Identification and characterization of springs in west-central Wisconsin

The objectives of this investigation were to locate springs within St. Croix County, to measure the discharge rate of each of these springs, and to estimate the recharge area for springs with discharge rates greater than or equal to 1.0 cubic foot per second (cfs). Eighty-seven springs were identified during this investigation, and 12 of these springs had a discharge rate greater than 1.0 cfs. Springs were found in each of the sedimentary rock groups younger than Precambrian age, but the Cambrian sandstone units had the greatest number of springs per unit area. Springs discharging at apparent interfaces between Cambrian rock units were especially common. A large number of springs were also found in the dolomitic Prairie du Chien Group, but since this unit is the uppermost bedrock for a large percentage of the county, the spring density was not especially high. The Prairie du Chien Group was significant for producing several of the larger springs, including the four springs with the highest discharge rates (≥ 2.0 cfs).

Conceptual models of groundwater flow were created to provide information about potential flowpaths to the springs located during this study. To accurately characterize groundwater tlowpaths, methods such as dye tracing and monitoring networks of piezometers are necessary; these methods were beyond the scope of this investigation. Instead, analyses of water chemistry data and geological information were used to infer likely groundwater flow mechanisms. For the springs in St. Croix County, the literature review and some chemical data suggested that flow through fractures or dissolution channels was probable. However, other chemical analyses, especially age dating of the spring discharge, showed that a significant portion of the water discharging from springs probably flowed as percolation through porous media. Groundwater residence times ranged from 22 to 35 years, showing that at least part of the spring recharge flowed through porous media or along deeper flowpaths within bedrock units. Residence time estimates probably reflect a mixture of water discharging from deeper flowpaths (older water) and water flowing through unconsolidated sediments (younger water), so wells extracting groundwater from either bedrock or soil units are likely to affect spring discharge.

For each spring with a discharge rate greater than 1.0 cfs, an estimated recharge area was defined. The estimated recharge area was based upon an iterative process of infiltration modeling and flow path prediction using maps of groundwater elevation. Each recharge area defined using this method represents only the area needed for groundwater infiltration/recharge to approximately equal spring discharge. Most likely, some groundwater flows to the spring from an area outside the delineated recharge zone, but a more definitive recharge model could not be generated with the data currently available. Readers are encouraged to remember that the recharge areas shown in this report should be viewed as minimum areas needed to protect springs from groundwater withdrawal, and groundwater extraction outside the recharge area boundaries may still adversely affect springs.