“Soil pollution” is a rather unfamiliar term to most people. In Hong Kong, the best-known recent incidence of large-scale soil pollution would be the shipyard pollution in Penny’s Bay, Lantau, in 2002. When Penny’s Bay was returned to the government for the construction of the theme park, serious pollution from the shipyard caused by various harmful substances such as heavy metals, dioxins and organic pollutants was exposed. The pollution had been going on for years. There were some 87,000 cubic metres of polluted soil. If it was not treated properly, enormous health risks would be posed to users of any facilities built on the land.
It is not easy to remove pollutants from soil. First, the area of the polluted site and properties and concentrations of the pollutants have to be known. Then it will be decided whether to carry out on-site or transfer treatment. The larger the polluted site, or the higher the concentration of pollutants, the greater the difficulty and the higher the cost.
If there are volatile pollutants, hot air can be pumped into the soil. Through heating up the soil, evaporation of the volatile pollutants is accelerated. The air containing the pollutants can then be extracted. If the pollutants can be dissolved by organic solvents, suitable solvents can be injected into the soil and then the dissolved pollutants can be extracted for further treatment. Soil contaminated by heavy metals is usually mixed with concrete to stabilize the pollutants and prevent seepage in the soil.
Chemical treatment is not the only way. There are quite a few “helpers” in nature that can treat pollution. For example, some microbes in the soil are pollutant “busters”. Through changing factors affecting the soil environment such as temperature, humidity or nutrients, these useful microbes can multiply, and help digest pollutants in the soil over time. On the other hand, some plants are good at “collecting” pollutants. Research has found that crucifers can rapidly absorb metals in the soil including chromium, lead and zinc. These crops can be planted and harvested to slowly remove soil pollutants. These biological methods of pollution treatment are more beneficial to the environment, but are limited by factors such as time, efficiency and concentration of pollutants.
It takes a short time to pollute the environment, but the treatment of pollution requires enormous time, human and other resources. It is crucial to discover and stop pollution at source. There are studies in other countries that make use of trees to monitor soil pollution. Trees absorb water and nutrients through their root systems. If the soil is polluted by heavy metals and organic compounds, the pollutants will enter and accumulate in the tree cores. Therefore, regular monitoring of the content of heavy metals and organic compounds in the tree cores is a rapid way to test soil pollution. The affected area and even time of pollution can be estimated by this method.
Compared with other types of pollution such as air, noise and water, soil pollution is much neglected in public awareness as there is no large-scale agriculture in Hong Kong anymore. However, soil - like air and water - is basic to our survival. Protecting the soil is equally important to the overall health of the environment and people!