Key Soil Contaminants

Why This Matters

Soil contamination sits at the intersection of several major Environmental Chemistry II concepts you'll be tested on: bioaccumulation, persistence, toxicity mechanisms, and remediation strategies. When you understand how contaminants behave in soil—whether they bind tightly to particles, leach into groundwater, or move up the food chain—you're demonstrating mastery of the chemical principles that govern environmental fate and transport. These concepts appear repeatedly in exam questions about risk assessment, ecosystem health, and human exposure pathways.

Don't just memorize a list of pollutants. For each contaminant below, know why it's problematic: Is it persistent? Does it bioaccumulate? What's its primary exposure route? The exam will ask you to compare contaminants, predict their behavior based on chemical properties, and evaluate remediation approaches. That's where this guide comes in—we've organized these contaminants by their defining characteristics so you can think like an environmental chemist.

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Persistent Organic Compounds

These contaminants share a critical property: chemical stability that resists natural degradation processes. Their carbon-based structures, often with halogen substitutions, make them resistant to microbial breakdown, photolysis, and hydrolysis—meaning they stick around for decades.

Persistent Organic Pollutants (POPs)

• Resist environmental degradation—their stable molecular structures allow them to persist in soil and water for years to decades
• Bioaccumulate through food chains, with concentrations increasing at each trophic level due to their lipophilic nature
• Examples include DDT, dioxins, and furans—regulated under the Stockholm Convention due to their global transport and toxicity

Polychlorinated Biphenyls (PCBs)

• Synthetic chlorinated compounds—historically used in electrical transformers and capacitors before bans in the 1970s-80s
• Highly lipophilic, meaning they accumulate in fatty tissues and biomagnify through aquatic and terrestrial food webs
• Associated with carcinogenic and endocrine-disrupting effects—their persistence makes legacy contamination an ongoing concern at industrial sites

Polycyclic Aromatic Hydrocarbons (PAHs)

• Formed during incomplete combustion—sources include vehicle exhaust, industrial emissions, and forest fires
• Known carcinogens that can cause DNA damage through metabolic activation to reactive intermediates
• Strongly sorb to soil organic matter—this binding makes them less mobile but also harder to remediate

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Inorganic Toxicants

Unlike organic pollutants, these contaminants cannot be broken down—they're elemental or simple compounds that persist indefinitely. Their toxicity stems from interactions with biological molecules, particularly proteins and enzymes.

Heavy Metals (Lead, Mercury, Cadmium)

• Cannot be degraded, only transformed between chemical species or physically relocated
• Bioaccumulate in organisms—lead disrupts neurological development, mercury causes $CH_3Hg^+$ (methylmercury) poisoning, cadmium damages kidneys
• Sources include mining, industrial discharge, and legacy contamination—urban soils often contain elevated lead from historical gasoline and paint use

Radioactive Materials

• Emit ionizing radiation (alpha, beta, or gamma) that damages DNA and cellular structures
• Sources include nuclear facilities, medical waste, and naturally occurring radon—exposure risk depends on isotope half-life and emission type
• Remediation requires specialized containment—often involves excavation and long-term storage rather than in-situ treatment

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Agricultural and Industrial Chemicals

These contaminants enter soil through intentional application or industrial processes. Their behavior depends heavily on chemical properties like volatility, water solubility, and sorption coefficients.

Pesticides and Herbicides

• Designed to be biologically active—this same property makes them potentially harmful to non-target species including pollinators and soil microorganisms
• Persistence varies widely depending on chemical class: organochlorines persist for years, while organophosphates degrade within weeks
• Can leach to groundwater if water-soluble, or bind to soil particles if hydrophobic—understanding $K_{oc}$ (soil organic carbon partition coefficient) predicts their fate

Industrial Solvents

• Many are volatile organic compounds (VOCs)—chemicals like trichloroethylene (TCE) and benzene evaporate readily and pose inhalation risks
• Dense non-aqueous phase liquids (DNAPLs) sink through groundwater, creating contamination plumes that are notoriously difficult to remediate
• High mobility in subsurface environments—their low sorption to soil particles allows rapid transport to drinking water aquifers

Petroleum Hydrocarbons

• Complex mixtures from crude oil—include alkanes, aromatics (like BTEX compounds), and heavier fractions
• Toxic to soil microbial communities—disrupts the organisms essential for nutrient cycling and soil structure
• Amenable to bioremediation—many petroleum compounds can be degraded by naturally occurring bacteria, making this a cost-effective treatment option

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Nutrient Pollution and Emerging Contaminants

Not all soil contaminants are toxic in the traditional sense. Some cause harm through ecosystem imbalance or represent newly recognized threats whose impacts are still being characterized.

Excess Nutrients (Nitrogen, Phosphorus)

• Result primarily from agricultural runoff—fertilizer application exceeding plant uptake leads to accumulation and transport
• Cause eutrophication when they reach water bodies, triggering algal blooms that deplete oxygen and create dead zones
• Alter soil microbial communities—excess nitrogen can shift bacterial populations and affect decomposition rates

Microplastics

• Particles less than 5mm derived from plastic breakdown or manufactured as microbeads in personal care products
• Accumulate in soil ecosystems—can affect soil structure, water retention, and organism health through physical and chemical mechanisms
• Act as vectors for other contaminants—hydrophobic pollutants like PAHs and PCBs sorb to plastic surfaces, potentially increasing bioavailability

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Quick Reference Table

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Self-Check Questions

1. Which two contaminant categories share the property of being impossible to chemically degrade, and how do their toxicity mechanisms differ?

2. A soil sample near an old electrical substation shows elevated levels of chlorinated organic compounds that bioaccumulate. What contaminant is most likely present, and what chemical property explains its persistence?

3. Compare and contrast PAHs and petroleum hydrocarbons: How do their sources differ, and why might petroleum contamination be easier to remediate?

4. An FRQ describes agricultural runoff entering a lake, causing fish kills. Which soil contaminant category is responsible, and what is the mechanism of ecological damage?

5. Why would an environmental chemist need to know a pesticide's $K_{oc}$ value when assessing contamination risk, and how would high vs. low values change their predictions?