Monday, January 16, 2012

Salty Language

Chicago is beautiful all dressed up in snow, but after a couple of days, the look becomes pretty dingy.  The snow pushed aside by plows reveals its underside, gross from the street dirt.  The snert (snow + dirt) then melts into brown slush pools that look solid but give way when you step on them.  Dogs also decorate the snow as they are wont to do in our neighborhood.  Then there's the salt grime, white or a rainbow of colors, depending on what variety is used.

A salted bike path by the Navy Pier.

It's remarkable how much salt is applied on this city when it snows or ices.  The streets and sidewalks are quickly covered in the stuff.  It does indeed lower the freezing point of water to about 0 degrees Fahrenheit, enabling the snow and ice to melt and run off the sidewalk, so it's a useful substance when it's not extremely cold.  I can't help but wonder what happens to all that salt as it is washed off the sidewalks and down into the drains or soil. 

Salt has bounced off the sidewalk onto the soil above this tree's roots.
Even though life is thought to have originated in the ocean, where there is plenty of salt, life on land has adapted to a low-salt environment.  Land plants are very vulnerable to excess salt because their cells shrivel and break when exposed to salt water.  Try watering a houseplant with very salty water.  It will die.  If the houseplant is a flimsy, soft one, it will wilt and shrivel in a matter of hours.  The tree in the picture above is going to be very stressed in the spring when rain washes the salt down to its roots.  I imagine the city has to replace many trees and use salt-tolerant varieties as much as possible.  It is not uncommon to see screens erected around street-facing gardens to keep the salt out.

The salt that doesn't seep into the soil washes down into the storm drains.  Some storm drains probably go straight to Lake Michigan, but most go to a sewage treatment plant, thank goodness.  Urban storm runoff contains not just rainwater and salt, but everything that drips off the underside of cars, dog waste, pollution particulates that have settled out of the air, and countless cigarette butts.  It contains more toxic compounds than regular flushed sewage. 

Salt isn't usually removed during sewage treatment, and it is released back into the environment with the cleaned sewage water, usually into a river or lake.  The salt then raises the salinity of the river, increasing mortality for aquatic life.

Road salt is the same chemical as what you sprinkle on your eggs in the morning at breakfast: sodium chloride.  It is a necessary compound, and without any salt we would die.  For us here in North America though, too much salt is usually the problem for humans as well as urban soils and river life.  Salt causes problems in our bodies the same way it does for fish downstream from winter stormwater runoff.  It's essentially a water-balance issue.  Have you noticed how thirsty you get after eating a very high-salt food like Fritos or a cheeseburger or almost any meal at a chain restaurant?  That thirst is a symptom of salt imbalance.  The salt has dehydrated our cells just like it does for plants, and our bodies become thirsty, causing us to drink more water to dilute the salt and unshrivel our cells before they break.  The excess salt and water in our bodies causes swelling and a 1-3 pound increase in body weight (also known as water weight), until our kidneys can filter out the whole mess. 

People who eat a high-salt diet long-term maintain that swelling and high kidney workload for years.  It can, depending on one's genetics and other lifestyle factors, contribute to chronic high blood pressure and eventual strokes or cardiovascular problems.  The tiny capillaries in the body can be degraded from constantly having an elevated amount of fluid in them, which disrupts function in the extremities of the body, the brain, the kidneys, the heart, the eyes, and everywhere else capillaries are essential to body function (which is, really, everywhere).  Fortunately, salt is usually only found in such high quantities in processed food and restaurant food, so if you cook for yourself most of the time and just salt your food at the table, you should be OK.

Very large quantities of salt are toxic in the short term, also known as acutely toxic.  Salt toxicity by oral ingestion has been tested on mice (thankfully not on humans).  A dose of 4g of salt per 1 kg of mouse will kill 50% of mice within one day of ingestion (those numbers are called the LD50, or lethal dose for 50%).  If that number is applicable to humans, let's figure out what it means.  A typical 150 pound person weighs about 68 kg.  4g times 68 kg equals 272 g.  So 272 grams of salt gives a person 50/50 odds of survival.  272 grams is about 15 tablespoons.  Ick.  Hypernatremia (salt toxicity) symptoms of thirst, irritiability, weakness and dizzyness kick in way before coma and death, so the problem can be remedied before it gets too bad.  Hypernatremia in humans is actually more commonly caused by removing water instead of adding salt, and we refer to it as dehydration.

Bacteria and other microbes are killed by excess salt the same way our cells are.  The salt causes their cells to shrivel.  Humans have taken advantage of this trait and used salt to prevent microbes from surviving on some foods and spoiling them.  Beef jerky, for example, is "cured" with lots of salt.  You can demonstrate this phenomenon by taking a piece of beef jerky and a piece of raw steak and leaving both out on the counter in your kitchen for a week.  Be sure to check on them daily to notice any smells or discoloration.  Or flies.

Now back to snow.  Seattle has decided it doesn't want to deal with salt pollution problems, and they handle snow clearing in a different way.  Seattle plows the snow with a rubber-coated plow, leaving some snow in a hard-pack on the streets.  Then they sprinkle sand on top of that.  The surface becomes less slippery, and the sand helps cars grip.  The streets are passable for front-wheel drive vehicles and all-wheel-drive vehicles, but they are tricky for rear-wheel-drive cars, such as the police use (oops).  Even though this solution has a slight logistical cost for getting around in winter, it really cuts down on the costs of environmental damage to Puget Sound.  Salt run-off harms fishing and tourism industries in the sound by reducing aquatic life, and it also slows natural nutrient cycling by killing bacteria, so the sound doesn't stay clean.  Seattle-ites would rather have a clean Puget Sound and drive a little slower in the winter.

1 comment:

  1. I enjoyed this. I always worried about the amount of salt. You see it in mountains at the DOT centers across the country.

    You should make this a front page article!