KARSTIC FLOW IN THE ST. LAWRENCE CONFINING UNIT

Southeastern Minnesota’s karst lands support numerous trout streams sourced by springs discharging from Paleozoic bedrock. Several of these bedrock units have been recognized as karst aquifers. Recently, field investigations have led to the discovery of sinking streams in the upper portion of the Cambrian St. Lawrence Formation. The St. Lawrence Formation has historically been viewed as a confining unit and in fact is recognized as such in the Minnesota Water Well Code. Stream sinks in the St. Lawrence Formation have been identified in four different streams. Dye tracing has been conducted at these four sites. At all sites, dye was recovered at springs emanating from the base of the St. Lawrence Formation. Travel velocities from the sinks to the springs are 100-300 meters/day. The exposed edge of the St. Lawrence Formation is found throughout southeastern Minnesota and extends into southwestern Wisconsin. The four dye tracing sites are geographically separated by over 60 Km which indicates that this is a regional phenomenon. The St. Lawrence Formation has significant dolostone beds; this, combined with rapid groundwater flow as determined by dye tracing, supports the conclusion that this so-called confining unit has a karstic flow component.