It’s going to rain all day today. The forecast is for half an inch or more.
Today, therefore, I must appreciate the little piles of salt that are left in the roadway.
Because tomorrow, they shall all be gone.
They will have begun their journey to return to Mother Ocean. Whence they originated, back when dinosaurs roamed the earth.
Think of it as a salty circle-of-life kind of thing.
Observation: There is (always!) excess salt left on the roadway.
We have reached the ugly end stage of this last snowstorm. Patches of white snow remain, but only here and there, on lawns and roofs. Parking lots remain clogged with great dirty piles of snow.
And roadways are littered with ugly little piles of salt. As they always are, after a snowstorm in this area.
Call those ubiquitous little piles of left-over road salt the “excess salt”. (Which is kind of judgemental, but for now, I just need something to call them).
I’m not talking about private parking lots, or walkways, or whatnot. Places where the property owner spreads salt in some catch-as-catch-can manner. I’m just talking about little piles of leftover salt, on the public roadways.
If you think about them, at all, it’s not as if VDOT set out to leave a bunch of salt lying around, when they salted the roads last week.
For one thing, those piles of excess salt are a deadweight loss. It takes money to buy and spread that salt. But the left-overs didn’t do anything useful. They just add to the subsequent salt burden on local rivers and, in my case, the water supply.
In fact, it’s fair to say that every state DOT is aiming to use less salt. Not just for the environmental impact, but because it rusts out cars, bridges, rebar embedded in concrete, and so on.
Paradoxically, part of this push to reduce the use of road salt is the now-common practice of “brining” the roads ahead of a snowfall. That is, spraying everything with salt water, which then evaporates to leave a microscopically thin, uniform layer of salt. This is used for anti-icing (to prevent snow from sticking to the road), as opposed to traditional de-icing (un-sticking already-frozen snow). That preventive approach to disbondment (see prior post) results in less salt being used, in total. In Virginia, brining the roads only came into general use in the 21st century, and was relatively uncommon as late as the mid-1990s (reference, .pdf).
Finally, as far as I have been able to tell, VDOT (and similar) aim for a uniform application of salt, just sufficient to achieve “debondment” between ice/snow and pavement.
And yet, little piles of salt in the “corners” of the roadway remain a fixture of the urban winter landscape. Post-snowstorm, we always end up with deposits of rock salt in the dead space next to the curb, sometimes between the lanes, and in general, on the road surface wherever tires do not routinely roll.
So that’s a little bit of a puzzle, isn’t it. Everybody involved is trying their damnedest to use as little road salt as possible. The salt is applied by professionals using the best known techniques. They aim for a uniform application of salt sufficient for debondment. But we still end up with little piles of excess salt everywhere.
Why? Mere carelessness is not an adequate explanation.
Why is there always excess salt, in little drifted piles, on the road, after a snowstorm?
This is a surprisingly hard question to answer. Around here, the excess salt piles show up at intersections. There’s no obvious explanation for that.
My best guess is that this occurs because a) the pavement of an intersection gets double-salted (one salt-truck pass for the main street, one for the side street), and b), as a matter of geometry, larger intersections effectively result in higher piles of excess salt at the curb.
Other explanations do not seem to hold water.
Mere sloppiness? No.
Naively, you might think that we see these excess salt piles because VDOT went at it with too heavy a hand. Or spilled some extra in those spots. You can try to dismiss it as the result of mere sloppiness.
But attributing those ubiquitous piles of excess salt to “good enough for government” work does not stand up to scrutiny. For one thing, even a bit of research shows you how seriously VDOT (et al.) take the the timing and calibration of their salt spreading and spread rates. It’s as close to a science as they can make it. For another, those little excess salt piles show up everywhere, pretty much, regardless of which government entity or contractor put the salt down on the public road. No matter which year. And they have done so, ever since I can remember.
Nor does the excess salt pile up everywhere, as it would if VDOT simply applied it with too heavy a hand. Around here, I see those excess salt piles at intersections, on highway overpasses, and where lanes merge. I specifically see none — no visible salt crystals whatsoever — along sections of road between intersections.
But what would be the systematic reason for them?
So, to the contrary, I’m going to start from the notion that there’s probably a reason that we always end up with little piles of left-over road salt. A reason those show up in the intersections. And I’m betting it’s something inherent about salting the roads that leads to this.
In other words, I’m betting that we always end up with little piles of excess salt because that’s an inherent and unavoidable part of salting the roads.
I can’t prove that, but here are my best guesses as to why we only see those excess salt piles in certain locations.
Guess 1: All VDOTs, everywhere, put extra salt down at intersections? And always have. Presumably on purpose, as a safety measure? To roughly the same level of excess. So, in effect, the excess salt was put there, on purpose by VDOT.
If so, they have all, collectively, been pretty quiet about doing that. I have found exactly zero evidence to suggest this is true. No directions to do so, no mention of doing that in a “best practices” manual, and so on.
(There may be some governments that only salt intersections, but VDOT is not one of them. Nor is the Town of Vienna. Near as I can tell, every street in Vienna got salted.)
In addition, most truck-mounted salt spreaders aren’t set up for a lot of fine-tuning on the fly. Like, turning them up, as you cross an intersection. For the simplest salt-spreader rigs, the only in-cab control is an on-off toggle switch (top, below). More modern ones let you control the flow of salt, on the fly (bottom, below). Neither style looks like it’s set up to encourage the truck driver to fiddle with it, while driving a piece of heavy equipment. Say, at night, in a driving snow, with traffic.
Source: A couple of installation manuals for truck-mounted salt spreaders.
So, while it’s not out of the question that VDOTs around the country somehow turn up the salt flow at intersections, this seems unlikely. I can find no mention of this as a standard practice, and it runs contrary to the fundamental notion that salt, for road use, is for disbondment of ice/snow on pavement.
Plus there’s no obvious way to do it, other than asking the salt truck drivers to slow down at every intersection.
Guess 2: The pavement of the intersection gets double-salted (once for salting the street, once for salting the cross-street).
That’s a fact. If you’ve set the salt spreader to provide X salt per 1000 square feet of road, then road intersections get 2X. Once for the main street, and again for the cross street. And, really, you might end up laying down extra salt anywhere salt trucks have to make multiple passes to completely salt a stretch of pavement (e.g., where there are an odd number of lanes, or where lanes begin or end, or merge areas.)
Guess 2A: Large intersections should generate more noticeable piles of salt at the curb line (compared to small intersections).
That’s also more-or-less a fact, owing to geometry. The area of an intersection increases with the square of the number of lanes. Salt is spread over the entire area. But the length of curb line is … roughly constant, at least until you get to the point where there’s a divider between the lanes. The upshot is that an intersection between six-lane roads ought to give curb-side excess salt piles that are nine times higher than you’d find at the intersection of a pair of two-lane roads.
Guess 3: Salt is applied uniformly on the roadway, but somehow migrates to the intersections? I don’t think this is true, but I can’t prove it.
For sure, salt migrates short distances on the roadways. It moves out of the path of actual tire contact, which is why the excess salt piles are always in the “dead” spots in or around the roadway. But that would account for salt moving a few feet, not hundreds or thousands of feet down a typical suburban street or arterial roadway. Which is the only way this hypothesis would explain salt at the intersections.
In addition, VDOT puts a lot of thought into keeping the salt where they put it. It is common practice to pre-wet the salt, both so that it stick better, and so that it starts melting snow faster. In addition, best practices call for applying road salt only after some snow has fallen, specifically so that the salt will stay put.
Finally, I think the contrast of intersections and surrounding roadways is too pronounced to have been produced by such a sloppy process as salt being pushed around by tires. With this last snowfall, I see no salt crystals at all on the road, until I’m literally at the intersection.
Upshot: Of the three mechanisms that would generate salt piles at intersections (only), and particularly at large intersections, I think it’s due to the inherent double-salting of the intersection pavement, coupled with simple geometry that results in higher excess salt piles at larger intersections.
Drinking water contamination, or, why do I even care about road salt?
I care because I’ll be tasting salt in my tap water sometime next week. Based on past experience, I’d bet that’ll start around Wednesday or so. That, as a result of the road salt being washed into the Potomac River by today’s rains.
There are enough reports of road salt contaminating drinking water that I’m pretty sure I’m not fooling myself about the salty taste of the water. This, even though my wife can’t taste the salt. Sensitivity to the taste of salt varies substantially across individuals. And, for an individual, it varies considerably with the amount of salt in the diet. (Unsurprisingly, the more salt-deprived you are, the more sensitive you are to the taste of salt).
Of more scientific interest, the fact that I can taste the snow-melt salt in my drinking water suggests that post-snowfall salt (chloride ion) content of my drinking water routinely exceeds 250 parts-per-million, the lowest threshold at which some people can taste the salt. At least a quarter-teaspoon of salt, per gallon. Not a health hazard for most, but enough that I can taste it.
But is that plausible? Do we really put down enough road salt to make the drinking water salty?
Yep. A simple and conservative back-of-the-envelope calculation says that if the Town of Vienna, VA applies road salt a standard rate, it would plausibly generate a brine salty enough to taste, as that salt mixes with half-an-inch of rain.
Further research shows that in a big snowfall, Vienna applies orders of magnitude more salt. The Town newsletter reported 600 tons of salt used for a January 2016 snowstorm that dropped about 30 inches of snow (Vienna Voice, March 2016, page 6). Or roughly 60 times as much salt as was assumed for the calculation above.
The upshot is that yes, a single application of road salt, at a middle-of-the-road rate, followed by half-an-inch of rain, should result in runoff from the Town of Vienna that has a noticeably salty taste. And that exceeds current EPA water-quality standards for salt in the water.
I may or may not be able to taste the salt, with this modest snowfall. After all, that will depend on the rate of salt use across the entire Potomac watershed.
I may yet invest $20 in a cheap salt meter, and track the chloride ion content of the water. Just to see if that’s accurate enough to document the wave of road salt that will be passing through my local water supply next week.
Conclusion?
This is a bit of an odd post, even for me.
You’ve probably seen little piles of excess salt on the roadway all of your life. And yet, each time, if you thought about them at all, you probably just tut-tutted them away. Dismissed them with some sort of ad-hoc explanation. Maybe your local DOT (or alternative) is just sloppy, or something, and over-applies the salt. Or maybe cars effectively sweep the salt along, until it gets to the intersections, where it stops. Or maybe VDOT specifically and purposefully over-salts intersections.
I don’t think any of those simple explanations is correct. They do not explain the consistency and ubiquity of these little piles of excess salt. No matter the year, or the jurisdiction, if they salt the roads, little piles of excess salt are a normal and expected part of the urban winter landscape.
My best guess is that the ubiquitous piles of excess salt are an inherent and inadvertent part of salting the roads in winter. The most plausible explanation of why the excess road salt is consistently where it is, around here, is a straightforward double-salting of the intersections. If you are salting the pavement adequately in all directions, then you are salting the pavement of each road intersection twice adequately.
And for larger intersections, that over-salting-rate gets amplified, as the salt strewn over the collection area of the intersection (which rises with the square of number of road lanes) gets squeezed against more-or-less the same length of curb. So that an intersection with twice the lanes should yield four times the concentration of excess salt pushed to the curb.
But in the end, it comes down to whether or not there’s anything actionable. Is there any way to avoid those piles of excess salt? And would it make any material difference, if you did?
And there, I think the answer is no. No, you can’t avoid over-salting intersections, with manually-controlled salt spreaders. For most, the driver simply turns the salt spreader on when the truck is moving, and off when the truck is stopped. (Having salt spreaders that are synced to vehicle speed is considered a big step up from the standard setup.)
When you get down to it, the problem is the use of road salt. Full stop. Modest over-use on intersections adds trivially to that problem, for the simple reason that intersection area is a small fraction of total road area. So the problem of excess salt in the intersections is not worth addressing, particularly if that would require use of more sophisticated (e.g., GPS-run) salt spreader controls, to avoid dumping salt twice on the same intersection.
My guess is that as long as we salt the roads, we’re going to see little piles of excess salt in the road intersections. I think that’s an inherent and unavoidable part of the process. But that, empirically, that additional salt in the intersections is a drop in the bucket, compared to the total amount of salt spread on the roads.
End of story.
The only followup will be to try to document the spike in salt in my drinking water, as a consequence of the runoff from this most recent storm. And that will depend on whether or not I feel like shipping $20 off to China, via an American oligarch, by the purchase of a cheap salt tester on Amazon.
