Surface water can soak into the ground and become groundwater. As water infiltrates into soil, it can be consumed by biological organisms, held in an unsaturated zone, or continue percolating down to the water table, reaching the saturated aquifer. Regionally, there is an upper aquifer (above the shale Eau Claire layer) and a lower aquifer (Mount Simon layer made of sandstone). Private domestic wells draw water from the upper aquifer, while municipal and other high capacity wells are deeper and draw from the lower aquifer. Withdrawals at all depths influence groundwater flow by redirecting water to replenish what is pumped out. Water within the aquifer flows towards locations of groundwater discharge, such as wells, springs, lakes and streams.
Recharge cycles surface water to groundwater where as discharge cycles groundwater back to surface water. This process is slower for deep versus shallow groundwater. Under natural conditions, regional aquifer flow generally reflects the surface topography above. Recharge occurs throughout the landscape but particularly at higher elevations. The top of the saturated aquifer is called the water table. From wherever recharge occurs, groundwater flows towards where the water table is lower. This results in groundwater discharging to replenish streams and other aquatic habitats. This continuous groundwater discharge, or baseflow, supports viable stream habitat during dry summer and cold winter months. Depth to the upper aquifer ranges from lake level to over 200 feet beneath the surface in the County's southwest. Water from the upper, unconsolidated aquifer is used by for shallow domestic wells.
The Mt. Simon aquifer is the most important aquifer in Dane County for high-capacity municipal wells. The aquifer ranges in thickness from about 100 feet to over 700 feet, with an average thickness of 500 feet. This aquifer consists of sandstone in the Mt. Simon and lower Eau Claire Formations. The lower boundary of the aquifer is Precambrian granite which is located about 1,0000 feet below the land 's surface. Impermeable shale in the Eau Claire formation forms the upper boundary. This layer is thicker in the western portion of the county. Because the Mt. Simon aquifer is bounded by relatively impermeable layers, it is called a confined aquifer and its contents can be under pressure. The "surface" of a confined aquifer is called the potentiometric surface which represents the height water would reach if a well were to be drilled into the formation. Direction and rate of flow within the confined aquifer depends on variation in this potentiometric surface. Flows go towards areas with lower surfaces.
Groundwater withdrawals for drinking water alters the speed of cycling between surface and ground water. Groundwater pumped from deep municipal wells is discharged as surface water after it is used and treated. This rapidly transports groundwater to the surface, without ensuring comparable recharge. Groundwater pumped from shallow domestic wells is generally treated by onsite septic systems after use and has a greater likelihood of replenishing groundwater.
The amount of precipitation that is able to infiltrate and recharge groundwater is influenced by land cover, slope, and soil type. The majority of Dane County has a recharge rate of 9 to 10 inches of water per year. The maps below the current estimated recharge rate and groundwater divides (groundwater flows away from divides as show by the arrows.
Recharge Rates and Groundwater Divides
- Groundwater divides generally follow surface watershed boundaries but explore their slight differences on the map below
- Click the "Layer List" icon to toggle layers off
- Click the "Legend" icon to see values for groundwater recharge rates
Hydrogeology of Dane County, Wisconsin (1999) This report describes and models regional groundwater conditions
Groundwater Recharge in Dane County, Wisconsin: Estimate Recharge Using a GIS-Based Water Balance Model (2012) This report quantifies recharge across the county
Groundwater Flow Model for Dane County (2016) A printed report and MODFLOW model to describe groundwater flow with 2010 typical withdrawals. This can be used as a decision-support tool to:
- Aid in the siting of new groundwater wells
- Improve well-head protection plans for water utilities and municipalities
- Assess short and long-term responses to climate change
- Determine the impacts of land-use change on groundwater resources
- Evaluate well and groundwater quality problems
- Develop more detailed groundwater models for site-specific evaluations
USGS National Water Information System (NWIS) Mapper: groundwater and spring monitoring data
Wisconsin Water Quantity Data Viewer: locations of USGS water quantity monitoring, groundwater protection features, wells, and springs
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