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Publication Date

October 2014


Jeffrey A. Green 1, John D. Barry 1, and E. Calvin Alexander, Jr. 2 1Minnesota Department of Natural Resources 2Department of Earth Sciences, University of Minnesota Introduction: Springs are the natural discharge points for groundwater. They provide baseflow for streams and in the case of trout streams are critical sources of isothermal water. They are commonly found emerging from the Paleozoic sedimentary rocks of southeastern Minnesota where river valleys cut deeply through the water-bearing bedrock layers. The different lithology and hydraulic properties (hydrostratigraphy) of the rock types makes some settings more likely for springs. Our research on springs focused on areas with cold-water streams that support trout populations within the bedrock-dominated landscape of southeastern Minnesota (Figure 1). In that region, carbonate (limestone, dolostone) and carbonate-cemented sandstone rock layers dissolve in slightly acidic groundwater and have developed a system of conduits that allow water to be routed quickly through the enlarged passages (karst). Other units, while not exhibiting all of the characteristics of traditional carbonate karst, do share some of the key hydrologic properties. A springshed is defined as "those areas within ground- and surface-water basins that contribute to the discharge of a spring" (Florida Geological Survey, 2003). Precipitation falling on the surface usually infiltrates through the soil. Where karst features are present, surface water enters the groundwater more quickly through sinkholes and stream sinks, the point at which a surface stream sinks into the ground (Figure 2). The boundaries of groundwater springsheds do not necessarily correspond to those on the surface. They are dynamic, changing as groundwater levels rise and fall. In order to conserve and protect springs and the surface water bodies they supply, it is necessary to understand their geologic setting and where they derive their water. The University of Minnesota (U of M), the Minnesota Department of Natural Resources (DNR), and a group of experienced local cavers have been actively working on mapping springsheds in southeastern Minnesota for several decades. Funding from the Environment and Natural Resources Trust Fund (ENRTF) has allowed these researchers to accelerate and formalize efforts to delineate springsheds by injecting fluorescent organic dyes into sinkholes or sinking streams to determine the general flow path to springs. This time- and labor- intensive method has only been applied to a small portion of the known springs in southeastern Minnesota. However, if combined with an understanding of the geology, dye-tracing provides experienced geologists and hydrologists with a powerful tool for interpreting undelineated springsheds. This ultimately improves our ability to assess the vulnerability of springs to activities on the land surface. Spring vulnerability Our focus is on the sources of water to the uppermost bedrock springs because they are most susceptible to degradation in water quality, flow rate, and temperature. Furthermore, the uppermost bedrock aquifer is more amenable to springshed extent estimation than the deeper systems. High-volume water appropriations can disrupt or decrease groundwater flow to springs depending on the number of wells, their distance to the spring, the pumping rate of the well, and the hydrogeologic characteristics of the particular unit. Landscape alteration from mining operations and road construction can disrupt the flow of groundwater to a spring by intercepting it (Green and others, 2003). An increase in impervious surface area in a watershed increases runoff and decreases infiltration, affecting water quality and temperature with potentially detrimental effects on biota (Wang and others, 2003). Agricultural nonpoint-source pollution (fertilizers, herbicides, insecticides and runoff) and point-source pollution from discrete chemical releases can impact a spring's chemistry and quality. Additional information on the impacts of human activities on springs is presented by Drew and Hotzl (1999). Contents: Geologic background: Physiography Bedrock geology Glacial and recent geology Hydrostratigraphy Porosity Aquifers and aquitards Surface to groundwater flow Groundwater age Groundwater-to-surface-water flow paths (baseflow) Hydrostratigraphic observations by physiographic region Groundwater flow direction Methods Surface-watershed delineation Locating karst features Field characterization Dye tracing Results Mapping and approximating springsheds Example of springshed estimation for the St Lawrence-Tunnel City aquifers Potentiometric-surface mapping Dye tracing Influence of geologic structure Discussion Bedrock unit properties and charactaristics Devonian Lithograph City Formation Devonian Little Cedar Formation Devonian Spillville Formation and Ordovician Galena Group Ordovician St Peter Sandstone and Shakopee Formation, Oneota Dolomite of the Ordovician Prairie du Chien Group Cambrian Jordan Sandstone Cambrian St Lawrence Formation and Cambrian Reno Member-Tunnel City Group Open Access - Permission by Author(s) See Extended description for more information.


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