The Silurian-Devonian Aquifer of Iowa, Wisconsin, and Michigan

The Silurian-Devonian Aquifer of Iowa, Wisconsin, and Michigan

Mike Sedlacek

April 21, 2010

GO 571: Hydrogeology

Dr. Marcia Schulmeister, Instructor

Hydrologic Setting
Hydrologic and Geologic Properties
Water Resources of the Silurian-Devonian Aquifer

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The Silurian-Devonian aquifer is one aquifer among many in its locality. The aquifer itself, located mostly in Iowa, as well as eastern Wisconsin, and central and southeastern Michigan, is unique in its own right--it is composed chiefly of limestone and dolomite rocks that were deposited in the U.S. Midwest during two geologic periods: the Silurian (440-417 million years ago), and the Devonian (417-354 million years ago)(Stanley, 2005). Each unit, though distinct from the others, has similar hydrologic and geologic properties, which is why they are all classified as one aquifer system (Olcott, 1992). Figure 1 shows the extent of the aquifer, as well as which parts are confined and unconfined.

Some basic information is discussed about the aquifer--various hydrological and geological properties that make the aquifer distinct from other aquifers that were created at different times. The aquifer as a water source is also discussed because it is important to understand the complex interactions between water usage, water quality, water quantity, and water abuse. We, as a society, need to be sure that we take care of our groundwater--one major piece of legislation, the Environmental Protection Act of 1987, addressed growing concerns about groundwater contamination, and ways to prevent contamination in the future (Environmental Protection Agency, 2002).

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Hydrologic Setting

The Silurian-Devonian aquifer is located in most of Iowa (excluding northern and western Iowa), eastern Wisconsin, and central and southeast Michigan. Figure 1 shows the extent of the aquifer in each of these states:

Figure 1: A map from Olcott (1992) shows a map of the extent of the Silurian-Devonian aquifer, which is in the Midwest region of the United States.

The climate for each of these areas does vary, but in general, Iowa, Wisconsin, and Michigan all have a similar climate. The average temperature range for Iowa is 45-52 degrees Fahrenheit (National Climate Data Center). Michigan has an average temperature of 49 degrees Fahrenheit (City-data, 2010), and Wisconsin has an average temperature of 39-50 degrees Fahrenheit. Average summer temperatures (all given in Fahrenheit) are: 61-82 (Iowa), and 72 (Michigan). Winter temperatures range between 15-22 in Iowa, and is averaged to be 23 in Michigan (City-data, 2010; National Climate Data Center). The average precipitation rates (recorded in inches) are: Iowa (34), Michigan (33), and Wisconsin (30-34)(University of Wisconsin Extension).

All areas are characterized as being temperate, with seasonal variations in temperatures and precipitation. Additionally, all areas are located in an area that is subjected to air mass collisions, which produces thunderstorms (National Climate Data Center). All areas are forested (mainly east) and/or grassy (mainly west). This of course, is due to the precipitation changes caused by increased elevation toward the west (Schulmeister, 2010). Overall, though, climate variations are minor across the Silurian-Devonian aquifer, as is noted by the climate data that was just mentioned.

Surface water bodies occur as a direct result of the regional, temperate, climate. Iowa, Wisconsin and Michigan all receive a good amount of precipitation each year (generally between 30 and 34 inches). Because of this, the area is capable of supporting surface water (National). Unless conditions are very dry and hot, such as a prolonged summer heat wave, the aquifers of the Midwest region are generally known to support gaining streams; streams that are replenished with groundwater (Schulmeister, 2010). Surface water in the Midwest region includes streams, lakes, wetlands, and ponds, just to name a few. The distribution of these surface water bodies is quite high--this being attributed to the relatively shallow water table of the area, which is generally a few meters below the ground surface (Olcott, 1992). According to Morton (2008), less than one percent of the state of Iowa is covered by water; yet surface water is still abundant at 1,458 square kilometers (563 square miles).

A general map showing the topography and recharge sites of the Silurian-Devonian aquifer is shown in figure 2 below:

Figure 2: A diagram from Olcott (1992) shows the regional dip of sedimentary beds in Iowa, to the southwest. The Silurian-Devonian aquifer is found in purple. Note how the aquifer crops out in eastern Iowa, as viewed from the right side of the diagram. Click on the link for a larger view.

Part of the Silurian-Devonian aquifer is overlain by Devonian-age (417-354 Million years old) shale in central Iowa, which acts to retard groundwater recharge from the surface (Olcott, 1992). Figure 3 below shows the extent of this confining layer.

Figure 3: A diagram taken from Olcott (1992) which shows the extent of the late-Denovian confining shale layer, which tops the Silurian-Devonian aquifer only in Iowa.

In eastern Iowa, such as the Cedar Rapids and Waterloo areas, rocks of the Silurian-Devonian aquifer crop out directly at the surface (Olcott, 1992), with Silurian beds (440-417 million years old) cropping (extenting above the sground surface) out in the northern area (Waterloo and Backbone State Park), and Devonian beds cropping out by Cedar Rapids and Iowa City (Olcott, 1992; Stanley, 2005). In addition to being confined by younger Devonian shale on top of the Silurian-Devonian aquifer, there is a confining layer of Ordovician-age (495-440 million years old) dolomitic shale called the "Maquoketa Shale" that underlies the aquifer, therefore allowing it to retain more water (Olcott, 1992; Stanley, 2005).

Recharge for the Silurian-Devonian aquifer of Iowa occurs where beds crop out at the surface in eastern and northeastern Iowa. From there, the aquifer drains groundwater southwest, where the aquifer becomes confined in central and western Iowa (Olcott, 1992). Similar stratigraphic patterns occur in the Wisconsin and Michigan parts of the aquifer as well--Figures 74-76, taken from Olcott (1992), show the distinct stratigraphic layering of each specific area of the aquifer. What should be noticed, though, is that when the aquifer is not cropping out at the surface, it is always overlain by a form of Devonian-age shale, and is always underlain by Ordovician-age shale. Aquifer recharge is primarily caused by precipitation that either 1.) infiltrates through Quaternary-age (1.8-0 Million years old) surficial deposits left over by the retreat of the Laurentide Ice Sheet during the last glacial period, or 2.) infiltrates directly through an outcrop. Surficial deposits can be found in yellow in Figure 2 above.

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Hydrologic and Geologic Properties

The Silurian-Devonian aquifer is composed primarily of the sedimentary rocks limestone and dolomite, mixed with interbedded layers of shale and evaporites (Olcott, 1992). Though similar hydraulically, the Siluruan deposits are made chiefly of dolomite, and the Devonian deposits are made of limestone and shale (Prior, et al., 2003). In most places, the aquifer is overlain by surficial deposits of Cenozoic age. Because of the chemical make-up of limestone and dolomite, CaCO3 (calcium carbonate), this aquifer is subject to direct dissoultion from the water itself--rainwater is slightly acidic. Rain water reacts with atmospheric CO2 (carbon dioxide) to create Carbonic acid (H2CO3)(Schulmeister, 2010).

As carbonic acid infiltrates into the carbonate-rich beds of the aquifer, the carbonic acid dissolves limestone and dolomite to create 1.) one charged calcium ion (Ca2+) and 2.) one carbonate molecule (CO3). Water then flushes both the calcium and the carbonate away from the original site, leaving voids, or caves, called karst (Schulmeister, 2010). The aquifer also contains shale, which as was mentioned before, can be found both above and below the aquifer--thus making it a confined aquifer (Olcott, 1992). But it is not that simple. the aquifer crops out in eastern Iowa, which means that it is also unconfined. This variation is due to the southwest directional dip of the beds, which causes the beds to surface in the northeast direction (northeast Iowa). Beds also dip eastward in Wisconsin toward Lake Michigan, and inward in Michigan toward the middle of Michigan, which structurally, is a syncline (bowl-shaped)(Olcott, 1992). The Silurian-Devonian aquifer has no confining layers in both Wisconsin and Michigan. Confining layers only occur in central and western Iowa (see Figure 77 of Olcott (1992).

The Silurian-Devonian aquifer ranges in thickness between 300 and 400 feet in Iowa, 400-500 feet in Wisconsin, and 1-200 feet in Michigan. In addition, the Iowa portion of the aquifer dips southwest from 1,000 feet above sea level in northeast Iowa, to 1,500 feet below sea level in southwest Iowa (Olcott, 1992). Permeability rates are usually medium due to the interconnected interstices of carbonate rocks, and the permeability of shale units is low. Areas where karst is present generally have high permeability. Porosity is generally medium in limestone and dolomite, and low in shale.

Transmissivity rates vary throughout the aquifer, as well as between Devonian and Silurian deposits. As an average, transmissivity of the aquifer is between 1,200 square feet per day when confined in Iowa, to 360,000 square feet per day where unconfined in eastern Iowa, Wisconsin, and Michigan (Olcott, 1992). Another estimate of transmissivity in Johnson County, Iowa (unconfined area) yielded an average transmissivity of 580 square feet per day (Tucci and McKay, 2005). Well yields can vary, but generally produce 100-500 gallons per minute throughout the aquifer. Secondary porosity can also have major impacts on groundwater flow; joints, fractures, faults, bedding planes, and dissolution cavities (karst) can all cause water to bypass the traditional flow path of soild rock (Prior, et al., 2003). Drawdown rates are low in unconfined areas, and high in confined areas. Additionally, the specific capacity of of the aquifer in Johnson County, Iowa yielded water movement of 2.1 gal./min./foot (Tucci and McKay, 2005). Average recharge rates also averaged 5.3 cubic feet per second in the (unconfined) Silurian-Devonian aquifer of Iowa City, Iowa (Tucci and McKay, 2005).

How does groundwater from the Silurian-Devonian aquifer affect surface water above? The aquifer serves as baseflow for many local streams (Prior, et al., 2003). Some recharge does come from streams that are flowing in the same direction as groundwater flow (Iowa Geological Survey). The aquifer also supports baseflow by adding water to streams. It is believed that in the unconfined section of the aquifer, rivers can receive between 30% and 70% of water volume from the aquifer below (Prior, et al., 2003). Luckily, Iowa, Wisconsin and Michigan all have some form of glacial deposit as an unsaturated zone, which means that the unsaturated zone is likely to have good permeability and porosity rates, allowing more precipitation to be captured and drained to the aquifer (Tucci and McKay, 2005).

Water quality is of high importance to everbody. Studies conducted in Iowa indicate that water quality varies by location. Water quality seems to become poorer with increased bed dipping (toward the southwest)(Olcott, 1992). Unconfined areas of Iowa contain fewer (less than 500 mg/L) dissolved solids (i.e. sodium chloride, phosphorus, nitrate, and sulfur, as well as dissolved metals) than confined areas, which can reach 500-5000 mg/L in groundwater (Olcott, 1992). In regard to Wisconsin and Michigan, groundwater can be quite saline at 160,000 mg/L. Addionally, due to the vast amounts of carbonate in the Midwest, groundwater in the Silurian-Devonian aquifer tends be high in dissolved calcium, which causes the local water pH to be higher than normal precipitation (Olcott, 1992).

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Water Resources of the Silurian-Devonian Aquifer

It is estimated that over 204 million gallons of water per day are pumped from the Silurian-Devonian aquifer (Olcott, 1992). Water usage from the aquifer varies by location though. Figure 4 below shows the calculated use of water from the aquifer in all three states.

Figure 4: A diagram taken from Olcott (1992) shows water usage rates from the Silurian-Devonian aquifer in three different areas: Iowa, Wisconsin, and Michigan.

The largest fraction of Iowa's water goes toward public drinking water supplies. Wisconsin uses almost 50% of their water for domestic and commercial purposes. Michigan uses over 45% of their water for industrial purposes (Olcott, 1992). The main sources of use include public drinking water/use, commercial use, industrial use, and agricultural use. As would be expected, agricultural use is higher in Iowa and Wisconsin (Olcott, 1992). Additionally, what really matters is how these different sectors (i.e. commercial vs. domestic water supply) will change in the future, and how those changes will affect water resources. As of 2010, the water resources of the Silurian-Devonian aquifer are being monitored closely (Prior, et al., 2003). Though not as threatened as nearby aquifers (such as the Galena-Platteville aquifer of Illinois), projections have been made as to how much water can be safely withdrawn from the aquifer (Prior, et al., 2003). As of 2003, there are no immediate threats to groundwater recharge in the aquifer.

The Silurian-Devonian aquifer is mainly susceptible to contamination from surficial deposits above, as well as outcrops (Olcott, 1992). Carbonates readily absorb contamination--especially if karst topography is present (Prior, et al., 2003). Luckily, contamination is minor within the aquifer, and even at that, the sources of contamination tend to be diffused, and (generally) below maximum EPA levels. Some common forms of contamination include (Prior, et al, 2003):

  • nitrate from fertilizer
  • pesticides, herbicides, and road salt
  • phospohrus from manure
  • leaky underground septic tanks and landfills
  • tile drianing
  • human activities and urban development

    Some other conditions within the aquifer can help retard contamination, such as the presence of fine-textured glacial till, and over fifty-feet of smaller-sized till in general (Prior, et al, 2003). As was mentioned before, the confined portion of the aquifer in southwest Iowa is far more suscptible to contamination because the confining layers do not allow contaminants to disperse (Prior, et al., 2003).

    The Groundwater Protection Act of 1987 provided funding for research, monitoring, remediation, and public education for environmental protection (Environmental Protection Agency, 2002). State natural resource agencies such as Iowa's Department of Natural Resources are responsible for protecting local groundwater; county Boards of Health are also responsible for providing clean, healthy water, and creating sewage treatment standards.

    The (Federal) Environmental Protection Agency, along with the state agencies just mentioned, are responsible for clean-up processes when contamination occurs, though the main role of the EPA is enforcement (Environmental Protection Agency, 2002). The EPA sets standards and deadlines for the polluter to follow. Once the contamination is at acceptable levels, the EPA monitors the site for a set amount of time. Contamination of groundwater is serious business; it is something that will affect every living organism within the aquifer's boundaries. With the creation of the Groundwater Protection Act of 1987, it is the hope of many that we will be able to preserve Earth's most precious resource: water.

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    Website created by Mike Sedlacek: April 21, 2010

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