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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

August 1999

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Understanding the resource

Groundwater on the move
Wisconsin's aquifers
Sand and gravel aquifer
Eastern dolomite aquifer
Sandstone and dolomite aquifer
Crystalline bedrock aquifer

Complete table of contents

"The idea that there are vast subterranean channels or caverns in which artesian waters flow like a river has been long since abandoned. These are matters of common scientific knowledge."
T.C. Chamberlin, "Geology of Wisconsin, Vol. 1," 1885

Water might be called our most recycled resource. The water you showered in this morning, for example, may have contained the same water molecules that caused a dinosaur hide to glisten in the prehistoric sun or carried the Nina, Pinta and Santa Maria across the Atlantic. The distribution of the earth's total supply of water changes in time and space, but the quantity has remained constant.

Wisconsin receives an average 30 to 32 inches of precipitation per year. Seventy-five percent evaporates or transpires through plants and never reaches surface water or groundwater. The six to 10 inches that do not evaporate immediately or get used by plants run off into surface waters or soak into the ground, depending on local topography, soil, land use and vegetation. For every inch of water that runs off the land to a stream or lake in gently rolling Dane County, three inches seep down to the water table. In the sandy plains of Portage County, nine inches are able to seep into the ground for each inch running off the land.

Water cycle illustration © Moonlit Ink

Water distribution is governed by a phenomenon known as the hydrologic, or water cycle, which is kept in motion by solar energy and gravity. Start with a spring shower. As the rain falls to earth, some flows downhill as runoff into a stream, lake or ocean. Some evaporates; some is taken up by plants. The rest trickles down through surface soil and rock. This water eventually reaches the water table – the top of a saturated zone of soil or rock, which is called an aquifer. The water contained in the aquifer is groundwater. Groundwater is discharged in wetlands, lakes and streams – the low places where the water table meets the land surface. The sun causes evaporation from these surface waters, and, as water vapor accumulates in the atmosphere and clouds begin to form, the water cycle begins anew.

Groundwater on the move

Geology controls the rate of groundwater movement. The size of the cracks in rocks, the size of the pores between soil and rock particles, and whether the pores are connected determine the rate at which water moves into, through and out of the aquifer.

Water generally moves more quickly in coarse sand, sometimes as much as several feet per day. Openings between the grains are large and interconnected, resulting in high permeability. Very fine-grained material like clay has many pores where water can be stored, but the pores are so small that moving water through or out is difficult. Clay formations are relatively impermeable – water may move only a few inches a year. Permeability in limestone, on the other hand, primarily depends not on pore spaces, but on the size, frequency and distribution of fractures and cracks.

Groundwater is always moving toward a surface outlet or "discharge" area, following the slope of the water table. In Wisconsin, the natural movement is from upland recharge areas (those places where rain or melt water infiltrates the ground and reaches the water table) to lowland discharge areas. Most precipitation seeping into the soil moves only a few miles to the point where it is discharged; in the vast majority of cases, it stays within the same watershed.

Groundwater baseflow replenishes rivers and streams.

© Don Blegen
Groundwater baseflow replenishes rivers and streams. © Don Blegen

Perhaps you've wondered why some streams continue to flow during dry periods and in winter, when there's no rainfall. Winter stream flow is largely groundwater discharge (called baseflow) that remains at a relatively constant temperature year 'round – about 50° F. Streams, and most lakes and wetlands, are constantly replenished during the winter by groundwater from the surrounding uplands. That same 50° F groundwater baseflow is the reason streams stay icy cold in the summer.

Wisconsin's aquifers

Wisconsin's aquifers. © Jeanne Gomoll

A rock or soil formation that can store or transmit water efficiently is called an aquifer. Wisconsin's groundwater reserves are held in four principal aquifers: the sand and gravel aquifer, the eastern dolomite aquifer, the sandstone and dolomite aquifer, and the crystalline bedrock aquifer.

Sand and gravel aquifer. © Jeanne Gomoll

Sand and gravel aquifer

The sand and gravel aquifer is the surface material covering most of the state except for parts of southwest Wisconsin. It is made up mostly of sand and gravel deposited from glacial ice or in river floodplains. The glacial deposits are loose, so they're often referred to as soil – but they include much more than just a few feet of topsoil. These deposits are more than 300 feet thick in some places in Wisconsin.

The glaciers, formed by the continuous accumulation of snow, played an interesting role in Wisconsin's geology. The 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." Pits were formed in the outwash where buried blocks of ice melted; many of these pits are now lakes. The sand and gravel aquifer was deposited within the past million years.

The sand and gravel outwash plains now form some of the best aquifers in Wisconsin. Many of the irrigated agricultural lands in central, southern and northwestern Wisconsin use the glacial outwash aquifer. Other glacial deposits are also useful aquifers, but 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.

Because the top of the sand and gravel aquifer is also the land surface for most of Wisconsin, it is highly susceptible to human-induced and naturally occurring pollutants.

Eastern dolomite aquifer. © Jeanne Gomoll

Eastern dolomite aquifer

The eastern dolomite aquifer occurs in eastern Wisconsin from Door County to the Wisconsin-Illinois border. It consists of Niagara dolomite underlain by Maquoketa shale.

These rock formations were deposited 400 to 425 million years ago. Dolomite is a rock similar to limestone; it holds groundwater in interconnected cracks and pores. 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.

Where the fractured dolomite bedrock occurs at or near the land surface, the groundwater in shallow portions of the eastern dolomite aquifer can easily become contaminated. In those areas (such as parts of Door, Kewaunee and Manitowoc counties), there is little soil to filter pollutants carried or leached by precipitation. Little or no filtration takes place once the water reaches large fractures in the dolomite. This has resulted in some groundwater quality problems, such as bacterial contamination from human and animal wastes. Special care is necessary to prevent pollution.

The Maquoketa shale layer beneath the dolomite was formed from clay that doesn't transmit water easily. Therefore, it is important not as a major water source, but as a barrier or shield between the eastern dolomite aquifer and the sandstone and dolomite aquifer.

Sandstone and dolomite aquifer. © Jeanne Gomoll

Sandstone and dolomite aquifer

Sandstone. © Tom Riewe

In sandstone aquifers, water can be found in the spaces between sand grains. © Tom Riewe

The sandstone and dolomite aquifer consists of layers of sandstone and dolomite bedrock that vary greatly in their water-yielding properties. In dolomite, groundwater mainly occurs in fractures. In sandstone, water occurs in pore spaces between loosely cemented sand grains. These formations can be found over the entire state, except in the north central portion.

In eastern Wisconsin, this aquifer lies below the eastern dolomite aquifer and the Maquoketa shale layer. In other areas, it lies beneath the sand and gravel aquifer. These rock types gently dip to the east, south and west, away from north central Wisconsin, becoming much thicker and extending to greater depths below the land surface in the southern part of the state.

The rock formations that make up the sandstone and dolomite aquifer were deposited between 425 and 600 million years ago. The sandstone and dolomite aquifer is the principal bedrock aquifer for the southern and western portions of the state. In eastern Wisconsin, most users of substantial quantities of groundwater, such as cities and industries, tap this deep aquifer to obtain a sufficient amount of water.

Crystalline bedrock aquifer. © Jeanne Gomoll

Crystalline bedrock aquifer

Granite bedrock. Cracks in layers of this dense stone can supply wells, but the quantity and quality of the water is not uniform. © Tom Riewe

Granite bedrock. Cracks in layers of this dense stone can supply wells, but the quantity and quality of the water is not uniform. © Tom Riewe

The crystalline bedrock aquifer is composed of a variety of rock types formed during the Precambrian Era, which lasted from the time the earth cooled more than 4,000 million years ago, until about 600 million years ago, when the rocks in the sandstone and dolomite aquifer began to be formed. 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.

The rocks that remain today have a granite-type crystalline structure. These are the "basement" rocks that underlie the entire state. In the north central region, they are the only rocks occurring beneath the sand and gravel aquifer.

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 homesite. The crystalline bedrock aquifer often cannot provide adequate quantities of water for larger municipalities, large dairy herd, or industries.

Many wells in the crystalline bedrock aquifer have provided good water. However, most of these wells do not penetrate deeply into the rock. Water samples from deep mineral exploration holes near Crandon and deep iron mines near Hurley have yielded brackish water.