Monitoring A Permeable Reactive Iron Wall Installation In Unconsolidated Sediments By Using A Cross-Hole Radar Method

By John W. Lane, Jr., Peter K. Joesten, and Jennifer G. Savoie

ABSTRACT

Cross-hole common-depth (CD) radar scanning was used at the Massachusetts Military Reservation (MMR) in Cape Cod, Massachusetts, to monitor pilot-scale testing of a hydraulic-fracturing method to install permeable reactive zero-valent iron walls in unconsolidated sediments. The pilot-scale study was undertaken to assess the feasibility of using zero-valent iron to remediate ground water that is contaminated with chlorinated solvents at depths exceeding the range of conventional iron wall installation methods. The pilot-scale test was conducted at the site near the source area of Chemical Spill 10 (CS-10), a chlorinated-solvents plume that underlies the MMR. Two iron walls 5 meters (m) apart and 12 m long were designed to intersect the contaminated ground water at depths ranging from 24 to 37 m below land surface.

A series of post-installation cross-hole CD radar-scanning surveys were conducted in boreholes installed on opposite sides of the walls. The presence of iron significantly reduces the radar-pulse amplitude and can be identified using CD radar scanning. Significant decreases in cross-hole radar-pulse amplitude were observed in field data after the iron walls were installed. Changes in cross-hole radar-pulse amplitudes observed in the field data were compared to results of two-dimensional finite-difference time-domain models used to predict the effects of holes in the wall and wall edges. Analysis of these data from the south wall indicates the presence of an irregularly shaped wall about 8 m wide, extending from about 27 to 37 m below land surface. Analysis of data from the north wall is presently underway.