Soil contamination is a form of land degradation brought by the spillage, migration, or burying of hazardous contaminants to the soil ecosystem from untreated industrial effluents, manufacturing wastes, local waste, agrochemicals, and other man-made substances or activities. Soil contaminants not only affect the soil but has an adverse effect on animal, plant, and human health as well. Exposure to contaminated soil may come from breathing, swallowing, touching, or ingesting food grown in contaminated lands. Soil contamination usually occurs in urban areas and former industrial and mine sites .
Types of soil contaminants
Contamination of soil from petroleum hydrocarbons is usually brought by crude oil and natural gas. Crude oil is present in lands nearby an oil well or pipelines. For natural gas, contamination typically occurs on urban pipeline system leaks.
Petroleum hydrocarbons cause nitrogen fixation and organic matter formation that has a pronounced effect on microbiological, physical, and chemical properties of soil. Increased organic matter in soil increases its water holding capacity to the point of deflocculation, resulting in loss of structure in soil and making it more prone to erosion .
Agrochemicals (pesticides, herbicides, and fertilizers)
Agrochemicals or agricultural chemicals are products used in agriculture, and mostly encompass pesticides, insecticides, herbicides, nematicides, and fungicides. It may also refer to hormones and synthetic fertilizers. Storing these chemicals improperly in bulk poses environmental risks in terms of accidental chemical spillage, leaks, or leaching. Pesticides that are sprayed by planes or tractors are often misdirected to other plants and animals, due to different wind patterns.
Overuse of pesticides can damage soil biomass, bacteria, fungi, and earthworms- organisms necessary to balance soil fertility and nutrient element cycle. Pesticides are often a short term solution as pests often develop immunity or resistance to it. Employing integrated pest management strategies are often found to be a better alternative than agrochemicals that causes soil and even groundwater contamination [3,4].
Polycyclic or Polynuclear aromatic hydrocarbons (PAH)
Polycyclic aromatic hydrocarbons or Polynuclear aromatic hydrocarbons (PAH) are carcinogenic micropollutant chemicals that are produced during the incomplete burning of organic substances, garbage, or fuels. It may also come from forest fires, volcanic eruptions, and industrial-scale burning of fossil fuels. PAHs are resistant to degradation due to its hydrophobic property and are often difficult to treat.
Soil contamination with PAH poses a toxic, carcinogenic, and mutagenic risk to human and ecological health more so when it enters the food chain. PAH alters grain size, water holding capacity, and porosity of the contaminated soil. PAH also affects soil properties such as volume, permeability, plasticity, as well as the microbe population and diversity [5,6].
Solvents such as halogenated non-polar aromatics, aliphatics, heterocyclics, and other polar organic compounds are found in many cleanup sites as it is used in many commercial and industrial processes as cleaning agents or as a product manufacturing ingredient. Halogenated solvents are mostly used for cleaning and degreasing metal surfaces, adhesive manufacturing, and dry cleaning. Chlorinated solvents such as trichloroethylene, trichloroethane, methyl chloride, and tetrachloroethylene are four of the widely used solvents. The improper storage, utilization, and disposal of solvents have resulted in soil contamination and groundwater pollution. Solvent contamination makes soil reduce its fertility and nitrogen fixation. Air stripping, carbon adsorption, soil venting, aeration, and bioremediation are options on the method of cleaning chlorinated solvents. [7,8]
Lead naturally occurs in soil but due to industrial activities and use of leaded products, lead concentrations rise above normal levels. Lead does not degrade over time and high exposure rate to the element leads to serious human health issues, especially in children where correlation on lead exposure and mental impairment and physical development was found apparent. Lead is widely used in the past as an additive in gasoline fuel for its antiknock properties. It is also used in residential painting. A large amount of lead traces can be found in heavy traffic areas .
Asbestos is a widely used construction material and is often found in roofing tiles and wall claddings. Asbestos soil pollution often comes from poor demolition practices and disposal management of asbestos cement sheets, fragments and fibres, thermal pipes, and other debris. Exposure to high concentrations of asbestos leads to lung scarring, lung cancer, and mesothelioma.
Heavy metal pollution in soils is often found in industrial areas, mining sites, milling plants, as well as metal, paint, and petrochemical plants. They can also come from fertilizers to supplement soil metal content for plant growth. Unlike organic compounds, metals do not degrade chemically or microbially. They persist through time and its accumulation pose health risks on humans and ecological life through ingestion, direct contact, or its introduction to the food chain. Heavy metals also affect the productivity and fertility of the contaminated land resulting in land use and agricultural production problems.
Some heavy metal remediation techniques are available such as soil washing, phytoremediation, and immobilization. Remediating sites with heavy metal content is important in order to ensure the reduction of associated health risks, enhance land productivity and security, and improve over-all agricultural yield .