Abstract
The three uppermost principal aquifer systems of the
Northern Great Plains—the glacial, lower Tertiary, and Upper
Cretaceous aquifer systems—are described in this report and
provide water for irrigation, mining, public and domestic
supply, livestock, and industrial uses. These aquifer systems
primarily are present in two nationally important fossil-fuel-producing
areas: the Williston and Powder River structural
basins in the United States and Canada. The glacial aquifer
system is contained within glacial deposits that overlie the
lower Tertiary and Upper Cretaceous aquifer systems in the
northeastern part of the Williston structural basin. Productive
sand and gravel aquifers exist within this aquifer system. The
Upper Cretaceous aquifer system is contained within bedrock
lithostratigraphic units as deep as 2,850 and 8,500 feet
below land surface in the Williston and Powder River structural
basins, respectively. Petroleum extraction from much
deeper formations, such as the Bakken Formation, is rapidly
increasing because of recently improved hydraulic fracturing
methods that require large volumes of relatively freshwater
from shallow aquifers or surface water. Extraction of coalbed
natural gas from within the lower Tertiary aquifer system
requires removal of large volumes of groundwater to allow
degasification.
Recognizing the importance of understanding water
resources in these energy-rich basins, the U.S. Geological Survey
(USGS) Groundwater Resources Program (http://water.usgs.gov/ogw/gwrp/) began a groundwater study of the Williston
and Powder River structural basins in 2011 to quantify this
groundwater resource, the results of which are described in
this report. The overall objective of this study was to characterize,
quantify, and provide an improved conceptual understanding
of the three uppermost and principal aquifer systems
in energy-resource areas of the Northern Great Plains to assist
in groundwater-resource management for multiple uses.
The study area includes parts of Montana, North Dakota,
South Dakota, and Wyoming in the United States and Manitoba
and Saskatchewan in Canada. The glacial aquifer system is contained within glacial drift consisting primarily of till,
with smaller amounts of glacial outwash sand and gravel
deposits. The lower Tertiary and Upper Cretaceous aquifer
systems are contained within several formations of the Tertiary
and Cretaceous geologic systems, which are hydraulically
separated from underlying aquifers by a basal confining unit.
The lower Tertiary and Upper Cretaceous aquifer systems each
were divided into three hydrogeologic units that correspond to
one or more lithostratigraphic units.
The period prior to 1960 is defined as the predevelopment
period when little groundwater was extracted. From 1960
through 1990, numerous flowing wells were installed near
the Yellowstone, Little Missouri and Knife Rivers, resulting
in local groundwater declines. Recently developed technologies
for the extraction of petroleum resources, which largely
have been applied in the study area since about 2005, require
millions of gallons of water for construction of each well, with
additional water needed for long-term operation; therefore, the
potential for an increase in groundwater extraction is high. In
this study, groundwater recharge and discharge components
were estimated for the period 1981–2005.
Groundwater recharge primarily occurs from infiltration
of rainfall and snowmelt (precipitation recharge) and infiltration
of streams into the ground (stream infiltration). Total
estimated recharge to the Williston and Powder River control
volumes is 4,560 and 1,500 cubic feet per second, respectively.
Estimated precipitation recharge is 26 and 15 percent of
total recharge for the Williston and Powder River control volumes,
respectively. Estimated stream infiltration is 71 and 80
percent of total recharge for the Williston and Powder River
control volumes, respectively. Groundwater discharge primarily
is to streams and springs and is estimated to be about 97
and 92 percent of total discharge for the Williston and Powder
River control volumes, respectively. Most of the remaining
discharge results from pumped and flowing wells.
Groundwater flow in the Williston structural basin
generally is from the west and southwest toward the east,
where discharge to streams occurs. Locally, in the uppermost
hydrogeologic units, groundwater generally is unconfined and
flows from topographically high to low areas, where discharge to streams occurs. Groundwater flow in the Powder River
structural basin generally is toward the north, with local variations,
particularly in the upper Fort Union aquifer, where flow
is toward streams.