One measure of when winter begins in the Class of 1918 Marsh is when ice cover first appears. This begins a winter tale of changes above and below the waters that are contributing to the degradation of this prized Preserve ecosystem. These changes follow from the pile up of snow on the campus and the subsequent application of road salt to streets, parking lots, and sidewalks, and to the storage of excess snow at the snow pile adjacent to the 1918 Marsh (see photo). Road salt, in various forms, includes ‘chloride’ ions that are toxic at high concentrations; chloride also provides an easy measure to follow the journey of road salt from roads, parking lots, and sidewalks to the waters and wetlands around the marsh, to the open water of the marsh, and to the storm drains and creeks that flow into Lake Mendota at University Bay. The City of Madison began using road salt as a deicer on streets in 1959, and presently applies 15,000 tons each winter. The concentration of chloride in Lake Mendota has reached 50mg/Liter. In the open waters of the marsh, it is much higher, up to 755 mg/L in winter, 2012-2013.
While wetlands and marshes are often credited with purifying the waters flowing towards the lakes and streams, they unfortunately do not remove or neutralize chloride. Chloride is passed freely downstream to surface and groundwater. Winter concentrations of chloride are much greater in the marsh and adjacent stream channels than they are in the lakes, where the chlorides are diluted by runoff in warm months when road salt is not applied either in our urban area or on major highways.
Throughout the winter of 2012-2013, the chloride content of waters in and around the 1918 Marsh was measured. The results are summarized in the map of chloride values below. Waters entering and leaving the Marsh, as well as the storm sewer from Parking Lot 60 and areas near the snow pile, were sampled.
The observed chloride concentrations were alarming. Maximum values were, with two exceptions, markedly above the acutely toxic concentration of 757 mg/L. The average concentrations with one exception were above the 230 mg/L no-effects levels set by the DNR. The highest concentrations are in the storm sewer from Lot 60 (Station 6). The concentrations towards the lake from the snow pile (Stations 4 and 5) are the next highest. Not shown are concentrations between the snow pile and the open waters of the 1918 Marsh where one extreme value of 12,700 mg/L was observed.
The present levels of road salt use by the City, the University, and other governmental, commercial, and private users are not sustainable.