Road Salt Study at Well 14

Drilled in 1960, Well 14 supplied high-quality water to the University Ave. corridor and Spring Harbor neighborhood for more than 40 years. But heavy road salt use has taken a toll. Levels of sodium and chloride, the two chemicals that make up salt, have more than doubled in the well's water since 2000. If current trends continue, levels will exceed the EPA's recommendation for chloride in the next 10-12 years.  

Madison Water Utility has launched a series of studies to help us better understand how salt is contaminating groundwater that supplies Well 14, evaluate potential impacts of well reconstruction (bore hole casing extension) on overall water quality, and investigate ways to reduce chloride levels.

Project Details

Monitoring wells

In December 2017, Madison Water Utility installed two monitoring wells (view map) in Spring Harbor Park between a nearby stormwater outlet and Well 14. The utility will use these small wells to monitor chloride levels in groundwater at the water table over an 18-month period. The purpose of the monitoring is to evaluate whether the stormwater runoff is contributing to chloride contamination at Well 14. Madison Water Utility began collecting samples from these monitoring wells once a month in January, 2018.

Monthly sampling results:

View monitoring well sampling results from January - August, 2018

Background

Chloride levels recorded at Well 14

2000:   58 mg/L
2005:   70 mg/L
2010:   94 mg/L
2015:   118 mg/L

Sodium levels recorded at Well 14

2000:   19 mg/L
2005:   25 mg/L
2010:   34 mg/L
2015:   42 mg/L

The EPA recommended drinking water guideline is 250 mg/L for chloride and 60 mg/L for sodium, based on taste and drinkability. There is no health-based required standard for chloride or sodium. 

Geology

The geology below Madison generally consists of four layers:  At the top is a layer of porous sandstone known as the upper aquifer, below that is a mostly impermeable layer called the Eau Claire shale, and finally, another sandstone layer called the lower aquifer sits below the shale. Beneath all three of those layers is granite bedrock.

The bore hole for Well 14 is feet 715 deep and extends into the Lower Aquifer. The top 117 feet of the well's bore hole is enclosed in a steel casing.

Recently completed studies

Chloride Source Assessment

Madison Water Utility worked to identify potential significant sources of chloride to Well 14. The utility looked at four principal candidates including a limited number of uncurbed streets and parking lots, the Village of Shorewood Hills salt barn (located off University Avenue), and the nearby stormwater outlet that receives runoff from a large drainage area extending as far away as West Towne Mall and roadways and parking lots located south of the Beltline. The outlet empties into Lake Mendota just across the street from Spring Harbor Park.

The Modeled Capture Zone shows the nearby area that contributes water to the well.

Conclusion: 

The short modeled travel time (two years) from the stormwater outlet to Well 14, combined with the volume and quality of water drained by the stormwater network, makes the outlet a likely chloride source worthy of additional investigation.

(Photo: Stormwater outlet near Well 14)

Borehole Investigation

In 2017, Madison Water Utility worked with the Wisconsin Geological Survey to gain a better understanding of underlying geology and in-hole dynamics at Well 14. The borehole investigation identified two primary rock fractures that carry groundwater to the well –  one near the base of the steel casing (~117 feet below the surface) and the other just above the Eau Claire shale (~230 feet below the surface); The water moving through both fractures contained similar levels of chloride. You can see the lower fracture by viewing the video below.

The investigation also found that the lower aquifer contributes little if any water pumped from the well. When the well pump is not operating, there is a downward flow of groundwater across the shale layer.

Geophysical Log Final 
Geophysical Log Final Stressed
Geophysical Logging Summary

Conclusions:

• Because chloride is entering the borehole through a fracture just above the shale, extending the bore hole's steel casing deeper into the upper aquifer alone would provide only limited short-term benefit.
• The downward gradient under non-pumping conditions transfers water from upper to the lower aquifer.
• Water quality in the lower aquifer is currently unknown due to transfer across the shale layer. Elsewhere in Madison, iron, manganese and radium have been found in the lower aquifer.

Wisconsin Salt Wise

Madison Water Utility is a proud member of the Wisconsin Salt Wise partnership, a coalition of organizations working together to reduce salt pollution in our lakes, streams and drinking water. Wisconsin Salt Wise is working to increase awareness about the impacts of salt on our water resources through strategic outreach, training, and the new Winter Salt Certification Program. The program teaches winter maintenance professionals how use the least amount of de-icing material necessary in order to keep parking lots, roads, sidewalks and driveways safe. 

More Information

Road Salt and Madison's Drinking Water
Road Salt and Our Lakes & Waterways