Is it safe to live close to polluted land?

If soil in your neighbourhood is contaminated, should you be afraid of eating carrots or apples grown there or even breathing the air? As far as contaminated soil is concerned, you should be aware of the possibilities that the contaminant can gain access to your body. As such, the mere existence of a chemical in soil does not cause health effects unless there is human exposure, in other words the chemical gains access to your body. This is very different in different cases, and most lay people are not able to recognize the difference.

Completely uncontaminated soil does not mean soil free of chemicals. Even natural chemicals are chemicals, many of them are toxic. Remembering this will place the concerns in their proper context.

The most likely routes of exposure are ground water, and direct handling of contaminated soil, leading to exposure through unwashed hands. Exposure by direct skin contact and lung exposure of contaminated dusts or evaporated chemicals are much less likely although sometimes that can occur and some extreme cases have been serious. Living plants take up relatively few chemicals but there are exceptions – some plants can pose health risks via contaminated food. One example is the heavy metal, cadmium.

Itai-Itai in Japan

One of the early examples of soil being responsible for mass poisoning occurred in Japan in the 1940s giving rise to the so-called itai-itai disease. This was caused by irrigation of rice paddies with water contaminated by cadmium from a nearby mine. Cadmium is one of the few metals to be absorbed by plants from the soil at toxic concentrations. Therefore it is very important that the use of waste water sludge is carefully monitored, if it is to be used for fertilization of fields with crops for human consumption.

Times Beach, Missouri, USA

Used oil was used at Times Beach, Missouri, for dust control on roads and in horse stables. However, some of the oil was contaminated by waste originating from an herbicide manufacturing factory, and it contained relatively high levels of dioxins. No human health effects were unequivocally documented, but horses suffered from various symptoms, and ultimately the whole area was evacuated. This is an example of the possibility that dust can cause exposure to sufficiently high levels of toxic compounds via the respiratory route.

Kärkölä, Finland

The Kärkölä chlorophenol incident is a good example of how differences between different compounds are important in causing real exposures. Large amounts of chlorophenols had been released by a local sawmill to the environment over a long time, several decades. These contained dioxins and furanes as synthetic by-products. The chlorophenols are reasonably water soluble but the dioxins are practically non-soluble. Therefore the villagers were exposed to chlorophenols which were detected in drinking water from a ground water well and in fish living in a downstream lake. Some chlorophenols were also detected in urine samples of the villagers. However, no evidence of dioxin contamination was detected despite measurements being taken from the villagers themselves (e.g. in mother’s milk and in fat samples taken from surgical patients) or samples of their food and water.

This case was very educational in emphasising the importance of the physicochemical characteristics of the chemicals such as their water solubility. It also demonstrated the importance of the detective work in screening the possible routes available for the particular chemical to cause exposure, and the time frame of plausible exposure. For example, a cancer detected soon after the contamination cannot be attributed to the chemical; cancer requires many years to develop.

There are hundreds or thousands of old sawmill sites scattered throughout the forested regions of the Northern hemisphere, similar to the one in Kärkölä, and many of them have contaminated their environment with wood preservatives. Not only are they a risk to the environment with the possibility of ground water contamination, but some such sites become a risk because of subsequent land development. If left alone, dioxin contamination is not usually a problem, because dioxins are not water soluble, though the risks posed by chlorophenols may be real. In many cases these compounds are flushed off with surface waters, but if they are confined to ground water reservoirs, unpleasant surprises can arise years later due to chlorophenols.

Toxic waste dump sites; Love Canal

Toxic waste causes other types of problems. Usually toxic waste also contains solvents, and these may be in barrels that gradually rust and start to leak. One of the most serious and best known examples is Love Canal near Niagara Falls, New York. A canal area had been used as a dumpsite for industrial toxic waste for years. Later, new housing and a school were constructed immediately adjacent to the covered dumpsite. This activity broke open at least a few of the waste barrels, and resulted in a public outcry. Several claims have been made about various health outcomes, including birth defects and low birth weight. As in all dumpsites, the problem is usually an unknown mix of miscellaneous waste including solvents in barrels or drums which start to leak and evaporate. Therefore any house or school built on covered old dumpsites must be considered as potentially hazardous.

Ground water reservoirs

The most serious problems are to be expected if the contaminated soil lies above an important ground water reservoir. Here the concerns of the public are not always in line with toxicological priorities. A good example is methyl-tertiary butylaether (MTBE) in petrol. Lead was used as anti-knocking agent for almost a century, and lead is certainly much more toxic than MTBE. However, if there is a leak from the tank of a service station, and petrol leaks into ground water, the bad smell and taste will reveal MTBE long before there is any possibility of toxic effects.

On the contrary, lead would be really dangerous in that kind of situation, because tetraethyl lead cannot be easily detected. In fact, there were no alarms as long as the anti-knocking agent was tetraethyl lead, but there have been many after it was replaced by MTBE. Also oil or gasoline as such are not considered dangerous by the public, even though experts know that both contain several carcinogenic components such as benzene.

Other chemicals prone to be found in ground water are two dry cleaning chemicals, trichloroethylene and tetrachloroethylene. These are very problematic in ground water, since they are heavier than water, and sink down to the bottom of the reservoir and cannot usually be removed by any means. Thus they may spoil the water source for decades.

The above examples illustrate some of the different risks of contaminated soils and sites. A typical cause of a problem is due to re-development or a change in the zoning for an area. Sawmill sites are usually not a problem unless there is a possibility of ground water contamination or they are disturbed by new construction. However, if industrial sites are to be developed for housing purposes, it may even be necessary to remove the topsoil and replace it with clean topsoil. It is very important that city planners know the risk areas so that they can avoid problems. Usually risk assessment is a complicated task, and expert advice should be sought. Limit values are almost useless in predicting health effects.

Contaminated soil usually entails very expensive clean-up procedures and renovation, and therefore prevention of contamination is by far the best and cheapest option. This requires also strict control by cities and other local authorities. Existing problem sites should be prioritized, because unless there is a possibility of human exposure, these sites are not usually a problem if left alone. Nonetheless those considered to pose a real risk are best treated sooner rather than later.

The crucial point with contaminated land is the possibility of exposure. One of the major problems is contamination of ground water. Insoluble and non-evaporating waste may not pose any major problems, if it is left alone, but the risks must be appreciated if there is to be any change in land use.

Notes and references

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