Hydrogeology of the Wisconsin River Valley in Marathon County, Wisconsin

A description and discussion of the hydrogeologic framework, history, groundwater movement, and potential for groundwater contamination in the Wisconsin River valley in Marathon County, Wisconsin.

Abstract: The sand-and-gravel deposits that fill the lower part of the Wisconsin River valley form the most important aquifer in Marathon County, called the Wausau aquifer in this report. This unconfined aquifer is the source of nearly all drinking, irrigation, and industrial water supplied to residents and industries in the valley near the Wisconsin River. Responsible management of the aquifer is therefore critical. Proper management requires a thorough understanding of aquifer characteristics and the groundwater flow system.

In this report we define the basic hydrogeology of the aquifer. Basic resource data have been collected and generated to apply to immediate management problems and future system modeling. These data include summary compilations of historical pumpage and other water-resource developments, hydrogeologic properties of the aquifer, and records of wells constructed in the area. We constructed maps of the water-table elevation, elevation of the base of the aquifer, and attenuation potential of soil. A steady-state model calibration verified that the data presented in this report are sufficient to construct a two-dimensional groundwater flow model at a regional scale.

The Wausau aquifer is bounded by hilly crystalline bedrock and irregularly distributed clay deposits. Pumping tests, piezometer tests, grain-size analyses, and well constructor’s reports demonstrate that the aquifer is extremely heterogeneous. Aquifer thickness ranges from about 160 ft in the north to 40 ft in the south, and hydraulic conductivity exhibits a trend ranging from about 10^-3 ft/s (10^-1 cm/s) in the north to 10^-5 ft/s (10^-3 cm/s) in the south.

Groundwater flow is generally southward and toward the Wisconsin River and its tributaries. Local exceptions that result from groundwater pumping can induce aquifer recharge from the river.

Because the soils overlying the Wausau aquifer provide little attenuation of contaminants, and because the sand-and-gravel deposits that compose the aquifer are extremely permeable, the Wausau aquifer is susceptible to contamination from surface sources. The heterogeneity of the aquifer material and the irregularity of its boundaries make site-specific investigations particularly difficult.