Road Salt Contamination

Deicing salt on the permeable pavement in front of Augustus Storrs Hall, a potential source of groundwater contamination causing mobilization of radium and radon.

Road salt is inescapable during a Northeast winter. Applied as a deicer, it helps prevent accidents, slips and falls. Salt lowers the freezing point of water, accelerating melting and keeping ice from forming when temperatures drop. Despite the benefits to transportation and safety, road salt has serious environmental impacts and presents hazards to human health. Researchers at UConn have recently completed two studies on the Storrs campus, examining how deicers interact with areas surrounding permeable surfaces and discovering a potential radioactive danger.

Mostly a combination of sodium and chloride, road salt chemicals can flow into surface and ground water impacting aquifers, wells, wildlife, flora and drinking water. While these effects have long been publicized, road salt continues to be heavily used due to its low cost and a lack of viable alternatives. The increased use of storm water management systems, particularly in urban settings, has renewed questions about how these contaminants travel and affect the neighboring environment.

A team of UConn researchers, including Professor Gary Robbins of the Department of Natural Resources and the Environment (NRE), Assistant Extension Educator Dr. Mike Dietz of the Department of Extension and Connecticut Sea Grant and NRE graduate students Derek Angel and Lukas McNaboe, investigated how the installation of one popular storm water management system, permeable asphalt, affects road salt contamination of groundwater. Connecticut Sea Grant funded the initial phase of the research.

Permeable pavements are porous, constructed of materials that allow water to move through its surface, which prevents flooding. Designed in multiple layers, permeable paving uses a special mix in its top layer with a prescribed air void content. The air void content determines the ability of air and water to penetrate the surface. Below this initial layer are varying strata of bedding, reservoirs and drains to meet the needs of the site.

Their initial study looked at chloride and alkali metal concentrations near a permeable asphalt site, a UConn parking lot in front of Augustus Storrs Hall. Monitoring wells were established both upgradient and downgradient of the location to determine the levels of these elements in groundwater before reaching the permeable pavement area surface and afterwards. Although other studies have explored permeable asphalt and chloride levels, this is the first research that moved sampling from directly below the site to surrounding groundwater locations.

Samples were collected from September 2014 through April 2015 to observe concentrations in the groundwater during the fall, before the frequent application of deicers, and after the end of their use in the spring.

The team found that during the winter months with substantial use of deicers, the levels of chloride and alkali metals rose significantly. However, the amount of chloride, though higher during times when road salt was deployed, was found to be lower during spring, summer and fall months when salt was not applied. This suggests that permeable pavement helps dilute groundwater for the most of the year, reducing the concentration of salt and other contaminants that might be present. These results are limited to urban environments and focused only on shallow groundwater, but these results intimate that areas with high chloride concentrations, such as the northeastern United States, could benefit from permeable pavement.

While the study demonstrated the ability of permeable asphalt to mitigate the negative effects of road salt raising certain levels of contaminants, Robbins, Dietz and McNaboe conducted another study at the same site looking at other compounds deicers interact with in the ground.

“The permeable pavement does its job: putting fresh water in the ground,” says Robbins. “The increases in salinity in the winter is a problem which got me thinking about what other issues might arise from the salt contamination of the groundwater.”

Robbins was concerned about cation exchange. Cations, positively charged ions, are present in rocks and soils. The influx of spreading deicers full of sodium and calcium cations into an environment could cause other cations, such as radium, a radioactive metal found naturally in the environment, to be released.

After conducting an additional study, they found that the mobilization of radium is enhanced by salinity. As radium decays, it releases radon gas, a known carcinogen. Exposure to this gas is the second leading cause of lung cancer after smoking. It is impossible to detect without testing because radon is odorless, colorless and tasteless.

At two of the monitoring sites, they detected levels of radium that exceeded EPA-recommended standards regarding maximum drinking water contamination levels. Their study indicates that the increased salinity of groundwater is causing radium dissolved in the groundwater to migrate more readily. The levels of radium were high enough to produce high levels of radon in the groundwater which, if volatized from the water table, could produce potentially dangerous amounts of radon gas. This raised the concern that buildings near areas where the groundwater becomes heavily salted can be impacted by levels of radon that exceed indoor air quality standards.

Their findings warrant further research, the groups says, to understand how factors such as temperature, groundwater flow and other properties might affect the relationship between salinity, radium and radon. Despite the focus on an area near permeable pavement, the team believes that any location where salted areas runoff and infiltrate groundwater quickly are susceptible to fostering potentially harmful exposure to radon. As their research continues, the team is looking at domestic wells across the state to help provide solutions for towns and homeowners dealing with increased salinity and to learn more about the mobilization of radium and radon.

Robbins believes that the geology of an area, specifically which rock types are present, is key to understanding the risks salting introduces to an environment in activating radium mobility and radon gas.

“The rock beneath the campus is Hebron Gneiss, which has naturally occurring radioactive materials. This likely contributed to our findings because as we’ve expanded our study into other wells in different towns, we haven’t always detected increased levels of radium and radon,” says Robbins.

They studied wells in Sherman, Connecticut, where marble rock is predominant, and did not find higher levels of radium and radon.

“While radium and radon mobilization is a serious concern, it all stems from these salt levels that we’ve seen increasing over time. I’ve examined a hundred years of records of salting in Connecticut. The levels of salt in the groundwater are rising and have been for years. Statewide they are ten times what the natural levels are, but in some places they are more than hundred times. Contamination of groundwater by salt is a very serious issue that we have not dealt with yet. It’s a real quandary,” says Robbins.

“We need to find the best way to deal with snow and ice that doesn’t contaminate the subsurface,” says Robbins. “It’s conservation of mass: it’s either going to go into a surface body of water or get put into the ground. It comes down to how and where you apply deicers and how you control the drainage.”

Dietz has been working with towns, homeowners and UConn’s Facilities Operations and Building Services to find ways to reduce the application of deicers.

“I’ve been working to figure out ways to address salting on campus with several UConn staff members. Other colleagues in NRE have been looking at ways to redirect water or to create a tank or system to store contaminated water, but these are not easy to accomplish.”

“There’s really no practical way to get the salt out once it’s in the groundwater. The solution is reducing what you’re applying,” says Dietz.

By Jason M. Sheldon