What Is Petroleum Hydrocarbons?

Petroleum hydrocarbons are not a single chemical. They are a broad class of organic compounds derived from crude oil and refined petroleum products such as gasoline, diesel fuel, kerosene, jet fuel, lubricating oil, heating oil, mineral spirits, and asphalt-related materials. In drinking water investigations, the term usually refers to measurable fractions of petroleum-related chemicals, including gasoline range organics, diesel range organics, oil range organics, volatile organic compounds, semi-volatile hydrocarbons, and polycyclic aromatic hydrocarbons.

The composition of petroleum contamination depends strongly on the product released. Gasoline contamination is typically rich in light, volatile compounds such as benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes, and oxygenates where present. Diesel and fuel oil contain heavier straight-chain, branched, and cyclic hydrocarbons, including compounds that are less volatile but more persistent in soil and groundwater. Crude oil and waste petroleum mixtures may also contain sulfur-, nitrogen-, and oxygen-containing organics, metals, additives, and degradation products.

Petroleum hydrocarbons are a high-risk drinking water concern because releases can create long-lived groundwater plumes, contaminate private wells, generate explosive or toxic vapors, and introduce carcinogenic constituents such as benzene and certain PAHs. Even when total petroleum hydrocarbon measurements are used as screening indicators, the actual health risk depends on the specific chemicals present, their concentrations, exposure duration, and whether volatile compounds are inhaled during showering or household water use.

Scientific Identity

Petroleum hydrocarbons are chemically diverse mixtures made primarily of carbon and hydrogen. Major groups include alkanes, isoalkanes, cycloalkanes, alkenes, aromatic hydrocarbons, and polycyclic aromatic hydrocarbons. Because there is no single formula or CAS number for “petroleum hydrocarbons” as a drinking water contaminant, laboratories usually report operationally defined fractions such as TPH, GRO, DRO, or specific target compounds. These categories are based on how compounds behave during extraction and gas chromatography, not on one uniform chemical identity.

Volatility, solubility, and biodegradability determine how petroleum compounds behave in water. Light hydrocarbons in gasoline can dissolve into groundwater and move with the plume, while also partitioning into soil gas. Benzene is especially important because it is relatively water soluble compared with many petroleum constituents and is a known human carcinogen. Heavier diesel and oil-range compounds are less soluble but can persist as separate-phase product, sorb to soil organic matter, and slowly release dissolved contamination over time.

At petroleum release sites, contamination often occurs as light non-aqueous phase liquid, or LNAPL. LNAPL floats on the water table but can smear through the aquifer as groundwater levels rise and fall. This residual petroleum trapped in soil pores becomes a continuing source of dissolved hydrocarbons. Microbial biodegradation can attenuate many petroleum compounds, but the process may consume oxygen and produce reduced groundwater conditions, methane, iron, manganese, sulfide, and other secondary water-quality problems.

How Petroleum Hydrocarbons Enters Drinking Water

The most common pathway is leakage from underground storage tanks, aboveground fuel tanks, pipelines, service stations, bulk fuel terminals, refineries, airports, rail yards, military installations, marine terminals, and industrial facilities. Small chronic leaks can be as important as large visible spills because petroleum can migrate downward through soil and reach groundwater before detection. Former dry wells, floor drains, disposal pits, and industrial lagoons can also introduce petroleum solvents and oily wastes into aquifers.

Private wells are vulnerable when they are located downgradient of a fuel release or constructed through contaminated shallow groundwater. Poorly sealed wells can act as conduits that move hydrocarbons from shallow contaminated zones into deeper aquifers. In rural areas, home heating oil tanks, farm fuel tanks, machine shops, auto repair properties, and abandoned commercial sites are frequent local sources. In urban settings, overlapping releases from service stations, old industrial parcels, sewers, and stormwater systems may create complex petroleum plumes.

Surface-water sources can be affected by petroleum spills to rivers, reservoirs, lakes, or stormwater channels. Petroleum may enter treatment plant intakes after tanker accidents, refinery incidents, pipeline breaks, highway crashes, firefighting runoff, or releases from industrial drainage systems. Although visible oil may be removed, dissolved aromatic compounds and taste-and-odor constituents can remain and require rapid monitoring and treatment adjustments.

Petroleum hydrocarbons can also create vapor intrusion concerns. Volatile petroleum compounds may move from contaminated groundwater or soil into soil gas and then into basements, crawlspaces, utility conduits, and buildings. For drinking water users, vapor exposure may occur not only from subsurface vapor intrusion but also from volatilization during showering, laundry, dishwashing, and other indoor water uses.

Occurrence and Exposure

Petroleum hydrocarbons are most often found in groundwater near known or suspected fuel-handling sites. Contamination may be localized around one leaking tank, or it may extend hundreds to thousands of feet depending on aquifer permeability, groundwater gradient, release volume, product type, and natural attenuation conditions. Wells drawing from shallow sand and gravel aquifers are generally more susceptible than deep, protected aquifers, although fractured bedrock wells can transmit petroleum contamination rapidly along cracks and bedding planes.

People encounter petroleum hydrocarbons through ingestion of contaminated water, inhalation of volatile compounds released during household water use, and dermal contact during bathing. Odor and taste may provide warning at some petroleum-contaminated wells, especially where gasoline-like or solvent-like compounds are present, but sensory detection is unreliable. Some hazardous constituents may be present below odor thresholds, while others may produce nuisance odors at concentrations not directly proportional to toxic risk.

Exposure patterns differ by product. Gasoline contamination can lead to short-term exposure to volatile aromatic compounds, including benzene, toluene, ethylbenzene, xylenes, and naphthalene. Diesel and heating oil releases may produce longer-term exposure to heavier hydrocarbons, fuel additives, and semi-volatile compounds. Waste oil and industrial petroleum mixtures may be more chemically complex because they can contain chlorinated solvents, metals, PAHs, used-oil degradation products, and manufacturing residues.

Health Effects and Risk

The health risk from petroleum hydrocarbons depends on the specific mixture. Benzene is the petroleum constituent of greatest regulatory concern in many drinking water cases because long-term exposure is associated with leukemia and other blood disorders. Ethylbenzene, naphthalene, and some polycyclic aromatic hydrocarbons also raise cancer or chronic toxicity concerns. Toluene and xylenes are more commonly associated with nervous system effects, irritation, dizziness, headaches, and organ effects at sufficient exposure levels.

Short-term exposure to high concentrations of volatile petroleum compounds may cause nausea, vomiting, dizziness, headaches, eye and throat irritation, and central nervous system depression. In homes using contaminated water, inhalation during showering can be a major pathway because volatile compounds transfer from water to indoor air. This is particularly important for gasoline-range contamination and for small bathrooms with limited ventilation.

Long-term exposure can affect the liver, kidneys, nervous system, blood-forming system, and immune function depending on the chemical profile. Children, pregnant people, older adults, and people with preexisting liver, kidney, blood, or neurological conditions may be more vulnerable. Infants may also receive higher dose per body weight when formula is prepared with contaminated water.

Petroleum contamination should never be evaluated only by odor or by a single “total petroleum hydrocarbons” number. TPH is useful as an indicator of petroleum impact, but health-based interpretation requires identifying specific constituents such as benzene, toluene, ethylbenzene, xylenes, methyl tert-butyl ether where relevant, naphthalene, and PAHs. A low total hydrocarbon result does not automatically exclude a high-risk compound, and a high TPH result may require fraction-specific risk assessment.

Testing and Monitoring

Testing petroleum hydrocarbons requires specialized laboratory analysis with correct sample containers, preservatives, and holding times. For volatile gasoline-range compounds, laboratories commonly use purge-and-trap gas chromatography or gas chromatography-mass spectrometry methods for VOCs. BTEX compounds, fuel oxygenates, and other volatile indicators are often measured separately because they are mobile and important for health risk assessment.

Diesel and oil-range contamination is usually analyzed by extraction followed by gas chromatography with flame ionization detection or mass spectrometry. Results may be reported as total petroleum hydrocarbons, gasoline range organics, diesel range organics, motor oil range organics, or carbon-range fractions. PAHs are typically measured with semi-volatile organic methods because several PAHs are carcinogenic or persistent. In complex industrial settings, testing may also include chlorinated solvents, metals, phenols, and other co-contaminants.

Field screening tools such as photoionization detectors, sheen observations, odor logs, and immunoassay kits can help guide investigations, but they do not replace certified drinking water laboratory testing. Proper sampling is critical. VOC samples must be collected without headspace, because trapped air can allow volatile compounds to escape before analysis. Samples should not be taken through carbon filters if the purpose is to determine raw-water contamination.

For private wells near a release, monitoring should include initial baseline testing, repeat testing after seasonal groundwater changes, and post-treatment verification if a treatment system is installed. At contaminated sites, well testing should be interpreted with groundwater flow direction, plume maps, product history, and nearby monitoring well data. A single clean result may not be enough if the plume is migrating or if the well is near the edge of contamination.

Treatment Methods

Treatment must be matched to the petroleum product and the chemicals detected. Activated carbon is often the most practical and effective treatment for many dissolved petroleum hydrocarbons, especially benzene, toluene, ethylbenzene, xylenes, naphthalene, PAHs, and odor-causing organic compounds. Granular activated carbon works by adsorbing hydrophobic organic molecules onto a highly porous carbon surface. It is widely used in municipal treatment, emergency response, private well systems, and point-of-use devices.

Activated carbon can be installed at the point of use, such as under a kitchen sink, or at the point of entry, where all household water is treated. Point-of-use carbon may be appropriate when the main concern is ingestion and cooking water, and when volatile concentrations are low. Point-of-entry treatment is preferred when volatile petroleum compounds are present because inhalation and skin contact can occur throughout the home during bathing, laundry, and toilet flushing. For significant VOC contamination, a whole-house system may combine aeration or air stripping with carbon polishing.

Activated carbon may fail if the petroleum concentration is high, if free product or oily droplets enter the unit, if the flow rate is too fast, if the water contains high levels of competing natural organic matter, or if cartridges are not replaced before breakthrough. Two carbon vessels in series with a sampling port between them are often used for higher-risk wells so that the first vessel can be monitored and replaced before contaminants pass through the second. Any treatment system should be verified by laboratory testing of treated water, not by taste or odor alone.

Regulations and Guidelines

Regulation of petroleum hydrocarbons in drinking water is complicated because “petroleum hydrocarbons” is a mixture category rather than a single regulated chemical. Many jurisdictions regulate individual petroleum constituents such as benzene, toluene, ethylbenzene, xylenes, styrene, naphthalene, and certain PAHs. In the United States, EPA has enforceable drinking water standards for several individual VOCs associated with petroleum, including benzene, while other petroleum fractions may be addressed through state cleanup levels, groundwater standards, health advisory values, or site-specific risk assessments.

WHO drinking-water guidance and national drinking water programs generally focus on individual chemicals rather than total petroleum hydrocarbons as a universal health-based standard. Some countries, states, provinces, or local agencies use TPH, GRO, DRO, or oil-and-grease values as screening levels for groundwater investigation, aesthetic impact, treatment triggers, or remediation goals. These limits vary by jurisdiction and by whether the water is a public supply, private well, environmental monitoring well, or cleanup-site compliance point.

Public water systems are typically required to monitor regulated VOCs under national or regional drinking water rules. Private wells are usually the owner’s responsibility, and testing is often triggered by nearby spills, real estate transfers, odor complaints, or environmental agency notifications. When petroleum contamination is detected, local health departments or environmental agencies may issue do-not-drink, do-not-use, or ventilation guidance depending on concentrations and vapor risks.

Because regulatory limits vary, the most defensible approach is to compare laboratory results with applicable local drinking water standards, groundwater protection criteria, and health-based guidance for each detected compound. A result reported only as “TPH” should be followed by compound-specific testing when drinking water use is involved.

Related Contaminants

Frequently Asked Questions

Does a gasoline or fuel odor mean my well is unsafe?

A petroleum odor is a serious warning sign and should be investigated immediately, but odor alone cannot determine safety. Some hazardous compounds, including benzene, may be present at concentrations that are not obvious by smell. Stop using the water for drinking and cooking until certified laboratory testing and local health guidance are obtained.

What should I test for if petroleum contamination is suspected?

A typical petroleum well investigation includes VOCs, BTEX, gasoline range organics, diesel range organics, and sometimes PAHs, naphthalene, fuel oxygenates, and lead scavenger compounds depending on the product history. At industrial or waste-oil sites, testing may also include chlorinated solvents and metals because petroleum releases can be mixed with other contaminants.

Is activated carbon enough to make petroleum-contaminated water safe?

Activated carbon is often effective, but only if it is properly sized, installed, and monitored for the specific contaminants and concentrations present. Small faucet filters are not appropriate for significant fuel releases. For volatile compounds, whole-house treatment may be needed to reduce inhalation exposure from showers and other indoor uses.

Can I boil petroleum-contaminated water?

No. Boiling is not a safe treatment for petroleum hydrocarbons. It can drive volatile chemicals into indoor air, increasing inhalation exposure, and it will not reliably remove heavier petroleum compounds. If petroleum contamination is suspected, use an alternative safe water source and seek professional testing.

How long can petroleum contamination remain in groundwater?

Petroleum plumes can persist for years or decades, especially where residual fuel remains trapped in soil or bedrock fractures. Natural biodegradation can reduce some compounds, but benzene, naphthalene, and heavier hydrocarbons may continue to leach from source areas. Long-term monitoring is often needed before a well can be considered reliably protected.