From Right Out of History...It's the Cretaceous Aquifer

 

Andy Holt

Professor Dr. Marcia Schulmeister

Introduction to Hydrogeology Term Project

 

Hydrologic Setting
Hydrologic and Geologic Properties
Water Resource
Ground-water Contamination
References

 

Hydrologic Setting

The Cretaceous aquifer is located primarily in the southwestern corner of Minnesota and the northwestern corner of Iowa. The area where the Cretaceous aquifer is located in Minnesota receives anywhere from 550mm to 750mm of precipitation a year. The average temperature varies from 14 to 74 degrees fahrenheit over the course of the year. It is not uncommon for temperatures of 100 degrees or more to be reached in southwestern Minnesota. Iowa tends to receive more precipitation, but has an average temperature slightly lower than that of Minnesota. Iowa receives 660mm to 910mm of precipitation a year with average temperatures ranging from 14 to 72 degrees fahrenheit (Encarta, 2006).

The Cretaceous aquifer can be recharged through precipitation and runoff, however, there are 2 other aquifers above the Cretaceous. One is called the Surficial aquifer and the other is called the Local aquifer. The Local aquifer is small and it does not cover the whole of the Cretaceous. Since there is no all encompassing confining unit above the Cretaceous aquifer, water leaks down into the Cretaceous from the Surficial and Local. In some cases water from aquifers below the Cretaceous like the Pennsylvanian and Mississippian flows upward. This occurs at places like the Big Sioux River and the Minnesota River where the hydraulic gradient is going in the opposite direction (USGS, 2006).

The area over the Cretaceous aquifer is covered with streams, rivers, and lakes. These bodies of water interact more readily with the Surficial than with the Cretaceous. After the water has percolated through the strata and past the Surficial and through the leaky confining unit then it mixes with the Cretaceous. When the Cretaceous comes into contact with the Big Sioux River water is pulled into the river system and out of the aquifer (USGS, 2006).

water map

USGS 2006

Hydrologic and Geologic Properties

The strata of the Cretaceous aquifer is compiled of sandstone, shale, and a little limestone making it heterogeneous. The sandstone comes from the Dakota formation and the grain size tends to get finer closer to the top of the aquifer. The aquifer is only partially confined as mentioned above. Water also leaks through the confining unit. The Greenhorn limestone, Carlile, Graneros, and Dakota shales make up the top confining unit and the bottom is enclosed by a Pennsylvanian shale and dolomite. As with any aquifer there is the possibility for artesian wells to occur, but this does not take place often with the Cretaceous aquifer. As is seen by the map above the aquifer is fractured into small pockets in some areas. This is explained by erosion that took place before more layers of limestone, shale, sandstone, and glacial deposits were piled up on top of the aquifer system (USGS, 2006).

The aquifer varies in thickness being somewhere between 89 and 162 feet thick. In order to tap the aquifer people have to drill 150 feet in some places and 400 feet in other places. The hydraulic conductivity of 35 to 50 feet per day. Transmissivity was from 3900 to 7600 square feet per day depending on where the sample was taken from. No values for storativity were found, but it is reasonable to conclude that the thicker the aquifer is the more water it is able to hold and in turn transmit through the sandstone. This aquifer is not as affected by atmospheric pressure as the Surficial and Local aquifers. This means that not as much water will be drawn up into the vadose zone when the atmospheric pressure declines and vice versa when dealing with increases in atmospheric pressure (USGS, 2006).

The Cretaceous aquifer is rich in calcium, magnesium, and sulfur. The aquifer also has a high number of total dissolved solids, or TDS. The calcium carbonate levels range from 22 milligrams per liter to 1600 milligrams per liter. The average calcium carbonate levels was 470 milligrams per liter. That is some "hard" water! Radium-226 and radium-228 have also been found in the water samples. The levels of radium were above acceptable levels for consumption by the public. The sulfur content is high due to water percolating through gypsum on its way down into the Cretaceous aquifer. The sulfur concentrations varied between 0.5 to 1700 milligrams per liter. The average concentration for sulfur was 460 milligrams per liter. The sodium levels in Iowa are not high enough to make the water unusable for irrigation, however, the sodium levels in Minnesota are much higher. These concentrations vary between 100 and 1000 milligrams per liter (USGS, 2006). It is believed that the sodium is coming from dissolving Cretaceous rock in North and South Dakota. See the table below for more information on TDS in the aquifer.

USGS 2006

Water Resource

water use

USGS 2006

The Cretaceous aquifer is used for agriculture for the most part, public consumption is second, commercial and domestic use comes in third, and industrial usage comes in fourth. This is true in both states, but Iowa used an average of 77 million gallons of water per day while Minnesota used only 10 million gallons of water per day. It is important to note that these figures are from a study done in 1985 and the present day figures have increased with time. It is difficult to say exactly how many people rely on the aquifer. The water is pumped

to rural water districts where it mingles with water from other aquifers, streams, rivers, and lakes. That water is then distributed through the districts to the public for consumption; for the most part this is how water use is regulated. I don't believe that the water in the Cretaceous aquifer is currently threatened. The Surficial aquifer is pumped much more heavily than the Cretaceous. It is foreseeable that if the Surficial runs low, then people will begin to dig deeper and use the Cretaceous more. People would have to dig wells 150 to 400 feet deep to reach the Cretaceous and they would not do that unless they really had to dig that deep for water (Sehl, 2006).

Ground-water Contamination

The Cretaceous aquifer is not currently contaminated. It has been in the past at sites like the Windom Dump in Cottonwood Co., Minnesota and the Adrian Municipal Field Well in Nobles Co., Minnesota. The Windom Dump had problems with volatile organic compounds and heavy metals. The Adrian Municipal Field Well only had contamination problems with volatile organic compounds like benzene and toluene (EPA Minnesota, 2006). Iowa has a site at the Vogel Paint and Wax Company in Sioux Co., Iowa. The site had been contaminated with volatile organic compounds and heavy metals. The site was cleaned up by the Vogel Paint and Wax Company under the supervision of the Iowa Department of Natural Resources. There has been no contamination leak to date, but the site is still being monitored for possible contamination of ground-water (EPA Iowa, 2006). There have been a couple other sites in Iowa over the years. They are no longer a threat to the environment. The sites include a Farmers' Mutual Cooperative in Sioux Co., Iowa and at Mid-America Tanning Company in Woodbury Co., Iowa. There is always the threat of a spill or contamination of the aquifer, but besides the Vogel Paint and Wax Company site in Iowa there is no immediate threat.

The Minnesota Pollution Control Agency is usually the state agency that is called upon to clean up a site in Minnesota. The Minnesota Department of Natural Resources, Department of Health, and the Department of Agriculture may also be called in if the situation warrants. Iowa is a little different. Iowa operates under the rules that the responsible party cleans up the site. In this case any number of different departments may be called upon to assist in contamination clean up (Sievers,2006). The Environmental Protection Agency can also become involved if the situation requires their participation in either state.

References

Encarta. Iowa. 2006.

Encarta. Minnesota. 2006.

EPA. Iowa. 2006.

EPA. Minnesota. 2006.

Fetter, C. W. Applied Hydrogeology. Fourth Edition. Prentice-Hall, Inc. Upper Saddle River, New Jersey. 2001.

Sehl, Jim, Minnesota Area Hydrologist. Personal correspondence. April 28, 2006.

Sievers, Julie, Senior Environmental Specialist Iowa Department of Natural Resources. Personal correspondence. April 28, 2006.

USGS. Cretaceous Aquifer. 2006.