747-759 GW S-O 04

Springs found in former lakebed wetlands in the glaciated Upper Midwest often provide a consistent source of mineralized water needed to sustain threatened wetland plant communities. For example, they are commonly associated with calcareous fens, one of the rarest wetland plant communities in North America (Eggers and Reed 1997). Flow from some springs in this region is extremely steady and does not increase significantly after precipitation events or seasonal recharge (Figure 1) (Domber 2000; Swanson 2001; Anderson 2002). Such steady spring flow can contribute to the generally stable water levels that are characteristic of fens (Mitsch and Goselink 1993). Therefore, identifying hydrogeologic properties that contribute to steady spring flow and predicting the response of springs to stresses will enable us to better manage threatened communities and restore native biodiversity. Springs emanating from shallow flow systems, particularly in fractured-carbonate or karst environments, often exhibit highly variable discharge in response to individual recharge events, as well as to seasonal patterns of recharge (Atkinson 1977; Dreiss 1982; Avery et al. 1999). Springs with extremely steady rates of flow are not widely documented in the literature. Historically, Bryan (1919) explained steady spring flow (i.e., spring flow not subject to seasonal or shorter-term temporal fluctuations) by faults that create conduits for the discharge of what he called “deep-seated ground water. ” Halihan and Wicks (1998) show that a constricted passage in an aquifer containing large voids can generate a steady response in well or spring hydrographs. Recent work, summarized in this paper, suggests that shallow sandstone aquifers can also generate springs with steady flow, even in areas where seasonal or higher frequency recharge occurs. Steady flow in such a system can result from diffuse recharge through unlithified deposits or sandstone, followed by focused flow through thin, laterally extensive, high-permeability zones in sedimentary bedrock. To test this conceptual model, we used (1) an analytical model and (2) numerical models that are Abstract Flow from some springs in former glacial lakebeds of the Upper Midwest is extremely steady throughout the year and does not increase significantly after precipitation events or seasonal recharge. Analytical and simplified numerical models of spring systems were used to determine whether preferential ground water flow through high-permeability features in shallow sandstone aquifers could produce typical values of spring discharge and the unusually steady rates of spring flow. The analytical model is based on a one-dimensional solution for periodic ground water flow. Solutions to this model suggest that it is unlikely that a periodic forcing due to seasonal variations in areal recharge would propagate to springs in a setting where high-permeability features exist. The analytical model shows that the effective length of the aquifer, or the length of flowpaths to a spring, and the total transmissivity of the aquifer have the greatest potential to impact the nature of spring flow in this setting. The numerical models show that highpermeability features can influence the magnitude of spring flow and the results demonstrate that the lengths of ground water flowpaths increase when high-permeability features are explicitly modeled, thus decreasing the likelihood for temporal variations in spring flow.