Soil Pollution

Excavation showing soil contamination at a disused gasworks.

Soil pollution is caused by the presence of man-made chemicals

or other alteration in the natural soil environment.This type of contamination typically arises from the rupture of underground storage tanks,application of pesticides,percolation of contaminated surface water to subsurface strata,oil and fuel dumping,leaching of wastes from landfills or direct discharge of industrial wastes to the soil.The most common chemicals involved are petroleum hydrocarbons,

solvents,pesticides,lead and other heavy metals.This occurrence of this phenomenon is correlated with the degree of industrializations and intensities of chemical usage.

The concern over soil contamination stems primarily from health risks,both of direct contact and from secondary contamination of water supplies.Mapping of contaminated soil sites and the resulting cleanup are time consuming and expensive tasks,requiring extensive amounts of geology,hydrology,chemistry and computer modeling skills.

It is in North America and Western Europe that the extent of contaminated land is most well known,with many of countries in these areas having a legal framework to identify and deal with this environmental problem;this however may well be just the tip of the iceberg with developing countries very likely to be the next generation of new soil contamination cases.

The immense and sustained growth of the People's Republic of China since the 1970s has exacted a price from the land in increased soil pollution.The State Environmental Protection Administration believes it to be a threat to the environment,to food safety and to sustainable agriculture.According to a scientific sampling,150 million mi (100,000 square kilometres) of China’s cultivated land have been polluted,with contaminated water being used to irrigate a further 32.5 million mi (21,670 square kilometres) and another 2 million mi (1,300 square kilometres) covered or destroyed by solid waste.In total,the area accounts for one-tenth of

China’s cultivatable land,and is mostly in economically developed areas.An estimated 12 million tonnes of grain are contaminated by heavy metals every year,causing direct losses of 20 billion yuan (US$2.57 billion).

The United States,while having some of the most widespread soil contamination,has actually been a leader in defining and implementing standards for cleanup.Other industrialized countries have a large number of contaminated sites,but lag the U.S. in executing remediation.Developing countries may be leading in the next generation of new soil contamination cases.

Each year in the U.S., thousands of sites complete soil contamination cleanup,some by using microbes that “eat up” toxic chemicals in soil,many others by simple excavation and others by more expensive high-tech soil vapor extraction or air stripping.Efforts proceed worldwide to identify new sites of soil contamination.

Ecosystem Effects

Not unexpectedly, soil contaminants can have significant deleterious consequences for ecosystems.There are radical soil chemistry changes which can arise from the presence of many hazardous chemicals even at low concentration of the contaminant species.These changes can manifest in the alteration of metabolism of endemic microorganisms and arthropods resident ina given soil environment.The result can be virtual eradication of some of the primary food chain, which in turn have major consequences for predator or consumer species.Even if the chemical effect on lower life forms 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 of egg shells,increased chick mortality and potentially species extinction.

Effects occur to agricultural lands which have certain types of soil contamination.Contaminants typically alter plant metabolism, most commonly to reduce crop yields.This has a secondary effect upon soil conservation,since the languishing crops cannot shield the Earth's soil mantle from erosion phenomena.Some of these chemical contaminants have long half-lives and in other cases derivative chemicals are formed from decay of primary soil contaminants.

Cleanup Options

Microbes can be used in soil cleanup

Cleanup or remediation is analyzed by environmental scientists who utilize field measurement of soil chemicals and also apply computer models for analyzing tra

nsport and fate of soil chemicals. Thousands of soil contamination cases are currently in active cleanup across the U.S. as of 2006. There are several principal strategies for remediation:

• Excavate soil and take it to a disposal site away from ready pathways for human or sensitive ecosystem contact. This technique also applies to dredging of bay muds containing toxins.
• Aeration of soils at the contaminated site (with attendant risk of creating air pollution)
• Thermal remediation by introduction of heat to raise subsurface temperatures sufficiently high to volatize chemical contaminants out of the soil for vapour extraction.Technologies include ISTD,electrical resistance heating (ERH), and ET-DSP^tm.
• Bioremediation,involving microbial digestion of certain organic chemicals.Techniques used in bioremediation include landfarming,biostimulation and bioaugmentating soil biota with commercially available microflora.
• Extraction of groundwater or soil vapor with an active electromechanical system,with subsequent stripping of the contaminants from the extract.
• Containment of the soil contaminants (such as by capping or paving over in place).

Types of soil pollution control

Body movement causes contamination and protective clothing such as hats, cleanroom suits and face masks are basic forms of contamination control. Apart from people, the other common way for contamination to enter is on the wheels of trolleys used to transport equipment.

To prevent airborne contamination, high efficiency particulate air (HEPA) filters, airlocks and cleanroom suits are used.HEPA filtration systems used in the medical sector incorporate high-energy ultra-violet light units to kill off the live bacteria and viruses trapped by the filter media.These measures restrict the number of particulates within the atmosphere, and inhibit growth in those that are viable.

Studies by 3M show that over 80% of contamination enters the cleanroom through entrances and exits, mostly at or near floor level.To combat this suitable flooring systems are used that effectively attract, retain and inhibit growth of viable organisms.Studies show that the most effective type of flooring system is one of polymer composition.

Polymer mats are particularly effective due to their suppleness as they allow for more contact with serration on shoes and wheels and can accommodate for more particles whilst remaining effective.An electrostatic potential adds to the effectiveness of this type of contamination control as it holds particles until being cleaned.This method of attracting and retaining particles is more effective than mats with an active adhesive coating which needs to be peeled and is often not as supple. As long as the tack level of the mat is greater than the donor (foot or wheel), the contamination touching the surface will be removed.Very high tack surfaces pose a contamination threat because they are prone to pulling off over-shoe protection.Polymeric flooring is produced to ensure a higher level of tackiness than the surfaces it comes into contact with, without causing discomfort and potentially damaging ‘stickiness’.