Dye Tracing to Camp Coldwater Spring, Minneapolis, MN

Abstract

Camp Coldwater Spring is a ~6.3 liter/sec spring that emerges from a Platteville Limestone ledge at the top of the west side of the Mississippi River gorge. It was the original water supply for Ft. Snelling in the early-mid 1800’s and is a registered Minnesota State Landmark. Potential impacts from nearby highway construction led to two successful dye traces to help define the groundwater basin feeding the spring. These traces are the first traces through the Platteville in the Twin Cities. Dye input trenches were dug with a backhoe to the top of the water table. Input B reached the top of the Platteville and the water table was a few inches above the bedrock surface. Eosin dye input into the trench reached Camp Coldwater Spring, 125 meters away in less than 1.5 hours. The minimum flow velocity in the fractured Platteville Limestone was 83 m/hr. Input C reached the water table while still in glaciofluvial sediments and was 305 meters from the spring. Fluorescein dye from Input C reached the spring in 16 days. The slower flow velocity is a combination of flow through the glaciofluvial sediments and through the fractured Platteville Limestone. These two positive traces demonstrate that Inputs B and C are inside the ground-watershed that supplies the Spring and support concerns about the potential impact of dewatering and construction activities on the Spring. The trace is ongoing.