COLDWATER SPRING, MISSISSIPPI NATIONAL RIVER AND RECREATION AREA: A WINDOW INTO A HUMAN IMPACTED SHALLOW URBAN AQUIFER

Coldwater Spring is located on the west edge of the Mississippi River Gorge in Minneapolis, MN and became part of the NPS Mississippi National River & Recreation Area in January 2010. The spring was the water supply for Fort Snelling from the 1820s until the 1920s. Between the 1950s and 2010 the site was occupied by the Bureau of Mines Twin Cities Research Center. Coldwater Spring has been declared a sacred site by some federally recognized Native American tribes. Coldwater is a gravity flow spring emanating from the near-horizontal Ordovician Platteville Limestone which is the first bedrock under southeastern Minneapolis. The spring is a window into groundwater flow and quality in a heavily urbanized area of a major city.

In addition to Coldwater Spring, a nearby spring fed-wetland and one major and two minor artificial culverts drain groundwater from the site. Recent work has demonstrated that the Coldwater Spring House itself is fed by at least three conduits whose water chemistry and temperatures are different. This is a complex, significantly altered system.

This presentation presents the results of ongoing efforts to determine and manage the human impacts on the water table aquifer that feeds Coldwater Spring. A few hundred meters from the spring two major four lane highways intersect and receive significant road deicing salt during Minnesota winters. The chloride levels in Coldwater have risen from ~ 4 ppm in 1880 (Maguire, 1880) to concentrations between 310 ppm in March 2013 and 410 ppm in January 2015 with superimposed annual peaks of about an additional 45 ppm between April and July of 2013 and 2014. Nicollet (1845) reported the temperature of Coldwater Spring as 7.8 °C in July 1836 and 7.5 °C in the winter of 1836-1837. Temperatures in the spring fluctuated sinusoidally between 10.7 and 13.1 °C in 2013 and 2014. This is a 3 to 5 °C increase in the water temperature since 1837. The lowest temperatures correlate with the April to July annual chloride peaks. It appears to take a few months for the cold salty road salt melt to reach the spring.

The water quality of Coldwater Spring has clearly been heavily impacted by human activities. The available data is not adequate to determine if water flow from the spring has been affected by surface and subsurface development in the area but that possibility is an ongoing concern.