Abstract
The Galena Group is mostly composed of carbonate rock with subordinate beds of fine siliciclastics, mainly shale. In southeastern Minnesota, it is a source of water for domestic and agricultural use. Hydrologic and geochemical characterization of a Galena Group spring, Bear Spring, began in 1979. Recent springshed delineation through dye tracing and continuous level, temperature, and nitrate monitoring has shed light on the aquifer’s hydrologic response to precipitation, but the degree of karstification in the Galena was unclear. Current and planned future modeling require additional insight into the pervasiveness of fractures and conduits, and their properties. A suite of characterization approaches, including karst mapping, lineament delineation, 2-D Electrical Resistivity Imaging (ERI), and quarry observations, was used to provide an enhanced 3-D view of the Galena Karst. Sinkhole and cave mapping determined that the conduits are bell-shaped and range from approximately 0.5 to 3.5 meters wide to 0.8 meters high. Two-dimensional ERI revealed distinct differences in resistivity signals consistent with clayey overburden, unsaturated karst, and saturated carbonate rock. Drought conditions allowed characterization of the karst fabric using a previously reported technique, crop line delineation. Relatively healthy green-hued vegetation growing in gaps was distinctly different than adjacent crops, allowing for GIS analysis of fracture line trends. Trends are dominated by northeast/southwest and northwest/southeast orientations. Observations from a quarry 2.6 kilometers north provide further context into the depth dimension and pervasiveness of fracturing, with centimeter to decimeter apertures penetrating a minimum distance of 7.3 meters from the bedrock surface to the quarry floor. This characterization of karst in the Bear Spring vicinity better constrains fracture orientations, penetration depths, and conduit size and shape to use in future modeling of the aquifer.
Rights Information
DOI
https://doi.org/10.5038/9781967518012.1009
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Characterization of the Galena Karst of East-Central Olmsted County, Minnesota
The Galena Group is mostly composed of carbonate rock with subordinate beds of fine siliciclastics, mainly shale. In southeastern Minnesota, it is a source of water for domestic and agricultural use. Hydrologic and geochemical characterization of a Galena Group spring, Bear Spring, began in 1979. Recent springshed delineation through dye tracing and continuous level, temperature, and nitrate monitoring has shed light on the aquifer’s hydrologic response to precipitation, but the degree of karstification in the Galena was unclear. Current and planned future modeling require additional insight into the pervasiveness of fractures and conduits, and their properties. A suite of characterization approaches, including karst mapping, lineament delineation, 2-D Electrical Resistivity Imaging (ERI), and quarry observations, was used to provide an enhanced 3-D view of the Galena Karst. Sinkhole and cave mapping determined that the conduits are bell-shaped and range from approximately 0.5 to 3.5 meters wide to 0.8 meters high. Two-dimensional ERI revealed distinct differences in resistivity signals consistent with clayey overburden, unsaturated karst, and saturated carbonate rock. Drought conditions allowed characterization of the karst fabric using a previously reported technique, crop line delineation. Relatively healthy green-hued vegetation growing in gaps was distinctly different than adjacent crops, allowing for GIS analysis of fracture line trends. Trends are dominated by northeast/southwest and northwest/southeast orientations. Observations from a quarry 2.6 kilometers north provide further context into the depth dimension and pervasiveness of fracturing, with centimeter to decimeter apertures penetrating a minimum distance of 7.3 meters from the bedrock surface to the quarry floor. This characterization of karst in the Bear Spring vicinity better constrains fracture orientations, penetration depths, and conduit size and shape to use in future modeling of the aquifer.