produced by heating coal in retorts or beehive ovens, carbon- izing the coal in the absence of air. The carbureted water gas process involved passing steam through burning coal to form a gaseous mixture referred to as water gas or blue gas. This mixture was then sprayed with oil as it passed through a su- perheater. The oil spray would generate additional gas, en- hancing the capacity of the overall gas mixture to generate heat and light. In each process, the gas was cooled and purified before dis- tribution. During cooling, an oily liquid known as coal tar would condense from the hot gas and settle in the bottom of gas holders, pipes, and other structures that were typically built belowground. As a result, these structures frequently
introduced MGP by-products directly into the ground. The
gas was then purified using wood chips treated with iron oxide to remove hydrogen cyanide from the gas.
Typically a reddish brown to black oily liquid, coal tar does
not readily dissolve in water and is commonly categorized as
a non-aqueous-phase liquid (NAPL). Although most coal tars
are slightly denser than water, the difference is slight. Consequently, coal tar can either float or sink when in contact with
water. Coal tar may be evident as sheen on a water surface or
as an NAPL in soil or water. At the former MGP site in Norwich, coal tar was found in the soils surrounding former MGP
structures and in down-gradient areas.
The site of the former manufactured
gas plant site in Norwich, New York,
is suitable for commercial use, and
participants in the remediation project
are working to obtain an environmental
easement that will make it possible
for the property to be redeveloped.
SEPTEMBER 2012 Civil Engineering  SEPTEMBER 2012 Civil Engineering