The geologic and hydrogeologic characteristics of Tertiary lithostratigraphic units that typically compose or underlie the High Plains aquifer system in southeastern Wyoming were described physically and chemically at a location on the Belvoir Ranch west of the city of Cheyenne in Laramie County, Wyoming. Evaluation of these characteristics indicated the groundwater system in Tertiary lithostratigraphic units was composed of, from shallowest to deepest, the High Plains aquifer system (high-yielding Ogallala aquifer only, composed of the saturated Ogallala Formation); the Brule confining unit composed of the Brule Formation of the White River Group and an underlying fine-grained depth interval with sediments that belong to either the Brule or Chadron Formation, or both; and the low-yielding Chadron aquifer (composed of poorly sorted coarse-grained sediments with substantial fine-grained matrix material assigned to the Chadron Formation of the White River Group).
The extent of brine contamination in the shallow aquifers in and near the East Poplar oil field on the Fort Peck Reservation was determined using geophysical methods and groundwater sampling. The brine contamination affects 15–37 billion gallons of groundwater within the shallow aquifers, the primary source of drinking water for the area’s residents. 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. The area of brine contamination is as much as 17.9 square miles and appears to be present throughout the entire saturated zone in contaminated areas. 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, however, it is not possible to identify discrete oil-related features as likely sources of brine plumes because several features commonly are co-located.
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