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Selecting Additional Stage 2 Compliance Monitoring Locations
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Water systems have two main ways to select Stage 2 DBP (TTHM and HAA5) compliance monitoring locations. Those systems that have historical DBP results from multiple locations in the distribution system should use this data to pick additional Stage 2 compliance monitoring locations. Systems that do not have historical DBP test data from more than one location will have to use the guidance below and their knowledge of the system to select appropriate Stage 2 monitoring locations.

Selecting Stage 2 Monitoring Sites Using Previous DBP Data
If you monitored at several locations under the Stage 1 DBP Rule, you can use your existing TTHM and HAA5 data to determine where you should monitor. Start by calculating the LRAA for TTHM and HAA5 at each Stage 1 DBP monitoring site.

The following electronic worksheet is provided to help you organize your existing data. The form allows you to enter your data and calculates your LRAAs for you.


To determine when to do your Stage 2 monitoring, see the Determining Your Stage 2 Monitoring Schedule section below.

Selecting Stage 2 Monitoring Sites Without Previous DBP Data
If you do not have TTHM or HAA5 data or if you need to select more monitoring sites for Stage 2 compliance monitoring than you have data for, you will have to use other knowledge about your system to identify high TTHM and high HAA5 locations in your distribution system that are appropriate monitoring sites. Each site type has certain characteristics that will help you locate appropriate monitoring sites. You may also want to consider the following when selecting your Stage 2 monitoring sites:

  • Geographic distribution of monitoring sites
  • Sites that are already used for compliance with other rules (e.g., Total Coliform Rule)
  • Site accessibility

Characteristics of High TTHM Monitoring Sites:
You should choose your high TTHM site(s) to represent areas in the distribution system where you expect to find higher levels of TTHM throughout the year as compared to other sites. TTHM formation is strongly influenced by residence time (the older the water, the higher the TTHMs). In addition, TTHM formation generally increases with increasing pH. Sites selected near dead ends must be located before the last customer or group of customers, not at the very end of the dead end line.

Good sites for high TTHM include:

  • Near the ends of the distribution system in areas physically located at the end of a water main or group of water mains without looping back to the main portion of the distribution system, at or before the last group of customers.
  • Sparsely populated areas with low flows or lightly developed areas in which there is little movement of water.
  • Downstream of storage facilities – especially those with a common inlet and outlet prior to the last fire hydrant.
  • Low or no residual – indicates likely advanced residence time.
  • Immediately after booster chlorination.

TTHM sample sites should not be located:

  • At a dead-end where there are no customers. The sampling should be representative of water that is being consumed, not stagnant water.
  • Immediately prior to booster disinfection.

Characteristics of High HAA5 Monitoring Sites:
Your high HAA5 site(s) should be chosen to represent areas in the distribution system where you expect to find higher levels of HAA5 throughout the year as compared to other sites. Different systems may find high HAA5 sites in locations with different characteristics. HAA5 formation and decomposition seems to follow a pattern that is different from that of TTHM in the distribution system. While TTHM concentrations are generally highest at the points in the system with the longest residence times, research suggests that HAA5 seem to form and then decompose. The consumption of HAA5 by microorganisms is known as biodegradation, which is more likely to occur when disinfectant residual levels are low or non-existent, particularly in warmer months. Therefore, a high HAA5 site will not necessarily be the site with the longest residence time, and may even be at a site with shorter residence time.

Areas of known biofilm growth should be avoided when choosing high HAA5 sites, although these sites may still be considered for high TTHM. Biofilm degrades HAA, so pipes with biofilm build-up may have water with low levels of HAA. Heterotrophic Plate Count (HPC) data may indicate where areas with biofilm build-up are located.

Good sites for HAA5 include:

  • In areas with low but detectable disinfectant residual (generally, disinfectant residual levels should be consistently above 0.2 mg/L for chlorine or 0.5 mg/L for chloramine) – indicates likely advanced residence time but not sites likely to have biofilm.
  • Near the ends of the distribution system, at or before the last group of customers.
  • In mixing zones where water from different sources combines within the distribution system.
  • Downstream of storage tanks.
  • Prior to the last fire hydrant.
  • After booster chlorination – where formation will have increased due to more available disinfectant and where any biodegradation will be halted.

HAA5 sample sites should not be located:

  • At a dead-end where there are no customers.
  • Immediately prior to booster chlorination.
  • Where there is no disinfectant residual or areas with difficulty maintaining a disinfectant residual (< 0.2 mg/L chlorine or < 0.5 mg/L chloramines).
  • Areas with biofilm problems.

Determining Your Stage 2 Monitoring Schedule
If you conducted standard monitoring or a system specific study for the IDSE, the schedule for your Stage 2 compliance monitoring was submitted as part of your IDSE final report. You should follow your approved Stage 2 monitoring plan.

If you received a 40/30 certification or a Very Small System (VSS) waiver for the IDSE, you must develop a Stage 2 Compliance Monitoring Plan (CMP). The Stage 2 DBP Rule requires that systems conduct monitoring during the peak historical month for TTHM or HAA5 levels. This is referred to as the "Controlling Month." It is meant to represent the "worst case" conditions when DBPs are expected to be at the highest levels during the year.

Since most small systems do not have TTHM and HAA5 data more often than quarterly, it is recommended that you use water temperature to determine your controlling month. Although there are no regulations that require you to measure water temperature in your distribution system or sources, many systems collect this information for process control. Other systems may have temperature data at different points in the treatment plant. If your system measures the temperature of water leaving the treatment plant, consider averaging these data for each summer month to identify the month of warmest water temperature. No matter which data you use, calculate the average for each summer month to identify the month of warmest temperature. This is your controlling month.

All systems are required to monitor during their controlling month, regardless of system size or monitoring frequency. If you monitor yearly, you will sample yearly during this month. If you monitor quarterly, you will sample during this month and every 90 days before and/or after the peak historical month to meet your monitoring requirements.

Note: You do not have to sample at exactly the frequency required for your system; sampling within the same week during each required month is sufficient. For example, if you are required to sample every 90 days, you could sample during the second week of every third month. If you are required to sample yearly, you could sample each year during the first week of your controlling month.