Geology & Aquifers and their Effect on Groundwater

Geology plays an important role in determining topography, the development of soils, and the location and availability of groundwater.  Geologic bedrock  forms the foundation that our environment and society are built upon, and eventually limits the depth of groundwater movement.  On top of the bedrock lies unconsolidated geologic materials that may contain aquifers that store, transmit, and yield reasonable amounts of groundwater to wells and springs.  The location and characteristics of the aquifer depend on geology and soil properties.  The following pages focus on:

To learn about the three main types of aquifers, click here.


Bedrock Geology

Bedrock of solid or fractured rock exposed at the surface provides the parent materials for soil formation.  According to Holt, Portage County bedrock consists of crystalline rock of the Precambrian Age and sandstone of the Cambrian Age.  Crystalline rock underlies all the aquifers in the County and limits the downward flow of groundwater. It consists mostly of granitic rock with some volcanic outcroppings in the northwest part of the County.  

In the northwest corner, crystalline rock is exposed at the land surface and weathered.   It slopes about 10 feet/mile from the northwest to the south and southeast directions.  Wells are usually drilled less than 30 feet deep and they yield less than 5 gpm (gallons per minute). 4

Sandstone from the Cambrian Age overlies the crystalline rock in the southern part of the County and near Stevens Point.  Sandstone consists of medium to coarse grains of cemented sand (also known as quartz cemented by silica and iron-oxide).  The thickness of this layer ranges from a few inches to several feet. 4 

A more detailed account of Portage County bedrock was provided by Greenberg and Brown.  They divided Portage County into four main bedrock types consisting of volcanic rock, intrusive rock from the Wolf River Batholith, granitic rock, and sandstone.  The map shows a detailed listing of the rock materials forming each type of  bedrock.14   To view this more detailed map of Portage County Bedrock along with a table describing the rock types, click here.  


(Clipped from Bedrock Geologic Map of Wisconsin 14)


To view the above map along with a table describing the rock type, click here


(Clipped from Bedrock Geologic Map of Wisconsin 14)


(Modified from Bedrock Geologic Map of Wisconsin 14)

Depth to Bedrock

Bedrock ultimately limits the depth of downward movement of groundwater.  It also helps determine  the depth to which a well should be drilled. The closer the bedrock is to the land surface, the closer the water table is to the land surface.  

The closer the bedrock is to the land surface, the less soil accumulation and geologic materials there are.  The shallower the unconsolidated geologic material is, the less potential there is for storing water and for attenuating or filtering out contaminants.  

For example, the Village of Junction City, located in the northwestern part of Portage County, must extract its water from fractures in the bedrock.  It struggles to meet the water needs of its residents.  A new well installed in 2000 is 340 feet deep and only yields 60 gpm (gallons per minute), which for them is a high flow rate

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Geology of Unconsolidated Materials

Bedrock provides the foundation for the unconsolidated materials that formed thousands of years ago during the Pleistocene Age.  During this period, the northern part of North America experienced large accumulations of snow and ice.  The ice masses, called glaciers, were up to two miles thick. 1  "Portage County was partly covered by at least one large continental ice sheet during the Wisconsin Glaciation, which advanced towards the south and west. (Thwaites, 1943, as quoted by Holt,1965) 4 "  

As this ice sheet moved, it transported and deposited rock sediment and debris forming moraines.  In Portage County,  four moraines that formed hills with ridges and one border drift (a moraine with no ridge) were formed. As the glacier melted, it also deposited well-sorted sand and gravel called glacial outwash, and unsorted sandy deposits called till.   These gently sloping outwash plains are the best storage container for groundwater.  The deposits left by the Ice Age  created the Drift Groundwater Province and Sand-Plain Groundwater Province in Portage County. 4

 

(Modified from C. L. R. Holt, Jr., 4)

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Groundwater Provinces

Groundwater provinces are areas with similar geologic and hydrologic characteristics such as rock materials, topography, surface drainage, and availability of groundwater. 4 Portage County has been divided into three groundwater provinces:  drift-crystalline rock, sand-plain, and drift.  Refer to the diagram for the location of these provinces.  Crystalline bedrock underlies all three groundwater provinces.

Drift-Crystalline Province

Occurs in the northwestern part of the County with all surface streams draining into the Wisconsin River.  The province consists of thin deposits of clay till, glacial outwash, and alluvium overlying  crystalline bedrock.  Small amounts of groundwater are stored in this thin layer or in fractures in the bedrock.  Generally, wells are less than 50 feet deep and they yield only small amounts of water.  Groundwater baseflow is small and surface runoff is large.  The Village of Junction City, located within this province in the northwest part of the County, has difficulty in acquiring adequate amounts of water to meet their needs. 4

 
Sand-Plain Province

Occurs in the central and southwestern parts of the County with all surface streams draining into the Wisconsin River.  The province consists of a thick and extensive aquifer of glacial outwash containing sand and gravel with a small amount of silt or clay overlying crystalline bedrock in the north and sandstone in the south.  The aquifer thickness ranges from a few feet to 250 feet.  Well depths range from 30 feet to 250 feet and they yield large quantities of water up to 2000 gpm (gallons per minute).  Groundwater baseflow is large and surface runoff is small. 4


Drift Province 

Occurs in the eastern part of the County with all surface streams draining into the Waupaca and Little Wolf Rivers and then into the Tomorrow River.  The province consists of thick sandy till, glacial outwash containing sand and gravel with small amounts of silt or clay, and drift overlying crystalline bedrock in the north and sandstone in the south.  The aquifer thickness ranges from a few feet to 250 feet deep.  The sandy till and alluvium contain small amounts of groundwater, whereas the outwash and drift contain large supplies of groundwater.  Domestic wells are 10-50 feet deep and yield up to 50 gpm, and high capacity wells yield up to 500 gpm.  Groundwater baseflow is large and surface runoff is small. 4

 

(GIS data from UWSP, College of Natural Resources digitized from the Surface Geology Map, Plate 1 of Paper 1796 by Holt, 4)


To view the above map along with a table describing the geology, click here


The Drift Province and Sand-Plain Province form the sand and gravel aquifer that covers most of the County.  The sand and gravel aquifer yields tremendous amounts of water up to 2000 gallons per minute (gpm).  In fact, the geologic material forming this aquifer is actually the land surface in many areas.   For this reason, human actions on the land surface can greatly impact the quality of the groundwater.  Groundwater contamination can result from contaminating water on the surface before it recharges the aquifer.

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To learn about soil and aquifer properties, click here.
To learn about the three main types of aquifers, click here.


(Italicized words defined in the glossary.)

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