Total Trihalomethanes and Haloacetic Acids in San Fernando Valley Tap Water: Stage 2 DBPR Compliance Data for LADWP and Las Virgenes MWD

What are total trihalomethanes and how do they form in tap water?

Trihalomethanes (THMs) form when a halogen-based disinfectant, most commonly chlorine or chloramine, reacts with naturally occurring organic matter (NOM) dissolved in source water. NOM originates from decaying plant material, algae, and humic acids that enter surface water reservoirs through runoff. The four compounds that the EPA regulates as total trihalomethanes are chloroform (trichloromethane), bromodichloromethane, dibromochloromethane, and bromoform. Their relative concentrations in a given water supply depend on the NOM content of the source water, the type of disinfectant used, the contact time, water temperature, and the bromide concentration in the raw water.

Colorado River water, which supplies a portion of the SFV tap water blend via the Metropolitan Water District, carries higher bromide concentrations from its passage through arid geological formations. Higher bromide shifts the THM distribution toward the brominated species (bromodichloromethane, dibromochloromethane, bromoform) rather than chloroform. The mix of these four compounds in LADWP-served SFV homes varies based on which MWD source water dominates in a given season, as described in the SFV water quality source comparison.

What are haloacetic acids (HAA5) and why does the five-acid grouping matter?

Haloacetic acids form through a parallel reaction pathway when disinfectants oxidize NOM in source water. The EPA's Stage 2 DBPR regulates a specific group of five: monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), monobromoacetic acid (MBAA), and dibromoacetic acid (DBAA). The grouping is defined by 40 CFR 141.65 and applies to any community water system that adds a disinfectant.

HAA5 and TTHM are both generated by the same oxidation process but through different reaction pathways, which is why the EPA tracks them separately. DCAA and TCAA are the most common HAA5 species in chlorinated and chloraminated water systems. Because HAA5 compounds are less volatile than TTHM, they do not dissipate from water at room temperature the way that chloroform, for example, can partially off-gas from a glass of water left open. This means a whole-house carbon filtration approach that reduces HAA5 at the point of entry addresses the concern across all household uses, not just at the drinking tap.

How does the EPA Stage 2 DBPR measure compliance for TTHM and HAA5?

The Stage 2 Disinfectants and Disinfection Byproducts Rule was promulgated in 2006 and required compliance by April 2012 for surface water systems serving more than 50,000 people. The rule changed the compliance metric from a system-wide running annual average (used under Stage 1 DBPR) to a Locational Running Annual Average (LRAA) at each individual monitoring location.

The distinction matters for SFV households because a distribution system is not chemically uniform. Water that travels through longer pipe runs, sits in storage tanks longer, or passes through warmer areas of the distribution network can accumulate higher DBP concentrations than water at the treatment plant outlet. Under Stage 1 rules, a low reading at one monitoring point could offset a high reading at another location when calculated as a system-wide average. Under Stage 2, each location must satisfy its own LRAA independently. Both the 80 µg/L TTHM limit and the 60 µg/L HAA5 limit must be met at each monitoring point.

California adopted the Stage 2 DBPR requirements into Title 22 of the California Code of Regulations, as maintained by the State Water Resources Control Board (SWRCB). The enforceable MCLs are the same as federal limits (80 µg/L for TTHM, 60 µg/L for HAA5). California's Division of Drinking Water conducts compliance review for utilities operating in the state, including LADWP and LVMWD.

What do LADWP Consumer Confidence Reports show for SFV monitoring locations?

LADWP publishes its annual Consumer Confidence Report at ladwp.com/water-quality. The report includes TTHM and HAA5 figures as Locational Running Annual Averages and flags any exceedance of the MCL. As a large surface water system, LADWP has been subject to Stage 2 DBPR compliance monitoring since 2012.

Because LADWP switched from free chlorine to monochloramine as its primary residual disinfectant in 2014, as detailed in the SFV chloramine article, the utility's TTHM formation profile shifted. Chloramine reacts with organic matter to form lower concentrations of TTHM than free chlorine does in the same source water. However, chloramine can form different byproduct classes, including some nitrogenous compounds, that are not currently part of the Stage 2 DBPR regulated list. The Consumer Confidence Report covers the regulated TTHM and HAA5 groups specifically. For current LRAA figures by monitoring zone, check the most recent annual report at the LADWP water quality page.

A limitation to keep in mind: the CCR reports distribution system monitoring point readings. Your specific address may receive water from any of several distribution segments depending on your location in the system. A free in-home water consultation measures TDS, pH, hardness, and chloramine residual at the tap, which supplements the CCR data with a point-specific reading.

What does Las Virgenes MWD report for Calabasas, Agoura Hills, and Westlake Village?

Most of Calabasas, Agoura Hills, and Westlake Village are served by Las Virgenes Municipal Water District, which also publishes an annual Consumer Confidence Report with TTHM and HAA5 data. LVMWD purchases treated water from MWD and distributes it to its service area. As a MWD purchaser, LVMWD's source water has a broadly similar origin profile to LADWP, drawing from State Water Project and Colorado River Aqueduct sources through MWD treatment facilities.

DBP formation in the LVMWD distribution system may differ from LADWP readings because the distribution pipe network, average pipe residence times, and storage configurations are distinct. Both utilities must maintain Stage 2 DBPR compliance at each monitoring location independently, so the regulatory standard is identical. For Calabasas and Agoura Hills homeowners, the relevant CCR is at lvmwd.com/water-quality, not the LADWP report. If you are uncertain which utility serves your address, check your water bill or search your address on the respective utility websites. Homes in the eastern portion of Calabasas near Valley Circle Boulevard may receive LADWP service rather than LVMWD.

For water softener and filtration installations in Calabasas, both TTHM and HAA5 figures from the LVMWD CCR are useful inputs for sizing a whole-house carbon stage if DBP reduction is a treatment goal alongside hardness.

Does using chloramine instead of free chlorine reduce disinfection byproducts?

At the same organic matter load, monochloramine forms lower concentrations of TTHM and HAA5 than free chlorine. This is a documented property of the disinfection chemistry: chloramine is a weaker oxidant than free chlorine, so it reacts more slowly with organic precursors and forms fewer halogenated byproducts in the Stage 2 DBPR-regulated groups. The EPA Stage 2 DBPR Q&A document notes this trade-off explicitly: chloramine can reduce TTHM and HAA5 formation while potentially increasing formation of byproduct classes not currently regulated under Stage 2, including iodo-trihalomethanes and N-nitrosodimethylamine (NDMA).

LADWP's switch to chloramine in 2014 was partly motivated by the Stage 1 DBPR compliance environment and the need to maintain treated water quality over long distribution pipe runs. The practical outcome for SFV households is that TTHM and HAA5 levels under chloramine treatment are generally lower than they would be under free chlorine with the same source water, but the regulatory CCR and annual monitoring data remain the authoritative record of what is actually in the distribution system.

Which filtration systems reduce TTHM and HAA5 at the tap?

The NSF International drinking water treatment unit certification database lists certified products by contaminant reduction claim. For TTHM and related volatile organic chemicals, the relevant NSF standards are:

• NSF/ANSI 42 (aesthetic effects): covers chlorine taste and odor reduction via activated carbon but does not certify VOC or TTHM reduction.
• NSF/ANSI 53 (health effects): covers specific contaminants including some TTHM components such as chloroform. A product's NSF 53 listing specifies the exact compounds it is certified to reduce.
• NSF/ANSI 58 (reverse osmosis systems): RO systems certified under NSF 58 are tested for volatile organic chemical reduction. Check the specific product's NSF listing for its certified TTHM and HAA5 reduction claims.

For whole-home DBP reduction, a catalytic or activated carbon whole-house filter installed at the point of entry addresses TTHM across all household uses, including the shower (where inhalation and skin absorption of volatile THMs occurs in addition to ingestion). Point-of-use RO addresses the drinking and cooking tap specifically. The right choice depends on the treatment goals, household water usage patterns, and what the tap test shows for TDS and other parameters.

San Fernando Valley water filtration system recommendations from Noohi Construction start with what the tap test shows at the specific address, not from system-wide CCR averages. For households prioritizing DBP reduction alongside hardness, reverse osmosis installation in the SFV combined with a whole-house carbon pre-filter addresses both at different points in the treatment chain.

Sources used in this article: 40 CFR Part 141 Subpart V (Stage 2 DBPR, eCFR); EPA Stage 2 DBPR Questions and Answers; California SWRCB Maximum Contaminant Levels and PHGs Table; LADWP Consumer Confidence Report (current year); Las Virgenes MWD Consumer Confidence Report (current year); Metropolitan Water District Water Quality Reports; NSF International Drinking Water Treatment Unit Certification Database.