Contamination of Soil and Its Control

After reading this article you will learn about the contamination of soil and its control.

Contamination of Soil:

(A) Contamination of Soil with Pesticides:

Types of Pesticides:

Pesticides can be classified into the following three broad groups:

(1) Insecticides:

An insecticide is a chemical prepared for controlling insect-pests. Organic insecticides may be classified into; Chlorinated hydrocarbons, Organophosphate, Carbamate and plant origin.

(i) Chlorinated Hydrocarbons:

This group of insecticides has low biodegradability. They persist in the soils and plants for long period and cause toxicity. Examples DDT, BHC, Lindane, Aldrin, Endrin.

(ii) Organophosphorus Compounds:

The organophosphorus insecticides such as parathion, malathion, metasystox, phosdrin are generally biodegradable and thus, less likely to build up in soils and water. Systemic insecticides of this group are relatively much more toxic to human and other mammalians. Great care should be taken not to inhale or expose any part of body to spray.

(iii) Carbamate Insecticide:

Insecticides like carbofuran, carbaryl, oxamyl, sevin are easily biodegradable and relatively low mammal toxicity.

(iv) Plant Insecticide:

The insecticides which are extracted from plants are termed as plant insecticides, viz., nicotine, pyrethrum, neemin. Such insecticides unlike synthetic insecticides are safer to use.

(2) Fungicides:

Fungicides are chemicals, used for controlling the diseases of crops, fruits and vegetables. Some fungicides containing mercury, arsenic and copper are poisonous to man and cattle. Organic fungicides such as dithiocarbamate (trade name, ziram, zineb, maneb, dithane, ferbam etc.) are less toxic.

(3) Herbicides:

The chemicals which kill or inhibit the weeds are called herbicides or weedicides. Generally, herbicides are less toxic to mammals in comparison to insecticides and fungicides. Some are quite toxic to fish. However, herbicides are generally, biodegradable. Example of herbicides; phenoxyl acid (2, 4-D), carbamate (Triallate), thiocarbamates (Eptam).

(B) Contamination with Toxic Inorganic Compounds:

Soils are also contaminated by a number of inorganic compounds. They are also toxic to human and other animals. Arsenic, cadmium, mercury and chromium are extremely poisonous; lead, fluorine and nickel are moderately; zinc, copper, boron and manganese are relatively less toxic.

Major Sources of Toxic Inorganic Compounds:

Highly Toxic:

Arsenic: Pesticides, coal and petroleum etc.

Cadmium: Fertilizer impurities, paints, batteries etc.

Mercury: Fungicides, metallurgy, evaporation of Hg etc.

Chromium: Brick manufacture, stainless steel etc.

Moderately Toxic:

Lead: Pesticides, fertilizers, gasoline, coal etc.

Fluorine: Pesticides, fertilizers, air pollution etc.

Nickel: Fertilizers, gasoline, batteries, coal etc.

Low Toxic:

Zinc: Fertilizers, pesticides, batteries, sewage waste etc.

Copper: Fungicides, fertilizers, industrial dust etc.

Boron: Fertilizers, gasoline, irrigation water etc.

(C) Contamination of Soil Due to Acid Rain:

Nitrogen and sulphur dioxides liberate from motor vehicles and industries. Nitrogen and sulphur gases dissolve in the water vapour of atmosphere and form nitric and sulphuric acid. These help acidify rain water, which falls on the soil as “acid rain”. The pH of acid rain may be around 4.0.

(D) Contamination of Soil Due to Organic Waste Disposal:

Soils are used as disposal reservoir for municipal refuse. The wastes include garbage, paper products and non-biodegradable materials like polythene, glass and metals. Leaching and run off from these sites can contaminate both surface and ground water. Contaminants include heavy metals as well as soluble and biodegradable organic materials.

Many organic wastes can improve soil physical and chemical properties and can provide nutrients for increased crop yields. However, when excess quantities are applied, nitrate accumulated in the soil and leach into ground water which may cause nitrate toxicity.

(E) Contamination of Soil with Salts:

Contamination of soils with salt is also a form of soil pollution. When the concentration of soluble salts increases to high level then it produces toxic effect directly to plants.

(F) Contamination of Soil with Chemical Fertilizers:

Nitrogen contamination, excess use of nitrogenous fertilizers add N to the soil and subsequently contaminate ground water. Nitrosamines are formed by the reaction, under highly acidic conditions of secondary and tertiary amines with nitrous acid anhydride (N₂O₃). Nitrosamines have been found to produce tumors in animals.

Nitrogen fertilizer contamination of water increases the possibility of hazardous nitrate concentrations. One effect of excess nitrate is blue baby disease (Methemoglobinemia) which occurs in baby mammals. Nitrate in the mammal intestine are converted into nitrites which attach to the blood hemoglobin forming methemoglobin, which carries no oxygen, Methemoglobin is not caused directly by nitrate (NO₃), but occurs when nitrate reduced to nitrite (NO₂) by bacteria found in the digestive tract of humans and animals.

Nitrite can then oxidize the iron of the blood (hemoglobin molecule), forming methemoglobin, which can not perform as oxygen carrier. The amount of hemoglobin that normally carries oxygen to the tissue is reduced, resulting in anoxia (internal suffocation). This can result in bluish colouration of the skin in infants, hence called blue baby syndrome. By surface run off, nitrogen fertilizers enriching lakes, ponds etc. by N causing depletion of dissolved oxygen and general degradation of water quality.

Ozone layer is also affected by Nitrogen. Nitrogen oxides (N₂O, NO) released to the atmosphere have been found to cause photo-oxidation of Ozone (O₃) in the atmosphere. This process reduces the capacity of the Ozone layer to protect the earth from ultraviolet rays emitted by the sun which contributing to global warming.

Phosphorus contamination is very little in comparison to Nitrogen contamination. In the mineral soils, only a small amount of phosphorous are leached but in organic soils, phosphorous becomes relatively mobile due to low content of iron and aluminum.

Super phosphate usually contains small quantities of cadmium, copper, nickel, zinc and manganese which ultimately accumulate in soils and causing contamination.

Toxic Effect of Nitrogen Contamination:

The major health concern with N is the potential carcinogenic effect of nitrosamine compounds. Nitrosamines are formed by the reaction, under highly acidic conditions (pH<4.0), of secondary and tertiary amines with nitrous acid anhydride (N₂O₃). They have produce tumours in animals.

Fertilizer nitrogen contamination of water increases the possibility of hazardous nitrate concentrations. One effect of excess nitrate is blue baby disease which occurs in baby mammals. Nitrate in the mammals intestine are converted into nitrite which attach to the blood haemoglobin forming methemoglobin, which carries no oxygen. This can result in a bluish colouration of the skin in infant mammals.

Toxic effect of lead:

Contamination of soil with lead comes primarily from airborne lead automobile exhausts. Some lead is deposited on the vegetation and some reaches the foil directly. Lead contamination is problem unique to urban soils. It primarily affects children, causing brain damage and even death. Lead contamination also contributes to high blood pressure in adults.

Toxic effect of mercury:

Mercury contamination of lake beds and of swampy areas, resulted in toxic level in fishes. Insoluble form of mercury converted into soluble form by the microorganism become available to plants and animals. The mercury moves up the food chain that may be toxic to human beings.

Toxic effect of arsenic:

Arsenic has generally accumulated in orchard soils following years of application of arsenic containing pesticides. Remain present in anionic form, it is absorbed by hydrous iron and aluminium oxides. The arsenic toxicity can be reduced by application of sulphates of zinc, iron and aluminium that tie up the arsenic in insoluble forms.

Toxic effect of heavy metals and trace elements:

Heavy metals are the metals in the periodic table that have a molecular weight greater than 55. While this includes Mn and Fe, heavy metals that are an environmental concern are usually regarded as chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), strontium (Sr), molybdenum (Mo), cadmium (Cd), mercury (Hg) and lead (Pb).

Some of these metals i.e. (Zn, Mo, Cu, Mn) are micronutrients absolutely required for plant and animal growth. Toxicities of Mo in livestock and Se in livestock and wildlife are two relatively common trace elements causing health problems. Selenium (Se) problems are much more widespread in semiarid and arid regions. High Se level is harmful to livestock.

Prevention and Control of Contamination of Soil:

There are following methods of preventing soil contamination by toxic inorganic compounds:

(i) Soil application of toxins through pesticides, fertilizers, irrigation water and solid waste should be reduced or eliminated.

(ii) The soil becomes reservoir for the toxins, immobilisation of toxins in the soil would check their availability to the plants. Most of the toxins becomes less mobile and less available if the soil pH near neutral (pH 7.0). Heavy metals such as copper, zinc, lead and cadmium showed highest adsorption in the soil colloid at around pH 7.0.

(iii) The draining of wet acid soils should be beneficial since the oxidized forms of elements such as NO₃^–,SO₄^{– –},Fe⁺⁺⁺, Mn⁺⁺⁺ are readily oxidized available to plants. This is because the reduced forms of iron (Fe⁺⁺) and manganese (Mn⁺⁺) are sometimes present at toxic levels in wet acid soil.

(iv) Leaching of some toxic elements (e.g., boron) may be effective in removing excess boron from the soil.

(v) Application of heavy phosphate in the soil reduces the availability of toxic cations but may have opposite effect on arsenic (anion form).

(vi) There are differences in the ability of crops/varieties to extract toxins from the soil. Sorghum crop absorbs more cadmium and nickel than rye and maize.

(vii) Forage crops should be harvested at the stage of maturity when the concentration of the toxin is lowest.

(viii) Fertilizers should be applied at a time when the plants can readily absorb the nutrients.

(ix) Liming of highly acidic soil, expedite the immobility of toxic elements.

(x) Reclamation of highly salt affected soil.

(xi) Soil organic matter can adsorb trace element pollutants e.g.; Pb, Cd, Cu which will reduce their chances of contamination of surface and ground water. Another advantage is the adsorption of pesticides and other organic chemicals. This reduces the possibilities of pesticide carry over effects, prevents contamination of the environment and enhances both biological and non- biological degradation of certain pesticides and organic chemicals. In addition, organic matter is known for its capacity to adsorb organic (e.g.; NO and NO₂, and organic gases (e.g.; CO).