DYE TRACING SEWAGE LAGOON DISCHARGE IN A SANDSTONE KARST, ASKOV, MINNESOTA

Shade (2002) documented an active karst region developed on the Mesoproterozoic Hinckley Sandstone in central Pine County, Minnesota and alerted resource managers to groundwater concerns in a previously unrecognised karst. Much of Shade′s fieldwork was conducted on public land in Banning State Park, Department of Natural Resources Forest Land and Askov City Property. Askov has been under a moratorium on new sewer hook-ups for several years because of limited capacity in their 40 year-old Waste Water Treatment Facility lagoon. Effluent that periodically discharges from the lagoons sinks into ″The Big Sinkhole″ lake about one kilometer down stream. Several sinkholes exist immediately adjacent to the lagoons. These conditions are helping Askov′s ongoing efforts to secure public funding for a new WWTF. The local karst hydrogeology limited the options available for additional sewage treatment but the added risk of groundwater contamination to local water supply wells has significantly increased Askov′s priority and visibility for public funding. As part of a receptor survey of private wells near the Askov WWTF, two fluorescent dye traces were initiated on 23 April 2004. Rhodamine WT was added to the approved discharge of lagoon effluent to the adjacent sinking stream and fluorescein was injected into a sinkhole immediately west of the lagoon. RhWT was first detected in a private well about a mile southwest from the terminal sinkhole lake three weeks later. As of this writing, RhWT is still present in that well, has been confirmed in a second private well, and has been detected two more wells. The fluorescein was detected in less than three days in three monitoring wells installed northeast of the sinkhole through the lagoon dikes and is still strongly present in two monitoring wells. In late May the fluorescein was confirmed in a private well between the lagoon and sinkhole lake, west of the sinkhole, and has been detected in two additional private wells. The implied groundwater flow velocities and dispersion patterns are much faster and completely incompatible with porous media models but are slower than would be expected from highly integrated conduit flow.

Shade, Beverly L. (2002) The Genesis and Hydrogeology of a Sandstone Karst in Pine County, Minnesota, Univ. of Minn. Masters Thesis, 171 p.