6.2.2.2 Installation of remediation boreholes
If the hot spot is not located close to a suitable existing injection point, such as a gas or
leachate well, then consideration should be given to drilling specific wells for the investigation and subsequent remediation of the hot spot. This has been undertaken with success at several sites, an example of which is given below.
The above case study highlights the long term nature of the dousing and monitoring that may be required to extinguish successfully a deep hot spot.
6.2.2.3 Disadvantages of dousing
Dousing needs to be carefully controlled to minimise all potential negative effects. The main disadvantage of this method is the introduction of liquid into the waste mass. Factors such as the availability of leachate, licensing requirements, the creation of leachate if water is used and the potential raising of the leachate head all need to be carefully considered before proceeding with hot spot dousing.
A range of other potential adverse effects are also cited in the literature, although their
frequency of occurrence was not identified during this research work. They include:
• the formation of cavities;
• reduction in waste stability; and
• channelling and possible wash out of fines leading to increased waste permeability.
The use of insufficient water may create additional hazards. Water gas, a mixture of
hydrogen and carbon monoxide, is generated when steam passes over hot carbon.
This situation may arise if the water is added in small amounts or so slowly that the hot
carbonaceous waste vapourises the water without being adequately cooled.
The hydrogen will form an explosive mixture that can be ignited by the residual hot spot. Hence, if dousing is used, a steady supply of large volumes of water and wide bore delivery pipes are essential to minimise the risk of hydrogen formation.