In Wisconsin, we enjoy kayaking our rivers, cooling off in our lakes, and bragging about our beautiful waters. Many residents know a great deal about their local lakes, rivers, and waterways — but how much do they know about the water under their feet?
Two out of three people in Wisconsin get their drinking water from groundwater – the water that moves deep underground within rock and sediment. Hydrogeologists at the Wisconsin Geological and Natural History Survey (WGNHS) study this subsurface water to support the management and protection of this critical resource.
If you want the scoop on what’s going on down below, here’s a simple look at how water moves underground in Wisconsin.
What does water do under the ground?
Groundwater starts as rain and snow. When precipitation lands, some stays at the surface and runs into streams and lakes. The rest sinks into the soil and travels deeper into the empty spaces or pores between particles of sediment or rock. Imagine spilling your drink at the beach and watching it sink into the sand. The liquid moved between the sand grains, almost like water into a sponge. This same phenomenon happens with rock, but some rock layers have more pore space than others. That is why hydrogeologists need to understand the geology across the state!
The precipitation will continue to move deeper into the rock or sediment, squeezing through any open spaces. It will eventually reach a depth where all the pores and cracks are completely filled with water, which hydrogeologists call the saturated zone.
The top of the saturated zone — where air-filled pores change to water-filled pores — is called the water table. To pump groundwater to the surface and eventually to our faucets, a well needs to cross the water table and reach into the saturated zone. Then, groundwater will flow pore by pore into the well. Remember finding water while digging a hole at the beach? That’s the water table, where it is very shallow.
How fast does groundwater flow?
Groundwater is always on the move, but very slowly.
In most parts of Wisconsin, groundwater moves only a few feet each day through the microscopic pore spaces. However, water can move much faster (more than 100 feet per day!) in areas with cracked or fractured bedrock. Mapping the geology of the state helps to identify these regions.
How does the water get to my well?
The sediment and rock layers that store and yield usable amounts of groundwater are called aquifers. Geologists and hydrologists in Wisconsin work to map out where different aquifers are in the state, and how deep the water table is underground.
How does a well get contaminated?
There are a few ways that groundwater can be contaminated, even though it is typically very deep underground. Some contaminants come from the surface, and some from the rocks themselves. It depends on the geology!
At times, the rock will have certain minerals that can add elements like iron to groundwater. In addition, the “hard” water we experience in southern Wisconsin is due to the carbonate, or limestone, rocks in the main aquifer.
Surface contamination can reach the aquifer when any pollutants at the surface travel quickly underground. In areas with highly fractured bedrock – such as limestone or dolomite – surface water reaches deep into the ground very fast. Similarly, in areas with a lot of sand at the surface – sandstone or sediment – the large pore spaces allow surface water to soak in quickly.
The rock layers holding our groundwater shape the amount and quality of drinking water, and these layers differ around the state. This is why hydrogeologists work hard to figure out what aquifers are available in different areas, and how susceptible it is to contamination.
What does this mean for me?
We can’t change the rock, but what we do on the land, such as farming, using septic systems, or construction, can increase the risk of water quality issues underground. We all rely on groundwater, even though we don’t see it.
Private well owners are tasked with managing their own well water and that can be challenging when it comes to knowing where to start, and what to test for. The Natural Resources Institute offers resources to help well owners manage their private well systems.
You can also explore the Wisconsin Geological and Natural History Survey’s catalog of groundwater maps and reports to learn more about groundwater in your area.