Chloride Monitoring at Coldwater Spring

Sophie Mitsuko Kasahara

Abstract


Coldwater Spring in Minneapolis, Minnesota was the original water supply for Fort Snelling in the 1840s; is a sacred spring to Native Americans and is now part of the National Park System. This project monitored the changes in salinity levels at Coldwater Spring to document human impacts on the spring’s water quality. Weekly testing for temperature, dissolved oxygen, conductivity, pH and anion levels, and monthly analyzing for cation and alkalinity levels were conducted at Coldwater Spring and the adjacent Wetland A from February, 2013 through April, 2014.  Coldwater Spring is classified as a gravity spring in which the spring’s water is supplied from recharge areas and travels through a permeable surface until it hits an impermeable surface that does not allow it to travel further into the ground. Coldwater Spring’s water flows through fractures in Platteville Limestone of Ordovician age. Studies on this formation have shown that it is ineffective at filtering out many of the contaminants from the recharge areas, in this case residential housing and major highways. The basic chemistry of Coldwater Spring is the calcium magnesium bicarbonate water typical of carbonate springs. However, on an equivalent basis Coldwater Spring’s water currently contain almost as much sodium as calcium + magnesium and more chloride than bicarbonate.  In 1880, a study conducted of Coldwater Spring by Army Captain Maguire found the chloride levels to be about 4.5 ppm. During the current study the chloride content in the spring fluctuated smoothly from about 320 ppm from March, 2013 climbed to a maximum of 430 ppm in June, 2013 and declined to 340 ppm in September, 2013 and then rose slowly to 385 ppm in April, 2014. These levels are about 100 times the levels from 1880. This major anthropogenic sodium chloride component has a chloride to bromide ratio of 2500 ± 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 throughout the late fall, winter and early spring. The several month delay in the peak salt concentration appears to be a travel time delay between the surface application and the spring.  The temperature of Coldwater Spring decreased smoothly from 12.7 °C in February to 11.2 °C in May and then increased to 13.0 °C by late November. This is not a temperature constant spring. The peak in the spring’s temperature is delayed by about six months from the air temperature pattern. The normal temperature of a spring in this area is about 8 °C, so the higher temperatures of the spring also indicate an outside source, such as underground utilities, that may be heating the spring.  The water also contains a significant nitrate-nitrogen component of 2.9 ± 0.3 ppm, which is additional evidence of anthropogenic impact.


Keywords


Karst Spring; Chloride; Temperature

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