INTRODUCTION
The Mississippian aquifer is present in Iowa, where it underlies about 60 percent of the State, and in Michigan, where it underlies much of the Lower Peninsula (fig. 59). The aquifer consists mainly of limestone and dolomite in Iowa and siltstone and medium-grained sandstone in Michigan. In both States, the aquifer is overlain either by Pennsylvanian or younger rocks that confine the aquifer and restrict ground-water circulation. Where the Mississippian aquifer forms the bedrock surface in Iowa and the Lower Peninsula of Michigan, the aquifer generally is overlain by the surficial aquifer system. In Iowa, the Mississippian aquifer generally is confined by fine-grained glacial deposits in this subcrop area. In Michigan, the Mississippian aquifer generally is unconfined in this subcrop area and is hydraulically connected with extremely permeable glacial deposits. Wells completed in the Mississippian aquifer in these subcrop areas yield 900 to 1,000 gallons per minute in both States. Dissolved-solids concentrations in water from the Mississippian aquifer in areas where it is confined generally exceed 500 milligrams per liter. Brine occurs in the deeper parts of the aquifer in confined areas.
HYDROGEOLOGIC UNITS
Mississippian rocks in Iowa are overlain by Pennsylvanian rocks,
Cretaceous rocks, and glacial deposits, whereas Mississippian
rocks in the Lower Peninsula of Michigan are overlain only by
Pennsylvanian rocks and glacial deposits. In Iowa, Mississippian
rocks dip gently southwestward from where they form the bedrock
surface and steeply southward along the western border of the
State; their altitude and thickness vary because of structual
flexures and erosion. In Michigan, Mississippian rocks dip and
thicken in a radial pattern toward the center of the Michigan
Basin.
Mississippian rocks in Iowa consist principally of limestone and dolomite and some sandstone and siltstone. The only significant shale is in the Warsaw Limestone (fig. 60) in the upper part of the Mississippian rocks in southeastern Iowa. Gypsum and anhydrite beds are present in the Mississippian rocks, especially in the St. Louis and the Spergen Limestones in southeastern Iowa (fig. 60). The entire Mississippian section in Iowa constitutes the Mississippian aquifer. It is a carbonate-rock aquifer that is characterized by solution openings and fracture permeability. The aquifer generally is confined by overlying fine-grained glacial deposits where it forms the bedrock surface along its northeastern edge and by Pennsylvanian and Cretaceous rocks in the southern and the southwestern parts of the State. The Mississippian aquifer is present in about 60 percent of Iowa and ranges in thickness from a featheredge to about 600 feet (fig. 61).
Mississippian rocks in the Lower Peninsula of Michigan generally consist of sandstone and some shale, limestone, and coal and range from a featheredge up to about 1,000 feet thick (fig. 61). The Mississippian aquifer primarily consists of the Marshall Sandstone (fig. 62); thin sandstone beds in the lower part of the overlying Michigan Formation and in the upper part of the underlying Coldwater Shale also might contribute some water. Mississippian rocks younger than the Marshall Sandstone form the overlying Bayport-Michigan confining unit. The Coldwater Shale, along with underlying older shales, forms a lower confining unit. The top of the Coldwater Shale generally is the base of the Mississippian aquifer, as well as the base of freshwater in most of the Lower Peninsula. The altitude of this surface ranges from about 200 to about 800 feet above sea level (fig. 63).
GROUND-WATER FLOW
Recharge to the Mississippian aquifer in Iowa is principally where
the aquifer forms the bedrock surface and is overlain by, and
hydraulically connected with shallower aquifers (fig.
64). In the north and the northeast, water enters the Mississippian
aquifer from the surficial aquifer system and moves either southward
along intermediate and long flow paths to the Des Moines and the
Skunk Rivers or westward along short flow paths to the Skunk River
(fig. 64). In the northwest, water
in the Cretaceous aquifer moves downward into the Mississippian
aquifer. The water then moves either southward into Missouri or
southeastward to the Des Moines River through solutionally enlarged
joints and fractures in the limestone under the confining Pennsylvanian
rocks.
Recharge to the Mississippian aquifer in Michigan is principally where the aquifer forms the bedrock surface and is directly overlain by and hydraulically connected with the surficial aquifer system to the north and the south of overlying Pennsylvanian rocks (fig. 65). Recharge from precipitation and from lakes and streams is through the surficial aquifer system. Water moves into the aquifer at high areas on the potentiometric surface to the north (1,100- and 1,000-foot contours) and to the south (1,000- and 900-foot contours), as shown in figure 65. Water moves from these recharge areas down the hydraulic gradient to Saginaw Bay on the east and to Lake Michigan on the west. Because of the minimal aquifer transmissivity and the occurrence of dense brine in the central part of the aquifer, little ground-water movement probably occurs. The principal movement of water in the Mississippian aquifer in Michigan, therefore, is probably largely restricted to peripheral areas of the aquifer.
WELL YIELDS
In north-central Iowa, where the Mississippian aquifer is overlain
by either the surficial aquifer system or the Cretaceous aquifer,
joints and fractures have been enlarged by dissolution, and the
Mississippian aquifer yields large quantities of water to wells;
the specific capacity of wells completed in this part of the aquifer
generally ranges from 1 to 5 gallons per minute per foot of drawdown
and can be as much as about 10 gallons per minute per foot of
drawdown (fig. 66). In the remainder
of the aquifer, the specific capacity generally is1 gallon per
minute per foot of drawdown or less (fig.
66).
Potential well yields from the Mississippian aquifer in the north-central and the southeastern parts of the area in Iowa where the aquifer forms the bedrock surface are summarized in table 4. Yields generally are greatest (about 5-20 gallons per minute) from the Gilmore City and the Hampton Formations and the North Hill Group in the north-central part of the area and are least (about 3-10 gallons per minute) from the Ste. Genevieve, the St. Louis, and the Spergen Limestones (table 4) in both parts of the area.
In Michigan, the Mississippian aquifer is one of the most important and productive aquifers in the State. Although much of the Mississippian aquifer is confined or semiconfined, unconfined conditions occur locally in the area where the aquifer forms the bedrock surface. Transmissivity values reported for the aquifer range from 2,700 to 67,000 feet squared per day depending primarily on differences in aquifer thickness and the size and the number of fractures in the aquifer. The aquifer ranges in thickness from a featheredge along the periphery to about 1,000 feet in the center of the Michigan basin (fig. 61). The aquifer has much larger transmissivity values near its periphery where it is thinner but contains larger and more numerous fractures than in the center of the basin. The Mississippian aquifer is used only in the southern part of the State and in the Saginaw Bay area because elsewhere either it contains water that is too salty for use or shallower aquifers are available.
GROUND-WATER QUALITY
In Iowa, water in the Mississippian aquifer meets recommended
drinking-water standards for public supply only in those areas
where the aquifer forms the bedrock surface (fig.
67). Dissolved-solids concentrations range from less than
500 milligrams per liter to about 5,000 milligrams per liter in
downdip areas. In areas where the Mississippian aquifer is overlain
by the Cretaceous aquifer, water in the Mississippian aquifer
has a dissolved-solids concentration of at least 1,500 milligrams
per liter; hardness concentrations range from less than 200 to
about 1,200 milligrams per liter (fig.
68); and sulfate concentrations range from less than 250 to
about 2,000 milligrams per liter (fig.
69). These constituents show a pattern of increasing concentrations
downdip except for the isolated case shown in figure
68 where hardness decreases in the central part of the downdip
area. This decrease might result from ion exchange where calcium
ions are selectively removed from the water and replaced by sodium
ions, thereby decreasing hardness. The ion-exchange phenomenon
is characteristic of water that has been in contact with shale
or shaly rocks, which might be the situation in Iowa.
Evidence of ground-water discharge from the Mississippian aquifer to the Des Moines River in Iowa is shown in figure 69. During periods of high flow, the flow of the river is largely surface runoff, whereas, during fair-weather or low-flow periods, the streamflow is supplied by ground-water discharge. The substantial sulfate concentration in the ground-water discharge imparts an increasingly greater sulfate concentration to the stream water as streamflow decreases and ground water comprises a greater percentage of the flow. The graph in figure 69 shows sulfate concentration of stream water as a function of discharge at five gaging stations. Although the data are scattered, there is a definite trend of increasing sulfate concentration with decreasing streamflow.
Water in the Mississippian aquifer in Michigan, principally in the southern and the eastern parts of the aquifer where it forms the bedrock surface, is typically a mixed ion type with dissolved-solids concentrations that range between 200 and 400 milligrams per liter (fig.70). In the central part of the aquifer, where it is overlain by the Bayport-Michigan confining unit, the aquifer contains brine with dissolved-solids concentrations that range from less than 1,000 to more than 300,000 milligrams per liter (fig. 71).
FRESH GROUND-WATER WITHDRAWALS
Withdrawal of freshwater from the Mississippian aquifer during
1985 totaled 67 million gallons per day; 22 million gallons per
day was withdrawn in Iowa; and 45 million gallons per day was
withdrawn in Michigan (fig. 72).
By far the largest use of freshwater in Iowa, 71.0 percent, was
for agricultural purposes, primarily stock watering, which totaled
15.6 million gallons per day during 1985. In the industrial State
of Michigan, 45 percent of the freshwater, or about 20 million
gallons per day, was used for industrial, mining, and thermoelectric-power
purposes. Public supply was the second principal use of freshwater
from the Mississippian aquifer in Michigan; 34.2 percent of the
total, or about 15.4 million gallons per day, was withdrawn for
that purpose.
