Chemistry, Toxicology, and Human Health Risk of Cyanide Compounds in Soils at Former Manufactured Gas Plant Sites
Cyanide-containing wastes are commonly found in soils at former manufactured gas plant (MGP) sites, also known as town gas sites. The complex forms of cyanide are responsible for the blue-stained soils and rocks found at these sites. Most concentrations of cyanide at MGP sites are below 2000 ppm, although concentrations greater than 20,000 ppm have been observed. An understanding of the chemistry of the MGP cyanide-containing compounds, their fate, and transport as well as their toxicology is critical to accurately assessing potential human health risks from these compounds. In this paper, the authors demonstrate that the most prevalent types of cyanide compounds found at former MGP sites are the relatively nontoxic iron-complexed forms, such as ferric ferrocyanide, rather than the highly toxic free cyanide forms. Moreover, the chemical conditions at most former MGP sites limit the extent to which free cyanide may be released into air and water from complex cyanides. Using a screening analysis, the authors estimate potential risks from a multiroute exposure to complex and free cyanides in soil, air, and groundwater at former MGP sites and demonstrate that such risks are likely to be insignificant. Unfortunately, the lack of readily available measurement techniques to characterize cyanides in soil can result in erroneous conclusions about potential risks from cyanide compounds in soils at former MGP sites, particularly if health-based soil criteria for free cyanide (e.g., the Massachusetts Department of Environmental Protection criterion for free cyanide is 100 ppm (MA. DEP, 1995)) are applied. The authors recommend development of routine methods for field sampling and laboratory testing techniques to demonstrate that cyanides in soil at former MGP sites are predominated by iron-complexed species and that free cyanide is less than levels of concern.