Dye Tracing Studies of the Fountain, Minnesota Sewage System

Abstract

Fountain, a small community in southeastern Minnesota, is located on a sinkhole plain developed in the Ordovician Galena Formation. Many of the approximately 100 houses in the town have sewer systems that empty directly into sinkholes. Qualitative dye traces using Fluorescein and a quantitative dye trace using Rhodamine WT indicate that effluent from the community's individual disposal systems resurges at a group of springs about a mile northwest of the community. These springs are located in the Galena aquifer which comprises the upper karst above the Decorah Shale aquitard. The travel time of the underground flow here is about one day. The citizens of Fountain are considering the construction of a community drainfield to alleviate the sewage disposal problem. The effluent from individual septic tanks would be collected and piped to a drainfield about two miles south of town. The proposed drainfield site is stratigraphically below the Decorah Shale in a valley underlain by limestones and dolomites of the Praire du Chien Group. Watson Creek, which flows through the valley, is a karst stream which loses water into the ground in the vicinity of the proposed site. The quantitative dye trace from the proposed site indicates that the water beneath it is moving southeast at a velocity of about 1.3 miles/year. Water in the lower karst aquifer (in the Prairie du Chien) is moving two to three orders of magnitude more slowly than in the upper karst aquifer (in the Galena). [Carbon-14] analyses of the water in the Prairie du Chien aquifer are consistent with the dye trace results and indicate residence times of less than 25 years. Flow in the upper karst aquifer is about 300 times faster than in the lower aquifer, and in the opposite direction.