New study shows deteriorating water quality in rivers and streams due to fertilizers, road chemicals and mining.
Salt concentrations are rising in North American streams and rivers because of road chemicals, fertilizers and other salt-containing substances that somehow get into water bodies. At the same time, the alkalinity of freshwater supplies is increasing.
The new study, called “Freshwater Salinity Syndrome,” was conducted by the University of Maryland at College Park with scientists from the Cary Institute for Ecosystem Studies, the University of Connecticut, the University of Virginia and Chatham University. The researchers evaluated, for the first time on a continental scale, long-term changes in salt content and hydrogen ion concentrations (pH, the hydrogen index) in freshwater. Based on data from 232 U.S. Geological Survey monitoring sites across the country, 50 years of increases in salt and alkaline concentrations were recorded.
“A lot of people think that if you sprinkle salt on the streets, it just washes away and leaves no trace. But it accumulates in the soil, in the groundwater, and it takes decades for it to wash out,” says Sujay Kaushal, a professor of geology at the University of Maryland.
According to the professor, most studies of freshwater salinization have evaluated the effect on water of sodium chloride, a table salt found in road chemicals. But in terms of chemistry, salt has a broader definition. It includes any combination of positively and negatively charged ions that dissociate in water. Some of the most common positive ions found in salts, including sodium, calcium, magnesium and potassium, can have damaging effects on fresh water in high concentrations.
Salt Cocktail
“These ‘cocktails’ of salts can be more toxic than a single salt because some ions can replace and release other ions from soils and rocks, exacerbating the problem,” Kaushal explained.
The current analysis simultaneously accounts for the effects of multiple salt ions on freshwater in the U.S. and southern Canada. Researchers have documented dramatic chemical changes in many of the nation’s major rivers – the Mississippi, Hudson, Potomac, Newse and others. They supply drinking water to nearby cities and towns, including some densely populated towns along the east coast. The analysis showed that 37% of the watersheds have significantly increased salinity. Alkalinity, which isn’t just increased by salt, has risen by 90 percent.
“Until now, we have not fully assessed the role that different salts play in changing the pH balance of stream and river water. Salt and alkaline content are fundamental aspects of water chemistry; they are major factors in changing the properties of freshwater,” said study co-author Gene Likens, president emeritus of the Cary Institute for Ecosystem Studies and professor at the University of Connecticut.
The causes of increased regional salt concentrations vary depending on the specific conditions. For example, in the Mid-Atlantic and New England, road chemicals were the main cause, and in the agricultural Midwest, fertilizers were the main cause. In other regions, mining wastes, weathered concrete, rocks, and soils are responsible for salt intrusion into adjacent waterways. In the southwest, in desert areas where salt concentrations have historically been high, in contrast, researchers have documented a decrease in salt concentrations. This is due to changes in land and water use and state and local government efforts to reduce salt use.
“We found that the pH balance in some rivers began rising in the 1950s and 1960s – decades before the Acid Rain Program regulations (aimed at reducing sulfur oxide emissions, began operating in 1995 – note +1). We have also seen elevated salt concentrations in the southeast, where no road salt is used. These are surprising trends; it’s a puzzle our team has been working on,” said Michael Pace, professor of environmental science at the University of Virginia.
Possible solutions
Professor Kaushal noted that there are already strategies to combat freshwater salt pollution. In particular, there are studies proving the effectiveness of granular salt for roads – it has the same effect with less reagent use. Pre-treating roads before heavy snowfalls also helps.
“Not all salt is equally effective for de-icing. Choosing the right salt compounds for certain temperature conditions can more effectively melt snow and ice with less salt. This can help solve the problem,” Kaushal added.
Precise selection of the amount of fertilizer in the right seasons can reduce salt leaching into water bodies, the researchers said. In cities, more careful building strategies away from waterways and designing efficient stormwater systems can solve the problem.
