GEOPHYSICAL AND HYDROLOGIC STUDIES OF SHALLOW AQUIFER CONTAMINATION, EAST POPLAR OIL FIELD AREA, NORTHEASTERN MONTANA

Abstract

Areas of high conductivity in shallow aquifers in the East Poplar oil field area are being delineated by the U.S. Geological Survey in cooperation with the Fort Peck Assiniboine and Sioux Tribes, in order to interpret areas of saline-water contamination. Ground electromagnetic methods were first used during the early 1990s to delineate more than 12 square miles of saline-water contamination in a portion of the East Poplar oil field area. An airborne electromagnetic survey was conducted during August, 2004, in a 106 square-mile area that includes the East Poplar oil field on the Fort Peck Indian Reservation. The electromagnetic equipment consisted of six different coil-pair orientations that measured resistivity at separate frequencies from about 400 hertz to about 140,000 hertz. The electromagnetic resistivity data were converted to six electrical conductivity grids, each representing different approximate depths of investigation. The range of subsurface investigation is comparable to the depth of shallow aquifers. Electrical induction conductivity and natural gamma logging was done during 1993, 2004, 2005 of selected boreholes to aid in interpretation of the airborne geophysical survey and to characterize electrical parameters of the lithology and ground water. Water-quality samples were collected from wells during 2003 to 2005 to correlate geophysical measurements with the chemical composition of water from shallow aquifers. The airborne, ground, and borehole conductivity data were used to delineate subsurface areas of high conductivity and correlated with hydrologic data to indicate areas of contamination. The U.S. Geological Survey determined that handling and disposal of brine produced with oil in the East Poplar oil field area has resulted in contamination of not only the shallow aquifers, but also the Poplar River. In the 10 years since the first delineation, the quality of water from wells completed in the shallow aquifers in the East Poplar oil field changed markedly. The current (2004) extent of saline-water plumes likely differs from that delineated in the early 1990s. The geophysical and hydrologic study is being used in ground water resource planning studies for the area.