What Is Coal Tar Contaminants?

Coal tar contaminants are not a single chemical. They are a complex mixture of industrial compounds derived from the destructive distillation, gasification, or high-temperature processing of coal. Coal tar historically came from manufactured gas plants, coke ovens, steelmaking, wood preservation, roofing materials, pavement sealants, coal-tar pitch production, and creosote manufacturing. In drinking water investigations, the phrase usually refers to coal-tar-derived organic contaminants that have migrated into soil, groundwater, sediment, or surface water.

The most important chemical group in coal tar is polycyclic aromatic hydrocarbons, commonly called PAHs. These include compounds such as naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene, and benzo[g,h,i]perylene. Coal tar may also contain phenols, cresols, quinolines, carbazoles, benzene, toluene, ethylbenzene, xylenes, heterocyclic nitrogen-, sulfur-, and oxygen-containing compounds, and trace metals depending on the source material and processing history.

Coal tar contamination is a high-risk drinking water issue because many of its components are persistent, toxic, hydrophobic, and capable of forming long-lived subsurface source zones. Some coal tar residues behave as dense non-aqueous phase liquids, or DNAPLs, meaning they can sink below the water table and remain in fractures, low-permeability layers, or deep aquifer zones for decades. Even when only small amounts dissolve into water, the resulting concentrations can exceed health-based screening levels for carcinogenic PAHs or volatile aromatics.

Scientific Identity

Coal tar contaminants are best understood as a chemically diverse industrial mixture rather than a substance with one formula, one molecular weight, or one CAS number. Individual constituents have their own identities and toxicological profiles. For example, naphthalene is a two-ring PAH that is more volatile and more water-soluble than heavier PAHs, while benzo[a]pyrene is a five-ring PAH that is much less soluble but far more significant for cancer risk assessment. Fluoranthene and pyrene are four-ring PAHs commonly used as indicators of coal tar, petroleum combustion, and creosote-related contamination.

The environmental behavior of coal tar is controlled by its mixture chemistry. Lower molecular weight aromatics, such as benzene, toluene, xylenes, phenols, and naphthalene, can dissolve into groundwater and migrate away from the source zone. Heavier PAHs tend to sorb strongly to organic carbon in soil and sediment, but they can still be important in drinking water if colloids, suspended particulates, or mobile coal-tar droplets are present. The most carcinogenic PAHs often occur at low dissolved concentrations, so sensitive laboratory methods are required.

Coal tar contamination can include both volatile organic compounds and semi-volatile organic compounds. VOCs are more likely to create vapor intrusion concerns, where contaminated groundwater or soil gas releases vapors that migrate into buildings. SVOCs, especially PAHs, are less volatile but persistent. The combined presence of VOCs, PAHs, phenolic compounds, and tarry residues is a key diagnostic pattern at former manufactured gas plant sites and creosote-impacted areas.

How Coal Tar Contaminants Enters Drinking Water

Coal tar contaminants enter drinking water primarily through industrial releases to soil and groundwater. Major source settings include former manufactured gas plants, coal gasification facilities, coking operations, steel plants, wood-treatment facilities using creosote, coal-tar storage areas, tar pits, industrial lagoons, rail yards, asphalt and pavement-sealant handling areas, and hazardous waste disposal sites. Historic facilities are especially important because many operated before modern hazardous waste controls, secondary containment, lined impoundments, or groundwater monitoring requirements existed.

Once released, coal tar can migrate downward through soil. If present as a dense tarry liquid, it may pass through the unsaturated zone and enter the saturated aquifer. Because coal tar can be denser than water, it may move downward through permeable sand, gravel, fractured rock, utility corridors, or buried stream channels until it encounters a lower-permeability layer. There it can pool, smear, or become trapped as residual droplets. These zones slowly release dissolved PAHs, phenols, and aromatic hydrocarbons into groundwater, creating plumes that can move toward private wells, municipal supply wells, springs, or surface water bodies.

Surface water can also be affected when contaminated groundwater discharges into rivers, lakes, or reservoirs, or when contaminated sediment is resuspended during storms, dredging, or flooding. Coal-tar-based pavement sealants and urban runoff can contribute PAHs to stormwater systems, although drinking water risk is usually highest where a water supply is hydraulically connected to an industrial source area. In older urban areas, buried coal tar residues may also intersect with water mains, sewer lines, or construction dewatering zones.

Vapor intrusion is relevant when coal tar contamination includes benzene, toluene, ethylbenzene, xylenes, naphthalene, or other volatile and semi-volatile compounds. Vapors may not directly contaminate finished tap water, but they can create an indoor air exposure pathway in homes built above contaminated groundwater or soil gas. In some cases, contaminated well water can also release volatile compounds during showering, laundry, or cooking.

Occurrence and Exposure

Coal tar contaminants are most often found near historic industrial corridors, former manufactured gas plant properties, wood-preserving sites, coking facilities, rail yards, ports, foundries, and older municipal utility sites. Many manufactured gas plants were located in city centers near rivers, rail lines, and gas distribution networks. As a result, coal tar contamination is frequently discovered during urban redevelopment, utility excavation, river sediment cleanup, or groundwater investigations.

Private wells are a particular concern because they may not be routinely tested for PAHs, phenols, or volatile aromatics unless a nearby contamination source has been identified. A well can appear clear and odorless even when trace levels of carcinogenic PAHs are present. Conversely, water affected by lighter coal-tar components may have a medicinal, tar-like, smoky, solvent-like, or mothball-like odor, especially when naphthalene or phenolic compounds are present. Odor is not a reliable safety indicator because health-relevant concentrations may be below the odor threshold for some compounds.

People can be exposed by drinking contaminated water, preparing infant formula, cooking, washing produce, inhaling vapors released from water during indoor use, or contacting contaminated water during bathing. For heavy PAHs, ingestion is often the primary drinking water pathway. For benzene and naphthalene, inhalation and dermal exposure may contribute meaningfully to total dose. Exposure can also occur through contaminated fish or sediment contact in waterways affected by coal-tar waste, although those pathways are usually managed under separate fish consumption and sediment advisories.

Health Effects and Risk

The health risk from coal tar contaminants depends on which compounds are present, their concentrations, exposure duration, and whether ingestion, inhalation, and dermal contact are all occurring. Coal tar mixtures are a major concern because they may contain carcinogenic PAHs. Benzo[a]pyrene is commonly used as a reference compound for PAH cancer potency, and several PAHs are evaluated using benzo[a]pyrene toxicity equivalency approaches. Long-term exposure to carcinogenic PAHs is associated with increased cancer risk, particularly through mechanisms involving metabolic activation and DNA damage.

Coal tar constituents can also affect non-cancer health endpoints. Naphthalene has been associated with hemolytic anemia, especially in susceptible individuals, and has toxicological concern for liver, respiratory, and developmental effects in certain exposure contexts. Phenols and cresols can irritate tissues and affect the nervous system, liver, and kidneys at sufficient doses. Benzene, if present, is a well-established human carcinogen associated with blood and bone marrow toxicity. Toluene, ethylbenzene, and xylenes can affect the nervous system and may contribute to taste, odor, and inhalation concerns.

Risk assessment for coal tar contamination is often more complex than for a single contaminant. A sample may contain dozens of detected and non-detected compounds, each with different detection limits and health benchmarks. Regulatory agencies and consultants often evaluate individual compounds, total PAHs, carcinogenic PAH equivalents, volatile aromatics, and site-specific exposure pathways. Sensitive populations include infants, children, pregnant people, individuals with compromised liver or kidney function, people with G6PD deficiency in the context of naphthalene exposure, and households using untreated private wells near known industrial sites.

Testing and Monitoring

Testing for coal tar contaminants requires specialized laboratory analysis. Standard household water quality tests for hardness, chlorine, nitrate, or bacteria will not identify coal-tar-derived PAHs or aromatic hydrocarbons. A proper investigation usually includes volatile organic compound analysis, semi-volatile organic compound analysis, and targeted PAH testing. Laboratories commonly use gas chromatography/mass spectrometry, or GC/MS, for VOCs and SVOCs, and high-performance liquid chromatography with fluorescence detection or mass spectrometry for low-level PAH analysis.

In the United States, laboratories may use EPA methods such as Method 524.2 or 524.3 for VOCs in drinking water, Method 525 series methods for organic compounds, Method 8270 for semi-volatile organic compounds in environmental samples, and Method 8310 for PAHs depending on the matrix and regulatory program. Method selection should be based on whether the sample is finished drinking water, raw groundwater, surface water, soil gas, sediment, or wastewater. Detection limits matter: carcinogenic PAHs may require reporting limits in the low nanogram-per-liter to low microgram-per-liter range to support health-based decisions.

Sampling technique is critical. VOC samples must be collected in preserved vials without headspace to prevent loss of volatile compounds. PAH samples should be collected in appropriate amber glass containers to reduce photodegradation and adsorption issues. If there is visible sheen, tar droplets, sediment, or turbidity, the laboratory and regulator should be consulted because unfiltered and filtered results may differ. For private wells near a known coal tar source, one-time testing is not always enough; seasonal groundwater changes, pumping patterns, and plume movement can change concentrations over time.

Treatment Methods

Activated carbon is the primary treatment technology for many coal tar contaminants in drinking water, especially PAHs, taste-and-odor compounds, and many hydrophobic organic chemicals. Granular activated carbon, or GAC, works by adsorbing organic molecules onto a high-surface-area carbon bed. It is particularly effective for many PAHs because they are hydrophobic and strongly sorb to carbon. Properly designed GAC can also reduce naphthalene, phenols, and some VOCs, although performance varies by compound, concentration, water chemistry, flow rate, empty bed contact time, and carbon age.

Activated carbon can fail if the system is undersized, the flow rate is too high, the carbon is exhausted, or the contaminant mix includes more mobile compounds that break through before heavier PAHs. Natural organic matter can compete for adsorption sites and shorten carbon life. High iron, manganese, sediment, or biofouling can clog beds and reduce contact. For this reason, coal tar sites often require lead-lag carbon vessels, scheduled media replacement, and post-treatment monitoring rather than relying on taste or odor as a warning sign. A lead-lag setup places two carbon units in series so the first unit can be monitored for breakthrough while the second provides a safety barrier.

Point-of-use activated carbon may be appropriate for a single drinking and cooking tap when contamination is low, limited to ingestion exposure, and confirmed by laboratory testing. However, point-of-entry treatment is usually more appropriate when VOCs such as benzene or naphthalene are present, because whole-house treatment reduces inhalation and dermal exposure during showering, bathing, and laundry. Point-of-entry GAC systems should be professionally sized and monitored. Pitcher filters and small refrigerator cartridges are not appropriate for confirmed coal tar contamination unless they are specifically certified for the detected compounds and replaced very frequently under a validated plan.

For homes affected by a documented industrial plume, treatment should be paired with source investigation and ongoing monitoring. Treatment protects users at the tap, but it does not remove the underground source. Municipal systems may use GAC, blending, wellhead treatment, or source abandonment depending on plume severity and regulatory requirements.

Regulations and Guidelines

There is usually no single drinking water standard called “coal tar contaminants” because coal tar is a mixture. Regulation is generally applied to individual chemicals or groups of chemicals, such as benzene, benzo[a]pyrene, total PAHs, carcinogenic PAHs, phenols, or site-specific indicator compounds. In the United States, the EPA has enforceable federal drinking water standards for some relevant individual contaminants, including benzene and benzo[a]pyrene. Other coal tar constituents may be addressed through health advisories, risk-based screening levels, state groundwater standards, cleanup criteria, or hazardous waste site orders rather than through a universal national drinking water limit.

Internationally, limits vary by country and jurisdiction. The World Health Organization has published guideline values for selected individual chemicals, including certain PAHs, but WHO guideline values are not automatically enforceable unless adopted by a national or local authority. The European Union, Canada, Australia, and individual U.S. states or provinces may regulate PAHs differently, sometimes using sums of selected PAHs, benzo[a]pyrene as an indicator, or risk-based site cleanup targets. Local rules can also differ for public water supplies, private wells, groundwater remediation, and contaminated land redevelopment.

At coal tar sites, regulatory concern often extends beyond finished drinking water. Agencies may require groundwater monitoring wells, soil gas sampling, vapor intrusion evaluation, sediment testing, ecological risk assessment, institutional controls, well-use restrictions, source removal, containment, or long-term pump-and-treat systems. Because standards and cleanup levels differ by jurisdiction and by exposure pathway, affected residents should rely on certified laboratory results and guidance from the relevant health department, environmental agency, or drinking water regulator.

Related Contaminants

Frequently Asked Questions

Is coal tar in drinking water the same as creosote?

Not exactly, but they are closely related in many contamination cases. Creosote is a coal-tar-derived wood preservative that contains PAHs, phenols, and other aromatic compounds. A former wood-treatment site may release creosote, while a former manufactured gas plant may release coal tar. Both can produce PAH-rich groundwater contamination and similar drinking water concerns.

Can I smell coal tar contamination in my water?

Sometimes, but smell is not reliable. Naphthalene may produce a mothball-like odor, phenolic compounds can smell medicinal or tar-like, and petroleum-like aromatics may smell solvent-like. However, carcinogenic PAHs can be present at levels of concern without a noticeable odor. Laboratory testing is required to determine safety.

Does activated carbon remove benzo[a]pyrene and other PAHs?

Properly designed activated carbon is one of the best available treatment options for many PAHs, including benzo[a]pyrene, because these compounds adsorb strongly to carbon. The system must be sized for the contaminant mix and flow rate, and the carbon must be replaced before breakthrough. For high-risk wells, lead-lag GAC with laboratory monitoring is preferred.

Should coal tar contamination be treated at one faucet or the whole house?

It depends on the compounds detected. If only nonvolatile PAHs are present at low levels, a certified point-of-use system for drinking and cooking water may be considered. If benzene, naphthalene, or other volatile compounds are present, point-of-entry treatment is usually more protective because it reduces inhalation and skin exposure during showering and other household uses.

What should I test for if my well is near a former manufactured gas plant or wood-treatment site?

A targeted test panel should include PAHs, VOCs such as benzene and related aromatics, semi-volatile organic compounds, phenols or cresols where appropriate, and possibly metals depending on site history. The best panel should be selected with a certified laboratory, environmental professional, or local health agency because coal tar contamination is site-specific.