Watching brief
It may be that even if the presence of contamination is proved, the material is static in the ground. If there is no viable pathway connecting the contaminant to a likely receptor, then the option with the lowest environmental risk may be to leave the material undisturbed. However, further surveillance of the situation will be necessary in case the situation changes.
Removal of the contaminant
Removal of the contaminated material and backfilling the resulting hole with an inert filler may be appropriate in some cases. If the material is simply being moved to landfill, this can be viewed as simply moving the problem (sometimes disparagingly referred to as ’dig and dump’).
In many cases, the changing value of land and of materials makes the excavation of contaminated sites and the extraction of the contaminant for re-use a viable option, e.g. some sites contaminated with coal mining spoil have been successfully reclaimed with the ‘small coal’ sold for power station consumption offsetting some of the renovation coasts.
Ex situ remediation
Some contaminants can be successfully removed from excavated material. The clean material can then be returned to the site (e.g. the more volatile hydrocarbons such as petrol and aviation fuel can be allowed to evaporate naturally from excavated soil if exposed to sun and wind for some time).
Excavated soil can be left in ‘windows’ on impermeable ground such as concrete runways on old airfields. The soil is regularly turned over and, when sufficient contamination has been removed, it is returned to the site for re-burial.
The disadvantages of this method are that the material is transported twice, and that the pollutant becomes airborne.
In Situ remediation
In recent years several technologies have been developed for remediation of contaminated land without excavation. Some success has been achieved with the deliberate introduction of micro organisms to digest hydrocarbons in situ. Often this method requires ‘air sparging’ – the introduction of an underground oxygen supply to keep the micro organisms alive.
Other techniques such as chemical treatment and solidification may be appropriate, depending upon the type of contaminant and the nature of the site.
Barriers
Vertical and horizontal barriers can be introduced to prevent movement of the material. This still leaves the need for ongoing surveillance.
Barriers can be sheet piles or materials which are injected and become solid in situ.