'A BLACK ART ' is the term used by bioremediation specialists to describe what still remains - despite a decade of use - a commercially secretive operation to destroy ground contamination by encouraging microbes in the soil to literally eat the pollutants.
The skill, says Mr Marshall, is to add a carefully measured amount of organic nutrient such as fertilizer to encourage the bugs to eat everything around them, including the contamination.
Mix in too little and the process takes too long; add too much and the treated soil could become too geotechnically weak to be recycled as a loadbearing material.
If the process is carried out by a subcontracted specialist it is possible the main contractor and the ultimate client are given - for commercial security - only scant details of the exact bioremediation technique adopted.
'We do it all ourselves, providing a seamless in-house process ' he says. 'It is in our interest that we offer the best value technique to both the client and to ourselves.'