USE OF TDR TO MONITOR CHANGES IN GROUND WATER LEVEL AND CRUDE OIL THICKNESS

Measurements were made in a well screened in a residual pool of crude oil near Bemidji, Minnesota to evaluate the use of TDR to monitor variations in oil thickness and groundwater levels. The pool is the result of a crude oil pipeline break in 1979 and rests on the water table. The U.S. Geological Survey maintains the site as a research facility and over 250 monitoring wells have been installed to characterize subsurface conditions. Data are being acquired automatically using TDR by interrogating an air-dielectric coaxial cable installed in one well. Independent measurements are being made manually with an oil-interface probe in several adjacent wells. Both types of measurements indicate a seasonal variation in apparent oil thickness within the wells; it varied from a minimum of 365 mm in June 1998 to a maximum of 655 mm in October 1998. These variations likely result from oil getting spread out vertically as the water level fluctuates in response to aquifer recharge and discharge. it is hypothesized that oil becomes entrapped in the unsaturated and saturated zones of the aquifer. This alternating entrapment and release of oil results in an apparent decrease or increase in oil thickness in the well. The relationship between thickness of oil in a well and the amount of oil in the adjacent aquifer is tenuous under these conditions. Nevertheless, this research demonstrates that it is now possible to continuously measure changes in the apparent thickness of light non-aqueous phase liquids (LNAPLs) using TDR, which may prove to be a useful tool in evaluating the long-term effects of on-site remediation of contaminated aquifers.

Kevin M. O'Connor
GeoTDR, Inc., Apple Valley, MN

Geoffrey N. Delin
U.S. Geological Survey, Mounds View, MN