HIGH-RESOLUTION GROUND PENETRATING (GPR) RADAR SURVEY IN THE SEARCH FOR DNAPLS, CASS LAKE, MN

For decades, St. Regis Paper Company operated a wood treatment facility in Cass Lake, MN, where creosote and copper-chromium-arsenic were used in the treatment processes. Spills and leaks associated with day-to-day operations resulted in significant contamination leading ultimately to the closure of the plant and placement of the site on the Superfund list. The site is situated on approximately 10 meters of glacial fluvial and glacial lacustrine sand overlying a till confining layer. Recently, an environmental review panel questioned whether dense non-aqueous-phase liquids (DNAPLs) residing in shallow depressions on the till surface might be recoverable.

To evaluate the microtopography of the till surface, a high resolution ground penetrating survey was conducted using a Pulse-Ekko 100 GPR unit equipped with 50 MHz antenna. A constant offset reflection experiment was acquired over a 40 x 40 meter grid that was laid out on private property adjacent to the site. The data were acquired using an antenna offset of 2.0 meters and an in-line and cross-line sampling interval of 0.5 meters. The elevation of each sampling point was determined using optical leveling in order that the radar survey could be registered to surface topography. A Common Mid-Point (CMP) experiment was also acquired in order to determine an appropriate velocity function for depth correction.

Following initial processing (editing, geometry, gain and filtering), a pseudo-seismic dataset was constructed from the data in order that it might be loaded onto a seismic workstation for interpretation. The interpreted data indicate that the surface of the till confining layer has considerable microtopography, exhibiting between 0.5 to1.5 meters of relief over the study area. Our results indicate that the till surface exhibits closed depressions in which DNAPLs might be expected to collect in significant quantities.