How much salt falls naturally out of the sky?

During the past several weeks, as snow has covered the roads of Washington and Oregon, salt is being used to prevent an ice layer from bonding to the concrete and to help melt the snow.  Clearly, salt can make travel far safer, prevent injuries and deaths, and keep our roadways functional. It can prevent a city from being crippled, like Seattle was in 2008 and Portland experienced last month.

But some folks wonder about the environmental impacts of roadway salt and the potential to corrode cars and other vehicles.   But to answer this question, one needs to start with another question.   How much salt is naturally in our environment, particularly salt that reaches the ground in precipitation or by dry deposition?

The answer is going to surprise you.  A lot of salt is falling out of the sky!  In fact, probably more than being spread by State and local departments of transportation.

Where is it coming from?  Breaking ocean waves!    The ocean is salty, of course, and when waves break or when wind produces spray, the air is filled with salt water droplets.  These droplets can evaporate, leaving small particles of salt in the air.   Here in the Northwest, salt particles can be easily blown inland by the

prevailing westerly winds and  brought back to the ground by precipitation or even dry settling (deposition).

To illustrate, here is a map created by the National Atmospheric Deposition Program, based on combining observations and models, that shows the wet deposition (by precipitation) of one component of salt, sodium ions.  (Remember,  from your basic chemistry that salt is sodium chloride, NaCl).   The red areas show large depositions (more an 4 kilograms per hectare).   And remember, a kilogram (kg) is 2.2 pounds and 1 square mile is 259 hectares.

You note there is more salt deposition near coastlines and particularly stormy coastlines.  That is why the Northwest is well salted but California is not.   You can see the effects of the Great Salt Lake as well.  Other years look very similar to 2015.

So based on such research it has been found that the Puget Sound lowlands get around 7 kg per ha per year of salt and the coast (closer to the breaking waves) receives about 30 kg per ha.

So there is considerable salt falling on the surface by natural processes, with little obvious problems for animals and plants.

OK, now let's have some fun, comparing the amount of salt falling on Seattle each year against the amount of salt resulting from roadway protection in the city.

Seattle encompasses 83.78 square miles.  Each square mile is comprised of 260 ha.  Thus, the city has 21,783 ha.    So if 7 kg  of salt falls per year per ha, then the total amount of salt reaching the ground in Seattle each year is 335,455 pounds of salt or 168 tons.

Mama mia!  We have a salty city!

Let's compare this total to how much salt is used to protect the roads for one snow storm, which seems to be the typical number per winter the last few years.  From a little digging, I believe that Seattle uses about 100 tons (200,000 pounds) for a single snow event--less than the 168 tons noted above.   But even I am off a bit, I suspect the bottom line is reliable:

The natural precipitation of salt from the ocean over Seattle is roughly equal to the salt spread for reducing icing during a single storm.  

Thus, I suspect roadway salt is not a significant environmental hazard. Furthermore, since the salt is spread over roads much of the salt goes into drains, some of which go directly to treatment plants and the Sound.  This concern is further reduced by the fact that Puget Sound is a salt-water estuary, with considerable salt content (although a bit less than the ocean, 2.9% versus 3.4%).

Roadway salt, spread during heavy snow periods, does not go into drains immediately, but rather over several days, thus reducing pulses of salt.    Furthermore, it is accompanied by a surge of melt water that dilutes its concentrations.  As an estuary expert I consulted stated:   "dilution is the solution".   You've got to love sayings like that.

It is interesting to note that the natural deposition of salt does not fall in a uniform way, but accompanies the major storm/rain events that also bring large waves and lots of sea spray.  I have confirmed this by looking at the weekly data available from the national deposition site.  Here is an example showing the weekly amounts of sodium from salt at La Grande, in Pierce County from January 2010 to now.  Lots of variability.

Thus, there are natural spikes in salt deposition, with only a few weeks each year providing much of the annual deposition.

I am not saying the salt on roadways is absolutely harmless, but that the risk is small compared to the alternative loss of life, injury, and economic damage. Like is about risk versus reward, costs versus benefits, and in this case the benefits of using road salt one or two times a year far outweigh the risks.

 Finally, modern vehicles have far better primers and paints, and thus are less susceptible to salt-associated corrosion.

Clearly, the environment implications far salt would be far greater for cities with a lot of snow that require many salt applications over the entire winter--such as Chicago or Buffalo.   Thus, Mayor Murray and the folks at SDOT can probably sleep soundly knowing the Seattle's occasional salt spreading is not undermining the environmental quality of the city.  A previous Seattle mayor learned the hard way the dangers of not using salt.

Acknowledgments:  I received guidance from Joel Thornton and Dan Jaffe, experts in atmospheric chemistry at the University of Washington.