A REFINED GROUNDWATER FLOW MODEL FOR THE NINE SPRINGS AREA NEAR MADISON, WISCONSIN

A refined groundwater flow model was developed for the Nine Springs area in southern Wisconsin. The overall modeling objective was to quantify past and potential land-use and groundwater withdrawal impacts on spring flow in the Nine Springs watershed, which is located on the urban fringe of the Madison metropolitan area. This information is essential in order to minimize further adverse environmental impacts to groundwater-fed wetlands in the watershed. The springs and their associated wetlands were not explicitly incorporated into a regional groundwater flow model for Dane County, Wisconsin (Krohelski et al., 2000). However, the regional model provided boundary conditions and starting parameter estimates for a Nine Springs inset model, which was able to more accurately represent local hydrologic features while maintaining a linkage between the regional flow system and the area of interest. Hydrogeologic characterization of shallow bedrock in the region suggested that thin, laterally-extensive, high-permeability zones may exist in the locally-defined Tunnel City aquifer and preferentially focus groundwater flow to springs at the margins of buried erosional bedrock valleys. Therefore, another modeling objective was to test whether this refined hydrostratigraphy and conceptual model can explain observed spring flow conditions in the Nine Springs watershed. The refined, steady-state model calibrated to average hydraulic head, spring flow, and stream flow conditions. Results of simulations suggested that spring flow is vulnerable to decreases in direct groundwater recharge that are expected to accompany continued urbanization. However, projected increases in municipal groundwater withdrawals may not have an immediate effect on spring flow in this watershed.