Northern Great Plains Aquifer System

by

Andy Vogelsberg

This webpage was created as a part of GO 571, Geohydrology at Emporia State University in April, 2007. The instructor of the course is Dr. Marcia Schulmeister.


Table of Contents


Introduction

The Northern Great Plains Aquifer System covers approximately 300,000 square miles. It is a culmination of five major aquifers. These aquifers include the lower Tertiary, upper Cretaceous, lower Cretaceous, upper Paleozoic and lower Paleozoic aquifers. The aquifer system lies underneath nearly all of North and South Dakota, as well as half of Montana. (See figure 1) It also covers nearly one third of the state of Wyoming. The Williston Basin along with the Powder River Basin are structural troughs that most of the system lies in. The Williston Basin covers much of Montana, North Dakota, South Dakota, and some parts of Canada (USGS, 2005c).


(Figure 1) An image of the Northern Great Plains Aquifer System taken from
http://capp.water.usgs.gov/gwa/ch_i/gif/I049.GIF (Click on link to enlarge)

Hydrologic Setting

Like all aquifers, climate plays a big part in the Northern Great Plains Aquifer System. Much of the recharge into the aquifer system can be attributed to both rainfall and snow melt. The regional climate of the area is considered Dfb. This classification stands for a humid continental climate, with a cool summer (NOAA, 2007). Nearly all of the recharge is taken in through areas of outcrop along the aquifers that have been exposed due to erosion. These areas are primarily due to folding and faulting. (See Figure 2) Streams also play a key role in the recharge of the aquifers. Water from the streams often seeps down through the stream beds into the aquifers near outcrops. Some recharge is due to over use of irrigation. This form of recharge is not sufficient and only occurs in few places. The porosity that is linked to the aquifers in the Northern Great Plains aquifer system is intergranular porosity. This means that their is open spaces between the particles of rock and gravel. Secondary porosity is seen in some parts of the aquifers (USGS, 2005c). The hydraulic conductivites of the aquifers range from about 10-6 to 10-4 cm/s (Fetter, 2001).

The lower Tertiary and upper Cretaceous aquifers in the system are both characterized by local flow systems. This type of flow system allows the water that moves in the aquifer to take short flow paths into nearby lakes, streams, and springs. Much of the discharge into the surface water bodies is highly mineralized with high salinities. However, other parts of the aquifer system are characterized by long, regional flow paths. This regional movement of water is from areas at high altitudes following the dip of the aquifers. A large majority of the water makes its way through the Williston and Powder River basins. The flow of water along the long flow paths is usually very deep, due to the great depth of the aquifers (USGS, 2005b).


(Figure 2) An image groundwater movement and recharge taken from
http://capp.water.usgs.gov/gwa/ch_i/gif/I064.GIF (Click on link to enlarge)

Geologic and Hydrologic Properties

The five different aquifers that make up the Northern Great Plains aquifer system have many different geologic and hydrologic properties. The five major aquifers are predominantly made up of sandstones that are Tertiary and Cretaceous in age and carbonate rocks that are Paleozoic in Age. There are different types of alluvial deposits that overlie the aquifer system, but they are not considered in the actual aquifers themselves because of the deep flow within the aquifers. These deposits are very permeable in some local locations. The base of the aquifer system is considered to be crystalline rocks. These rocks give little water to the system (Downey and Dinwiddie, 1988).

The lower Tertiary aquifers, which are the shallowest of the five aquifers, are composed of sandstone beds that are Eocene and Paleocene in age. These beds occur in the Wasatch and Fort Union Formations. Both of these formations contain large amounts of coal. This area is known as one of the nation's largest reserves for coal. The thickness of the lower tertiary aquifers is very variable. It can be as thick as 3600 feet in the Powder River Basin and as thin as 300 feet in the Fort Union. Water movement in the lower Tertiary aquifers is from the north and northeast. All areas in the Fort Union Region yield water. This is not the case in the Wasatch Formation. Only some of the sandstones in the formation yield water (Downey and Dinwiddie, 1988). Water in the lower Tertiary aquifers is said to be unconfined. However, in some places clay beds act as confining units. This takes place where glacial deposits overlay the aquifers (USGS, 2005c).

The upper Cretaceous aquifers surround and lie just beneath the lower Tertiary aquifers. The upper Cretaceous aquifers are also composed of sandstone beds. These beds are in the Lance and Hell Creek Formations, as well as the Fox Hill Sandstone. The thickness of the Lance and Hell Creek formations is nearly the same. They range from 350 to 3400 feet thick. However, the Fox Hills Sandstone is around 300 to 450 feet thick. The regional waters of the upper Cretaceous aquifers flow in a northerly direction. Some flow may go in northeastward direction in the Williston Basin. The upper Cretaceous aquifers are generally unconfined. Water often leaks down from the lower Tertiary aquifers to act as a source of recharge the upper Cretaceous aquifers. Most of the water in the aquifers contains less than 3000 milligrams per liter dissolved solids. There are areas in North and South Dakota that can have dissolved solid concentration over 10000 milligrams per liter. Even though most areas of the upper Cretaceous aquifers contain minimal dissolved solids, the water is still very high in sodium concentrations (USGS, 2005c).

The lower Cretaceous aquifers are separated from the upper Cretaceous aquifers many layers of shales. These shales act as a confining unit. Some of the shales are very thick. The Pierre Shale can be as thick as 3000 in some areas of North and South Dakota. Very little water is stored in the shales due to their composition. There are a few sandstones that are intermixed with the shales, but they also provide very little water. The water that is supplied in the sandstones is very mineralized. Below the confining units, several different sandstones make up the lower Cretaceous aquifers. The Newcastle Sandstone and the Inyan Kara Group provide the largest amount of water in the aquifers. These are both located in the Powder River Basin. The lower Cretaceous aquifers are confined in most areas, and semi-confined in local areas (Downey and Dinwiddie, 1988). Although the lower Cretaceous aquifers are larger than all other aquifers in the Northern Great Plains aquifer system, they contain very little fresh water. The water of the aquifers usually has dissolved solid concentrations that are greater than 3000 milligrams per liter. Pleistocene glaciation is thought to be the main source of water in the aquifers (USGS, 2005c).

The upper Paleozoic aquifers are made up of Madison Group Limestone. The thickness of the Madison Limestone is very variable. It can be under a foot thick to as much as 2800 feet thick. The limestone is exposed to the land surface in some areas. Karst topography is not uncommon in these areas. Large springs are also not uncommon to see coming out of the upper Paleozoic aquifers in these karst conditions. Groundwater movement in the aquifers moves in a northeastward direction (USGS, 2005c). The upper Paleozoic aquifers are confined aquifers. The only places that are not confined are where structural uplift has created a place of recharge. These small areas of recharge are the only source of fresh water in the upper Paleozoic aquifers. As the water moves down through the aquifer, the dissolved solid concentration becomes much greater. Brine water is not uncommon in these aquifers (Downey and Dinwiddie, 1988).

The lower Paleozoic aquifers are primarily made up of carbonate rocks as well as sandstones. These rocks are buried very deep and serve as oil and gas reservoirs. Brine and saline water are very common in these rocks. The sandstones that make up the aquifers can be as thick as 2000 feet. However, much of this sandstone bed is not considered part of the lower Paleozoic aquifers because there is little to no permeability through much of it. Water moves in a northeastward direction through most parts of the aquifers. Fresh water is very hard to come by in these aquifers. Only small parts of Wyoming contain fresh water from the lower Paleozoic aquifers. Like the upper Paleozoic aquifers, the lower Paleozoic aquifers get their recharge from areas of uplift that have been exposed due to erosion (USGS, 2005c).

Water as a Resource

The water that is contained in the five aquifers that make up the Northern Great Plains aquifer system is used in many different ways. The main uses of the water include: public supply, domestic and commercial, agricultural, and industrial. Irrigation is where the largest portion of the water goes from the aquifer system. This is important for the longevity of the aquifers, because excess irrigation is a key way of recharge for all of the aquifers. Each of the four states that make up the aquifer system uses over 50 percent of its water for agricultural use. Wyoming uses much more water than any of the other three states. (See figure 3) Every county in Wyoming uses over five million gallons per day on average. North and South Dakota only use about two to three million gallons of water per day per county. On average, Montana uses about 3 million gallons of water per day per county. The aquifer system as a whole looks to be in good shape. The amount of fresh water that comes out of the aquifers is a concern. Even though the amount of water that is supplied by the aquifers is sufficient, there is a lack of fresh water. The resource is managed by the state that it lies within. The following image helps to understand how the water from the aquifers is used (USGS, 2005a).


(Figure 3) An image of the uses of groundwater taken from
http://capp.water.usgs.gov/gwa/ch_i/gif/I021.GIF (Click on link to enlarge)

Contamination in Aquifer System

Contamination in the Northern Great Plains aquifer system is a concern. There is a shortage of the amount of freshwater that is taken from the aquifers. Freshwater is considered to have less than 500 milligrams per liter of dissolved solids. In some areas, water is used for human consumption that has dissolved solid concentrations over 2000 milligrams per liter. The reason for this is because of the lack of fresh water. The main sources of contamination come from agricultural fertilizers and pesticides. Many of the Northern Great Plains aquifers are affected by these chemicals through their recharge processes. The shallower aquifers are particularly affected because of the lack of time that the contaminant has to be absorbed. Point source contamination is also a problem in this region. Petroleum spills is often times a problem due to underground storage tank spills (USGS, 2005a).

References

NOAA Satellites and Information. Observed Data. 2007. http://ols.nndc.noaa.gov/plolstore/plsql/olstore.prodspecific?prodnum=C00095-PUB-A0001#TABLES Accessed April 20, 2007.

USGS, 2005a Regional Summary. GROUND WATER ATLAS of the UNITED STATES Montana, North Dakota, South Dakota, Wyoming. http://capp.water.usgs.gov/gwa/ch_i/I-summary.html Accessed April 16, 2007.

USGS, 2005b Principal Aquifers. GROUND WATER ATLAS of the UNITED STATES Montana, North Dakota, South Dakota, Wyoming. http://capp.water.usgs.gov/gwa/ch_i/I-text1.html Accessed April 18, 2007.

USGS, 2005c. Regional Aquifer Systems. GROUND WATER ATLAS of the UNITED STATES Montana, North Dakota, South Dakota, Wyoming. http://capp.water.usgs.gov/gwa/ch_i/I-text2.html Accessed April 13, 2007.

Downey, J. and G. Dinwiddie, 1988. The regional aquifer system underlying the northern Great Plains in parts of Montana, North Dakota, South Dakota, and Wyoming--summary.

Fetter, C.W., 2001. Applied Hydrogeology / C.W. Fetter. 4th Ed. p.cm.


This webpage project was created to meet the requirements of GO 571, Geohydrology at Emporia State University. For more information,contact Andy Vogelsberg, andyvogelsberg@yahoo.com.

Created April 2007, from the Earth Science Department, Emporia State University: http://www.emporia.edu/earthsci/, at Emporia State University, Emporia, KS http://www.emporia.edu/.