Soil Pollution

Introduction

Soil is a complex, living system made of minerals, organic matter, water, air, and organisms. In environmental chemistry, soil pollution refers to the undesirable chemical contamination of this system to such an extent that its natural functions are impaired: plant growth is hindered, groundwater quality is threatened, and ecosystems and human health are put at risk. This chapter focuses specifically on the chemical aspects of soil pollution: typical pollutants, their behavior in soil, and consequences and basic remediation approaches.

Sources and Types of Soil Pollutants

Soil pollutants are introduced from a variety of diffuse and point sources. Many originate from human activities; some also have natural contributions that become problematic when intensified by anthropogenic inputs.

Heavy Metals and Metalloids

Heavy metals and toxic metalloids are among the most persistent soil pollutants, as they are elements and cannot be degraded.

Typical contaminants include:

• Lead (Pb): from leaded gasoline residues (historically), paints, mining, smelting, industrial emissions, and shooting ranges.
• Cadmium (Cd): from phosphate fertilizers, metal refining, batteries, and waste incineration.
• Mercury (Hg): from coal combustion, chlor-alkali electrolysis plants, and some pesticides and industrial processes.
• Arsenic (As): from wood preservatives, mining, ore processing, and historically from pesticides.
• Chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn): from metal plating, waste disposal, fertilizers and manure, and industrial emissions.

While some metals such as Cu and Zn are essential micronutrients at low concentrations, elevated levels become toxic. Because these elements are not destroyed in the environment, pollution is cumulative and often long-term.

Organic Pollutants

Organic contaminants in soil originate mainly from industrial chemicals, agriculture, and incomplete combustion processes. Important groups include:

• Persistent organic pollutants (POPs), e.g.:
• Polychlorinated biphenyls (PCBs) from electrical equipment and industrial uses.
• Polycyclic aromatic hydrocarbons (PAHs) from fossil fuel combustion, coking plants, and vehicle exhaust.
• Organochlorine pesticides (e.g. DDT, lindane) from historical agricultural use.
• Modern pesticides and herbicides:
• Insecticides, fungicides, herbicides that may sorb to soil or leach into deeper layers.
• Petroleum hydrocarbons:
• From leaking storage tanks, oil spills, refineries, and traffic (diesel, gasoline, lubricants).
• Solvents and industrial chemicals:
• Chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE) from degreasing and dry cleaning.
• Pharmaceuticals and personal care products:
• Introduced via sewage sludge application, irrigation with treated wastewater, or manure from medicated livestock.

Persistence, volatility, solubility, and susceptibility to biological degradation vary widely across these substances, determining their environmental fate.

Excess Nutrients: Nitrogen and Phosphorus

Nutrients essential for plant growth can become pollutants when applied in excess:

• Nitrogen compounds:
• Ammonium salts, urea, and nitrates from mineral fertilizers and animal manure.
• Excess nitrogen can lead to accumulation of nitrate ($\ce{NO3^-}$) in soil and leaching into groundwater.
• Phosphates:
• From phosphate fertilizers, manure, and sewage sludge.

While primarily associated with water pollution and eutrophication, nutrient overload also alters soil chemistry, salinity, and biological activity.

Salts and Acidifying Substances

Some contaminants primarily affect soil by changing its pH or salinity:

• Salts from irrigation (especially in arid regions), de-icing road salts (NaCl), and some industrial waste lead to soil salinization and sodification.
• Acidifying substances:
• Deposition of acid-forming air pollutants (e.g. sulfur and nitrogen oxides) and acidifying fertilizers can cause soil acidification.
• Acid soils increase the mobility and toxicity of some metals such as Al and Mn.

Solid Waste and Microplastics

Improper waste disposal contributes both chemical and physical contaminants:

• Landfilled or illegally dumped waste:
• Releases leachates containing metals, organic pollutants, and salts.
• Microplastics:
• Fibers and fragments from plastic mulches, tire abrasion, litter, and sewage sludge.
• Serve as carriers for hydrophobic organic pollutants and additives (plasticizers, flame retardants).

Chemical Behavior of Pollutants in Soil

The impact of a soil pollutant is determined not only by its quantity, but also by its chemical form and interactions with soil components. Important processes include sorption, speciation, transport, and transformation.

Sorption to Soil Components

Sorption (adsorption and, in some cases, absorption) describes the binding of pollutants to soil solids, especially:

• Clay minerals:
• Provide charged surfaces and cation exchange sites.
• Organic matter (humus):
• Provides hydrophobic domains and functional groups capable of binding both metals and organic molecules.
• Oxides and hydroxides of Fe, Al, and Mn:
• Offer specific binding sites for anions and metal cations.

Consequences:

• Strong sorption:
• Reduces immediate bioavailability and mobility of contaminants.
• Can immobilize some pollutants, but also leads to long-term “storage” in soil.
• Weak sorption:
• Increases risk of leaching into groundwater or uptake by plants.

The extent of sorption depends on pollutant properties (charge, polarity, hydrophobicity) and soil properties (pH, organic carbon content, clay content).

Speciation and Redox State

For metals and metalloids, “speciation” (the distribution among different chemical forms) strongly influences mobility and toxicity.

Examples:

• Chromium:
• $\ce{Cr(III)}$ (e.g. $\ce{Cr(OH)3}$) is less mobile and less toxic.
• $\ce{Cr(VI)}$ (e.g. $\ce{CrO4^{2-}}$) is highly mobile and toxic.
• Arsenic:
• Reduced forms (e.g. $\ce{As(III)}$) are often more toxic and mobile than oxidized forms ($\ce{As(V)}$).
• Mercury:
• Can be transformed to methylmercury by microorganisms, which is more bioavailable and toxic.

Redox conditions in soil (oxidizing vs. reducing) are influenced by water content, oxygen availability, and microbial activity, and they control which species dominate.

pH Effects

Soil pH is a central factor in pollutant behavior:

• At low pH (acidic soils):
• Many metal cations (Pb, Cd, Zn, etc.) become more soluble and mobile.
• Protonation of functional groups on soil organic matter weakens binding of some cations.
• At higher pH (neutral to alkaline):
• Some metals precipitate as hydroxides or carbonates:
  $$\ce{M^{2+} + 2OH^- -> M(OH)2 (s)}$$
• Some anions (e.g. arsenate, chromate) may become more mobile.

Adjusting pH is therefore an important tool in managing metal mobility.

Transport and Leaching

Pollutants can move vertically and horizontally in soil:

• Dissolved species:
• Move with soil water; risk of leaching into deeper layers and groundwater.
• Particulate-bound contaminants:
• Move with eroded soil particles via surface runoff, contributing to contamination of nearby water bodies.
• Volatile pollutants:
• Some organic solvents and hydrocarbons can volatilize and move as soil vapors.

Soil texture, structure, and water regime influence transport:

• Coarse-textured, sandy soils with low organic matter allow faster percolation and less sorption.
• Fine-textured, clay-rich soils generally have higher sorption capacity but may develop preferential flow paths (cracks, worm channels).

Degradation and Persistence

Organic pollutants can undergo transformation in soil:

• Biodegradation:
• Microorganisms use some organic contaminants as energy or carbon sources, ultimately converting them to $\ce{CO2}$, water, and biomass.
• Degradation rates depend on chemical structure, temperature, moisture, oxygen availability, and nutrient balance.
• Abiotic degradation:
• Hydrolysis, oxidation, photodegradation (near the surface) and other chemical reactions can alter pollutant structures.

Persistent organic pollutants are characterized by:

• Resistance to degradation.
• High sorption to organic matter.
• Often high hydrophobicity and potential to bioaccumulate.

Metals and metalloids are not degradable and remain in the environment, although their speciation and distribution can change.

Effects of Soil Pollution

Impacts on Soil Organisms and Soil Functions

Soil biota—bacteria, fungi, protozoa, nematodes, earthworms, and others—carry out central soil functions (decomposition, nutrient cycling, aggregation).

Soil pollution can:

• Inhibit microbial activity and diversity, reducing decomposition and nutrient turnover.
• Harm soil fauna (e.g. earthworms) through direct toxicity, affecting soil structure and aeration.
• Disrupt symbiotic relationships such as mycorrhizae and nitrogen-fixing bacteria.

Overall, the soil’s ability to provide ecosystem services (fertility, water regulation, pollutant buffering) is impaired.

Plant Uptake and Food Chain Transfer

Plants absorb water and nutrients from soil; many pollutants can be taken up simultaneously:

• Metals such as Cd, Pb, and As can accumulate in roots, leaves, or edible parts.
• Some organic pollutants (e.g. certain pesticides, PAHs of lower molecular weight) can be taken up and translocated within plants.
• Elevated salinity and some toxic ions (e.g. $\ce{Na^+}$, $\ce{Cl^-}$, $\ce{B}$) damage plant tissues and reduce yields.

Contaminated crops can transfer pollutants into the food chain, affecting livestock and humans, and may lead to chronic exposure.

Groundwater and Surface Water Contamination

Leaching and erosion link soil pollution to water pollution:

• Nitrate from excess fertilization easily leaches into groundwater, posing health risks (e.g. methemoglobinemia) and contributing to eutrophication.
• Leaching of pesticides, solvents, and metals contaminates groundwater used for drinking and irrigation.
• Pollutant-laden eroded soil particles contribute to contamination and eutrophication of surface waters.

Thus, polluted soils often act as long-term sources of diffuse water pollution.

Human Health and Ecosystem Health

Exposure pathways for humans include:

• Ingestion of contaminated food or drinking water.
• Ingestion or inhalation of soil and dust (particularly relevant near industrial sites, for children, and in urban areas).
• Dermal contact in contaminated areas.

Possible effects:

• Acute toxicity in cases of severe contamination.
• Chronic effects (e.g. neurotoxicity, carcinogenicity, kidney damage, endocrine disruption) from long-term low-dose exposure to metals and organic pollutants.

Ecosystems are affected as toxic substances alter species composition, reduce biodiversity, and weaken resilience to additional stresses (e.g. climate extremes).

Examples of Typical Soil Pollution Scenarios

Agricultural Soils

Relevant pollution issues include:

• Overuse of mineral fertilizers:
• Accumulation of nitrate and, in some cases, phosphate; risk of leaching and eutrophication.
• Application of pesticides:
• Residues and metabolites may persist and affect non-target organisms.
• Use of sewage sludge and manure:
• Potential accumulation of metals (e.g. Cd, Cu, Zn), organic pollutants, and pharmaceuticals.

Long-term, even low-level inputs can result in significant accumulation.

Industrial and Urban Areas

Common problems:

• Former industrial sites (brownfields):
• High levels of metals, PAHs, solvents, and other industrial chemicals from spills and past disposal practices.
• Mining and smelting areas:
• Deposition of dust and tailings enriched in metals and metalloids.
• Traffic-related pollution:
• Accumulation of PAHs, metals (e.g. from brake and tire wear), and microplastics near roads.

Such areas often require detailed investigation and targeted remediation before reuse (e.g. for housing or playgrounds).

Investigation and Assessment of Soil Pollution

Environmental chemistry uses specific methods to identify and evaluate soil contaminants.

Sampling and Analysis

Typical steps:

• Sampling:
• Collection of soil cores or surface samples at defined depths and locations.
• Composite samples may be prepared to represent larger areas; hot spots may require individual samples.
• Sample preparation:
• Drying, sieving, digestion or extraction depending on the target analytes.
• Chemical analysis:
• Metals and metalloids are commonly determined after acid digestion using techniques like atomic absorption spectroscopy (AAS) or inductively coupled plasma methods (ICP-OES, ICP-MS).
• Organic pollutants are extracted using suitable solvents and analyzed by gas chromatography (GC), high-performance liquid chromatography (HPLC), often coupled with mass spectrometry (MS).

Results are compared with guideline or limit values set by regulatory frameworks to assess risk.

Bioavailability and Risk Assessment

Total concentrations do not always reflect actual risk. Important considerations:

• Bioavailable fraction:
• Portion of the total pollutant that is accessible to organisms, influenced by sorption, speciation, and soil conditions.
• Site-specific factors:
• Soil type, pH, organic matter content, land use (e.g. residential vs. industrial) and exposure scenarios.

Risk assessment integrates chemical data with toxicological information and exposure pathways to determine whether measures are needed.

Approaches to Managing and Remediating Polluted Soils

When soil pollution is confirmed and risk is unacceptable, various management and remediation strategies are used. These rely heavily on understanding the underlying chemistry.

Preventive Measures

Prevention aims to avoid new contamination:

• Optimized fertilizer and pesticide use:
• Matching application rates to plant needs; using less persistent substances.
• Controlled waste management:
• Proper disposal, treatment, and recycling of industrial and household waste.
• Emission control:
• Reducing atmospheric deposition from industry and traffic.

These measures reduce the input of pollutants to soil and are often the most cost-effective.

Containment and Immobilization

If removing contaminants is not feasible, their mobility and bioavailability can be reduced:

• pH adjustment:
• Liming acidic soils to raise pH and decrease metal solubility.
• Addition of amendments:
• Phosphates, iron oxides, clay minerals, or biochar to immobilize metals or organic pollutants by sorption or precipitation.
• Capping:
• Covering contaminated soil with clean material to reduce exposure and erosion.

These techniques rely on chemical processes such as precipitation, sorption, and complexation.

Removal and Treatment

In some cases, pollutants are physically removed or degraded:

• Excavation and off-site treatment:
• “Dig and dump” to secure landfills or specialized treatment facilities.
• Soil washing:
• Using water and sometimes surfactants or chelating agents to extract contaminants from soil particles; cleaned soil may be returned.
• Thermal and chemical treatments:
• High temperatures for organic contaminants, or chemical oxidation/reduction to transform pollutants into less harmful forms.

These methods can be effective but are often technically complex and costly.

Biological Remediation

Biological processes can support or accomplish pollutant removal:

• Bioremediation:
• Stimulation of native microorganisms or introduction of specialized strains to degrade organic contaminants under controlled conditions.
• Phytoremediation:
• Use of plants to extract, stabilize, or degrade pollutants.
• For example, some plant species accumulate metals in their biomass, which can then be harvested and removed.

While often slower than physical-chemical methods, biological approaches can be more sustainable and less disruptive to soil structure.

Summary

Soil pollution is a central topic of environmental chemistry because soil acts both as a sink and a potential source of contaminants. Key aspects include:

• A wide range of pollutant types from heavy metals and persistent organic chemicals to excess nutrients, salts, and microplastics.
• Complex chemical behavior in soil, governed by sorption, speciation, pH, redox conditions, and biological activity.
• Far-reaching effects on soil organisms, plant growth, water quality, human health, and ecosystems.
• The need for careful investigation, risk assessment, and tailored remediation strategies based on an understanding of soil chemistry.

Managing soil pollution is essential to maintain soil as a functional, life-supporting component of the environment and to protect linked compartments such as water and air.