This might make a good science fair project for somebody, so I’m giving this topic one final, compact write-up.
If you live in an urban area that draws its drinking water from a local river, or other nearby flowing surface water …
… and you live in a climate where they salt the roads for winter storms,
… and the weather cooperates, in the form of some distinct road-cleaning rain or melt event following a winter storm,
… you can easily infer the presence of road salt, in your drinking water,
… with a cheap ($6) total-dissolved-solids (TDS) meter, a water glass, and some patience.
In my area — where the Potomac River is the main source of drinking water — it takes about ten days from the time the rain washes the salt off the roads and parking lots, until that salt shows up in the drinking water. YMMV.
See posts 2085, 2086, 2088, 2089, 2090, 2091, and 2092 for background.
The required background, as a series of true statements.
We use a lot of road salt in the U.S. Google’s AI tells me we use 20 million metric tons of it a year. The same AI tells me we have about 230 million licensed drivers. So I make that out to be just under 200 pounds of road salt, per licensed driver, per year.
The accepted EPA threshold for “salty taste” in the drinking water is 250 parts-per-million chloride ion. Assuming I did the math right, 200 pounds of salt (60% chloride by weight) is enough to impart a salty taste to more than 50,000 gallons of water. Or, enough to impart a salty taste to 0.7″ of rain, on your standard suburban quarter-acre lot.
That’s all by way of saying that, “outdoors” is a big place, but that’s still a lot of salt, even when spread outdoors. Enough salt that you ought to be able to notice it, in the environment.
The negative effects of road salt use are well-known, including corrosion (of cars, bridges, rebar in concrete …) and pollution of surface and ground waters with the salty runoff. In particular, nothing that lives in your local fresh-water environment really likes being subjected to a salty water.
There has been a prolonged push in the U.S. to use less road salt. Seems like that started in the late 1990s in New Hampshire, where they were discovering problems with water wells that had been, in effect, poisoned by prolonged use of salt on nearby roadways.
State DOTs and others do not use salt to melt the snow off the roads. They plow the snow off the roads. The salt is just there to achieve “disbondment”, that is, to prevent the packed snow and ice from freezing solidly to the pavement. So that they can plow down to bare pavement.
The desire to use less road salt led to the now-common practice of brining the road surfaces prior to snowfalls, one of a set of techniques known as “anti-icing” (as opposed to after-the-fact de-icing). If weather conditions are right (e.g., no rain prior to the snowfall), spraying the roads with a thin layer of salt water, then allowing that to dry, achieves “disbondment” of the initial snowfall with minimum use of salt. Brining uses roughly one-quarter of the salt that would be required to achieve the same road-clearing result, if spread as rock salt. (Source: Brine Fact Sheet, 2016, American Public Works Association.)
That thin layer of salt creates a weak spot in the snow/ice layer that forms on the road. That weak layer is what creates the “disbondment” of the ice and the underlying pavement. That “disbondment” allows the plows to scrape the snow off the road, to get down to bare pavement. Rock salt is also there for the disbondment, it just achieves it less efficiently.
Some of the sodium in salt tends to stay local. This is what “burns” greenery near salted areas such as sidewalks. But the chloride in salt travels along with the runoff, plausibly (around here) in the form of calcium chloride, formed as sodium was exchanged for calcium in the soil.
A “total dissolved solids” meter measures the electrical resistance of water, and so indirectly measures the concentrations of ions in the water. Around here, in normal times, that would be mostly calcium and carbonate ions, as that’s the main dissolved mineral contributing to our roughly 10 grains of water hardness in this area. But ions are ions, whether they be from calcium carbonate or sodium chloride. And so, a total dissolved solids meter will react to salt in the water, as it would to any other ions in the water.
As a result, to the extent that road salt gets into my drinking water, this should generate a predictable rise in total dissolved solids, as measured in my tap water. Each time the salt is flushed off the roads (by rain, say), I should see a rise in TDS in my tap water, with the appropriate lag.
In Fairfax County, it takes about a week for water to work its way from the filtration plants to the furthest taps in the system. This is known, because Fairfax flushes the system annually (switching from chloramine to chlorine during that period), and it warns citizens about the resulting change in the smell and taste of the water, annually. And in that warning is the factoid that it takes about a week.
All you need to track TDS in your drinking water is a cheap ($6 via Amazon) total-dissolved-solids meter, and patience. The patience is required because, with a cheap meter, you’ll only get stable results if you allow the tap water to sit long enough to come up to room temperature. (The underlying conductivity measurement is quite temperature-sensitive, and the cheap TDS meter that I bought takes forever to adjust to the water temperature.)
And, voilà:
As noted, these peaks in tap water TDS are ten days after some weather event that flushed a lot of road salt into the local creeks. (Typically, a rainy day.)
Although the timing and magnitude are right, I have not proven that this is purely the effect of salt. Maybe TDS goes up after every rainstorm, salt or no salt? I think that’s unlikely, but I can’t rule it out until weather conditions are right, and we have a rainy day with no remaining salt on the roads.
Conclusion
I’m pretty sure the peaks in tap-water TDS, shown above are driven by road salt being washed off the roads. Water filtration (short of reverse-osmosis) does not remove salt (or chloride) from the water. And, because we drink river water, not well (ground) water or water stored in large reservoirs, that salt then shows up, in short order, in the water.
All of which tells me that these peaks look about right.
I’d like to have double-checked that it is salt, by being able to taste the saltiness in the water, but the increase in TDS was not large enough to cross the commonly-accepted threshold for salty taste (250 ppm chloride ion in the water).
Ultimately, all that’s left to show is to show that such TDS peaks don’t appear, 10 days after a rainy day, when there isn’t salt on the roads. That way I can rule out that these TDS peaks are simply related to rainstorms. Leaving salt (moved by rainstorm) as the only plausible explanation.
Again, the beauty as a science fair experiment is that all it takes is a cheap TDS meter, a water glass, and patience.
