After some digging:
http://www.ineosnitriles.com/media/files/HCN%2098.pdf
2.2.6 Volatilization from Water and Soil: Volatilization is expected to be an important (if not dominant) fate process for hydrogen cyanide. At pH less than 9.2, most of the free cyanide should exist as hydrogen cyanide, a volatile form of cyanide. Wide variations in the rate of volatilization are expected since this process is affected by a number of parameters including temperature, pH, wind speed and cyanide concentration (ATSDR, 1995).
Agency for Toxic Substances and Disease Registry (ATSDR), 1995. Toxicological Profile for Cyanide. - available via an internet search
The answer would seem to be that the amount of HCN liberated from a soil is dependent on many factors including pH, temperature and various other soil properties and noone has really studied it in any detail.
I suppose you could measure the HCN evolution from a soil containing cyanides using a flux box, the concentration being compared to STEL in EH40 (available via the internet). Any additional risks then arising following development would require the existing soil conditions to change in some way that increased the evolution of HCN, such as a reduction in pH (as might be caused by a careless person pouring car battery acid away in their garden); but actually putting number on paper to describe this is going to be difficult. One would lhave to take account of the background cyanide exposure from a cassava eating resident smoking 20 fags a day just to be safe.
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