Soil along busy roadsides and near industrial sites often is polluted by heavy metals and persistent hydrocarbons that have accumulated there over decades, posing environmental and health risks. This article explores the sources of these contaminants, their behavior in soils, and the dangers they pose to ecosystems and human health. It also discusses precautions – especially for urban gardeners – to reduce exposure and grow food safely in affected areas.

Sources and History of Soil Contamination

Industrial Activities and Transportation Emissions: Industrial operations (such as mining, metal smelting, and manufacturing) and vehicular traffic are primary sources of soil contamination. Smokestack emissions and particulate fallout from factories deposit toxic metals onto surrounding land, while cars and trucks emit pollutants that settle into roadside soils. For much of the 20th century, automotive exhaust was a major contributor to lead contamination due to the use of tetraethyl lead in gasoline. Lead was added to fuel in the 1920s as an anti-knock agent, and for decades leaded fuel was burned worldwide. Lead particles drifted and settled on nearby soil, creating long-lasting contamination along highways and urban areas. Although leaded petrol was phased out (banned in the U.S. in 1996 and eliminated globally by 2021), the toxic lead it left in topsoil remains a pressing issue.

Brownfields – The Remnants of Past Pollution: The term brownfield refers to abandoned or underused properties where redevelopment is complicated by real or perceived contamination. These sites, often former factories, gas stations, or dry cleaners, may have soil and groundwater polluted with hazardous substances from prior use. According to the U.S. Environmental Protection Agency (EPA), a brownfield is any property where reuse is hindered by the presence or potential presence of hazardous pollutants. There are hundreds of thousands of brownfields in the United States alone. Cleaning up and revitalizing these lands is important not only to remove environmental health hazards but also to return the land to productive use. Brownfields are less severely contaminated than “Superfund” sites (which contain more acute toxic waste) but still pose significant long-term risks if not addressed.

Other Sources of Contaminants: Other historical practices have added to soil pollution. The use of lead- and arsenic-based pesticides (for example, lead arsenate in old orchards) left residues in agricultural soils. Pressure-treated lumber preserved with chromated copper arsenate (CCA) can leach arsenic into adjacent soil over time. Creosote (used on railroad ties and telephone poles) is a source of polycyclic aromatic hydrocarbons (PAHs). Leaks or spills of petroleum products (oil, diesel, gasoline) at industrial facilities or gas stations can introduce a range of hydrocarbons into the subsurface. Over time, these contaminants persist unless actively remediated, contributing to the patchwork of polluted soils in many urban and industrial areas.

Heavy metals are dense metallic elements that are toxic even at low concentrations. Once deposited in soil, they do not biodegrade; instead, they remain indefinitely, cycling between soil, water, and living organisms. Among these pollutants, lead, cadmium, mercury, and arsenic are most concerning in roadside and industrial soils. The EPA classifies them as priority contaminants due to their toxicity and prevalence. Each metal has unique sources, behavior in soil, and health effects:

1. Lead (Pb) Contamination in Soils

Lead is the most common heavy metal contaminant found in soils​. It can originate from many sources: leaded gasoline (petrol) exhaust, peeling lead-based paint from old buildings, emissions from smelters, and past use of lead in products such as pesticides and plumbing. Chemically, lead is an element, so it cannot break down any further into anything else, and does not break down over time. Once lead particles settle into soil, they tend to bind to soil particles and remain near the surface, especially in the top few inches of soil​. How far lead spreads in soil depends on soil type and environmental conditions, but generally it doesn’t leach deeply unless the soil is very acidic or certain chelating (binding) agents are present.

Lead is the most common heavy metal contaminant found in soils​. It can originate from many sources: leaded gasoline (petrol) exhaust, peeling lead-based paint from old buildings, emissions from smelters, and past use of lead in products such as pesticides and plumbing. Chemically, lead is an element, so it cannot break down any further into anything else, and does not break down over time. Once lead particles settle into soil, they tend to bind to soil particles and remain near the surface, especially in the top few inches of soil​. How far lead spreads in soil depends on soil type and environmental conditions, but generally it doesn’t leach deeply unless the soil is very acidic or certain chelating (binding) agents are present.

Lead Uptake in Plants: Roots, Leaves, and Fruits

Most plants do not readily absorb large amounts of lead into edible tissues, but the degree of uptake varies by crop part. Lead tends to bind strongly in the root zone, resulting in much higher concentrations in roots than in shoots or fruits​. In fact, studies show root vegetables like carrots can accumulate significantly more lead than other crops when grown in contaminated soil​. Leafy greens (e.g. lettuce, spinach) generally take up some lead into their leaves, though typically less than root vegetable. By contrast, fruiting crops (tomatoes, peppers, beans, etc.) usually have the lowest lead levels in their edible fruit, even when grown in the same contaminated soil​. This is because lead is largely retained in roots and only a limited amount translocates (moves) to above-ground parts​. However, “limited” translocation does not mean no translocation – even small lead amounts in leaves or fruits can contribute to human exposure, especially with frequent consumption​. Moreover, any edible root that has accumulated lead (e.g. a carrot or beet) poses a direct ingestion risk if grown in lead-rich soil.

• Root Vegetables: Root crops exhibit the highest lead uptake. For example, carrots readily absorb and store lead in their roots​. Such root crops can even exceed health-based lead limits in moderate to highly contaminated soils (e.g. soil lead >100–200 mg/kg). Because of this, some plants with extensive roots have been studied for phytoremediation (using plants to clean soil), but lead’s strong binding to soil and roots means most food crops are not effective “lead removers”​. Any roots used to accumulate lead would be considered hazardous to eat.
• Leafy Greens: Leafy vegetables have moderate lead uptake. Leaves receive some translocated lead from roots, so crops like lettuce or kale can build up measurable lead if soil levels are high​. They usually accumulate less lead than root vegetables, but elevated soil lead can still result in unsafe lead levels in leafy tissues​. For instance, one study found that leafy greens grown in soil with >100–200 ppm lead sometimes exceeded food safety standards​.
• Fruiting Vegetables: Fruiting crops tend to accumulate the least lead in their edible parts. Edible fruits (tomatoes, peppers, cucumbers, beans, etc.) tend to contain minimal lead even when grown in contaminated ground​. Research on old orchard soils (historically treated with lead arsenate pesticide) showed that tomato and bean fruits had much lower lead concentrations than carrots or lettuce​. In many cases, lead in fruits was undetectable or well below health limits​. Nonetheless, low uptake into fruits is not a free pass – precautions are still needed, because soil contact can coat even fruits with contamination (and overall garden exposure still matters, as explained below).

How Lead Enters the Food Chain from Soil

Even if internal uptake by plants is limited, lead in soil can still enter the human food chain through external contamination.

• Soil particles containing lead tend to cling to the surfaces of produce – especially root crops and leafy greens – and are difficult to fully remove without thorough washing. For example, potatoes, carrots, and radishes can have fine soil residue trapped in crevices or peel, and leafy greens can have dust on their leaves. If produce grown in lead-contaminated soil is not washed and peeled properly, people may ingest this lead-rich dirt directly along with the food​. Studies and authorities consistently find that the main route of exposure from garden soil is consuming soil/dust, not the plant’s internal uptake​
• Another route is inhalation or incidental ingestion of wind-blown dust containing lead. Dry, lead-contaminated soil can generate dust that settles on home-grown produce or is breathed in. This is why even fruiting vegetables (with low internal lead) can become coated with lead-bearing dust. In urban gardening studies, researchers found that people are far more likely to get lead exposure by eating or breathing soil particles than by eating the small amount of lead taken up inside carrots or lettuce. Simply put, lead in soil easily becomes lead on our hands, produce, and lungs.

In other words, a greater risk of lead exposure comes from eating unwashed vegetables (with soil on them), inhalation of wind-blown dust, and hand-to-mouth contact with contaminated soil, than from the plant tissue itself.

Risks of Growing Food in Lead-Contaminated Soil

Growing food in lead-rich soil is risky because even relatively low soil lead levels can contribute to lead exposure. Soil with a total lead concentration of a few hundred parts per million (ppm) can result in produce that, if not properly handled, adds to lead ingestion. Since there is no physiological need for lead in our diet and any additional burden is harmful, conservative exposure limits are set. In the U.S., the EPA’s current guidance is to avoid planting food crops in soil that exceeds 400 ppm lead. Many university extensions recommend that if your soil tests high for lead, you should not use it for growing vegetables. Instead, mitigation measures include:

• Use Raised Beds or Clean Soil: The EPA and USDA advise building raised garden beds filled with clean soil (lead levels <50 ppm) if you wish to grow edibles in an area with contaminated ground. This creates a barrier between plant roots and the native soil. Similarly, you can lay geotextile fabric and put fresh topsoil/compost on top of contaminated sites to reduce plant contact with the original soil.
• Grow Safer Crops: If moderate contamination is present (around 100–400 ppm range), favor fruiting crops in-ground (tomatoes, corn, squash, etc.) which uptake very little lead​, and avoid root and leafy crops directly in that soil. More sensitive crops (leafy greens, root vegetables) should be grown in containers or raised beds with clean soil in such scenarios​. This way you minimize the lead that ends up in edible parts.
• Soil Amendments: Adding organic matter (compost) and maintaining neutral pH (around 6.5-7) can further reduce lead bioavailability to plants​. The application of phosphorus fertilizers can also immobilize lead. While these steps don’t remove lead, they help lock it in the soil and make it even less likely to be absorbed by plants or cling to produce. Good compost has the added benefit of diluting the concentration of lead in soil and improving overall crop yield​.
• Garden Hygiene: In any suspect soil, practice strict hygiene to prevent ingesting or spreading lead. Always wash produce thoroughly (scrub roots, rinse greens, peel root crops) to remove soil particles. Wash hands after gardening, and avoid bringing soil indoors on tools or shoes​. Keep the soil surface mulched to reduce dust. These steps are crucial to cut down the transfer of lead-laden soil into your home and diet.

For more detail on practical risk reduction and remediation (correction) measures, see the section “Best Practices for Growing Food in Contaminated Soil” towards the end of this article.

Using Bioremediation to Remove Lead Contamination from Soils: It’s important to note that phytoextraction (bioremediation using plants) is not a quick fix for lead in soils. While certain plants (like some Brassica species or sunflowers) can uptake more lead than typical crops, lead’s limited bioavailability means remediation is a slow, difficult process​. Most of the lead remains in the ground or confined to roots, even after many growing cycles. Thus, trying to “clean” a vegetable plot by growing lead-accumulating plants would take many years and still leaves you with contaminated plant matter to dispose of. For home gardeners, removing the soil or covering it is usually more reliable than attempting to extract lead with plants​.

In short, if your soil lead is high, the safest course is not to grow food in it without remediation – use safe soil or find a different site for your vegetable garden. If you suspect high lead levels in your soil, get it tested and err on the side of caution.

Lead Toxicity and Health Risks

Lead is a a toxic heavy metal and potent neurotoxin with no safe level of exposure that can affect almost every organ system, with the nervous system being its main target—especially in developing children. Even very low blood lead concentrations—just a few micrograms per deciliter—have been linked to developmental problems in children. Low levels of lead exposure can cause developmental delays, learning and behavior problems, reduced IQ, and attention disorders in children. In adults, chronic exposure to lead can contribute to high blood pressure, kidney damage, and reproductive problems.

Importantly, the effects of lead are cumulative; lead stored in the body (for example, in bones) continues to cause harm over time. When produce grown in lead-contaminated soil is eaten, especially unwashed root or leafy vegetables, small amounts of lead can be ingested. Over time, continued exposure may contribute to elevated blood lead levels. The human body can accumulate lead by storing it in bones and organs, and even low doses may build up to harmful levels with repeated exposure. Children are particularly vulnerable because their developing brains and bodies are more sensitive to lead’s effects, and they absorb a higher fraction of ingested lead than adults. For example, young children playing in a lead-contaminated yard or garden may pick up lead-rich dirt on their hands and then ingest significant amounts of lead by putting their hands in their mouths, even if they are not consuming garden produce. This cumulative toxicity is why legacy lead in soil from sources such as past gasoline (petrol) use or paint remains a significant public health concern long after the original sources have ceased.

2. Cadmium (Cd) Contamination in Soils

Cadmium is a naturally occurring metal, but industrial activities have greatly increased its concentration in certain soils. Major sources of cadmium pollution include metal mining and smelting (cadmium is often a byproduct of zinc and lead refining), combustion of fossil fuels (coal burning can release cadmium), and application of phosphate fertilizers and sewage sludge to fields (both can contain cadmium). Cadmium was also used in some pigments and coatings. Along roadsides, cadmium can be deposited from tire and brake wear and from diesel emissions.

In soil, cadmium strongly binds to organic matter and clay particles. It does not volatilize or degrade, though it can change chemical forms. Cadmium can travel through air as fine particles and eventually settle onto soil and water bodies, sometimes far from the source, due to wind transport. Importantly, cadmium is more readily taken up by plants than lead. Plant roots can absorb cadmium from soil, and it may accumulate in leaves, grains, and fruits to some extent. In fact, cadmium enters the food chain primarily through agriculture: leafy vegetables, grains, and legumes grown in cadmium-rich soils can have elevated cadmium levels​. For example, spinach, lettuce, and rice are known to sometimes accumulate cadmium from contaminated soils. Smoking tobacco is another significant exposure route, since tobacco plants readily uptake cadmium and smokers inhale it with cigarette smoke.

Cadmium Toxicity and Health Risks

Cadmium is highly toxic to kidneys. Long-term exposure, even at low levels, leads to cadmium accumulation in the body (particularly in the kidneys), which can result in kidney damage or kidney disease. Cadmium also causes bones to become fragile (a condition called itai-itai disease was observed in Japan due to cadmium-polluted rice causing severe osteoporosis and fractures). Inhalation of cadmium (e.g., occupational exposure in factories or from smoking) can damage the lungs. Cadmium is classified as a human carcinogen – it has been linked to lung cancer and is considered a probable human carcinogen by the EPA. Due to these risks, even relatively low concentrations of cadmium in soil are concerning if that soil is used to grow food.

3. Mercury (Hg) Contamination in Soils

Mercury is unique among heavy metals for being liquid at room temperature (in its elemental form) and for its ability to vaporize and travel through the atmosphere. Mercury has been released into the environment from coal burning, mining (especially gold mining, which historically used mercury to extract gold), and industrial processes like chlor-alkali production. Along roads, mercury from car exhaust (small amounts from fuel impurities) and wear of certain components can contribute, but generally mercury soil hotspots are associated with industrial sites or spills.

In the environment, mercury cycles through different forms. Elemental mercury can evaporate into air and travel long distances before redepositing. In soil, mercury tends to adhere strongly to soil and sediment particles. It does not degrade and can remain in topsoil or sediment for many years. Certain bacteria can transform inorganic mercury into organic forms, such as methylmercury, especially in waterlogged soils or sediments (like wetlands). Methylmercury is of particular concern because it can bioaccumulate in living organisms. Plants can absorb some mercury (especially methylmercury) from soil, and it can also deposit on plant surfaces from air. However, most human exposure to mercury comes not directly from soil, but from the consumption of fish and seafood in which mercury has bioaccumulated. That said, a mercury spill or high mercury content in soil is still dangerous: mercury vapor from soil can be inhaled, and mercury can leach into groundwater under certain conditions.

Mercury Toxicity and Health Risks

All forms of mercury are toxic, with the nervous system and kidneys being primary targets. Methylmercury, which accumulates in the food chain, causes neurological damage – impairing coordination, vision, and cognitive development (famously illustrated by the Minamata mercury poisoning disease outbreaks in Japan in the 1950s). Elemental mercury vapors can also cause tremors, memory loss, and organ damage with chronic exposure. Pregnant women and young children are most vulnerable, as mercury can cross the placenta and affect fetal brain development. While garden plants are unlikely to contain dangerous levels of mercury, contamination of soil with mercury requires careful management to prevent human contact and further environmental spread.

4. Arsenic (As) Contamination in Soils

Arsenic, which is actually a metalloid, is often discussed alongside heavy metals because of its similar toxicity and persistence. It occurs naturally in many soils at low concentrations (background levels are typically 1–40 ppm), but certain human activities have greatly increased arsenic levels in some locations. Historically, arsenic-based compounds were widely used in pesticides and herbicides – for example, lead arsenate was a common insecticide in fruit orchards in the early 20th century. Arsenic is also used as a wood preservative in chromated-copper-arsenate (CCA) treated timber and in some industrial processes such as glass manufacturing. Mining and smelting operations can release arsenic as well, since arsenic is often present in metal ores. In some regions, arsenic contamination is natural: certain geological formations lead to high arsenic groundwater which, when used for irrigation, can deposit arsenic into agricultural soil over time (which is why some groundwater shouldn’t ever be tapped into).

In soil, arsenic tends to remain in inorganic forms (combined with oxygen, chlorine, or sulfur). It does not vaporize from soil, and while it can change form (for instance from one mineral form to another or to an organic form in plants), it doesn’t break down or disappear. Arsenic is relatively immobile in many soils – it often binds to iron oxides and clays and stays in the upper layers. However, changes in soil chemistry (like flooding or shifts in pH) can mobilize arsenic into water, causing it to move through the soil. Plants can take up arsenic from soil; certain ferns are known arsenic hyperaccumulators, and rice paddies (waterlogged soils) have been notorious for causing arsenic uptake into rice grains. Most crops, however, accumulate only limited arsenic in edible parts under normal conditions. The bigger risk is direct ingestion of arsenic-contaminated soil or dust, and use of arsenic-contaminated water.

Arsenic Toxicity and Health Risks

Arsenic is a well-known poison and carcinogen. Ingesting high doses of inorganic arsenic can be acutely (short-term) lethal. Chronic (long-term) exposure to lower doses causes a range of problems: skin pigmentation changes, keratosis (small corn-like growths on skin), peripheral neuropathy (a “pins and needles” feeling in hands and feet), and various organ damage. In the long term, arsenic in drinking water or food increases the risk of cancers, particularly skin, bladder, lung, and liver cancer. Health agencies like the EPA and WHO have set very low limits for arsenic in water (10 μg/L, or 0.01 ppm, in drinking water) because of these cancer risks. Children exposed to arsenic may suffer developmental effects; some studies suggest lower IQ and increased child mortality with prolonged exposure to even moderate levels of arsenic. Unlike lead or mercury, arsenic does not preferentially attack the nervous system, but its systemic (whole body) effects and cancer risk make it extremely dangerous.

In summary, heavy metals (and arsenic) persist in soils for centuries if not longer. They can be taken up to varying degrees by plants or move into groundwater, and they accumulate in animal and human bodies, causing chronic toxicity. Lead, cadmium, mercury, and arsenic each pose serious health hazards, from neurological damage and kidney failure to cancer. The presence of these metals in roadside and industrial soils is a cause for careful concern and management.

Persistent Hydrocarbon Contamination in Soil

Alongside metals, many polluted soils – especially near roads, factories, or fuel storage areas – contain organic contaminants known as hydrocarbons. Hydrocarbons are compounds made up of hydrogen and carbon atoms. The ones of concern in contaminated soils typically come from petroleum and combustion processes. They can be broadly grouped into polycyclic aromatic hydrocarbons (PAHs) and petroleum hydrocarbons (PHCs). These substances are notable for their persistence (resistance to breakdown) and potential toxicity.

1. Polycyclic Aromatic Hydrocarbons (PAHs)

PAHs are a class of complex organic molecules consisting of multiple fused aromatic rings (flat, cyclic structures composed of carbon atoms with alternating double bonds and delocalized π electrons, giving them extra stability). They are generally produced by incomplete combustion of organic material. Common sources of PAHs include vehicle exhaust, coal and wood burning, waste incineration, and even charbroiling of food. They also occur in crude oil, tar, and asphalt. There are over 100 different PAH compounds; examples include naphthalene, benzo[a]pyrene, and anthracene. PAHs tend to be semi-volatile – they can exist attached to airborne particulates or as residues in soil and sediment.

Chemical structures of common polycyclic aromatic hydrocarbons (PAHs). (Source: Recent Advances in the Extraction of Polycyclic Aromatic Hydrocarbons from Environmental Samples, 2020)

In soil, PAHs are quite persistent. Because they are hydrophobic (water-repelling), PAHs strongly bind to soil organic matter and clay particles. This means they don’t readily dissolve and wash away; instead, they remain in the soil, especially heavier PAHs with larger ring structures. Microorganisms can slowly degrade some PAHs, breaking them down over weeks to months, but the rate of breakdown decreases as the PAH size increases. High-molecular-weight PAHs (with four or more rings, like benzo[a]pyrene) are particularly resistant to biodegradation and can persist for years in the soil​

Lighter PAHs (two to three rings, like naphthalene or phenanthrene) are somewhat more prone to degradation or even evaporation. Sunlight can also help break down PAHs on soil surfaces (a process called photodegradation) over time.

PAHs can migrate in the environment under certain conditions. While they mostly stick in place, some PAHs can leach through soil to contaminate groundwater – especially the lighter, more soluble ones. However, the majority tend to stay in the topsoil or move into sediments if carried by erosion. PAHs also have a tendency to bioaccumulate (build up in living organisms): plants and animals in contaminated areas can end up with PAH concentrations higher than those in the surrounding soil or water. For example, shellfish living in PAH-polluted waters concentrate PAHs in their tissues, and crops grown in PAH-contaminated soil may have PAH residues on or in them (mostly on surfaces or in fatty portions of plants).

PAH Toxicity and Health Risks

Many PAHs are toxic and several are known or suspected carcinogens. One of the most studied, benzo[a]pyrene, is classified as a human carcinogen and has been linked to lung and skin cancers in exposed populations. In general, PAHs can cause cellular damage through the formation of reactive metabolites that bind to DNA. Environmental exposure to mixtures of PAHs (for example, breathing highway soot or ingesting soot-contaminated soil) has been associated with increased cancer risk. Besides cancer, PAHs can also harm aquatic life and soil microorganisms. They can cause developmental and reproductive problems in wildlife. It’s worth noting that typical environmental PAH levels from urban soil or air are low, and the health effects of chronic low-level exposure are still being studied. But at contaminated sites (like old gas manufacturing plants or creosote-treated areas), PAH levels in soil can be very high and pose acute risks to health if not managed.

2. Petroleum Hydrocarbons (PHCs)

Petroleum hydrocarbons (PHCs) refer broadly to the mixture of hydrocarbons found in crude oil and petroleum products (gasoline/petrol, diesel, kerosene, etc.). PHCs in soil typically come from oil spills, leaking underground storage tanks, pipeline ruptures, or chronic drips and emissions from vehicles. This category includes a wide range of chemicals, from light volatile compounds like butane and benzene to heavy oils and tars.

Two important subcategories of PHCs are aliphatic hydrocarbons (which include alkanes and alkenes) and aromatic hydrocarbons (like benzene, toluene, and larger PAHs discussed above).

The chemical structure of typical alkanes, cycloalkanes, alkenes and alkynes (Source based on: United States Forest Service, 2002)

• Alkanes: Alkanes are saturated hydrocarbons, meaning they consist solely of carbon-carbon single bonds in a chain (general formula C_nH_{2n+2}). Examples include methane, hexane, and the longer chains in motor oil or asphalt. Alkanes tend to be chemically less reactive. In soil, smaller alkane molecules can evaporate (volatilize) or be degraded by microbes under aerobic conditions relatively easily. Larger alkanes (the waxy or oily components) can persist longer but are still susceptible to slow biodegradation. Alkanes are generally less toxic than many other hydrocarbons. They do pose risks (for instance, inhaling volatile alkanes like hexane can affect the nervous system), but in soil contamination scenarios their toxicity is often lower relative to other components of fuels.
• Alkenes: Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (general formula C_nH_{2n}). Examples in fuels are fewer (most volatile fuel components are alkanes or aromatics), but alkenes can form during the breakdown of other hydrocarbons or be present in some fuels. Alkenes are more chemically reactive than alkanes due to the double bond. This can make them more toxic and also sometimes more amenable to degradation (because microbes and chemical processes can attack the double bond). However, reactivity can also mean certain alkenes form harmful byproducts or react to form polymers that linger in soil. In general, as a class, unsaturated hydrocarbons (including alkenes) tend to exhibit higher toxicity than saturated hydrocarbons. For instance, studies indicate that, other factors being equal, alkenes are more toxic to organisms than their alkane counterparts​. This is one reason why the composition of a petroleum spill influences its environmental impact.
• Alkynes: Alkynes (included in the table below) are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (general formula C_nH_n), are not a primary concern in PHC contamination. This is because they only occur in trace amounts in petroleum and are more reactive, generally makes them less persistent in the environment, as they are more likely to undergo chemical reactions (such as oxidation) that transform them into other compounds. Consequently, they do not typically accumulate to the levels or persist long enough to pose a significant long-term risk.
• Cycloalkanes: Often referred to as naphthenes—are also a component of the petroleum hydrocarbon (PHC) mixture found in contaminated soils. Cycloalkanes are saturated hydrocarbons formed into a ring-type structure. This class of hydrocarbons includes such compounds as cyclopentane, cyclobutane, and methylcyclopentane. They tend to be less volatile than their straight-chain alkane counterparts and are somewhat more resistant to biodegradation, which means they can persist in soils under certain conditions. While cycloalkanes are not entirely benign, they typically exhibit lower acute toxicity compared to many aromatic hydrocarbons. As a result, environmental and human health concerns are often more heavily weighted toward tend to prioritize the aromatics and volatile compounds, the more toxic or mobile components of PHCs—such as BTEX compounds and PAHs—because these tend to present a higher immediate risk.

Alkanes and alkenes differ in chemical structure (Source: Naming Alkanes Worksheets)

Aromatics vs. Aliphatics: In petroleum, aromatic hydrocarbons (like BTEX compounds benzene, toluene, xylenes, and PAHs) are often the most toxic components. Benzene, for example, is a known carcinogen. Aromatics are more soluble in water and more bioavailable than large alkanes, which means they can more readily contaminate groundwater and enter living organisms​. They also tend to be more resistant to biodegradation if they have complex structures. Aliphatic hydrocarbons (straight-chain or branched alkanes, and alkenes) can be degraded by soil bacteria under the right conditions – indeed many oil-degrading microbes will preferentially consume simpler alkanes first. However, heavy oil residues (long-chain alkanes) can remain for years as well, especially if oxygen is limited.

Typical aromatic compound benzene with ring structure (shown by two different schemes) shown on the left, and phenanthrene, an example of a polycyclic aromatic hydro-carbons (PAH) on the right. (Source based on: United States Forest Service, 2002)

What Are BTEX Compounds?

BTEX is an acronym that stands for benzene, toluene, ethylbenzene, and xylenes. These compounds are classified as aromatic hydrocarbons. Each of the compounds contains one or more benzene rings, which is the defining characteristic of aromatic compounds. These volatile organic compounds (VOCs) are commonly found in petroleum products and are often associated with petroleum hydrocarbon contamination in soil and groundwater.

Here are some key points about BTEX compounds:

• Benzene: Known for its carcinogenic properties, benzene is a significant concern due to its association with an increased risk of leukemia and other blood disorders.
• Toluene: Often used as an industrial solvent, toluene can affect the central nervous system and has other potential health impacts.
• Ethylbenzene: Typically used in the production of styrene, ethylbenzene exposure can affect the respiratory system and has been linked to other adverse health effects.
• Xylenes: These are a group of isomers used in various industrial applications; exposure to xylenes may lead to neurological and respiratory effects.

BTEX compounds are particularly concerning because of their toxicity, their ability to migrate easily through soils and into groundwater, and their potential to volatilize into the air. Due to these factors, they are often monitored in environmental assessments and are a key focus in remediation efforts at contaminated sites.

Persistence of PHCs

The persistence of a given petroleum hydrocarbon in soil depends on its molecular weight, structure, and environmental conditions. Light hydrocarbons (short-chain alkanes, small aromatics) might evaporate or be biodegraded in days to months. Heavy hydrocarbons (like those in crude oil or bunker fuel) can persist for many years, especially if they penetrate into subsurface soil where less oxygen is available. A spill of oil can leave behind an “aged” contamination where the lighter fractions have evaporated or degraded, leaving a tarry mix of heavy alkanes and PAHs that can persist for decades. Even when biodegradable, PHCs can create long-term issues because they can kill vegetation and soil life upon initial contamination – for example, a diesel spill might sterilize soil microbes in the area, reducing soil fertility. Over time, natural attenuation (biodegradation) often reduces the concentrations, but the intermediate breakdown products might still be harmful.

PHC Toxicity and Health Risks

Regarding human health, the risk depends on the specific compounds present: for instance, benzene in soil can evaporate and be inhaled, posing cancer and blood-disorder risks; PAHs in soil dust can be carcinogenic if ingested or inhaled; and even some aliphatic hydrocarbons can cause skin irritation or neurological effects with sufficient exposure. Many petroleum products also have additive chemicals (like solvents, or lead in the old leaded gasoline) that compound the hazard.

In essence, the mixture of hydrocarbons in contaminated soils, whether from exhaust particles or oil spills, tends to stick around and can slowly leach or emit vapors. Both heavy metals and these persistent hydrocarbons often co-occur in soils near roads and industries (for example, lead and PAHs from traffic, or arsenic and diesel-range organics at old industrial sites). This combination can create complex risks for the environment and human health.

One should also note that contaminated sites often have mixed exposures. For example, an industrial site might have both lead and PAHs in soil; the combination can be particularly harmful because multiple organs and systems are targeted by different toxins. Moreover, socioeconomic factors play a role – low-income communities are often closer to industrial areas or busy highways and thus more likely to be exposed to these soil hazards, compounding health disparities.

When heavy metals and hydrocarbons accumulate in soil, they can significantly disrupt local ecosystems. Soil is not just an inert medium – it’s a living environment filled with microbes, fungi, insects, and plant roots.

Impact of Heavy Metal Contamination

Heavy metal contamination can reduce soil biodiversity by poisoning sensitive microorganisms and soil fauna. For example, elevated mercury or cadmium levels can alter the soil microbial community, inhibiting beneficial bacteria and fungi that are crucial for nutrient cycling​. Plants growing in contaminated soil may experience stunted growth or yield reductions due to metal toxicity interfering with physiological processes (such as enzyme function and nutrient uptake). Some plants, however, tolerate or even accumulate heavy metals – these differences in plant response are exploited in phytoremediation strategies (using plants like certain ferns or alpine pennycress to extract metals from soil). Nonetheless, in a natural setting, heavy metal presence generally leads to lower plant diversity and can allow only metal-tolerant species to thrive, reducing overall ecosystem health.

Impact of Hydrocarbon Contamination

Hydrocarbon pollution in soil similarly harms soil life. Fresh petroleum hydrocarbons can be particularly damaging – they may coat soil particles and create oxygen-poor conditions, killing aerobic microbes and small invertebrates (like earthworms) in the contaminated zone. The result is often a patch of soil where normal decomposition and nutrient cycling are halted, leading to what is sometimes called “soil sterilization.” In aquatic ecosystems, runoff carrying PAHs or oil can be toxic to fish, amphibians, and aquatic invertebrates. Many PAHs cause mutations or cancers in wildlife; for instance, PAH-contaminated sediments have been linked to tumors in bottom-dwelling fish in polluted waterways. Hydrocarbon contaminants can also bioaccumulate: small organisms like worms may accumulate PAHs or other chemicals, which then move up the food chain to birds or mammals.

Water Source Contamination

Contaminants in soil also threaten groundwater and surface water quality. Rainwater percolating through contaminated soil can carry soluble contaminants downward. Arsenic, for instance, can leach into groundwater under certain geochemical conditions, leading to contaminated wells. Nitrates and salts typically leach more readily than heavy metals, but metals like cadmium and zinc can move if the soil pH changes or if they form soluble complexes.

Some petroleum hydrocarbons, notably the benzene, toluene, ethylbenzene, and xylene (BTEX) compounds in gasoline (petrol), are moderately soluble in water and can migrate to groundwater – this is a common problem at leaking underground fuel tank sites. Even heavier oil components can reach groundwater in non-aqueous phase form, creating a lasting source of pollution that slowly dissolves into the water over time​. Once contaminants reach groundwater, they can spread out as a plume, potentially tainting drinking water wells or discharging into streams and lakes. Ecosystems downstream may then be exposed to these pollutants. For example, arsenic or lead in groundwater can accumulate in sediments of a creek, affecting aquatic organisms; or a PAH plume seeping into a river can harm fish and make the fish unsafe for predators (including humans) to eat.

Effects of Heavy Metals and Hydrocarbons on the Food Chain and Bioaccumulation

Another environmental risk is the transfer of soil contaminants into the food chain. Plants growing in contaminated soil can take up certain heavy metals (especially cadmium and arsenic) into their roots, shoots, or fruits. When herbivores eat those plants, the metals enter their bodies, and can then move on to predators. Mercury provides a classic case of this: inorganic mercury from soil or water is converted by bacteria into methylmercury, which then accumulates in aquatic organisms and biomagnifies in each step of the food chain, reaching high concentrations in top predators like large fish. Humans who eat those fish are then exposed to potentially harmful mercury levels. Lead and arsenic do not biomagnify to the same extent, but chickens foraging on lead-contaminated soils, for instance, can have elevated lead in their eggs; cows grazing on polluted pastures could accumulate cadmium in their liver and kidneys over time.

Similarly, animals can accumulate hydrocarbons. Grazing animals may ingest soil along with grass – if the soil contains PAHs or oil residues, some of that ingested material can be absorbed. Insects and worms in contaminated soil may concentrate PAHs, and insect-eating birds or mammals can thus dose themselves with those chemicals. While most hydrocarbon compounds will be metabolized or excreted by higher animals (unlike metals, they don’t permanently accumulate in tissues as much), continuous exposure can still cause chronic poisoning or increased cancer risk in wildlife.

Best Practices for Growing Food in Contaminated Soil

Urban agriculture and gardening in areas with a history of industrial or roadside contamination require thoughtful planning and proactive management. Although soils may contain heavy metals or hydrocarbons, effective strategies exist to minimize exposure and and make gardening safer.

Below are some key best practices to help you manage soil contamination while growing healthy, edible plants.

1. Test Your Soil: Before planting edible gardens in urban or suspect areas, it’s wise to test the soil for common contaminants. Contact a soil testing laboratory (many government departments or university extension services offer this) and request analyses for heavy metals like lead, cadmium, and arsenic. If the site is near a roadway or former gas station, testing for petroleum hydrocarbons or PAHs may be warranted as well. Knowing your soil’s contaminant levels is the first step in managing risk.

2. Use Raised Beds with Clean Soil: One of the simplest and most effective strategies for growing food safely is to use raised garden beds filled with clean, uncontaminated soil or compost. By importing clean topsoil/compost and elevating the growing area above the native soil, you create a barrier between plant roots and the contaminated ground​. Make sure to line the bottom of raised beds with landscape fabric or another barrier such as root barrier plastic if there’s concern that roots might penetrate or that soil might mix. Raised beds also have the benefit of being easier to manage and can improve drainage.

3. Cover Bare Soil and Reduce Dust: If you have contaminated soil areas in your yard, keep them covered to prevent windblown dust and direct contact. Apply a layer of mulch around 5-10cm (2-4 inches) thick, or plant groundcover vegetation on bare soil​. This is particularly important around play areas for children. By covering the soil, you minimize the chances of contaminated dust sticking to vegetables or being inhaled. Also avoid tracking soil into the house – remove gardening shoes outside and use doormats.

4. Soil Amendments – Compost and pH Adjustment: Mixing substantial amounts of organic matter (compost) into soil can help dilute contaminants and bind heavy metals, making them less bioavailable.

• Compost-rich soil tends to sequester (bind) lead and other metals onto organic particles, reducing uptake by plants and lowering exposure risk.
• Maintaining a neutral to slightly alkaline pH (around 6.5–7.0) also reduces the availability of many heavy metals​. Acidic soils (low pH) can make metals like lead and cadmium more soluble and more easily absorbed by plant roots; thus, if your soil is acidic, consider adding garden lime or dolomite lime to raise the soil pH.

Note: While compost and pH control help, they do not remove soil contaminants – they simply stabilize them. The produce grown should still be handled carefully.

5. Crop Selection and Gardening Practices: If your soil has elevated heavy metals or PAHs but you still wish to garden in it, be strategic about what and how you grow:

• Grow fruiting crops rather than root or leafy crops. Fruits like tomatoes, peppers, beans, and squash generally uptake far fewer contaminants than root vegetables (carrots, potatoes) or leafy greens that directly contact soil. Plus, the edible part (fruit) is further away from soil in many cases (staked tomatoes, trellised beans, etc.).
• Avoid planting food crops directly at the drip line of old painted buildings or near high-traffic roads, since those areas often have the highest lead concentrations.
• Consider using containers for vegetables if the ground soil is questionable. Container gardening with clean potting mix completely sidesteps the issue of existing soil contamination, though you still need to be mindful of dust from the surrounding area.
• If hydrocarbons are the concern (e.g., near a garage or spill site), know that many will degrade over time with microbial action. You can encourage this by keeping the soil aerated (tilling in compost), inoculating the area with beneficial soil microbes by adding worm farm liquid leachate (‘worm wee’) and planting vigorous ornamental plants to stimulate microbial activity (not for food use until levels have subsided). For serious petroleum contamination, professional remediation may be necessary before any gardening.
• Avoid using recycled or used railway sleepers (railroad ties) in the garden, as these pieces of timber used beneath train tracks are treated with creosote.

6. Hygiene and Harvest Handling: Practice good hygiene to avoid ingesting soil particles:

• Wash all produce thoroughly with running water. This is critical for root and leafy vegetables. Peel root crops (like carrots, radishes) since peels can hold soil residues. For leafy greens, wash them in several changes of water and consider discarding the outer leaves.
• Wash your hands after gardening and always before eating. Also supervise children to ensure they don’t eat soil or put dirty objects in their mouths.
• Wear gloves while working in the garden, and consider using a mask on dry windy days if you’re kicking up a lot of dust.
• Remove and launder work clothes after heavy gardening sessions to avoid bringing dust into your home.

Remediation for the Long Term: If testing shows very high contamination (for instance, lead levels in the thousands of ppm, or a large oil spill), you should look into remediation options:

• Soil Removal and Replacement: For small areas like residential yards, digging out the top layer of contaminated soil and replacing it with clean soil is a direct (if costly) solution. The removed soil should be disposed of according to local government hazardous or contaminated waste regulations.
• Phytoremediation: Some plants can slowly uptake contaminants. Certain hyperaccumulator plants (like alpine pennycress for zinc/cadmium or some mustard species) can be grown to extract metals, after which the plants are harvested and disposed of as hazardous waste. Keep in mind this process can take many crop cycles and is not very effective for lead without soil amendments. It’s more useful for cadmium or arsenic in some cases. For hydrocarbons, planting contaminant-tolerant grasses and legumes can support microbial communities that degrade pollutants – a process called phytoremediation (plant remediation) or rhizoremediation (root remediation).
• Professional Cleanup: For large or heavily polluted sites (old industrial lots, brownfields), professional environmental contractors can employ techniques like soil vapor extraction (for volatile hydrocarbons), bioremediation (adding specialized microbes/nutrients), or immobilization (mixing soil amendments that lock up metals in insoluble forms). Community gardens on brownfields often go hand-in-hand with such cleanup efforts supported by government programs.

By following these recommendations, the risks of gardening in contaminated urban soil can be greatly reduced. In fact, many community gardens have been successfully established on remediated brownfields or filled with clean soil over contaminated ground. The key is to know what’s in your soil, to keep contaminants out of your plants and mouth, and improve the soil conditions to minimize contaminant availability.

In conclusion, heavy metal and hydrocarbon contamination is a lasting legacy of industrial and automotive activities that continues to pose risks to ecosystems and human health. Contaminants like lead, cadmium, mercury, arsenic, and PAHs persist in soils for decades, interfering with plant growth, reducing soil health, and entering the food chain. Despite progress in reducing such pollution, legacy contamination remains a serious concern. Through awareness, testing, proper gardening practices, and remediation where needed, it is possible to manage these risks. Healthy gardening means using safe soil to protect both our environment and public health.

References

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• Agency for Toxic Substances and Disease Registry (ATSDR). (1999). ToxFAQs for Mercury. U.S. Department of Health and Human Services. (Mercury fate and health effects). https://webarchive.library.unt.edu/eot2008/20080916043537/http:/www.atsdr.cdc.gov/tfacts46.html
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• Reuters. (2021, August 30). Use of leaded petrol eliminated in ‘milestone’ for health, environment, U.N. says. Reuters News. (Historical context on tetraethyl lead in gasoline and its global phase-out). https://news.trust.org/item/20210830154500-iopb6
• Understanding Brownfields | US EPA. (2025, February 21). US EPA. https://www.epa.gov/brownfields/understanding-brownfields
• U.S. Environmental Protection Agency, U.S. Consumer Product Safety Commission, & U.S. Department of Housing and Urban Development. (2013, September). Protect your family from lead in your home (EPA-747-K-12-001) [Fact sheet]. https://semspub.epa.gov/work/07/30304965.pdf
• United States Environmental Protection Agency (EPA). (2013). Polycyclic Aromatic Hydrocarbons (PAHs) Fact Sheet. EPA/CDC Joint Fact Sheet. (Overview of PAH sources, exposure, and health information) https://www.epa.gov/sites/default/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf
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