Used in winter to melt ice from roads, salt seeps deep into groundwater until it reaches surface waters. A University of Delaware study illustrates some of the dynamics that affect the phenomenon
by Matteo Cavallito
Salt has always been used on roads to help ice melt during winter periods. This widespread practice, however, could impact groundwater not only during the cold months, according to a study from the University of Delaware. The investigation, led by Rachel McQuiggan, a researcher with the Delaware Geological Survey, an agency associated with the same university, was based on monitoring groundwater in a infiltration basin. That is, one of those large basins placed on the side of the road that allows rainwater to be collected.
The basic idea, McQuiggan explains, is to protect surface water from salt seepage. The basins are designed accordingly “with the idea that stormwater benefits from a natural ‘filtering’ of contaminants as it infiltrates through soil, and contaminants dilute as that recharge mixes with existing groundwater.” But this operation, in any case, does not come without impact.
“Stormwater infiltration best management practices aim to prevent the salt from polluting streams and waterways, but this may shift pollutants to groundwater resources,” the research states. “In response to limited field studies investigating groundwater quality impacts caused by input of salt from stormwater infiltration best management practices, we monitored water levels and quality of groundwater at various depths in an unconfined aquifer around a stormwater infiltration basin using in situ sensors coupled with grab sampling.”
The study, which ran from mid-May 2019 through February of last year and was published in the Journal of Environmental Quality, a publication of the American Society of Agronomy, the Crop Science Society of America and the Soil Science Society of America, shed light on what dynamics – from geologic features to soil properties – influence the movement of saltwater deep in the ground. A phenomenon, the researchers point out, that ends up in the long run spilling over to surface streams as well. To which Delaware’s groundwater contributes as much as 80 percent of the total.
Salt is present year-round
Most significant is the ability of salt to infiltrate year-round, not only in winter. What is crucial is the ability of the soil to hold it even though the movements of its basic elements change: chloride, for example, moves more easily in water, while sodium tends to bind to soil particles. “Water–matrix interactions in the vadose zone beneath the basin affected the transport of sodium (Na) into groundwater following non-winter recharge,” the research explains.
“Sodium movement through the aquifer was delayed relative to chloride (Cl), indicating a longer residence time of Na in the vadose zone. Radium (Ra) concentrations were correlated with Cl concentrations, suggesting salt-impacted recharge caused desorption of Ra into groundwater because of increased salinity.”
The climate variable is also important. “If it’s a particularly dry spring and summer,” McQuiggan explains, “then the sodium can take longer to reach groundwater. And in Delaware, snowfall typically melts and runs off the roads within a few days of falling. In colder climates it can stay frozen for months.”
Searching for the best solution
The dynamics that affect salt seepage have long been a subject of interest. Groundwater, McQuiggan says, provides nearly half of all the world’s drinking water. Saltwater intrusion into groundwater, according to FAO, favours excessive salinization of soils, a phenomenon that may affect 20 percent to 50 percent of the Planet’s agricultural fields.
“In central and southern Delaware, groundwater is the only source of potable drinking water,” McQuiggan says. ” Hopefully the results of this project will encourage best management practices for deicer use to protect groundwater resources.”
Today, the study states, there are many ways to melt ice while making roads safe. Sand, for example, increases traction on the surface and has minimal impact on groundwater even if the operation requires additional maintenance activities. Other carbohydrate-based deicers are available even if they are often used in combination with salt. Each option, the authors note, has pros and cons in terms of environmental impact and cost.