Karst conduit flow in the Cambrian St. Lawrence Confining Unit, southeast Minnesota, USA

Abstract

Southeastern Minnesota’s karst lands support numerous trout streams created by Paleozoic bedrock springs. Several of the Paleozoic bedrock units are recognized as karst aquifers. Recent field investigations have discovered sinking, and one losing, streams in the Cambrian St. Lawrence Formation. The siliciclastic-dominated St. Lawrence Formation has historically been viewed as a confining unit and is designated as such in the Minnesota Water Well Code. Stream sinks in the St. Lawrence Formation have been identified in five different streams, and there are numerous other streams crossing the St. Lawrence that have yet to be investigated. Dye traces have been conducted on the five streams. At all of the sites, dye was recovered at springs emanating from the base of the St. Lawrence Formation. The initial dye breakthrough occurs quickly, but the breakthrough curves have tails that continue for months to over a year. Breakthrough travel velocities from the sinks to the springs are 35–750 m/day. The five dye tracing sites are geographically separated by 65 km. The subcrop of the St. Lawrence Formation is found throughout southeastern Minnesota and southwestern Wisconsin. The St. Lawrence Formation has well-developed macroporosity; most of the formation is moderately to well cemented with dolomite and contains abundant horizontal and vertical fractures in outcrop and subcrop conditions. Volumetrically minor dolomite beds with centimeter-scale solution cavities are common in the lowermost part of the formation. These attributes, combined with rapid groundwater flow as determined by dye tracing, supports the conclusion that this purported confining unit has karst conduit flow properties in subcrop conditions.