 100 percent purchasing vs. producing systems); the number of treatment plants in producing systems; and other system-specific characteristics.
Water systems will need to conduct a Standard Monitoring Program (SMP) or a System-Specific Study (SSS) in order to comply with the IDSE requirements.

The SMP entails a one-year distribution system monitoring effort to determine the locations that routinely show higher DBP concentrations. The general approach for selecting SMP monitoring sites is to use available data sources and analysis tools to select a large number of potential locations (referred to as preliminary sites). From this group of sites, systems will need to consider several distribution system factors such as geographic coverage, storage facilities, and booster chlorination to narrow down preliminary sites to final SMP sites. After conducting one year of routine monitoring at the final SMP sites, the results will facilitate selection of Stage 2B DBPR compliance monitoring sites.

Unlike an SMP, an SSS does not require one year of routine monitoring. An SSS can be done by water systems that:

•  Have sufficient historical THM4 and HAA5 data.

•  Use a well-calibrated water distribution system model for hydraulic conditions and/or water quality, and conduct at least one round of new sampling to confirm model results.

Systems can use alternative options to conduct an SSS (e.g., historical data combined with new data), but will need to evaluate their methodology carefully and get authorization from the primacy agency. In general, data from an SSS must be equivalent or superior to data from an SMP.
The IDSE is probably among the most complex distribution system evaluations faced by water utilities in order to meet drinking water regulations. Utilities need to determine whether or not they need to conduct an IDSE, understand their system classification, and develop their IDSE sampling and monitoring plans. Data analysis, compilation, and interpretation are complex, and extensive IDSE reports and records are required. As a result, utilities may not have the resources and experienced staff necessary to undertake such an extensive evaluation. Carollo has assisted water utilities with numerous related projects and has the necessary staff and knowledge to assist you with your IDSE.