Disinfection Byproducts Rule
Some people who consume water with DBPs in excess of the Maximum Contaminant Level (MCL) over many years may experience problems with their liver, kidneys, or central nervous system, and may have an increased risk of getting cancer. The EPA has set the MCLs at 80 micrograms per liter (µg/L) for TTHMs and 60 µg/L for HAA5. These numbers are computed for your water system on a locational running annual average (LRAA). For water systems sampling quarterly, the LRAA is an average of the last 4 quarters of data at each sampling location. Because of this, your water system may have single detections above the MCL without having an MCL violation.
Chlorine has been noted in numerous scientific and historical studies as being one of the most important health advances in modern human history. However, there is a balance between proper chlorine dosage as a microbial disinfectant versus the adverse effects of improper dosage. The Disinfection Byproducts Rule is the regulatory response to managing this balance.
Disinfection Byproducts (DBPs) are a group of contaminants that can form when disinfectants used to control microbial pathogens combine with naturally occurring organic matter in the source water. There are more than 500 DBPs that have been detected in treated drinking water and many more that continue to be identified and quantified.
Montana DEQ requires water systems that use chloramines or chlorine to monitor for the following DBPs*:
- Total Trihalomethanes (TTHM)
- Five Haloacetic acids (HAA5) *Water systems that use ozone as the primary disinfectant are required to monitor for Bromate
Maximum Contaminant Level (MCL)
The EPA has set the MCLs at 80 micrograms per liter (µg/L) for TTHMs and 60 µg/L for HAA5.
Protecting Your Water/Health
DBPs are mainly driven by source water quality and treatment effectiveness of the water system. Consumers should request information from their water supply on their specific results.
Compliance with the Stage 2 Disinfection Byproduct (DBP) Rule is required by Community (C) and Non-Transient Non-Community (NTNC) Public Water Systems that use a disinfectant other than ultraviolet light. This rule complies with the Administrative Rules of Montana (ARM 17.38.213), which adopts the Code of Federal Regulations (40 CFR 141.130) and (40 CFR 141.620 for Stage_2 Subpart V).
Sampling requirements depend on source water type (groundwater or surface water), population served, and historical concentrations. Since surface waters systems typically have significantly higher organic matter, the monitoring for surface water systems is weighted heavier than groundwater systems that don’t typically have high organic content. See below tables showing the routine and reduced monitoring requirements:
Please keep in mind that the samples are due in the peak historical week and month - meaning the week of highest historical detections or the week with highest average temperature. Most reduced monitoring schedules are for the 2nd week of August due to temperature, which is a factor in DBP formation.. If you would like to access your online monitoring schedule, please follow this link:
Public Water Supply Monitoring Schedule
Maximum Redisual Disinfection Limit (MRDL)
To ensure that your consumers are not consuming water with chlorine levels above the appropriate level (4.0 mg/L on a running annual average), public water systems are required to monitor their free chlorine residuals in the distribution system monthly (at same time and place as their total coliform sample). To fulfill this requirement, please fill out the form in the link below and send it to the email address on the form within 10 days after each quarter. Alternatively, simply provide the free residual chlorine level on the chain-of-custody you provide to the laboratory with each total coliform sample, and the lab will provide these values to DEQ.
Monthly DBP Reporting Form
Understanding Your Samples
DBPs represent a chronic risk, which means the adverse health effects are based on a lifetime exposure. Water systems and water consumers should look at long-term trends and keep an eye on long-term improvements for reduction of DBPs.
EPA website with many helpful links and documents
Quick Reference Guide for Disinfection Byproducts
Disinfection Byproducts Rule Manager
PO Box 200901
Helena, MT 59620
Phone: (406) 541-9014
Fax: (406) 444-1374
DEQ Mainline: (406)444-4400