Abstract
Coldwater Spring in Minneapolis, Minnesota was the water supply for Fort Snelling from the 1840s to 1920. The spring site has been declared a sacred site by some federally recognized Native American tribes. The site is managed by the National Park Service. This project has monitored the water chemistry of Coldwater Spring to document human impacts on the spring’s water quality. Temperature, dissolved oxygen, conductivity, pH and anions were monitored weekly and cations and alkalinity monitored monthly at Coldwater Spring and the adjacent Wetland A from 15 February 2013 through 18 January 2015. Coldwater Spring’s water flows through fractures in Platteville Limestone of Ordovician age. The basic chemistry of Coldwater Spring should be the calcium magnesium bicarbonate water typical of carbonate springs. However, on an equivalent basis, Coldwater Spring’s water currently contains almost as much sodium as calcium + magnesium and more chloride than bicarbonate. The chloride concentrations are about 100 times the levels from 1880. Maguire (1880) reported the chloride levels of Coldwater Spring were about 4.5 ppm. During the current study the chloride content in the spring increased from about 320 ppm from March 2013 to about 410 ppm in December 2014. In April, May and June of 2013 and 2014, the chloride rose about 100 ppm in three month-long pulses. The chloride concentration of the water in Wetland A ranges from about 400 ppm to over 600 ppm with a pattern that is a mirror image of the Coldwater Spring pattern. This major anthropogenic chloride component has a chloride to bromide ratio of 2,500 ± 300, well within the range of chloride to bromide ratios of road salt, 1,000 to 10,000. Road salt is applied to two major multi-lane highways close to the spring and is used extensively in this heavily urbanized area throughout the winter. The temperature of the spring is variable and higher than its pre-settlement temperature. Nicollet (1841) recorded the temperature of Coldwater Spring multiple times in summer of 1836 as 46 °F (7.8 °C) and multiple times in winter of 1837 as 45.5 °F (7.5 °C). More recently the temperature of Coldwater Spring fluctuates smoothly between 10.7 and 13.1 °C. The higher temperature of the springs’ discharge also indicates an anthropogenic source of heat within the spring-shed or spring recharge area. The spring water is coldest in May and June and warmest in October and November. The temperature of the water in Wetland A fluctuates from 6.4 to 13.8 °C – in a pattern that is opposite of that in Coldwater Spring. Coldwater Spring also contained a significant, increasing nitrate-nitrogen component which ranged from 2.5 to 5.2 ppm – with dips at the same times as the chloride pulses. Wetland A’s nitrate-nitrogen level varied between 0.2 to almost 6 ppm with large pulses at the same time as Coldwater Spring’s dips. A 2014 study performed by the U.S. Geological Survey came to the conclusion that increasing chloride levels in lakes and streams are likely driven by increasing road salt application, rising baseline concentrations, as well as an increase in snowfall in the Midwestern area of the U.S. during the time of the study (Corsi 2014). The significant chloride, temperature and nitrate levels are likely to be driven by anthropogenic sources.
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DOI
http://dx.doi.org/10.5038/9780991000951.1015
Human Impacts on Water Quality in Coldwater Spring, Minneapolis, Minnesota
Coldwater Spring in Minneapolis, Minnesota was the water supply for Fort Snelling from the 1840s to 1920. The spring site has been declared a sacred site by some federally recognized Native American tribes. The site is managed by the National Park Service. This project has monitored the water chemistry of Coldwater Spring to document human impacts on the spring’s water quality. Temperature, dissolved oxygen, conductivity, pH and anions were monitored weekly and cations and alkalinity monitored monthly at Coldwater Spring and the adjacent Wetland A from 15 February 2013 through 18 January 2015. Coldwater Spring’s water flows through fractures in Platteville Limestone of Ordovician age. The basic chemistry of Coldwater Spring should be the calcium magnesium bicarbonate water typical of carbonate springs. However, on an equivalent basis, Coldwater Spring’s water currently contains almost as much sodium as calcium + magnesium and more chloride than bicarbonate. The chloride concentrations are about 100 times the levels from 1880. Maguire (1880) reported the chloride levels of Coldwater Spring were about 4.5 ppm. During the current study the chloride content in the spring increased from about 320 ppm from March 2013 to about 410 ppm in December 2014. In April, May and June of 2013 and 2014, the chloride rose about 100 ppm in three month-long pulses. The chloride concentration of the water in Wetland A ranges from about 400 ppm to over 600 ppm with a pattern that is a mirror image of the Coldwater Spring pattern. This major anthropogenic chloride component has a chloride to bromide ratio of 2,500 ± 300, well within the range of chloride to bromide ratios of road salt, 1,000 to 10,000. Road salt is applied to two major multi-lane highways close to the spring and is used extensively in this heavily urbanized area throughout the winter. The temperature of the spring is variable and higher than its pre-settlement temperature. Nicollet (1841) recorded the temperature of Coldwater Spring multiple times in summer of 1836 as 46 °F (7.8 °C) and multiple times in winter of 1837 as 45.5 °F (7.5 °C). More recently the temperature of Coldwater Spring fluctuates smoothly between 10.7 and 13.1 °C. The higher temperature of the springs’ discharge also indicates an anthropogenic source of heat within the spring-shed or spring recharge area. The spring water is coldest in May and June and warmest in October and November. The temperature of the water in Wetland A fluctuates from 6.4 to 13.8 °C – in a pattern that is opposite of that in Coldwater Spring. Coldwater Spring also contained a significant, increasing nitrate-nitrogen component which ranged from 2.5 to 5.2 ppm – with dips at the same times as the chloride pulses. Wetland A’s nitrate-nitrogen level varied between 0.2 to almost 6 ppm with large pulses at the same time as Coldwater Spring’s dips. A 2014 study performed by the U.S. Geological Survey came to the conclusion that increasing chloride levels in lakes and streams are likely driven by increasing road salt application, rising baseline concentrations, as well as an increase in snowfall in the Midwestern area of the U.S. during the time of the study (Corsi 2014). The significant chloride, temperature and nitrate levels are likely to be driven by anthropogenic sources.
