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Haloacetic Acids (HAA5): Regulation, Toxicity, Occurrence, Trends & EPA Violations

Farin Tasnuva Dhara and Andrew Easterling

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What are HAAs?

Haloacetic acids

Family of disinfection byproducts (DBPs)

Potential carcinogens

    • Infants, children, and pregnant women especially vulnerable

Regulated by National Primary Drinking Water Regulations (NPDWR)

Cl

O

OH

Cl

O

OH

Cl

Br

O

OH

Br

Br

O

OH

O

OH

Cl

Cl

Cl

Monochloroacetic acid

Dichloroacetic acid

Trichloroacetic acid

Monobromoacetic acid

Dibromoacetic acid

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HAA Classification

  • HAA5 – regulated by NPDWR
    • Monochloroacetic acid
    • Dichloroacetic acid
    • Trichloroacetic acid
    • Monobromoacetic acid
    • Dibromoacetic acid
  • HAA9
    • Monochloroacetic acid
    • Dichloroacetic acid
    • Trichloroacetic acid
    • Monobromoacetic acid
    • Dibromoacetic acid
    • Tribromoacetic acid
    • Bromochloroacetic acid
    • Bromodichloroacetic acid
    • Chlorodibromoacetic acid

​

​

​

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HAA MCL and MCLG

  • Total MCL: 0.060 mg/L
  • No collective MCLG
    • Dichloroacetic acid: 0 mg/L
    • Trichloroacetic acid: 0.02 mg/L
    • Monochloroacetic acid: 0.07 mg/L
    • Bromoacetic acid and dibromoacetic acid: no MCLG

66.8%

33.2%

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History of HAA Regulation

Dec 1998 – Added to the NPDWR alongside disinfectants and other DBPs (THMs) – 0.060 mg/L

    • Stage 1 Disinfection and Disinfection Byproducts Rule (DBPR)
    • 2002 deadline for systems serving at least 10,000
    • 2004 deadline for systems serving fewer than 10,000

Jan 2006 – Revision to the DBPR – HAA5 and TTHM

    • Stage 2 DBPR
    • Provides MCLGs for monochloro-, dichloro-, and trichloroacetic acid
    • Increased monitoring requirements
    • Revised Stage 1 to cover every sampling site in a distribution system, instead of a system-wide average

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HAA Regulations

  • Applies to water systems using any disinfectant other than UV light
  • Initial Distribution System Evaluation (IDSE)
    • Evaluation of distribution system for locations with high DBP concentration
    • Running annual average at these individual locations, not just system-wide
  • Utilities must measure short-term peaks in DBPs, and review operations to address – action handled on a state-level
  • Quarterly monitoring, depending on system size
  • Stage 2 estimated to reduce risk of bladder cancer by up to 182 cases per year, at an average household cost of $0.51/year
    • Reduction of reproductive and developmental diseases were also anticipated but not quantified

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HAA5 Toxicity

HAA Species

Minimum RfD (mg/kg/day)

Cancer Data Available

Drinking Water Equiv. Level (mg/L)1

Calculated MCLG (mg/L)2

True MCLG (mg/L)

Chloroacetic acid

0.002

​

0.07

0.014

0.07

Dichloroacetic acid

0.004

X

0.14

0.028

0

Trichloroacetic acid

0.02

X

0.7

0.14**

0.02

Bromoacetic acid

0.0017

​

0.0595

0.012

N/A

Dibromoacetic acid

0.0003

X

0.0105

0.002

N/A

 

 

  • 70 kg body weight
  • 2 L/day intake
  • RSC assumed to be U.S. EPA default 20%
  • Reference dose (RfD) data taken from CompTox Chemicals Dashboard

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Occurrence Aspect 1: National Distribution

  • Majority of measurements fall below 40 µg/L → low chronic risk for most systems.
  • Many samples cluster 40–60 µg/L, meaning utilities often operate near compliance limits.
  • Violations usually fall between 60–80 µg/L, indicating moderate exceedance rather than catastrophic failure.
  • Rare extreme values >120–200 µg/L occur during operational upsets.

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Detection Limit Distribution

  • Most labs use DL <1 µg/L, sufficient for regulatory compliance.
  • Ensures reliability of detecting values near MCL.
  • A few DL >3 µg/L could under detect lower values → possible variability in low-end occurrence.

​

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Occurrence Aspect 2: Population Size

  • Over 30,000 violations originate from very small systems (<1,000 people).
  • Small systems face the highest risk due to:
    • limited operator training
    • outdated treatment technology
    • high water age in distribution
  • Large systems show fewer violations but occasional large spikes.

System size is a major predictor of DBP risk.

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Occurrence Aspect 3: Source Water Type

  • Surface water (SW, SWP) systems consistently show higher HAA5 levels → more NOM.
  • Groundwater systems have lower HAA5, but more MR violations.
  • SWP (purchased surface water) systems show high violations due to lack of direct treatment control.
  • Source water type is a core determinant of HAA risk.

​

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Geographical Hotspots: State-level Occurrence

  • High-HAA states often:
    • depend heavily on surface water
    • have warm climates (higher temperature → more DBP formation)
  • Clear hotspots appear in Southeast, Midwest, and some coastal states.
  • State-year heatmap shows persistent “hot” regions with chronic elevation.

​

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Monthly & Seasonal Occurrence Trends

  • Slightly elevated HAA levels in summer & early fall.
  • Seasonality correlates with:
    • warming → faster DBP formation
    • algal blooms → more NOM
    • lower chlorine stability
  • Yearly seasonality pattern reinforces DBPR need for year-round monitoring.

​

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Yearly Trends

  • Violations increased sharply after 2013–2014 → Stage 2 DBPR monitoring.
  • Peaks around 2022–2023 → post-COVID + operational disruptions.
  • Decline in 2024 likely due to incomplete reporting.
  • Long-term HAA5 time-series shows:
    • episodic spikes
    • increasing range
    • strong seasonal modulation

​

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Trends by Population Group

  • Very small systems show erratic spikes → operational instability.
  • Mid-size systems show seasonal behavior.
  • Large systems show more stable patterns → better control technologies.

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Trends by Source Water Type

  • SW and SWP systems show sharper peaks in summer.
  • GW systems remain low but occasionally spike due to blending or operational changes.
  • Confirms that source type strongly modulates DBP formation kinetics.

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What Happens When a Utility Exceeds the MCL?

  • Utilities must issue Tier 2 Public Notice.
  • Increased sampling frequency, compliance schedules.
  • EPA or state may impose penalties.
  • Long-term exceedance → increased cancer risk.
  • Communities lose trust → pressure on local government.

​

Treatment & Mitigation Strategies:

  • Content:
  • Enhanced coagulation → TOC reduction
  • Activated carbon (GAC/POE) → removes precursors
  • Switch to chloramines → reduces HAAs (but produces other DBPs)
  • Optimize distribution system flushing
  • Install mixers in storage tanks
  • Source water protection

​

HAA5 risk is manageable but requires investment, training, and continuous monitoring.

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Why We Should Care:

    • HAA5s pose long-term cancer risks, and millions of people are served by systems that still exceed or approach regulatory limits.
    • Small and resource-limited systems have the highest violation rates, meaning vulnerable communities face disproportionate risks.
    • Surface water systems and warm-season peaks increase exposure, making treatment and monitoring consistency essential.
    • Rising national violations and frequent monitoring/reporting failures indicate gaps in compliance rather than chemistry alone.

Conclusion:

    • HAA5 exceedances are driven by system size, source water type, and seasonal factors, all of which are predictable and manageable.
    • Most violations are preventable, with better monitoring, enhanced NOM removal, and targeted support for small systems.
    • Data suggests increasing yearly violations, highlighting the need for proactive treatment improvements and regulatory reinforcement.
    • Ensuring compliance protects public health, reduces long-term risks, and strengthens trust in drinking water systems.

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