Groundwater Tracing in Orion, Marion and Eyota Townships of Olmsted County, Minnesota

Abstract

Orion, Marion, Eyota and Pleasant Grove Townships are located in Olmsted County southeast of Rochester, Minnesota and host a significant number of mapped karst features including the Orion Sinkhole Plain. The area is intensively farmed and is increasingly residential as part of Rochester’s exurbs. The area bedrock is composed of a relatively flat upland underlain by limestone, dolomite, and silty limestone of the Middle Ordovician Stewartville, Prosser and Cummingsville Formations. The carbonates are underlain by the Decorah Shale, which forms a partial aquitard; though the Decorah is truncated by incised stream valleys. Numerous karst springs emerge just above the Decorah and form tributaries which flow to the Zumbro and North Branch Root Rivers. These regional base level rivers are incised into the St. Peter Formation and Prairie du Chien Group. Thin, discontinuous glacial sediments and loess overlie the bedrock. Groundwater is the sole source of potable water in the area.

Groundwater tracing research was initiated in the area due to environmental issues related to the presence of confined animal feedlots. Dye tracing has been successfully used to define connections between water running into sinkholes or losing streams and water emerging from springs, wells or along gaining reaches of streams in the karst of southeast Minnesota for 75 years. Dye traces have proven to be singularly effective tools in helping elevate discussions among communities, landowners and farmers about surface water-groundwater interactions. Information gained helps landowners implement strategies to minimize nutrient and bacteria losses into vulnerable karst aquifers.

We initiated nine traces between 2014 and 2016. Eight of the traces were positive, in that at least one connection was confirmed between the sinkhole input of the dye and a monitored spring or creek. The positive results are drawn as springshed maps illustrating these connections, and are overlain on a base map of the previous dye tracing work. The base springsheds map will be updated as new traces are completed and the map is available on-line at: http://files.dnr.state.mn.us/waters/groundwater_section/mapping/springshed/springshed_map.pdf