GEOLOGIC CONTROL OF TAR MIGRATION AT A FORMER MANUFACTURED GAS PLANT

A remedial investigation at a former manufactured gas plant (MGP) site in New York City determined that geology controlled the lateral and vertical migration of dense non-aqueous phase liquid (DNAPL) tar (a by-product of the gas production process). The MGP operated between circa 1857 and the mid- to late- 1950's. In order of increasing depth, geologic units encountered were: fill, alluvial deposits, the Harbor Hill terminal moraine and ground moraine, and saprolitic Manhattan Schist. The alluvial and terminal moraine deposits were generally medium to coarse-grained sand and gravel mixtures. The upper portion of ground moraine consisted of a dense silt to silt-sand mixture which where present, was on order of 50-feet thick. The lower portion of the ground moraine unit included stratified sands interbedded with the silt; these materials extended to approximately 125 feet below ground surface, where saprolite (weathered Manhattan Schist) was encountered.

The permeable nature of the alluvial and terminal moraine deposits allowed DNAPL tar to migrate downward with minimal lateral spreading from specific release points until encountering the ground moraine till. The ground moraine acted as a confining layer to the downward mobility of tar from two overlying former gas holder structures (tar release points). Tar was not encountered beneath the top of the silty ground moraine (44 feet deep) at these two locations. At a third gas holder location the silty confining ground moraine till was absent and was encountered up to 100 feet deep.