Non-point Evaluation Studies
- Project Team
- William R. Selbig, Research Hydrologist (Nonpoint Studies Lead)
- Steve R. Corsi, Research Hydrologist, Chemistry
- Rob Waschbusch, Hydrologist
- David W. Owens, Hydrologist
- Todd D. Stuntebeck, Physical Scientist
- Judy A. Horwatich, Hydraulic Engineer
- Mari E. Danz, Hydrologist
- Austin Baldwin, Hydrologist
- Matthew J. Komiskey, Physical Scientist
- Troy D. Rutter, Hydrologic Technician
USGS IN YOUR STATE
USGS Water Science Centers are located in each state.
Road Salt Frequently Asked Questions (FAQs)
(updated Jan. 14, 2015)
What was the purpose of this study?
- The primary objectives of this study were to define temporal trends in chloride concentrations in streams.
- Define these trends in the context of chloride dependency on streamflow rates,
- Compare temporal chloride trends among seasons, and
- Compare these trends to changes in urban land cover, aquatic life criteria, and road salt sales patterns.
What led you to this study topic?
- This was a follow-up study from our previous study examining impacts of road salt on aquatic toxicity and water quality. We wanted to define how concentrations have been changing over the years and if concentrations were still increasing over time.
What geographic areas did you end up studying?
- A detailed local study in the Milwaukee Metropolitan area showed that concentrations were increasing rapidly, so we also expanded the study to other sites in the northern U.S. to give a broader perspective.
How did you study the effects of pavement deicer runoff?
- We used the extensive historical databases from USGS and USEPA (http://www.waterqualitydata.us/) as well as Milwaukee Metropolitan Sewerage District.
- Data was compiled from numerous streams in the northern U.S. with a range of urban influence in their watersheds. We also collected data from one site in Texas to examine the data in an area without road salt applications.
- We analyzed water-quality data from 30 monitoring sites on 19 streams near cities in Wisconsin, Illinois, Colorado, Michigan, Ohio, Pennsylvania, Maryland, Texas and the District of Columbia.
- Trends were determined for all sites during all four seasons of the year and put in the context of variable streamflow rates.
- Analysis of data to determine trends was done using a modern statistical modeling technique called Weighted Regression on Time Discharge and Season (WRTDS) within the software package Exploration and Graphics for RivEr Trends (EGRET; https://github.com/USGS-R/EGRET/wiki).
What did you find?
- In 84 percent of urban streams analyzed, chloride levels increased substantially during the study period.
- Twenty-nine percent of the sites exceeded the U.S. Environmental Protection Agency’s chronic water-quality criteria (230 milligrams per liter) by an average of more than 100 days per year from 2006 through 2010, which was almost double the amount of days from 1990 through 1994.
- The lowest chloride concentrations were in watersheds that had little urban land use or cities without much snowfall, such as Dallas, Texas.
- In 16 of the streams, winter chloride concentrations increased over the study period.
- The greatest chloride concentrations occurred during winter periods.
- In 13 of the streams, chloride concentrations increased over the study period during non-deicing periods such as summer. This finding suggests that chloride infiltrates the groundwater system during the winter and is slowly released to the streams throughout the year.
- Chloride concentrations increased more rapidly than development of urban land near the study sites.
Did any of these findings surprise you?
- The fact that there were elevated chloride levels was not surprising. The surprise was:
- Chloride Concentrations appear to be increasing more rapidly than the increase in pavement that requires deicing.
- These rapid chloride increases were likely caused by a combination of these possibilities:
- The rate at which salt is applied on any given surface has increased.
- The baseline conditions (the concentrations during summer and early fall low-flow periods before deicing begins) have increased over time.
- Greater snowfall in the Midwest during the latter part of the study.
Could other sources be the cause of high chloride levels?
- In this study, we specifically targeted streams and time periods where the dominant source would be road salt, but in a general sense, there are other sources such as:
- Wastewater treatment effluent
- Septic systems
- Farming operations
- Natural sources
- Geographic comparison, seasonal comparison, land use comparison, site selection (most likely streams with road salt runoff)
- Analysis suggests road salt as the most likely source
Who primarily uses deicing salt?
- City maintenance crews
- County/highway departments
- Commercial applicators for private and public parking lots, driveways, and sidewalks
- Private individuals for driveways and sidewalks
Are there alternative deicers that could be used?
- Yes, but each has their own impact.
- Some may have toxic impacts as well
- Some have oxygen demand when introduced to a water body causing potential for low dissolved oxygen levels that can be harmful to aquatic life
Are there ways to reduce road salt applications and maintain the same level of safety?
- In many cases, yes. There are a host of different salting practices that can be adopted to make most efficient use of the salt that is applied. Some examples include:
- Pre-wetting the pavement is one example. A liquid salt brine is applied to the pavement before the storm arrives so that snow and ice does not bond as well to the pavement. This makes plowing more effective and reduces the need for chemical deicers.
- There are also new plow-blade designs that make plowing more effective. Some act like a squeegee and others work by breaking adhesion to the pavement more effectively. The result is the same for both. After plowing, the pavement is actually cleaner, so less chemical deicers are needed.
- Many more, some of which are included in this report: Strategies to Mitigate Impacts of Chloride Deicers on the Natural Environment (Transportation Research Board/National Cooperative Highway Research Program, http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_449.pdf)