COUNTY ATLAS SERIESATLAS C11, PART B, PLATE 7 OF 10
Bedrock Hydrogeology
STATE OF MINNESOTADEPARTMENT OF NATURAL RESOURCESDIVISION OF
WATERS
GEOLOGIC ATLAS OF MOWER COUNTY, MINNESOTA
Dclp
Digital base map composite:Roads and county boundariesMinnesota
Department of Transportation GIS Statewide Base
Map (source scale 1:24,000)Hydrologic featuresU.S. Geological
Survey Digital Line Graphs (source scale 1:100,000)Digital base map
annotationMinnesota Geological Survey.
Project data compiled from 1997 to 2001 at the scale of
1:100,000. Universal TransverseMercator projection, grid zone 15,
1983 North American datum. Vertical datum is mean sealevel.
GIS data and metadata available through the DNR Waters website:
http://www.dnr.state.mn.us/waters
BEDROCK HYDROGEOLOGY
By
Moira Campion
2002
MAP EXPLANATION
Well Label
If shown, ground-water age in years, estimated bycarbon-14
30,000
Map Symbols
TM
This information is available in an alternative format on
request.
This map was compiled and generated using geographic information
systems (GIS)technology. Digital data products are available from
DNR Waters.
This map was prepared from publicly available information only.
Every reasonableeffort has been made to ensure the accuracy of the
factual data on which this mapinterpretation is based. However, the
Department of Natural Resources does not warrantthe accuracy,
completeness, or any implied uses of these data. Users may wish to
verifycritical information; sources include both the references
here and information on file in theoffices of the Minnesota
Geological Survey and the Minnesota Department of NaturalResources.
Every effort has been made to ensure the interpretation shown
conforms tosound geologic and cartographic principles. This map
should not be used to establish legaltitle, boundaries, or
locations of improvements.
Equal opportunity to participate in and benefit from programs of
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2002 State of Minnesota,Department of Natural Resources, and
theRegents of the University of Minnesota
U.S. 63
Waltham
Brownsdale Racine
Lansing
Maple View
I-90Grand
Meadow
U.S. 218
Elkton
Adams
U.S. 218
Lyle
Le Roy
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)
E
leva
tion
(fee
t abo
ve s
ea le
vel)Cedar River
Cedar River
Cedar River ( )
Cedar River ( )
CedarRiver ( )
Cedar River
I-90RobinsonCreek ( )
Dexter
RootRiver
RoseCreek
Roberts Creek ( )
UpperIowa River ( )
Austin
I-90
Dubuque Formation
Upper Cedar Valley aquifer
Lower Cedar Valley aquifer
Spillville-Maquoketa aquifer
Galena Group aquifer
Chickasaw shale
Potentiometric contour of aquifers in feetabove mean sea
levelContour interval is20 feet. Dashed where contour is
uncertain.Arrow indicates general direction of ground-water
movement.
Upper Cedar Valley
Spillville-Maquoketa
Lower Cedar Valley
Upper Cedar Valley
Lower Cedar Valley
Spillville-Maquoketa
Wells measured for static water level
Dcvu
Dcvl
Dsom
Ogal
Greater than 75 feetof till cover
CROSS-SECTION EXPLANATION
RecentWater entered the ground since 1953 (10 or more tritium
units).Well screen color shows recent water.
MixedWater is a mixture of recent and vintage waters(0.8 to less
than 10 tritium units). Well screen color shows mixed water.
VintageWater entered the ground before 1953 (less than 0.8
tritium units).Well screen color shows vintage water.
Upper Cedar Valley aquifer
Sand and gravel deposits
Undifferentiated till
Spillville-Maquoketa aquifer
Lower Cedar Valley aquifer
Dubuque Formation
Chickasaw shale
Galena Group aquifer
St. Peter-Prairie du Chien-Jordan aquifer
Decorah-Platteville-Glenwood map unit*
Very oldWater with carbon-14 age greater than 10,000 years
before present.Well screen color shows vintage water.
In feet above mean sea level.Contour interval is 20 feet.
Arrowindicates general direction of ground-water movement
If shown, ground-water age in years,estimated by
carbon-14Vintage water shallower than mixed waterSV
18,000
Spring associated with streamGround-water flow into the cross
sectionGround-water flow out of the cross section
Odub
Dclc LOCATION DIAGRAM
Upper Cedar Valley
Spillville-MaquoketaLower Cedar Valley
*The Decorah Shale and Glenwood Formation both act as confining
units, but the interveningPlatteville Formation is a thin aquifer.
Combined, these units are treated as a confining unit.
28,000
9000
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5
7
3
90
90
218
5
6
56
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1
20
7
63
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90
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218
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14
1
8
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1
105
56
14
16
3
3
4
56 7
105
5
56
16
16
16
16
56
T 102 N
T 104 N
R 18 W R 17 W R 16 WR 15 W R 14 W
T 101 N
T 103 N
R 14 WR 15 W
R 16 WR 17 WR 18 W
T 104 N
T 103 N
T 102 N
T 101 N
9245'9300'
4345'
4330'
4345'
4330'9230'9245'9300'
9230'
Racine
MeadowGrand
Le Roy
Taopi
Adams
CreekRose
Austin
Maple View
Brownsdale
Waltham
Dexter
Elkton
Lyle
Lansing
Varco
Sargeant
UDOLPHO
WALTHAM
SARGEANT
PLEASANTVALLEY
RACINE
FRANKFORD
GRANDMEADOW
DEXTER
AUSTIN
WINDOM
LE ROY
LODINEVADALYLE
CLAYTON
RED ROCK
LANSING
MARSHALL BENNINGTON
Cedar
R
iver
Mill PondRamsey
Wolf Creek
Dobb
ins
Cre
ek
MurphyCreek
Orc hard
Cr eek
Woodbury Creek
Ceda
r
Rive
rRos e Creek
Otter Creek
Little
Cedar
River
Upper
Iowa
R iver
L ittl
e
Iow
a
Rive
r
Nor
thB
ranc
hUp
p
er I
ow
aR
iver
South
Branch
Roo
t
River
Deer Creek
Deer
Cree
k
South Fork
Bear
Creek
North
Cree
kBearFork
Creek
Robin
son
Creek
North
Bra
nch
Root
Riv
er
Ro berts Creek
C
ar
ey
16
31 36
1
36
616
31 36 31
1661
3631
1
36
6
31 36 31
1616
31 36 3136
1616
31363136
16
31
1
36
6
31
1
313631
6
31
6
1
36
6
31
1
31
36
36
3636 31
31
1 6 1
36
61
31
6
Upper Cedar Valley
Spillville-Maquoketa
Lower Cedar Valley1200
1200
1200
SV
SV
SV
SVSV
Characterization of Bedrock Hydrogeology
This plate displays the Mower County aquifer system in map and
cross-sectionviews. This presentation provides practical
information about the hydrogeology inthe county, which is crucial
to understanding ground-water supply and managementissues. The
information also was essential to the development of the
pollutionsensitivity map on Plate 9. Concepts that require more
detailed explanation arediscussed on Plate 8 and in the Technical
Appendix.
The interpretation of the bedrock hydrogeology was modified from
a historicalone-aquifer system to a sequence of several aquifers
and confining units. Thisapproach has been used by hydrogeologists
in Iowa and Minnesota (see Plate 2, PartA). The presence of
regional shale confining units provides protection from
surfacecontamination in areas with limited, low-permeability
glacial cover. Mapping theshallow confining units at a countywide
scale provides an opportunity to avoiddrilling unnecessarily deep
wells for drinking water supply. Regulators who knowthe location of
the regional shale units can target aquifers that may be
hydrologicallyprotected from surface contamination. This can reduce
drilling costs for municipalitiesand residents who depend on ground
water for drinking water, irrigation, and industry.
The bedrock aquifers, confining units, and major mapped faults
in the countyare shown on the map, and a pattern overlies the area
where till thickness is greaterthan 75 feet. Cretaceous deposits
are thin and discontinuous; they are not shown onthe map or cross
sections. The potentiometric surfaces for the three
uppermostaquifers are shown with 20-foot contours of different
colors, as well as the locationsof wells used to develop the
potentiometric contours. The locations of wells sampledfor
geochemistry and for residence time information from tritium and
carbon-14 agedating are also displayed. The cross sections show the
arrangement of aquifers andconfining units at depth and colored
residence time interpretations based on the age-dating results from
the sampled wells. All sampled wells have been projected ontothe
closest cross section. No well is more than 2 miles from a cross
section. SeePlate 8 for a description of radiometric dating of
ground water.
The map shows potentiometric contours of the Upper Cedar Valley,
LowerCedar Valley, and Spillville-Maquoketa aquifers. The small
data points showing thewell locations used to develop these
surfaces are displayed in colors correspondingto the contours.
There are few locations where wells from shallower and
deeperaquifers are close enough to one another to be used as nests
for a point-by-pointcomparison of water-level information from
different aquifers. Enough data pointsexist, however, to show that
the potentiometric surface of each aquifer is different.The
potentiometric differences between the Upper Cedar Valley and the
Lower CedarValley aquifers are subtle in the southeastern part of
the county. Greater differencesappear in the center of
cross-section EE. The difference between the potentiometricsurfaces
of the Lower Cedar Valley and the Spillville-Maquoketa is most
obviousin the center of the map and in the center of cross-section
CC. Generally, groundwater flows from east-central Mower County to
the south and west in the Upper andLower Cedar Valley aquifers. In
the northeast, ground water in the Spillville-Maquoketa aquifer
flows from the center of the county to the north. In the
northwest,ground water in the Spillville-Maquoketa aquifer flows
south through potentiometrictroughs toward the Cedar River
discharge area. Generally, potentiometric differencesare smallest
at the Cedar and Little Iowa rivers, which are regional discharge
areas.
Recent ground water was found in samples from 12 wells based on
tritiumdating. Nine of the wells with samples of recent water have
till cover less than 75feet thick. For most of the central and
northeastern parts of the county, ground watergets progressively
older with depth. This is a typical way for water to infiltrate
fromthe land surface into the bedrock aquifers. However, there are
areas shown on cross-sections AA and CC where samples of vintage
waters were found shallowerthan the samples of mixed waters in
deeper nearby wells. In southwest and southeastMower County, the
distribution of recent, mixed, and vintage samples on the
crosssections is different from the rest of the county. Recent and
mixed waters are foundat great depths. Recent and mixed waters
found at depth indicate focused recharge.Faulting and sinkholes are
also found in the western portion of CC and DDand the eastern
portion of EE and FF. The presence of faulting and karst
featuresmay be a factor in focused recharge. These residence time
patterns are discussed inmore detail on Plate 8.
Mower County has an abundant supply of high-quality drinking
water in thebedrock aquifers found throughout the county. Ground
water in the two uppermostbedrock aquifers flows from the
east-central part of the county to the south and west.In the
northeast, ground water in the Spillville-Maquoketa aquifer flows
from thecenter of the county generally to the north. Results of age
dating show that till thickerthan 75 feet can restrict infiltration
from the land surface to the bedrock aquifers.The faults and karst
features in the west and southeast may contribute to the
focusedrecharge of the bedrock aquifers from the land surface.
REFERENCES CITED
Green, J. A., Mossler, J. H., Alexander, S. C., and Alexander,
E. C., Jr., 1997, Karsthydrogeology of Le Roy Township, Mower
County, Minnesota: MinnesotaGeological Survey Open-File Report
97-2, 2 pls., scale 1:24,000.
Libra, R. D., and Hallberg, G. R., 1985, Hydrogeologic
observations from multiplecore holes and piezometers in the
Devonian carbonate aquifers in Floyd andMitchell counties, Iowa:
Iowa Geological Survey Open File Report 85-2, Part1, p. 119.
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Tritium units: 1Casing depth: 695 feetDepth completed: 911
feet
2500
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Tritium units: less than 0.8Casing depth: 930 feetDepth
completed: 1035 feet
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Uppermost Bedrock Aquifers and Confining Units
Potentiometric Contour
1 0 1 2 3 4 5 MILES
SCALE 1:100 000
1 0 1 2 3 4 5 6 7 8 KILOMETERS
Aquifer orconfining unit SymbolWell Symbols
Shape indicates aquifer type
Spillville-Maquoketa
Upper Cedar ValleyLower Cedar Valley
St. Peter-Prairie du Chien-JordanGalena Group
Color indicates tritium age
RecentWaters with tritium concentrations of 10 tritiumunits (TU)
or more entered the ground water since 1953.
MixedWaters with 0.8 to 10 TU are a mixture of recentand
vintage.
Quaternary
Spillville Maquoketa
Cedar Valley UpperCedar Valley Lower
St. Peter-Prairiedu Chien-JordanGalena
Quaternary
Aquifername
AquiferCode
Lithologiccode on map
Quaternary
DSOM
DCVUQ
DCVL
Fault line
Spillville Maquoketa
Cedar Valley UpperCedar Valley Lower
St. Peter-Prairiedu Chien-JordanGalena
Quaternary
Aquifername
AquiferCode
Lithologiccode on map
Quaternary
DSOM
DCVUQ
DCVL
Spring
VintageWaters with less than 0.8 TU enteredthe ground water
before 1953.
MULTIAQUIFER SYSTEM IN DEVONIAN ANDUPPER ORDOVICIAN BEDROCK IN
MOWER COUNTY
Most of the previous hydrogeologic studies in Mower County
describe a single, thick bedrock aquifer underlying the county.
This studypresents the Devonian and upper Ordovician bedrock as a
sequence of carbonate aquifers separated by shale units.
Hydrogeologists in Iowa(Libra and Hallberg, 1985) first classified
the rocks as regionally extensive carbonate and shale units that
compose a regional system ofaquifers and confining units. John
Mossler used this approach to classify the stratigraphy of these
carbonates and shales on Plate 2, Part A.Green and others (1997)
also described these rock units as aquifers and confining units
similar to those in Iowa. Pump test data and residencetime
information in this study supported this approach to characterizing
the carbonate and shale units. This approach also has
practicalbenefits. With approval from the Minnesota Department of
Health, wells may be completed in some shallow carbonate units
beneath regionalshale units. This change would reduce required
depths of well completion by 300 feet to 500 feet and reduce
construction costs by severalthousand dollars per well.
The information below includes the rock unit name, aquifer
symbol (where applicable, boldface in parentheses), bedrock symbol
(inparentheses), and details about the rock units. A geologic
column and more information about these aquifers are in the
Technical Appendix.
Upper Cedar Valley Aquifer (Dcvu) (Dcuu and Dcum). The
Lithograph City Formation, the Coralville Formation, and the
Hinkleand Eagle Center Members of the Little Cedar Formation form a
shallow bedrock aquifer. The Lithograph City unit contains many
joints,fractures, and bedding planes enlarged by solution resulting
in a well-developed conduit network and high permeability (Green
and others,1997). The Coralville unit contains shale beds that may
be several feet thick but are discontinuous (Libra and Hallberg,
1985). Mostpermeability in this aquifer is fracture related, with
water moving through joints or bedding planes. The lower part of
the aquifer has lesssolution-related permeability than the
Lithograph City. This aquifer is up to 110 feet thick.
Chickasaw Shale (Dclc). This silty shale has low permeability
and hydrologically separates the Upper Cedar Valley above from
theLower Cedar Valley underneath. It has been mapped on Plate 2,
Part A, and is clearly shown on the gamma log in the Technical
Appendix.In western Mower County, buried stream valleys dissect it.
This unit is 36 feet to 43 feet thick in southeastern Mower County
and 15 feetto 20 feet thick in western Mower County.
Lower Cedar Valley Aquifer (Dcvl) (Dclp). The Bassett Member of
the Little Cedar Formation forms a thin aquifer (Green and
others,1997) and is 40 feet to 70 feet thick. The upper and lower
parts of the Pinicon Ridge Formation contain shale and shaly
dolostone that mayact as confining units. The Pinicon Ridge
Formation is 20 feet to 47 feet thick. These shaly units were not
mapped in Part A, and the distributionof data for this study is
inconclusive regarding the ability of the Pinicon Ridge shales to
hydrologically separate the Lower Cedar Valleyaquifer from the
underlying Spillville-Maquoketa aquifer. However, results from a
pump test in Le Roy provide local evidence that the shalesfrom the
Pinicon Ridge do separate the Lower Cedar Valley aquifer from the
underlying Spillville-Maquoketa aquifer (Green and
others,1997).
Spillville-Maquoketa Aquifer (Dsom) (Dspl and Omaq). The
Spillville and Maquoketa formations form a single aquifer unit in
MowerCounty. The porosity and permeability in the upper part of the
Spillville is due to fractures and in the lower part is due to
fossil features (Plate2, Part A). The upper part of the Maquoketa
has extensive fracturing possibly because of exposure at the
surface before the deposition ofthe overlying Spillville Formation.
The lower part of the Maquoketa has many thin shale units and
grades into the underlying DubuqueFormation, which together
hydrologically separate the Spillville-Maquoketa aquifer from the
Galena Group aquifer. The Spillville andMaquoketa formations
together are up to 170 feet thick.
Dubuque Formation (Odub). This formation consist of limestone
interbedded with thin to medium beds of calcareous shale. It is
aconfining unit that separates the overlying Spillville-Maquoketa
aquifer from the underlying Galena aquifer and is 23 feet to 35
feet thick.
Galena Group Aquifer (Ogal) (Ogal). The Galena Group, which is
an important regional aquifer, comprises the Stewartville
Formation,Prosser Limestone, and Cummingsville Formation. The
Stewartville and Prosser both have more well-developed porosity and
permeabilitythan the Cummingsville and form an aquifer. The
greatest permeability of the group occurs where the aquifer is near
the surface and haswell-developed karst features. Total thickness
of this group is 200 feet to 205 feet.
Decorah ShalePlatteville FormationGlenwood Formation (Odpg).
These three formations are treated collectively because theyare
found between two extensive regional aquifers. Only the Decorah
Shale and Glenwood Formation have predictable
low-permeabilitycharacteristics. They effectively separate the
Galena Group aquifer from the underlying St. Peter Sandstone and
have a combined thicknessof 45 feet to 50 feet. The Platteville
Formation is a thin limestone aquifer (30 feet thick) between the
Decorah Shale and the GlenwoodFormation. Although it is not heavily
used for water supply, the Glenwood Formation can transport large
quantities of water.
Geologic Units and Aquifers
Potentiometric Contour Cross-Section Symbols
Dcvu
Dclc
Dclc
DcvuDcvl
Dcvu
Dcvu
Dclc
Dclc
Dcvl
Dcvu
Dclc
Dsom
Dclc
Dcvl
Dsom
Dsom
OdubDsom
Dcvl
Dsom
Odub
Dsom
DsomDcvl
Dcvl
Dclc
Dcvu
Dcvu
Dclc
Dcvu
Dclc
Dcvu
Dcvu
Dcvu
Dcvl
Dclc
Dcvu
Dcvu
Dcvu Dcvu
Dclc
Dcvu
Dcvu
DcvuDcvl
Dclc
Dcvl
Dsom
Dsom
Dsom
Odub
Ogal
Dsom
Dsom
Dsom
Dcvl
Ogal
Odub
Dsom
Dsom
Dsom
Dsom
DcvuDclc
Dcvl
Dcvu
Dsom
Dcvu
Dcvu
Dclc
Dcvu
Dsom
Ogal
Dcvl
Upper IowaRiver ( )
1 0 1 2 3 4 5 MILES
SCALE 1:146 000
0 2 4 6 8 KILOMETERS
VERTICAL EXAGGERATION X 40
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DNRWaters
