GEOLOGIC CONTROL OF MGP NAPL DISTRIBUTION AND REMEDY SELECTION

The approximately eight-acre Niagara Mohawk Gloversville Hill Street MGP Site was operational from 1898 to 1952. Peak production of over 200 million cubic feet/year of water gas and carburetted water gas occurred in the 1930s. The southward draining site is located in the southern Adirondacks and is underlain by glaciofluvial and possibly glaciolacustrine sediments dominated by a silty sand unit underlain locally by a coarse gravel, likely a glaciofluvial channel deposit. These materials are underlain by a confining and laterally extensive silty clay to clay unit that pinches out to the south.

Shallow groundwater discharges to Cayudutta Creek, the local sensitive receptor, at the southwest corner of the site. The creek in this area was relocated prior to 1900 probably during construction of a rail line. The former relief holder was located approximately 500 feet north-northeast of the creek. DNAPLs, likely originating from the former relief holder or associated facility piping, have migrated in the subsurface and discharge to Caydutta Creek. DNAPL migration appears to be controlled by the distribution of the coarse fluvial gravel units. A 2-inch diameter monitoring well (with a 20-slot screen) located within or near the coarse gravel unit has produced over 4,000 gallons of NAPL during an 18-month period.

Geologic and hydrogeologic factors are controlling the selected remedial alternative for this site. Rather than employ traditional techniques to prevent DNAPL discharge (e.g. a physical barrier and hydraulic gradient manipulation), the selected remedy is proposed to use the current preferential pathways and the creek system as a NAPL control structure. The creek is proposed to be relocated to near its original configuration, maintaining a hydraulic gradient toward the current creek bed through groundwater and DNAPL collection.