December 2001

Wait 'til the Dust Settles

Wait ’til the Dust Settles

By Bill Langer

Editor’s Note: This article is the final installment in a 12-part series focusing on how geology can lessen the “surprises” and help overcome the challenges posed by nature during the process of aggregates extraction.

A while back, my wife and I were driving from Denver to visit our daughter and son-in-law in Phoenix. It was a beautiful autumn day—clear skies and somewhat breezy. We were headed towards Albuquerque, N.M., when my wife pointed to a large plume of dust about a mile off and proclaimed, “I bet that’s one of your aggregate quarries!”

For some reason my wife likes to taunt me by implying that every quarry or gravel pit that is creating a disturbance somehow belongs to me. I responded, “There are many activities that can create dust. Maybe it is a construction project.”

It took us about a minute to reach the source of the dust, and as each second passed I became more and more uneasy. It was rapidly becoming obvious that my wife was right and I was wrong. (I hate it when that happens.)

A xeriscape garden at an aggregate operation reflects the biodiversity of the area. Photo courtesy of Lafarge.

About 50 yds. from the side of the road, sat a portable crusher and screen cranking out what appeared to be equal parts of crushed stone and dust. The breeze carried a thick cloud of dust away from the crusher in a plume just above the treetops that ran parallel to the ground for nearly 200 yds. beyond the quarry boundaries. When the plume approached a hill, it was forced upward where it gradually dispersed into a thin, white haze. It appeared that this had been going on for some time because the nearby trees were covered with a gray dust and looked really sickly.

It was obvious that the operation was taking no precautions to prevent or control the dust. I hastily made a feeble comeback. “Aggregate operations don’t have to create that kind of mess. Obviously, they don’t have a good handle on what they are doing!”

As if to prove my point, we passed a very large aggregate operation about a mile farther down the road. A major producer ran that operation; almost all of you would recognize the company name. Not a speck of dust was leaving the site. It was my turn to proclaim, “Now that’s my kind of quarry!”

That single episode nicely summarizes many of the points about aggregate extraction and the environment that this column has addressed over the last year:

• Geology and other natural conditions control the types of environmental impacts and the areas affected by those impacts—the rocks in the area were limestones with some shale interbeds. They are prone to creating dust when crushed. This is no surprise. Dust even comes from the disintegrated rock on natural outcrops. The wind, topography and other natural conditions determine the area impacted.
• Many environmental impacts are caused by engineering activities and can be controlled by engineering techniques—the type of crusher and the way the crusher was set up created an inordinate amount of uncontrolled dust. The well-run operation demonstrated that dust generation can be limited and that the transport of dust from the site can be controlled.
• Some activities create cascading impacts—the dust had spread over the landscape and covered nearby trees, which smothered leaf surfaces, which reduced photosynthesis and blocked leaf stomata, thus inhibiting gas exchange. The vegetation could die, and the area would become more prone to erosion. Sediment load would increase in streams. And on and on.
• Aggregate operations can degrade or improve biodiversity—the poorly run operation clearly degraded biodiversity. The well-run operation has a xeriscape landscape that reflects the biodiversity of the area and the ground cover, parking lot, pavement, signage and building make use of aggregate—all helping to educate the public.

And speaking about education, how many people saw the dust plume that my wife and I saw? We did some quick calculations and estimated that about 50 cars pass that point in one minute; 3,000 cars per hour with an average of 2.5 people per car meant that 7,500 people per hour pass that location. We figured that most of the people saw the plume of dust coming from the poorly run operation, but only a few noticed the well-run operation. A potential audience of about 7,500 people per hour learned that quarries create large amounts of dust—dust that goes uncontrolled across the landscape and can harm nearby trees.

Dust is one of the most visible impacts associated with quarrying and its visibility often raises concerns that are not directly proportional to its impact on human health and the environment. Fortunately, dust can be controlled and rarely presents a serious threat to people or the broader environment. In the majority of situations, the dust generated by aggregate operations is chemically inert. Most dust that is generated is contained at the site, and dust that does leave the site disperses rapidly away from the source. Nevertheless, 7,500 people per hour received a lesson on how poorly a quarry can be operated. The next time there is a quarry being proposed in their hometown, how many of those people do you think will show up at the public meeting to discuss the dust and share their experience?

William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.