Chloride in Starkweather Creek
For more than a decade, Public Health Madison & Dane County has monitored chloride concentrations along Starkweather Creek.
Ivy Street, at the downstream end of Starkweather Creek's East Branch, has the highest summer chloride concentrations of the six monitored sites. Winter conditions can reach chronically toxic concentrations.
Overall, the East Branch of Starkweather Creek (Zeier Rd and Ivy St) has higher concentrations than the West Branch (Hwy 51, Anderson St, and Fair Oaks Ave). This monitoring continues so as to document how baseflow chloride concentrations change over time. Starkweather Creek flows into Lake Monona, which has likewise seen chloride concentrations increasing.
Since 1970, chloride concentrations have increased approximately 1 mg/l per year in the Yahara chain of lakes. Water with salt in it is more dense and sinks to the bottom, which prevents lake mixing in the spring and fall. Without this mixing, oxygen-depleted water at the bottom of a lake is not replaced with oxygen-rich water from the surface and conditions for aquatic life towards the bottom of the lake degrade. Therefore, steadily increasing chloride concentrations is a concern for the lake ecosystems.
The interactive display below shows changes in chloride concentrations in the Yahara Lakes and in Fish Lake, a small rural lake in northwest Dane County. The display also shows depth profiles of chloride concentrations in Fish, Mendota, Wingra, and Monona. Increasing chloride concentrations with depth have been documented in Lake Wingra and Lake Monona. Click Lake Monona on the map to display high winter chloride concentrations towards the bottom of the lake. At the surface, chloride concentrations in Lake Monona have increased from approximately 20 mg/l in the 1970's to 70 mg/l in the 2010's. Concentrations at the bottom of Lake Monona have reached five times that, with 373 mg/l recorded in February 2017.
Click on Lake Monona to how chloride concentrations change with depth on the graph
Of these lakes, the highest chloride concentrations are detected in Lake Wingra, which has the most urbanized watershed (65% developed). Lake Mendota, with only 27% of its watershed developed, has the lowest chloride concentrations within the chain of lakes. The other three (Monona, Waubesa and Kegonsa) all have similar chloride concentration trends and have watersheds that are roughly 34% developed. Chloride concentrations are much lower and have increased at a slower rate in Fish Lake, located in rural northwestern Dane County. Only 6% of the Fish Lake watershed is developed, meaning less salt use compared to the urbanized Madison watersheds.
Chloride in Other Streams
Chloride concentrations in Starkweather Creek have increased more quickly compared to other area streams, especially those with more rural watersheds. Across the region, however, concentrations in other streams are going up too.
As long as salt is applied to land surfaces, chloride will continue to reach our surface and ground waters.