杏吧原创

SLAG heaps, rubbish tips, illegal dumps: the words 鈥渃ontaminated land鈥� seem to conjure up the essence of the 20th-century environmental nightmare. But localised sites are only part of the story: almost all of our planet鈥檚 surface is contaminated to some degree by airborne pollution. It is, however, the dramatic and visible localised instances that grip us-and do much of the serious damage to the environment.

Land is contaminated when it contains substances that, when present in sufficient concentrations, may harm humans, animals or the environment. When untreated sites are subsequently used, serious problems can arise. In 1980, reclaimed land at Lekkerkerk in the Netherlands, where some 270 houses had been built, was found to have been contaminated by 1600 drums of illegally dumped toxic waste. With their drinking water and the voids beneath their floors affected, 870 people were forced to leave their homes. Afterwards, 150 000 tonnes of contaminated soil had to be removed from the site. The toxic wastes included toluene and other harmful organic chemicals that are used in the textiles industry.

Lekkerkerk was an unfortunate echo of Love Canal, the contamination story of the 1970s. This area of upstate New York became notorious when solvents and other chemicals leaked from a disused chemical waste burial site. Some 2500 homes had to be abandoned.

Even a country as seemingly untouched by the ravages of industrial pollution as New Zealand has not escaped the taint. A task group from the New Zealand Ministry of the Environment identified up to 600 sites where pentachlorophenol (PCP)-an organochlorine pesticide-may have been used to treat timber, leaving harmful residues in its wake. Sediment from Lake Rotorua was also found to contain dioxin-a by-product of herbicide and disinfectant manufacture-at levels normally associated with heavily contaminated waterways in Europe and North America. Dioxin is highly toxic, particularly to animals but also to humans.

The exact scale of this kind of problem is often hard to calculate, however. For example, Germany鈥檚 national environmental agency estimates that there are more than 143 000 suspected contaminated sites within its borders, and the final figure is said to be probably nearer 250 000.

Laying waste

Law of the land

CONTAMINATION of the solid outer crust of the Earth-the lithosphere-is by no means a recent phenomenon. In Roman Britain, during the 1st century BC, people were already mining and smelting metals, which polluted the surrounding countryside with mine waste and also deposited it further afield as airborne dust. Really serious contamination did not happen until the Industrial Revolution was under way-a phenomenon vividly chronicled by Charles Dickens and other writers. The forerunners of today鈥檚 chemicals industry produced appalling pollution around their works and shaped British and French landscapes in ways still evident today.

More recent industrial development has laid waste large tracts of Eastern Europe. Shortly before the Soviet Union鈥檚 fragmentation in 1991, the Kremlin said that 3.5 million square kilometres of the country were polluted to the point of risk to human health; and in Poland, the country鈥檚 own academy of sciences has estimated that a quarter of the soil there is so contaminated, safe food cannot be grown on it.

As a result of high-profile cases like Lekkerkerk and Love Canal, many countries such as the US, Canada and the highly industrialised nations of Europe have enacted laws to control the development of any such sites in the future and to make land contamination illegal. Unfortunately, there are no truly international guidelines on limits and procedures: the laws have often been developed in isolation because soil contamination, unlike pollution of the atmosphere and other resources, tends to be localised. The European Union, for example, has not yet evolved policies for soil, though it has published numerous directives on air and water.

In Southeast Asia and the less industrialised European countries, especially those in Eastern Europe, the development of policy is at a preliminary but fast-moving phase. But, before legislation can be introduced or the contamination cleaned up, it is essential to answer the question, 鈥淗ow clean is clean?鈥� In some industrialised countries, including the Netherlands, a concept of multifunctionality has prevailed until recently, meaning that sites are cleaned up, or remediated, to almost pristine conditions regardless of how the land is to be used afterwards. Such thoroughness is hardly surprising in the Dutch, considering how hard they have had to work to reclaim the land from the encroaching sea in the first place. The end-use approach, which Britain currently favours, sets the level of cleanliness according to whether, say, a car-park or a children鈥檚 play area is the site鈥檚 ultimate fate.

Then there are the criteria set out in the legislation and guidelines. These have tended to set what are called prescriptive levels for individual contaminants, or classes of them, present on a site. But this isn鈥檛 always the best route. Governments are now beginning to realise that an approach based on risk can be more appropriate (see Inside Science No 33, 鈥淩isky business鈥�). To pose a risk, three factors must be present: the source of the contamination, the pathway it takes, and the target, such as a child, an animal or a plant. If any one of these is absent, there is no risk.

If there is risk, and a site becomes contaminated, clean-ups don鈥檛 come cheap. In the US, as a result of the 1980 Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), a billion-dollar 鈥淪uperfund鈥� was established to deal with hazardous sites (see Figure 2). Under the act, landowners are strictly liable for any contamination on their property, even if they did not cause it.

This tough approach is unlikely to find favour in Britain. The House of Lords ruled in December 1993 that Eastern Counties Leather, a leather treatment company, could not be held responsible for pollution caused in the early 1970s to the Cambridge Water Company鈥檚 water supplies. The Lords said that the company could not reasonably have foreseen the damage caused by the spillage of perchloroethene (PCE) solvent, a cleaning agent. When drums of the solvent were tipped into cleaning tanks, small amounts regularly spilled around them onto the concrete floor, and eventually seeped through the concrete. Pools of neat PCE then collected near the base of a chalk aquifer beneath the works and travelled through it at a rate of about 8 metres a day. Eventually this groundwater travelled some 20 kilometres away, where it contaminated Cambridge Water鈥檚 supplies.

Airborne ills

Pollution鈥檚 payoff

WHAT, however, of the widespread deposition of contaminants from the air? While mainly concentrated around industrialised regions in the northern hemisphere, airborne pollution moves freely across international boundaries. Metals and sulphates that result from the burning of coal in Britain, for instance, eventually find their way to Sweden and Norway. Similarly, industrial emissions originating in the northern US reach Canada as acid rain.

The effects of airborne pollution can be serious. In 1988, part of China ended up in the Canadian Arctic. Around 4000 tonnes of contaminated soil had blown over from China and settled over 20 000 square kilometres of land, turning the snow brown and leaving appreciable quantities of organic micropollutants such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). And sheep in otherwise idyllic pastures in Snowdonia, Wales, are still affected by radioactive fallout from the 1986 Chernobyl accident.

The dietary demands of a heavily populated planet have led to a rise in the use of a major soil contaminant: agricultural chemicals such as pesticides and fertilisers. Residues of pesticide, particularly of the organochlorine variety, which break down very slowly, can be found in land, water and the organisms that come into contact with each. The overuse of fertilisers leaves high levels of nitrates in the soil and the crops grown on it (see Inside Science No 37, 鈥淣itrates in soil and water鈥�). After we have eaten the nitrate-laden food, it often makes its way back onto the land as sewage sludge. Combined with treated sludge from industrial processes-which is often discharged into the same sewers-the resulting mix, if used on prime agricultural land, can then contaminate it with heavy metals as well as nitrates. Even the by-products of this cycle are implicated in contamination. The packaging and leftovers that make it into the bin in municipal incinerators can, when burnt, release dioxins into the air and onto the land.

Industry is the most obvious source of land contamination, and industrial sludges are the mere tip of the iceberg. The contaminants are as varied as the products made. Old gasworks, for example, produced gas right up to the middle of this century by carbonising coal, releasing a Pandora鈥檚 box of land contaminants including coal tar (containing PAHs), phenols, cyanides, sulphur, sulphates and sulphides. The chemicals and petrochemicals industries are also obvious culprits and at service stations everywhere, corroding tanks and pipes seep their contents into the surrounding earth-a potential source of explosions, fire and contamination of groundwater.

The contaminants in this array affect the environment, and us, in a variety of ways. Metals, particularly zinc and cadmium from industrial sewage sludge, can harm the microorganisms in soils. The phytotoxic elements boron, copper, nickel and zinc adversely affect plant growth; and aluminium is particularly toxic to fish.

The toxicity of contaminants to humans is more difficult to pin down, but can be estimated by epidemiological studies. A case in point is the research on diseases of workers exposed to asbestos. Toxicologists can also estimate the toxicity of a chemical contaminant via animal experiments. Apart from ethical considerations, however, there are well-known dangers in extrapolating data from these experiments to humans. For example, when the US Environmental Protection Agency ordered tests on laboratory animals, it found that the dioxin 2,3,7,8-tetrachlorodibenzodioxin was so potent that 1 microgram (one-millionth of a gram) is sufficient to kill a guinea pig. Yet epidemiological studies on workers exposed to the dioxin suggest that the toxic dose for humans would be much greater than the difference in size between guinea pigs and humans would suggest.

What is it that makes a molecule toxic? It must first be capable of finding a way into the body. Small, nonpolar molecules such as those of benzene, found in petrol, are volatile and so enter the body through inhalation. Asbestos fibres, used in a number of industries, often end up in soil and may then be released into the atmosphere to be inhaled into the lungs, where they can cause serious diseases such as asbestosis and mesothelioma. PCBs and PAHs are lipophilic, or have an affinity for fats, and their molecules can therefore be absorbed through the skin. Since they are insoluble in water, or hydrophobic, they must be converted metabolically into polar products to be excreted from the body. But this is a rarity, and lipophilic molecules tend instead to 鈥�bioaccumulate鈥� in certain body tissues. Hydrophilic, or soluble, molecules, such as phenols, can penetrate body tissues through the alimentary tract.

Once in the body, a molecule of a toxic chemical can interfere with fundamental physical processes in several ways. Some molecules can modify the transmission of signals by the nerves by mimicking natural transmitter molecules in structure, size and polarity. They can then fit into a receptor site, say, on a protein or enzyme, much like a key in a lock. Other toxins may act as systemic agents, affecting the body as a whole rather than any one particular site. Mercury, for example, acts on the brain, kidneys and bowels.

Mobile metals

Testing, testing . . .

MERCURY is only one of the complex and diverse group of metallic contaminants that bioaccumulate. Lead is a very common metallic contaminant in land, and its toxicity has been studied in detail. Although still widely used, for example in car batteries and some paints, lead is gradually being eradicated from many areas. Like other metals, it can form salts and other compounds with varying degrees of solubility and hence mobility in the soil, which inevitably means an ability to reach groundwater and ultimately drinking water supplies. Metals can contaminate drinking water in other ways. Many mining operations-particularly those that work sulphide ores or coal where iron pyrites are present-release acidic, metal-laden effluent, to catastrophic effect on the ecology of streams and rivers.

Aside from solubility, a decreasing, or more acid, soil pH aids the mobility of metals. Some of the metals in soils are available, or taken up by plants; so clean-ups can target these if site investigators analyse soil especially for them, as well as for total metals present. The amount of organic matter, the redox conditions-that is, the balance of oxidation-reduction-and the cation exchange capacity (CEC) also determine the ability of a soil to hold metals. Soils made up of large particles tend to have a lower CEC, so sandy soils have a lower CEC than clayey ones.

When combined with organic contaminants, mercury, lead and tin pose even greater threats to our health. Such combinations can arise naturally, via biotransformation or metabolism in the body, which can convert chemicals into compounds; or through manufacture, as with tetraethyl lead, the antiknock product added to petrol and now banned from many countries (see Figure 3).

Discovering the exact source of such contamination is all part of the art of site investigation. Sites may be investigated for a number of reasons, such as concerns about leaching of contaminants into groundwater, or assessment of an industrial site鈥檚 potential liabilities by a purchaser. A desk study is then begun, in which records and maps of the site are scrutinised for clues to its history and any developments that might have given rise to the pollution. Local people are often a good source of information, especially if they worked on the site in the past.

The second phase begins after all interested parties are consulted. Site investigators sample and test soil (and gas and water, if encountered) and any other items likely to pose a risk, such as asbestos. Mechanical diggers may be brought in to dig trial pits, and drills to probe the ground. As technology advances, more equipment for on-site testing is becoming available; but often there is no substitute for the full laboratory testing of samples by skilled analytical chemists.

Safety considerations are paramount during this phase. Workers must not be exposed unnecessarily to the substances. Nor must any contamination be spread, either on the wheels of a dumper truck leaving the site, downwards into the groundwater during test drilling, or as dust carried by the wind to surrounding areas.

What does the site investigator test for? The desk study is a useful first guide. But just like the technologies used in remediation, many of the tests used are still evolving-and some may not be completely appropriate for the purpose. For example, analytical chemists normally test a soil for metal contamination in the laboratory by using an aggressive acid digestion such as aqua regia (a mix of nitric and hydrochloric acid), then measuring the levels of any metals dissolved in solution. Government guidelines in Britain and the Netherlands give safe levels based on these so-called total metal concentrations, above which action is recommended.

In many cases, total concentrations overestimate the risk. A problem is that the acid solution used in the lab is more potent than most acids in nature, even highly acid rain; so any metals in the soil would remain insoluble-and safe. Much of the metal in the soil may, in any case, be bound up in insoluble forms. Barium sulphate, for example, is insoluble; and in this form, the normally toxic barium poses little threat.

Mistakes in assessing risk can also occur at the start, with soil sampling: small areas where contamination is concentrated can be missed altogether. What is needed are more rugged and dependable screening tests, which will enable more samples to be tested. Meanwhile, scientists at the Institute of Terrestrial Ecology at Monks Wood in Britain are pursuing some interesting experiments using earthworms to assess risk on contaminated sites. These so-called 鈥�biomarker鈥� animals are let loose on the site and their body fluids are examined for signs of damage.

Once the site investigator has established the major threats, and liaised with the regulators, developers and other interested parties, the task of remediation, or making the site 鈥渇it for purpose鈥�, remains. Aside from the site鈥檚 intended use, the time scale, funds, the cost of landfill, the technology available and the site鈥檚 geology and hydrogeology all affect the task.

Site remediation does not always involve cleaning. Contaminated soil may simply be covered over, for instance by tarmac; isolated by barriers such as impermeable clay or slurry walls to prevent leaching of the contaminants into groundwater; or excavated and removed to a landfill site. But a landfill only transfers the problem to somebody else鈥檚 back yard, thus storing up the problem for future generations.

As landfills are costly in environmental terms, other ways of remediating sites are becoming attractive. Germany, for example, where regulations on establishing and managing landfill make it an expensive option, has invested in alternative technologies, as has the US.

The technology available for tackling contamination can be divided into five categories: physical, chemical, thermal, solidification and biological. Two main methods come under the first category: ex situ, or carried out away from the site, and in situ, carried out on it. Ex situ methods include separating particles of soil out, as toxic materials often stick to the smaller ones, and physical extraction of the contaminants. In situ methods include soil vapour extraction and the similar air sparging, which help to remove volatile organic compounds, and electroremediation. Electrokinetic fencing (see Figure 4) is a novel related approach now used in the Netherlands. Another method is soil washing, which can be done either on or off the site.

Oxidation, reduction, chlorination, neutralisation, precipitation, hydrolysis and more come under the chemical category. The thermal category encompasses in situ steam/hot-air stripping, combustion, infra-red incineration, thermal desorption, and even freezing. With this technique, liquid nitrogen is used to freeze contaminants in the ground and to prevent them from migrating into groundwater while further investigation or remediation is carried out.

Solidification is used when it is not possible to render certain chemicals, such as toxic metals, harmless. The toxicity of most toxic metals cannot, unlike many organic compounds, weaken naturally. In these cases, the culprit chemicals are entombed in glass, or vitrified, or in cement, or even encapsulated in thermoplastic. And finally, there are the biological methods (see Box).

While the development of new techniques continues apace, remediation is still in its infancy as a technology, and will only be encouraged and developed if economics-and governments-work in its favour. As always, prevention is better than cure, and we must adopt environmental best practice to avoid contaminating the soil in the first place. In the meantime, the skills of chemists, biologists, toxicologists, civil engineers, planners, lawyers, environmental scientists, ecologists, geologists, hydrogeologists and more will be needed for many years to come.

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Weird and wonderful: bioremediation

PERHAPS the most intriguing of the emerging technologies are those based on biology. In 1989, when the oil tanker Exxon Valdez ran aground off the Alaskan coast, 41 million litres of Alaskan North Slope crude oil were spilled in Prince William Sound, contaminating some 2000 kilometres of intertidal shoreline with oil. In the clean-up, fertilisers containing nitrogen were spread on the shoreline to stimulate the hydrocarbon-degrading microorganisms present to multiply and accelerate the natural degradation of the oil.

More advanced methods have now emerged, such as the isolation and culturing of 鈥渟uperbugs鈥� in the laboratory. These are seeded into the appropriate contaminated material, either ex situ in specially designed reactors, or in situ using landfarming in which microorganisms, nutrients and bulking agents, for aeration, are added to the soil.

A team of scientists from Imperial College, London, has even used bacteria to separate metal contamination from mine runoff, using the anaerobic, or oxygen-hating, bacterium Desulfovibrio desulfricans; the metal is then recycled. D. desulfricans, also known as sulphur-reducing bacteria, can turn sulphates into hydrogen sulphide (H₂S), which precipitates iron as sulphide compounds. They also release bicarbonates, which neutralise acids.

Fungi are also proving doughty warriors in the fight against contamination. One site in Finland was choked with pentachlorophenol that had been used, ironically, to kill fungi in timber. Another fungus, Phanerochaete chrysosporium, came to the rescue. These so-called white-rot fungi contain enzymes called peroxidases that destroy lignin, the structural polymer in wood. They also degrade toxic organic compounds by converting chlorine bound to organic molecules into the harmless ionic form found in sodium chloride, and they degrade aromatic hydrocarbons to carbon dioxide and water.