Contaminant Guide

Mercury in Well Water

Mercury in drinking water is primarily inorganic mercury, which causes kidney damage — distinct from the methylmercury in fish that affects neurological development. The EPA MCL is 2 µg/L and, unusually, the MCLG was set equal to the MCL at 2 µg/L — one of the few contaminants where EPA concluded a non-zero MCLG is protective. Mercury in well water is uncommon but occurs near industrial sites, former chlor-alkali plants, and historic mining areas.

What is mercury in well water?

Mercury exists in several forms. In drinking water it is primarily inorganic mercury — either elemental mercury (Hg⁰) or ionic mercury (Hg²⁺, Hg²²⁺). This is chemically different from methylmercury (CH₃Hg⁺), the organic form that bioaccumulates in fish and is associated with neurological damage. The health effects and exposure routes differ substantially.

Sources and where it occurs

Mercury in groundwater near private wells typically comes from:

  • Industrial contamination: chlor-alkali plants (used mercury cathodes historically), thermometer factories, fluorescent lamp manufacturing
  • Historic mining: mercury was used in gold and silver mining (amalgamation process); former mining districts in the western U.S. have mercury-contaminated soils and groundwater
  • Improper disposal of mercury-containing products
  • Atmospheric deposition from coal combustion (contributes to soil but rarely elevates groundwater significantly)

Mercury above the MCL in well water is uncommon without a nearby contamination source. Most affected states include New Jersey, Pennsylvania, Michigan, Ohio, and Texas — areas with industrial history.

Health effects

  • Kidney damage from inorganic mercury — Inorganic mercury accumulates in the kidney proximal tubule cells. Chronic exposure above the MCL can cause proteinuria and renal dysfunction. The 2 µg/L MCL is based on this kidney toxicity endpoint.
  • Important distinction: the neurological effects of methylmercury (developmental neurotoxicity in fetuses and children, via fish consumption) are caused by the organic form and are NOT the primary health concern from drinking water mercury. Do not conflate fish consumption advisories with drinking water mercury risk — they involve different compounds and exposure routes.

The EPA limit: MCL = MCLG = 2 µg/L

Mercury is one of the few health contaminants where EPA set the MCLG equal to the MCL at a non-zero value (2 µg/L). This reflects EPA's judgment that inorganic mercury kidney toxicity has a threshold — a level below which adverse effects do not occur — unlike carcinogens where MCLG is set at zero. The MCL is based on a reference dose for kidney effects, not on carcinogenicity.

Testing

Mercury is measured by cold vapor atomic absorption spectrometry (CVAAS, EPA Method 245.1) or ICP-MS (Method 200.8). Ultra-low detection limits are required; certified labs are essential. Standard water panels often include mercury; request it specifically if you are near an industrial site or former mining area. Note that total mercury analysis does not speciate inorganic vs. organic forms — speciation requires separate methods.

Find a certified lab and learn how to collect a sample

Treatment

  • Reverse osmosis (RO) — effective for inorganic mercury at point of use
  • Activated carbon adsorption — moderate effectiveness for mercury; GAC can adsorb Hg²⁺ but efficiency varies with water chemistry and carbon type
  • Ion exchange — can remove ionic mercury using specialized resins
  • Lime softening at high pH — precipitates mercuric hydroxide; used at municipal scale

Compare mercury treatment systems for private wells

Regulatory framework

MCL: 2 µg/L. MCLG: 2 µg/L. Phase II Rule (1992). MCL=MCLG at 2 µg/L — unusual because most health contaminants have MCLG below or at zero. EPA's determination that inorganic mercury kidney toxicity has a threshold (RfD-based derivation) allows a non-zero MCLG. This regulatory structure is similar to fluoride and differs from arsenic, lead, and PFAS where MCLG=0.

Detection

Cold vapor atomic absorption spectrometry (CVAAS, EPA Method 245.1): mercury reduced to elemental Hg⁰ with stannous chloride or sodium borohydride, purged from solution, measured by atomic absorption at 253.7 nm. Detection limit typically 0.1 µg/L or lower. Cold vapor atomic fluorescence spectrometry (CVAFS) achieves sub-ng/L detection limits. ICP-MS (Method 200.8) also used. All require rigorous contamination control; mercury is notorious for sample contamination from laboratory glassware and reagents.

Geochemistry

Mercury speciation in groundwater includes Hg⁰ (elemental, volatile), Hg²⁺ (mercuric ion), Hg₂²⁺ (mercurous), and organomercury complexes. Inorganic mercury mobility is controlled by sulfide chemistry: Hg²⁺ forms extremely insoluble HgS (cinnabar) under reducing sulfide-rich conditions — limiting mobility in many anaerobic aquifers. Oxidizing conditions or low-sulfide environments increase Hg mobility. Methylmercury (CH₃Hg⁺) can form in anaerobic sediments via sulfate-reducing bacteria methylation; this process is more relevant in surface water and sediments than in deep groundwater.

Data access

Access our data API and methodology

References

  1. Counter, S.A., & Buchanan, L.H. (2004). Mercury exposure in children: A review. Toxicology and Applied Pharmacology, 198(2), 209-230. https://doi.org/10.1016/j.taap.2003.11.032
  2. Mergler, D., Anderson, H.A., Chan, L.H.M., et al. (2007). Methylmercury exposure and health effects in humans: A worldwide concern. Ambio, 36(1), 3-11. https://doi.org/10.1579/0044-7447(2007)36[3:MEAHEI]2.0.CO;2