DELINEATION OF BRINE CONTAMINATION IN AND NEAR THE EAST POPLAR OIL FIELD, FORT PECK INDIAN RESERVATION, NORTHEASTERN MONTANA, WILLISTON BASIN

For more than half a century, millions of barrels of brine have been produced with oil from the East Poplar oil field, located on the western flank of the Williston Basin. The brine, which contains 37,900 to 210,000 milligrams per liter (mg/L) dissolved solids, was placed in pits and ponds or injected into subsurface geologic units through disposal wells. The extent of brine contamination in the shallow aquifers in and near the East Poplar oil field is as much as 17.9 square miles and appears to be present throughout the entire saturated zone in contaminated areas. The brine contamination affects 15–37 billion gallons of groundwater. Brine contamination in the shallow aquifers east of the Poplar River generally moves to the southwest toward the river and then southward in the Poplar River valley. The likely source of brine contamination in the shallow aquifers is brine that is produced with crude oil in the East Poplar oil field study area. Brine contamination has not only affected the water quality from privately owned wells in and near the East Poplar oil field, but also the city of Poplar’s public water-supply wells.