By Sanjenbam Jugeshwor Singh
Soil contamination or soil pollution as part of land degradation is caused by the presence of Xenobiotic (human-made) chemicals or other alteration in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals, and improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons (such as Naphthalene and Benzo (a) pyrene), solvents, pesticides, Lead and other heavy metals. Contamination is correlated with the degree of industrialization and intensity of chemicals substance. Mapping of contaminated soil sites and resulting cleanup are time consuming and expensive task, requiring extensive amounts of geology, hydrology, and chemistry, computer modelling skills and GIS in environmental contamination as well as an appreciation of the history of industrial chemistry.
Soil pollution can be caused by oil spills, mining and activity by other heavy industries,corrosion of underground storage tanks, Acid rain, intensive farming, Agrochemicals such as pesticides, herbicides and fertilizers, industrial accidents, road debris, drainage of contaminated surface water into the soil. Ammunitions, chemical agents, waste disposal, oil and fuel dumping, nuclear waste to the soil, discharge of sewage, landfills and illegal dumping, coal ash and electronic waste also help to contaminate the soil.
The most common chemicals involved are petroleum hydrocarbons, solvents, pesticides, Lead and other heavy metals. Any activity that leads to other forms of soil degradation (erosion compaction etc.) may directly worsen the contamination effects in that soil remediation become more tedious. Historical deposition of coal ash used for residential heating as well as for industrial process such as ore smelting were a common source of contamination in areas that were industrialized before about 1960.
Coal, naturally concentrates Lead and Zinc during its formation as well as other heavy metals to a lesser degree. When coal is burned ,most of these metals become concentrated in the ash( the principal exception being Mercury) coal ash and slag may contain sufficient Lead to quality as a characteristic hazardous waste ,defined in the U.S.A as containing more than 5mg/l of extractable Lead using the TCLP procedure. In addition to Lead, coal ash typically contains variable but significant concentration of polynuclear aromatic hydrocarbon (PAHs,). These PAHs are known human carcinogens and acceptable concentration of them in soil are typically around 1mg/kg.
Coal ash and slag can be recognized by the presence of off-white grains in soil, grey heterogeneous soil or (coal slag) bubbly, vesicular pebble-sized grains. Treated sewage sludge, known in the Industry as bio solids has become controversial as fertilizers. As it is the by-product of sewage treatment, it generally contains more contaminants such as organisms, pesticides and heavy metals than other soil.
A pesticide is a substance or mixture of substance used to kill a past .A pesticide may be a chemical substance, biological agent (such as a virus or bacteria), antimicrobial, disinfectant or device used against any pest. Pest include; insects, plant pathogens, weeds, mollusks, birds, mammals, fish, nematodes(roundworm) and microbes that compete with humans for food, destroy property, spread or are a vector for diseases or cause nuisance.
Although there are benefits to the use of pesticides, here are also drawbacks, such as potential toxicity to humans and other organisms. Herbicides are used to kill weeds especially on pavements and railways. They are similar to auxins and most are biodegradable by soil bacteria. However, one group derived from trinitrotoluene (2:4 D and 2:4:5 T) have impurity dioxin, which is very toxic and causes fatality even low concentrations. Another herbicide is Parquet. It is highly toxic but it rapidly degrades in soil due to the action of bacteria and does not kill soil fauna. Insecticides are used to rid farms of pests which damage crops.
The insects damage not only standing crops but also stored ones and in the tropics it is reckoned that only third of the total production is lost during food storage. As with fungicides, the first insecticides used in 19th century were inorganic e.g Paris green and other compounds of arsenic.
Nicotine has also been used since the late 18th century. Organochlorine include DDT, Aldrin, Dildine and BHC. They are cheap to produce, potent and persistent. DDT was used on a massive scale from 1930s with a peak for 72,000 tons used in 1970. The usage fell as the harmful environment effect were realized. It was found worldwide in fish and birds and was even discovered in the snow of Antarctica. It is only slightly soluble in water but is very soluble in the bloodstream. It affects and cause the eggshells of birds to lack calcium causing them to be easily breakable. It is thought to be responsible for the decline of the numbers of birds preys like Ospreys and peregrine falcons in the 1950s- they are now recovering. As well as increased concentration via the food chain, it is known to enter via permeable membrane, so fish get it through their gills.
As it has low water solubility, it tends to stay at the water surface, so organisms that live there are most affected. DDT found in fish that formed part of human food chain caused concern.Organophosphates e.g parathion, methyl parathion and about 40 other insecticides are available nationally. Parathion is highly toxic, methyl-parathion is less so and Malathion is generally considered safe as it has low toxicity and rapidly broken down in mammalian liver. This group works by preventing normal nerve transmission as cholinesterase is prevented from breaking down the transmitter substance acetylcholine, resulting uncontrolled muscle movement.
Contaminated or polluted soil directly affects human health through direct contact with soil or via inhalation of soil contaminants which vaporized; potentially greater threats are posed by the infiltration of soil contaminants into groundwater, aquifers used for human consumption, sometimes apparently far removed from any apparent source of above ground contamination. This tends to result in the development of pollution related diseases.
Health consequences from exposure to soil contamination vary greatly depending on pollutant type, pathway of attack and vulnerability of the exposed population. Chronic exposure to chromium, Lead and other metals, petroleum, solvents and many pesticides and herbicide formulation can be carcinogenic can cause other chronic health conditions. Industrial or man-made concentrations of naturally occurring substances such as nitrate and ammonia associated with livestock manure from agricultural operations have also been identified as health hazards in soil and groundwater.
Chronic exposure to Benzene at sufficient concentrations is known to be associated with higher incidence of Leukemia. Mercury and cyclodiens are known to induce higher incidence of kidney damage and some irreversible diseases. Not unexpectedly, soil contaminants have significant deleterious consequences for ecosystem.
There are radical soil chemistry changes which can arise from the presence of many hazardous chemicals even at low concentration of the contaminants species. These changes can manifest in the alteration of metabolism of endemic microorganisms and arthropods resident in a given soil environment. The result can be virtual eradication of some of the primary food chain which in turn could have major consequences for predators or consumer species. Even if the chemical effect on lower life form is small, the lower pyramid levels of the food chain may ingest alien chemicals which normally become more concentrated for each consuming rung of the food chain. Many of these effects are now well known, such as the concentration of persistent DDT materials for avian consumers, leading to weakening egg shells, increased chick mortality and potential extinction of species.
Effects occur to agricultural land which have certain type of contamination. Contaminants typically alter plant metabolism, often causing a reduction in crop yields. This has a secondary effect upon soil conservation, since the languishing crops cannot shield the Earth’s soil from erosion. Some of these chemicals contaminants have long half- lives and in other case derivative chemicals are formed from decay of primary soil contaminants.
(The writer is a faculty at NIELIT, Imphal)