Because the department, like many others, can switch intakes and mix in water with lower salinity, the final product contains lower levels, said Chris Crockett, director of planning and research.
If current trends continue in the coming decades, however, experts say that aquatic life will suffer and water supplies could be threatened. Sodium is a concern for people with medical conditions such as hypertension.
"This cannot go on indefinitely. It is not sustainable," said Jonathan Husch, chair of Rider University's department of geological, environmental and marine sciences, which has been researching salt issues locally.
Just since Sunday, PennDot has spread 8,000 tons of salt in Southeastern Pennsylvania. The City of Philadelphia put out an additional 7,000 tons. Hundreds of smaller municipalities, thousands of businesses, and millions of residents tossed more.
Unlike the pollutants that are typically removed by water-treatment plants, getting the salt out can require entirely different technologies such as reverse osmosis.
Eventually, said Crockett, governments may need to decide on which end of the process to spend precious public funds: more environmentally friendly deicers for the roads or new treatments for the water. Both cost more.
Officials - especially those in more northern areas - have been aware of the problems with salt for more than a decade. But it's only been in the last few years, with increased public focus on the environment, that significant innovations have emerged.
Highway crews in both Pennsylvania and New Jersey, for example, have been spraying ahead of time with a salty brine solution. The liquid - look for the stripes down the lanes before a storm - stays on a bare road better than salt, it delays the formation of ice and, when salt is spread later, it speeds melting.
Spreaders have been reengineered and recalibrated to reduce overshoot and to keep the salt from bouncing onto the shoulder.
In some cases, workers can clean the same amount of snow with half the salt that they once used.
"We are extremely aware of the situation with salt," said PennDot's Nick Martino, who is in charge of road maintenance for the five southeastern counties.
Officials employ elaborate calculations to project nuances of temperature and precipitation as storms approach and intensify.
This year, PennDot is piloting a sophisticated storm-fighting computer system - with touch screens in the trucks - that helped Indiana reduce salt use by a third. Using radar, it forecasts road conditions and fine-tunes how much salt should be spread.
Manufacturers are coming out with new deicers, including one made from beet juice. Transportation officials in Maryland, New York, and Chicago are trying it.
Marketed under brand names such as GeoMelt and IceBite, it is less corrosive for bridges and cars - another issue surrounding salt. Also unlike salt, it doesn't cause potholes.
This, like many other salt "alternatives," is really just an additive. It helps a brine solution stay put and enhances the melting effect.
But some have complained that the beet juice stains and stinks like rotting vegetables. It has unwanted environmental effects, too. Bacteria that break down the organic chemical consume oxygen - and low oxygen levels are another problem in many urban streams.
And a Madison, Wis., study found that substituting the beet product for the salt brine it used in 2008-09 would have cost more than 10 times as much.
So good old road salt is still the cheapest thing going and the primary deicer. Although anything that melts in water will lower its freezing temperature, no other broadscale substitutes have taken hold.
"I don't know that salt will ever go away, given its effectiveness and the price," said William Hoffman, a Nevada transportation official who chairs an American Association of State Highway and Transportation Officials snow and ice task force.
Nationally, road-salt use took hold after World War II, when new prosperity led to more cars, more paved roads, and, over the years, higher expectations.
"Now it snows, and we want clear roads the next day," said Rider's Husch.
In 1940, an estimated 149,000 tons of rock salt were sold in the United States for highway use. Now, we're up to about 18 million tons in a bad winter.
PennDot statistics for the southeastern region show that in the 1980s, annual salt use never topped 43,000 tons. But in the last decade, half the winters have led to the use of 80,000 tons or more. Last year was a record-breaker - 142,738 tons.
Meanwhile, study after study has found that from the Great Lakes to mountain streams, salinity in water bodies has been rising. In isolated cases, municipal water wells have had to be shut down because of contamination from road salt.
Wetlands have been affected. Salt-tolerant species have become more common along highways with high salt use.
In 2009, a U.S. Geological Survey study found that 40 percent of streams in and around Northern U.S. cities underlain by certain kinds of aquifers - Philadelphia is not among them - had salt levels high enough to damage aquatic life.
Earlier this year, USGS researcher Steve Corsi and others collected water fleas and flathead minnows in streams around Milwaukee. They found that during winter deicing, water in more than half the streams sampled was toxic to the organisms or affected their growth and reproduction.
Eventually, salt can change not only a stream's plants and aquatic organisms, but its entire ecosystem, said Philadelphia's Crockett.
"You go from things that are not tolerant of a salty environment to things that can handle that kind of shock."
Crocket suspects that, from an ecosystem standpoint, the Schuylkill, which has seen steep salt increases, is headed that way. "I think, in my lifetime, we'll see the Schuylkill hit its carrying capacity," he said.
For more than 10 years, Hongbing Sun, a professor of geological and environmental sciences at Rider University, has led studies looking at salt in the Delaware River.
They concluded that the primary source was road salt.
Sun said that levels of sodium and chloride traced to road salt spike not only after a winter storm, as might be expected, but also in summer.
So now, his team is studying how salt accumulates in soil and how long it stays there.
"The bottom line is that . . . you can't spread thousands of gallons of chemicals on the environment and not have them create some sort of impact," said Corsi, the USGS researcher.
But Corsi, who lives in Wisconsin, grasps the dichotomy.
He figures he has a high tolerance for snowy roads because he knows the problems that deicing can cause.
Then again, he said, "I have two kids in the backseat of my car, and I want them to be safe."
New York City and its airports are struggling to dig out. A3.
Despite delays, Downingtown
band will march in Pasadena. B1.
Philadelphia International Airport getting back to normal. C1.
Contact staff writer Sandy Bauers at 215-854-5147 or firstname.lastname@example.org.
Visit her blog at http://go.philly.com/greenspace.