Environmental pollution is a widely discussed topic with air, water, and soil being polluted in today’s rapidly progressing society. Soil, being a “universal sink,” is significantly at risk of the effects of environmental pollution. Soil pollution occurs when chemicals or substances which are not naturally present are found in soil or when naturally occurring compounds and materials are present but in abnormally high concentrations. Because soil pollution cannot be visually perceived or directly assessed, it poses a significant threat to soil fertility and productivity. Hence, the urgency in controlling soil pollution.
The soil is the outer layer of the earth’s crust composed of organic and inorganic materials. It is the basis of agriculture that supports the growth of rooted plants and crops for human and animal consumption. Due to soil pollution, food security is greatly affected due to the presence of toxic levels of contaminants resulting in a reduction in crop yield and with crops deemed unsafe for human and animal consumption.
Assessment of soil to identify contaminants present is a difficult task nowadays due to the evolution of these contaminants caused by agrochemical and industrial innovations. In addition, the soil is often on private lands, unlike aquatic and gaseous environments, making ecological risk assessment relatively challenging. That is why soil pollution is becoming an alarming issue. It has been identified as the third most crucial threat to soil function in Eurasia and Europe, fourth in North Africa, fifth in Asia, seventh in Northwest Pacific, eighth in North America, and ninth in Latin America. In order to determine the full extent of soil pollution and contamination, an environmental characterization study is necessary.
Exposure Route from Soil to Human Intake
There are three main routes where soil can directly enter the body: inhalation, eating, or through skin contact.
This route mostly affects people, like workers, who are continually working with soil or those who reside nearby such areas. These types of environments have fine dust particles floating around, which can be inhaled and eventually absorbed by the body.
Adults consume soil through accidental ingestion. An example of this is when the ingested food, like vegetables, still has some soil attached to it. However, in some parts of the world, the soil is deliberately consumed due to cultural reasons.
On the other hand, children, especially those under the age of three, are at high risk for soil contamination exposure as they tend to eat soil while playing outdoors. In addition, their biological makeup is more likely to absorb more of the toxic chemicals than that of an adult.
Also known as “dermal absorption” or “cutaneous absorption,” this route is most applicable to volatile organic compounds. However, some heavy metals do cause skin contact problems.
This occurs when soil contaminants transfer from the soil to surface water contaminating the drinking water. Aside from that, the contaminated water may also be absorbed by plants and, in turn, contaminating the said plant, which may be consumed by livestock or humans, thereby allowing the contaminants to enter the human food chain .
Indirect consumption of these contaminants becomes dangerous when it accumulates in the body resulting in abnormally high levels, exhausting the body’s natural detoxification system. When this happens, the body cannot rid itself of the contaminants, which result in adverse effects on the tissues and organs that are affected .
This happens through the use of pesticides, herbicides and fertilizers, domestic and industrial waste disposal through land burial, use of sewage sludge to agricultural lands, chronic discharge from motor vehicles like oil leaks, and accidental discharges or spills from storage tanks or transportation of toxic substances.
Particulates from the atmosphere get mixed into the soil through dry deposition, wet deposition, or gas partitioning. Dry deposition occurs when particulate matter, or the contaminant, settles out of the atmosphere and onto plant and soil surfaces. Wet deposition is when the pollutants attached to airborne particles get washed off to the soil by rain or snow. Gas partitioning happens when contaminants diffuse out from the air and diffuse into the ground.
Contaminated Water Use
Irrigating farmlands, lawns, or gardens with contaminated water supplies can result in the accumulation of contaminants in the soil.
The source of contamination can also be the soil itself. Internal contamination happens when biological organisms naturally present in the soil produce toxic chemical byproducts or are health-threatening themselves.
Discussed below are the most common pollutants of the soil. The categories listed below are according to the Food and Agriculture Organization of the United Nations (FAO) .
Heavy Metals and Metalloids
Heavy metals and metalloids are those that have a relatively high atomic mass. These include lead (Pb), cadmium (Cd), mercury (Hg), strontium (Sn), and zinc (Zn). There are also non-metals considered together with heavy metals: arsenic (As), Antimony (Sb), and selenium (Se). They are naturally present in soils and are essential micronutrients for plants, animals, and humans.
These include but are not limited to herbicides, insecticides, fungicides, nematicides, molluscicides, rodenticides, and plant growth regulators. They are applied to crops to prevent losses due to pests, pathogens, and weeds.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs are a group of semi-volatile organic pollutants that have low water solubility and slow mass-transfer rates from solid, causing them to be retained in the soil for more extended periods of time. The most prevalent PAHs are benzopyrene, pyrene, naphthalene, anthracene, fluoranthene, and phenanthrene.
Persistent Organic Pollutants (POPs)
These are organic, hydrophobic, lipophilic substances that bioaccumulate through the food chain and pose adverse effects on human health and the environment. Examples include polychlorinated biphenyls (PCBs) and organochlorine pesticides such as DDT.
The most common radionuclides found in soil are the following: 137Cs, 40K, 90Sr, 232Th, 238U.
Emerging Pollutants (EPs)
EPs are chemicals, natural or synthetic, that have recently appeared in the environment and are not usually monitored. These chemicals may be from pharmaceuticals, hormones and toxins, biological pollutants like bacteria and viruses, and endocrine disruptors.