The extent of saline-water contamination in Quaternary deposits in and near the East Poplar oil field may be as much as 12.4 square miles and appears to be present throughout the entire saturated zone. The saline-water contamination affects 9-60 billion gallons of ground water. Saline- contaminated water moves westward through Quaternary glacial deposits and merges with southward-flowing water in Quaternary alluvium in the Poplar River valley. Saline ground water discharges into the Poplar River, and increases the dissolved-solids and chloride concentrations of the river. The probable source of saline-water contamination in the Quaternary deposits is brine that is a byproduct of the production of crude oil in the East Poplar oil field study area.