GROUNDWATER FLOW AND CONTAMINANT MIGRATION IN THE COHOES MELANGE LITHOTECTONIC UNIT: INFLUENCE OF A THRUST FAULT

At a former manufactured gas plant (MGP) in Cohoes, New York, tar, occurring as dense non-aqueous phase liquid (DNAPL), migrated into fractures in a lithotectonic unit known as the Cohoes Melange. This unit is composed of shale with some lenses/blocks of siltstone and sandstone surrounded by a shale matrix. These rocks, now structurally positioned directly below the westernmost Taconic Allocthon, were deposited as continuous bedded strata, but were subsequently deformed such that on a local scale they display: folded and disrupted/discontinuous bedding; closely-spaced cleavage; small-scale reverse faults; and high-angle normal faults intersecting earlier rock fabrics.

DNAPL migration and groundwater flow in this unit are largely controlled by an open, low-angle fracture zone associated with a thrust fault. The fault subcrops beneath the MGP, thus facilitating movement of DNAPL and impacted groundwater into bedrock. The fault dips ±5 to10+º eastward and extends under the adjacent Mohawk River. Indications of DNAPL were identified along the fault directly down-dip of the MGP at depths up to ±100 feet. Groundwater recharges the fault near the subcrop and then flows approximately parallel to the fault strike. Concentrations of dissolved constituents associated with the DNAPL decrease in the downgradient direction. An upward vertical hydraulic gradient exists from the thrust fault under the river, but there are no indications of dissolved constituents migrating upward toward the river.

Systematic patterns of open fractures that may control groundwater flow were not readily apparent in cores or outcrops adjacent to the site. Thus, this assessment required a phased, weight-of-evidence approach. Initially, cores, packer pressure testing and geophysics were used to characterize the rock and the vertical profile of hydraulic conductivity changes, and to select well screen intervals. Equipotential mapping, pumping tests, groundwater quality data and an understanding of regional structure were then used to assess water-bearing zone continuity and three dimensional flow.