Water Quality
Trihalomethanes (THMs) and haloacetic acids (HAAs) are disinfection byproducts that form when chlorine-based disinfectants react with naturally occurring organic matter in source water during drinking water treatment. The U.S. Environmental Protection Agency regulates them under the Stage 2 Disinfectants and Disinfection Byproducts Rule (40 CFR Part 141, Subpart L): the Maximum Contaminant Level for total trihalomethanes (TTHM) is 80 micrograms per liter, and for five haloacetic acids (HAA5) it is 60 micrograms per liter, both measured as locational running annual averages. LADWP uses chloramines rather than free chlorine as its primary residual disinfectant across the San Fernando Valley distribution system. Chloramine chemistry suppresses some THM formation compared to free chlorine but generates different HAA species at varying ratios depending on source water organic content. LADWP's 2024 Consumer Confidence Report, published at ladwp.com, reports TTHM and HAA5 averages for SFV distribution zones that remain below federal MCLs. For households seeking to reduce DBP exposure beyond compliance levels, NSF/ANSI 53-certified activated carbon block filters and NSF/ANSI 58-certified reverse osmosis systems provide independently verified reduction for these compounds.
What trihalomethanes and haloacetic acids are in treated drinking water
Trihalomethanes are a class of four regulated compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. They form when free chlorine or chloramine reacts with naturally occurring organic material (humic and fulvic acids dissolved from soil and vegetation) during water treatment and distribution. The EPA regulates them together as TTHM, the sum of all four. Haloacetic acids are a related class: the five regulated species in HAA5 are monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. They form through a parallel set of reactions with the same organic precursors.
Both compound classes are regulated because epidemiological studies, reviewed by the EPA in setting the Stage 2 DBPR, have associated long-term exposure at high concentrations with increased risk of bladder cancer and adverse reproductive outcomes. The full regulatory background is covered in our article on the Stage 2 DBPR and SFV disinfection byproducts. At the concentration levels typical in compliant utility water, documented risks are substantially lower than the risks of inadequate disinfection, which is why EPA sets MCLs as a balance rather than treating any detectable DBP as a violation.
Why LADWP uses chloramines as its primary disinfectant in the San Fernando Valley
LADWP converted most of its SFV distribution system from free chlorine to chloramines as part of a broad industry shift that followed the 1998 Stage 1 Disinfectants and Disinfection Byproducts Rule, which tightened the TTHM MCL from 100 to 80 micrograms per liter and established the first federal HAA5 MCL at 60 micrograms per liter. Chloramines, formed by combining chlorine and ammonia in controlled ratios, are a more stable residual disinfectant than free chlorine in large distribution systems. Free chlorine degrades quickly over the distances involved in a system as large as LADWP's SFV network, leading to inadequate residual at remote taps. Chloramines maintain effective residual across longer pipe runs and larger storage volumes.
The tradeoff is that chloramines interact differently with organic matter in source water. While TTHM formation is generally lower under chloramine disinfection than under free chlorine at comparable contact times, chloramines generate iodinated THMs and nitrogen-containing byproducts (such as nitrosamines) not regulated under TTHM or HAA5. For context on how chloramine affects the taste and odor of SFV tap water, see our companion article on LADWP's chloramine system and water smell in the SFV.
LADWP 2024 Consumer Confidence Report: TTHM and HAA5 levels in SFV distribution zones
LADWP publishes annual Consumer Confidence Reports for each service area under Safe Drinking Water Act requirements. The CCR for SFV areas reports TTHM and HAA5 as Locational Running Annual Averages (LRAAs): 12-month rolling averages calculated at each monitoring location across the distribution system. This approach, required by the Stage 2 DBPR since 2012 for large utilities, is more sensitive than the older systemwide average method because it prevents high and low zones from averaging each other out.
| Regulated Compound | EPA MCL | Measurement Basis | SFV 2024 Status |
|---|---|---|---|
| Total Trihalomethanes (TTHM) | 80 µg/L | Locational Running Annual Average (LRAA) | Below MCL per 2024 CCR |
| Haloacetic Acids, 5 Species (HAA5) | 60 µg/L | Locational Running Annual Average (LRAA) | Below MCL per 2024 CCR |
| Total Chloramines (as Cl2) | 4 mg/L MRDL | Running Annual Average | Within MRDL per 2024 CCR |
Source: LADWP 2024 Consumer Confidence Report at ladwp.com. EPA MCLs from 40 CFR Part 141, Subpart L. For the source water blend serving SFV neighborhoods, see our article on LADWP's groundwater and imported water blend.
How chloramine disinfection compares to free-chlorine systems for DBP formation
The shift from free chlorine to chloramine changes the DBP profile in predictable ways. EPA technical support documents for the Stage 2 DBPR note that chloramine disinfection typically reduces TTHM formation compared to free chlorine at similar doses and contact times, because chloramine reacts less readily with organic precursors to form chloroform and bromodichloromethane, the dominant TTHM species in free-chlorine systems. HAA5 formation patterns differ: some HAA species are reduced under chloramine while others remain at comparable concentrations. Chloramine also generates nitrogen-containing byproducts (cyanogen chloride, haloacetonitriles, nitrosamines) that are not regulated under TTHM or HAA5. The Stage 2 DBPR addressed this complexity by requiring monitoring at each distribution system location rather than a single systemwide average, pushing utilities to control DBPs throughout the network, including zones where water age is higher.
SFV homeowners who want the complete monitoring dataset, including all individual location results, can request this information from LADWP under Safe Drinking Water Act public information provisions. The full Stage 2 DBPR framework is documented at epa.gov/dwreginfo.
Health context for disinfection byproducts at SFV concentration levels
EPA's MCLs for TTHM and HAA5 represent a judgment about the balance between DBP exposure risk and the risk of inadequate disinfection. At concentrations below the MCLs, where SFV water has consistently fallen based on LADWP CCR reporting, EPA considers the water in compliance with federal health standards. The International Agency for Research on Cancer classifies chloroform as a Group 2B possible carcinogen based on limited human evidence and animal data. Dichloroacetic acid (a HAA5 species) is a Group 2A probable carcinogen based on animal studies at high doses. At the concentration levels reported in LADWP CCRs for SFV zones, long-term risks are substantially lower than the microbial risks that would result from inadequate disinfection. For comparison with other regulated contaminants in SFV water, see our articles on PFAS in SFV and Santa Clarita water and chromium-6 in SFV zone compliance.
Which filter certifications remove THMs and HAAs from SFV tap water
Two NSF International certification standards apply to DBP reduction at the residential level. NSF/ANSI 53 governs point-of-use and point-of-entry systems that reduce health-effects contaminants: filters certified under this standard for "Volatile Organic Compounds" or "Trihalomethanes" have passed independent challenge tests at rated flow rates using chloroform as the test contaminant. Carbon block filters meet this standard; standard granular activated carbon (GAC) pitcher filters generally do not and have limited effectiveness against chloramine as well. NSF/ANSI 58 governs reverse osmosis drinking water systems and includes reduction claims for dissolved compounds including HAA5 species. A compliant RO system certified under NSF/ANSI 58 will reject HAA5 compounds alongside other dissolved material.
For SFV homes using LADWP's chloramine-treated water, catalytic carbon is the recommended media for chloramine reduction; it also maintains effectiveness for THM reduction. A whole-house catalytic carbon system addresses DBPs at every tap and appliance. An under-sink carbon block or RO addresses the drinking and cooking tap specifically. For a full walkthrough of how these systems are installed and sized for SFV homes, see our article on how whole-house water filtration works in the SFV. NSF certification records are searchable at nsf.org.
Do trihalomethanes form in residential plumbing?
THMs in tap water are formed primarily during the utility's treatment and distribution process, not in residential plumbing. LADWP adds chloramine at treatment plants and DBP formation occurs as water moves through large distribution mains. Residential pipe contact time is short enough that additional THM formation in home pipes is not a significant factor for most SFV households.
Does LADWP's San Fernando Valley tap water contain trihalomethanes?
Yes, at low concentrations that remain below the EPA MCL of 80 micrograms per liter. LADWP reports TTHM locational running annual averages for SFV distribution zones in its annual Consumer Confidence Report at ladwp.com. Chloramine disinfection, which LADWP uses across most SFV service areas, generally produces lower TTHM concentrations than free-chlorine systems at comparable doses and contact times.
Does a carbon pitcher filter remove trihalomethanes from SFV tap water?
Only if it carries an NSF/ANSI 53 certification for trihalomethane or VOC reduction. Standard granular activated carbon in most pitcher filters is not certified for THM reduction and has limited effectiveness against chloramine. Carbon block filters with an explicit NSF/ANSI 53 THM claim provide more consistent reduction. Verify certification status for any product at nsf.org before purchasing.
Can chloramine produce THMs and HAAs in drinking water?
Yes. Chloramine reactions with natural organic matter in source water produce both TTHM and HAA5, though generally at lower TTHM concentrations than free chlorine at comparable doses. Chloramine also generates nitrogen-containing byproducts including nitrosamines and haloacetonitriles that are not regulated under TTHM or HAA5 but are subject to ongoing EPA research.
What is the EPA limit for trihalomethanes in drinking water?
The EPA Maximum Contaminant Level for total trihalomethanes (TTHM) is 80 micrograms per liter, measured as a locational running annual average under the Stage 2 Disinfectants and Disinfection Byproducts Rule (40 CFR Part 141, Subpart L). The MCL for five haloacetic acids (HAA5) is 60 micrograms per liter, measured the same way. Both limits apply to all community water systems in the United States.
Do reverse osmosis filters remove haloacetic acids from tap water?
Yes. Reverse osmosis membranes certified under NSF/ANSI 58 reject a broad range of dissolved compounds, including HAA5 species. A maintained RO system can reduce HAA5 concentrations significantly at the point of use. Cartridge replacement on the manufacturer's schedule maintains reduction performance over time. Most SFV households address HAA5 at the drinking tap with an under-sink RO unit.
