Chip sealing--and how dust affects our lakes

Today, most concerns about air pollution focus on global warming.  Yet for those of us working for the health of our lakes, air pollution remains an important issue.

It's hard enough to convince people that dirty stormwater runoff is an important issue.  So why even bother with dust in the air?

It all adds up

Ever since the big industrial polluters were forced to stop dumping to our waterways, nonpoint sources have been the problem.  It's very, very difficult to control the myriad of tiny sources of water pollution about the city and countryside.

But to be successful, we have to try to control them all--starting with the most significant ones.  You know the drill--pick up after your pet, less fertilizer on lawns....

Looking at it that way, air becomes one of the nonpoint sources.  Activities that add dust to the air become nonpoint sources.  That's because dust falls to earth as rain.  And dust on the pavement becomes water pollution, when it flushes to the stormsewers.

Giant snow pile on campus, at end of the winter.
Much of this dirt is fallout from the air.

It started with the glacier

When the glaciers invaded Wisconsin, they ground  rock into dust.  This dust was deposited in layers over 50 feet thick in places--deposits called loess.  It makes wonderfully fertile soil--that's why Wisconsin has productive farms.

But when that soil washed into lakes, they became over fertilized.  Weeds and toxic algae choked the lakes.  That dust from the glacier has contributed to problems in our lakes--though misuse of the landscape makes the problem far worse.

Dust blowing from the valley of the Copper River, which drains glaciers in Alaska. NASA

When you compare Wisconsin to Cape Cod, you can see the influence of glacial dust.  Imagine visiting the two places, just as the glaciers start to recede.  In Wisconsin, clouds of dust take to the air as winds blow across streams of meltwater flowing from the ice.  Any dust that blows away is replaced by dust arriving from further upwind.

But in Cape Cod, any dust in the meltwater washes or blows out to sea.  Since Cape Cod is on the coast, with winds often coming from the sea, there's not much glacial dust blowing in from elsewhere.  So Cape Cod has poor soil--farms will be abandoned by 1900, and the lakes will stay pure (until recently).

Left: Kettle pond on Cape Cod. Note clear water.

Right: Kettle pond in Wisconsin. Note algae bloom in Fish Lake.

Click to enlarge.

Local sources of dust

Madison is the 24th most polluted city, in terms of short-term particulate pollution in the air.  So it's a significant health issue, potentially harming tens of thousands of sensitive people--children, the elderly, people with asthma or lung disease.

Much of the dust originates at construction sites...

Photo above at MG&E.
...or bare ground at industrial yards, dumps, or quarries.  

Enormous amounts of mud are tracked out of construction sites.

Far-away wildfires

One day while swimming in a pristine pond on Cape Cod, I noticed a layer of fluffy dust on the surface, catching light from the afternoon sun.

I knew this was unusual, so I looked for the source.  The news was reporting wildfires to the north in Quebec, and the wind had been blowing towards Cape Cod.

So the dust on the Pond's surface was probably wildfire ash, bringing unwanted phosphorus to overfertilize the pond.

An earlier fire in Quebec sends smoke over Cape Cod.
The Cape's outline is visible on the far right.

And now, dust from chip sealing

The City has been sealing the surface of Madison's streets, using a combination of asphalt and a black, gritty substance called "bottom slag."

Slag is coal ash, from coal-burning power plants.  The ash contains phosphorus, salts, and toxic heavy metals.  But spreading it on roads is considered acceptable, because these harmful components aren't soluble, and the grit is supposedly held in place by the asphalt.

Using slag, instead of putting it in a landfill, has environmental benefits.  And using slag to maintain roads can mean big savings for the City.  But do the benefits outweigh the costs, over the long haul?

Slag in pavement will be continually pounded into dust.

Since Engineering is planning to REAPPLY chip sealing numerous times, this means a large fraction of the slag layer will--over the years--disappear into the air and water.  The salts, metals, and P in slag dust will be much more mobile, and leachable.

Anything you put on the pavement winds up in the lakes.  So, chip sealing is acceptable ONLY IF we test the dust of the slag being used, to make sure it doesn't release it's harmful load.

Mud and dust are two sides of the same coin.

Both are bad for the lakes.

1 comment:

  1. The dust which comes from power plants, factories, fertilized soils and construction sites pollutes our clean air and water. A number of countries from different parts of the world have been adversely affected by dust problems .It may came from ongoing constructions or infrastructures or even dirty roads. I hope the government will impose penalties for contractors or industries that emit dust or pollute our air and water.
    Monica Barnes


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