Report Summary:

Deicer is applied on critical roads, like Interstate-5 (I-5) in the Siskiyou Pass, to make them safer for winter travel by preventing buildup of snow and ice. Oregon Department of Transportation (ODOT) commonly uses magnesium chloride (liquid deicer) and wanted to test out combining the liquid application with solid sodium chloride deicer (road salt) on problematic highways, like the Siskiyou Pass, near the California/Oregon border. The addition of road salt offers cost and safety advantages compared to just using liquid deicer alone. In this study, USGS scientists examined groundwater quality in the area to help ODOT assess the effects of this combination of deicers and inform decisions on how those chemicals are used.  

During 2016 through 2020, ODOT applied up to 16,000 gallons per mile of liquid deicer and 143,000 pounds per mile of road salt along an 11-mile stretch of I-5 through the Siskiyou Pass. Scientists collected groundwater samples during July 2018 – February 2021.  

Despite the benefit of safer driving conditions, some possible negative effects are associated with the use of chloride-based deicers. Other studies have shown that chloride deicers can increase the salt content in surface water and groundwater, which can harm aquatic life and affect the safety of drinking water infrastructure. High salt levels in groundwater can affect the taste of drinking water from domestic wells. Additionally, excess salt in soils and shallow groundwater can worsen the impact of drought on roadside plants and make them more susceptible to disease. 

This study found that groundwater downstream from I-5 had higher levels of dissolved salt compared to groundwater in areas upstream from I-5. The highest salt concentration was near the Siskiyou Pass, indicating likely contamination from the highway. Elevated salt levels were also found in streams downstream from the highway where shallow groundwater is discharging into springs and streambeds, with some sites exceeding the recommended EPA surface-water secondary maximum level of 250 mg/L for chloride. 

None of the sampled domestic wells had high salt concentrations near the EPA recommended limits - indicating that salt had not migrated that deeply into the groundwater system. Results show that salt is likely getting into the groundwater mainly through stream drainages where highway runoff seeps into the ground, less so between drainages. Scientists also found that after rain events, the salt levels in surface water near the highway increased because rainwater washed more salt into the streams. This pattern was less noticeable in a spring with a concrete cistern, where surface water had little influence. 

This report provides a snapshot of dissolved salt concentrations in the Siskiyou Pass area during 2018–21. The use of salt deicers on the highway is seasonal and depends on weather conditions. The study wasn't long enough to predict how salt levels will change or stabilize in the future. Future study could include extended monitoring of spring sites to confirm trends in salt migration in shallow groundwater. The approach and methods used in this study can be applied in other areas of chloride deicer use to determine if local groundwater is affected.