Wisconsin Groundwater
A view underneath Wisconsin
The Water Cycle
Hydrogeology (hydro-water and geology-the study of the Earth) studies how groundwater moves and is distributed in the soil and rocks below the Earth’s surface.
Pictured here is the water cycle, which shows how water moves across and through the Earth’s surface and how land use can affect water movement through the subsurface.
Groundwater is water that fills up pores and cracks in the soil and rocks underneath our feet. Groundwater can be found almost everywhere you look – beneath oceans, hills, valleys, mountains, lakes, and even deserts.
Intro to Hydrogeology
Groundwater starts as precipitation (either rain or snow) that falls on the Earth’s surface and slowly seeps downward through unsaturated soil and rocks until it reaches the saturated zone. This process is known as groundwater recharge, and the top of the saturated zone is called the water table (see figure on right or click here ). The water table can be very shallow, even at the land surface in some cases, or very deep, sometimes hundreds of feet below the land surface.
Once recharge reaches the water table, it becomes part of the groundwater system. Groundwater flows toward discharge areas such as streams, lakes, springs, and wetlands. That movement of groundwater is very slow, often moving only inches per day, and sometimes even as slow as inches per decade.
The rocks and soils where groundwater flows are called “aquifers”. The basic properties of the aquifer control how quickly and how easily groundwater moves through the aquifer. Aquifers with a lot of open pore space or large fractures allow groundwater to move more quickly than tight, compacted aquifers.
Wisconsin has four main aquifer formations: the Sand and Gravel Aquifer, the Silurian-Devonian Dolomite, the Cambrian-Ordovician Sandstone and the Precambrian Bedrock.
Sand and Gravel
The Sand and Gravel formation covers most of the state, except for some parts of southwest Wisconsin. It consists mostly of sand and gravel deposited in river valleys and/or from past glaciations. The glacial deposits are loose or unconsolidated , so they often are referred to as soil – but they include more than just a few feet of topsoil. These deposits can exceed 300 feet thick in some places in Wisconsin.
Glaciers , formed by the continuous accumulation of snow, played an interesting role in shaping Wisconsin's geology. Snow turned into ice, which reached a maximum thickness of almost two miles. The ice sheet spread over Canada, and part of it flowed in a general southerly direction toward Wisconsin and neighboring states. This ice sheet transported a great amount of rock debris, called glacial drift.
As the ice melted, large amounts of sand and gravel were deposited, forming "outwash plains." Kettles were formed in the outwash where buried blocks of ice melted; many of these are kettle lakes. Evidence of this can be seen in the Northern Kettle Moraine State Forest of Sheboygan and Fond Du Lac County. The youngest glacial deposits were placed during the most recent glaciation, which ended about 10,000 years ago.
Sand and Gravel
The sand and gravel outwash plains now form some of the best aquifers in Wisconsin. Many of the agricultural lands in central, southern and northwestern Wisconsin use the glacial outwash aquifer for irrigation. While some glacial deposits are useful aquifers, in some places large glacial lakes were formed and over time, accumulated thick deposits of clay. These old lake beds of clay do not yield or transmit much water.
Material deposited directly by glacial ice is called till. Because of how it was deposited, till is typically poorly sorted, meaning it contains a mix of fine-grained and coarse-grained materials, which does not transmit groundwater as readily as a well-sorted outwash deposit.
A steam shovel moving sand and gravel in a quarry, Circa 1940 (Credit: Wisconsin Historical Society)
Silurian-Devonian Dolomite
The Silurian-Devonian dolomite occurs in eastern Wisconsin from Door County to the Wisconsin-Illinois border. These rocks are part of what is called the Niagara Escarpment – a wide arc stretching from Wisconsin, through Michigan’s Upper Peninsula, and across Ontario, Canada to Niagara Falls, New York.
These rock formations were deposited about 430 million years ago as a lime mud on an ancient sea floor. Dolomite is a carbonate rock similar to limestone, it holds groundwater in interconnected cracks.
The extent of the Niagara Escarpment (marked in red)
Silurian-Devonian Dolomite
The water yield from a well in this aquifer mostly depends on the number of fractures the well intercepts. As a result, it's not unusual for nearby wells to vary greatly in the amount of water they can draw from this layer. The fractures in the dolomite, along with the shallow soils above the rock, also make this aquifer highly susceptible to contamination.
The Maquoketa shale layer beneath the dolomite was formed from clay minerals that do not transmit water easily. Therefore, it is important as an aquitard, or barrier, between the Silurian-Devonian aquifer and the Cambrian-Ordovician sandstones below.
Silurian Dolomite Sponge Fossil, Door County Wisconsin (Credit: DNR)
Cambrian-Ordovician Sandstones
The Cambrian-Ordovician sandstone aquifer is actually a series of interbedded sandstones, shales, limestones, and dolomites. But groundwater primarily flows through the sandstone units. This aquifer stretches across the upper Midwest, from Minnesota and Iowa to Michigan’s Lower Peninsula. The rocks are slightly tilted that are at the land surface in southern Wisconsin are over 15,000 feet below ground in central Michigan.
The Cambrian-Ordovician rocks were deposited between 600 and 425 million years ago when the current day central continental US was covered by a shallow sea.
Leaping the Chasm at Stand Rock, Cambrian Sandstone, 1886, Sauk County, Wisconsin (Credit: H.H. Bennett)
Precambrian Bedrock
The Precambrian Bedrock of Wisconsin is composed of various rock types formed during the Precambrian Era, which lasted from the time the Earth cooled more than 4 billion years ago, until about 600 million years ago.
During this lengthy period: sediments, some of which were rich in iron and now form iron ores, were deposited in ancient oceans; volcanoes spewed forth ash and lava; mountains were built and destroyed, and molten rocks from the Earth's core flowed up through cracks in the upper crust. These are the "basement" rocks that underlie the entire state of Wisconsin.
The Precambrian Bedrock located in north part of Bayfield and Douglas counties is Precambrian Sandstone dating back to 1.1 billion years ago. This Precambrian Sandstone can be viewed in and around the Apostle Islands National Lakeshore.
Precambrian Crystalline Bedrock
The Precambrian Crystalline rocks remaining today have a texture of interlocking mineral crystals, meaning groundwater does not flow through pores in these rocks. Instead, groundwater flows through cracks and fractures.
Some cracks and fractures are found within this formation and store water. The cracks and fractures storing and transmitting water in these dense rocks are not spaced uniformly. Some areas contain numerous fractures while others contain very few. To obtain water, a well must intersect some of these cracks; the amount of water available to a well can vary within a single home site. The crystalline bedrock aquifer often cannot provide adequate quantities of water for larger municipalities, large dairy herds, or industries.
Many wells in the Crystalline Bedrock aquifer have provided clean water. However, most of these wells do not penetrate deeply into the rock. Water samples from deep mineral exploration holes near Crandon, Wisconsin, and deep iron mines near Hurley, have yielded brackish (salty) water.
Devil's Doorway, Precambrian Quartzite, Sauk County, Wisconsin (Credit: DNR)