Polluted Soil: Definition, Sources, and Remediation

Definition of Soil Pollution

Soil pollution, as a part of land degradation, occurs when xenobiotic (human-made) chemicals or alterations in the natural soil environment degrade soil quality. This pollution is primarily caused by industrial activities, agricultural chemicals, and improper waste disposal.

Sources of Soil Pollution

Soil pollution originates from various natural and anthropogenic sources, which can be categorized into airborne, waterborne, and solid waste-based sources.

A) Pollution Through Soil

• Airborne Pollutants Contaminants from industrial exhausts, vehicular emissions, and chemical processes settle on the soil. Acid rain containing sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) alters soil pH and disrupts nutrient balance.
• Drainage System Contamination Pollutants enter the soil via wastewater from industries, sewage treatment plants, and agricultural runoff.
• Dumped Solid Materials Waste from mining activities, animal slurries, sewage sludge, and municipal solid waste introduces toxins into the soil.

B) Pollution Through Water Water’s solvent properties enable it to dissolve industrial by-products, pesticides, and chemical fertilizers, which infiltrate and contaminate the soil through irrigation and precipitation.

Kinds of Soil Pollution

1. Pesticides Excessive use of insecticides, herbicides, and fungicides leads to the accumulation of toxic residues in soil.
2. Inorganic Contaminants (Heavy Metals) Toxic elements like lead (Pb), cadmium (Cd), mercury (Hg), and arsenic (As) accumulate in the soil due to industrial waste, mining activities, and electronic waste dumping.
3. Organic Wastes (Municipal and Industrial Wastes) Unregulated disposal of organic waste from households, factories, and landfills results in soil degradation.
4. Fertilizers and Salts Overuse of chemical fertilizers leads to salinization, nutrient imbalances, and the leaching of nitrates into groundwater.
5. Radionuclides Radioactive contamination from nuclear plants, medical waste, and improper disposal of nuclear material causes long-term soil toxicity.

Acid Rain and Soil Pollution

Acid rain occurs when pollutants like sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) react with water vapor in the atmosphere, forming sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These acids:

• Reduce soil pH, making it more acidic.
• Leach essential nutrients like calcium and magnesium.
• Mobilize toxic metals, making them more bioavailable to plants and animals.

Chemical Reactions Involved in Acid Rain Formation

1. SO₂ + H₂O → H₂SO₃ (Sulfurous Acid)
2. 2SO₂ + O₂ → 2SO₃ (Sulfur Trioxide)
3. SO₃ + H₂O → H₂SO₄ (Sulfuric Acid)
4. NO₂ + H₂O → HNO₃ (Nitric Acid)

These acids lower the soil pH, causing severe damage to plant growth and microbial communities.

Major Soil Pollutants

• Agricultural Pesticides – Herbicides, fungicides, and insecticides accumulate in the soil.
• Organic Waste – Municipal and industrial waste contributes to organic pollution.
• Heavy Metals – Lead (Pb), Cadmium (Cd), Arsenic (As), and Mercury (Hg) accumulate in the soil.
• Fertilizers – Overuse of nitrogen and phosphorus-based fertilizers disrupts the soil’s nutrient balance.
• Radionuclides – Radioactive contamination from nuclear waste affects soil quality.
• Acid Rain – Alters soil pH, leaches essential nutrients, and mobilizes heavy metals.

Preventive and Remediation Measures

To mitigate soil pollution, various preventive and remedial strategies can be adopted.

Preventive Measures

• Reduce Chemical Fertilizer and Pesticide Use Shift to organic farming practices. Promote integrated pest management (IPM) to minimize pesticide usage.
• Use of Bio-Fertilizers and Manures Composting, vermicomposting, and bio-fertilizers enhance soil fertility naturally.
• Adopt Biological Pest Control Methods Introduce natural predators and microbial pesticides (e.g., Bacillus thuringiensis) to reduce chemical dependency.

Soil Remediation Techniques

When soil pollution has already occurred, remediation techniques are essential to restore soil quality.

A) Physical Remediation

• • Soil Washing Removes heavy metals and organic pollutants using water or chemical solutions.
  • Soil Vapor Extraction (SVE) Extracts volatile organic compounds (VOCs) from contaminated soils through vacuum pumps.

B) Chemical Remediation

• • Soil Stabilization Binds contaminants to prevent their mobility using amendments like lime, gypsum, and phosphates.
  • Electrokinetic Remediation Uses electric fields to extract heavy metals and organic contaminants.

C) Biological Remediation (Bioremediation)

• • Phytoremediation Plants like mustard, sunflower, and vetiver grass absorb heavy metals from contaminated soils.
  • Microbial Bioremediation Bacteria and fungi (e.g., Pseudomonas, Rhizobium, and Mycorrhizae) break down toxic pollutants.