| Springs in the Nine Springs watershed near Madison, Wisconsin contribute a consistent source of water to remnant, but locally-diverse, sedge meadows and fens while groundwater-fed wetlands throughout much of the watershed have been degraded by the effects of agricultural practices and recent urban expansion. The springs discharge water at rates of up to one cubic feet per second and typically show little or no response to precipitation and/or seasonal groundwater recharge events. This research aimed to test conceptual models of the hydrogeology that contributes to the abundance of springs in the region and their unique flow characteristics using geochemistry, field-based hydrologic measurements, and numerical modeling approaches. Results of the research suggest that springs may develop where laterally-extensive, high-permeability zones due to bedding plane partings or conglomeratic beds in the Tunnel City Group sandstones are intersected by buried erosional bedrock valleys. The spring flow is vulnerable to the decrease in groundwater recharge that often accompanies urbanization; however, spring flow may not be immediately impacted by proposed increases in municipal groundwater withdrawals.; Major ion geochemistry in association with age dating by the tritium/helium 3 method initially constrained spring source waters and called for more detailed characterization of heterogeneities within the regionally-defined unlithified and upper bedrock aquifers as opposed to a lower confined aquifer. High-permeability zones were identified in the shallow bedrock aquifer (Tunnel City Group sandstones) through the use of borehole geophysics in association with short-interval packer tests. The zones appear to be continuous across the Nine Springs watershed and may be laterally-extensive for tens of miles.; Generic numerical and analytical models evaluated the effects of high-permeability features in the shallow bedrock and/or a heterogeneous unlithified aquifer on the magnitude and steady nature of spring flow. Numerical models of a hypothetical spring system adequately simulate the steady nature of flow under transient recharge conditions; however, only those models that include thin, high-hydraulic conductivity layers simulate a reasonable magnitude of spring flow. A one-dimensional analytical solution for periodic groundwater flow through an unconfined, equivalent-stratified aquifer suggests that under the assumed hydrogeologic conditions, measurable fluctuations in spring flow are not expected. |
