|
In the particulate matter spectrum, particles
that are less than 2.5 micrometers (microns) in diameter — about
one-twenty-eighth the diameter of a human hair — are called fine
particles (PM2.5). Particles 10 micrometers microns or less — about
one-seventh of the diameter of a human hair — are called coarse
particles (PM10). While soot from coal-fired power plants is classified
as a fine particulate matter, dust is considered a coarse particle.
The EPA has had national air quality standards
for PM2.5 since 1997, and for PM10 since 1987. The agency’s 1987
standards for PM10 coarse particles and smaller are a 24-hour standard
not to exceed 150 micrograms per cubic meter of air, and an average
annual standard of 50 micrograms per cubic meter.
Scientific studies have linked particulate
matter pollution with a series of significant health problems,
including:
-
Increased respiratory symptoms, such as
irritation of the airways, coughing, or difficulty breathing.
-
Decreased lung function and aggravated asthma.
-
Development of chronic bronchitis and
irregular heartbeat.
-
Premature death in people with heart or lung
disease.
Particle pollution can cause coughing, wheezing,
and decreased lung function even in otherwise healthy children and
adults.
“PM pollution settles on soil and water and
harms the environment by changing the nutrient and chemical balance,”
the EPA states in its particulate matter guidance. “Particle pollution,
unlike ozone, can occur year-round.”
Tighter coarse PM regs?
New proposed revisions published in January 2006
would change the definition of PM10 so that it covers only particles
between 10 and 2.5 micrometers in diameter, to be known as PM10-2.5, and
tightens urban standards for inhalable coarse particles.
Of interest to the road industry is that the
tighter standard would apply to airborne mixes of coarse particles that
come from sources such as high-density traffic on paved roads and from
industry, but not to mixes of coarse particles that do not pose much
risk to public health, such as windblown dust and soils, and
agricultural and mining sources.
This would be a 24-hour (daily cumulative)
PM10-2.5 standard “qualified so as to include any ambient mix of
PM10-2.5 that is dominated by resuspended dust from high-density traffic
on paved roads, and PM generated by industrial sources and construction
sources,” the EPA said.
The standard “excludes any ambient mix of PM
10-2.5 that is dominated by rural windblown dust and soils and PM
generated by agricultural and mining sources ... [a]gricultural sources,
mining sources, and other similar sources of crustal material shall not
be subject to control in meeting this standard.”
Even though the proposed regs would impact only
urban coarse particulate matter, rural interests are alarmed because
they see establishment of this regulation as a potential, incremental
step toward rural regulation. Rural interests want to see no tightened
coarse particulate matter standard, period.
“There is not a sound or adequate basis for the
adoption of a coarse PM standard in rural or urban areas at this time,”
says the National Cattlemen’s Beef Association. “Rural or urban, it’s
ridiculous to consider regulation of dust in the wind. You manage
fugitive dust with proactive dust control measures. It is impossible for
dust in the wind to be regulated in this manner.”
Exceeding PM10 in the Southwest
Arizona’s Maricopa County, home to Phoenix and
its surrounding suburbs, is one of the few counties not in compliance
with existing EPA PM10 standards. The others include the Mojave Desert
counties of California.
In Maricopa County’s desert location, little can
be done about the prodigious amounts of desert-generated dust, so to
reduce the count of PM10 material, the county is looking very closely at
man-made (anthropogenic) sources, including activity-related and
windblown dust from unpaved roads and parcels, and construction and
agriculture.
The county urges residents to reduce travel on
days with poor air quality, and drive slowly on unpaved roads and other
dirt surfaces.
In New Mexico, on certain days some counties can
suffer dust levels that exceed both state and federal standards for
PM10. The State of New Mexico identifies sources of dust which include:
-
Unpaved roads, parking lots, and playgrounds.
-
Soil disturbance during construction projects.
-
Disturbed land areas that are cleared and
vacant.
-
Windblown emissions from tilled fields.
-
Military training exercises.
-
Unpaved equipment yards.
-
Undisturbed desert areas during the highest
winds.
Dust control agents save roads
So if dust is a problem for unpaved roads and
construction sites, what can be done about those sites?
A time-proven solution to fugitive dust from
roads is use of a palliative which forms a hygroscopic — or
moisture-absorptive — layer on an unpaved road surface which will
suppress blowing dust. It’s one element of a toolbox of solutions that
works. But these palliatives do more than just improve air quality; they
also keep your unpaved road from blowing away.
“When you see dust coming up from your roads,
you’re really seeing dollars thrown to the wind,” says the Rhode Island
Technology Transfer Center, part of the Local Technology Assistance
Program funded by the Federal Highway Administration.
“Road dust is made up of fine particles that are
important to the stability of the road,” the center states. “These fines
are small enough to pass through a No. 200 sieve and feel like powder
when rubbed between your fingers. When fines blow away, the gravel road
begins to break down. Traffic scatters the coarser aggregate, causing
potholes, ruts, washboards, loss of profile, loss of ditch lines, and
other problems.”
Periodic blading of an unpaved road surface
several times a year, though needed to maintain a crown and eliminate
ruts and holes, has the effect of loosening aggregate and dust as well.
Water as dust suppressant
Water is the traditional dust-suppressant and is
required on most road projects and in gravel pits and quarries
throughout the summer. Moisture wets the dust particles, effectively
increasing their mass and cohesion. Moisture helps fines adhere to each
other and to aggregates, allowing for optimum compaction under traffic.
But frequent water applications are labor-intensive and the money saved
by use of simple water can be absorbed by higher labor costs.
Also, it’s never possible to keep all unpaved
road surfaces moist at all times, and in arid climates the moisture can
evaporate in minutes. And if water is used excessively, the excess may
create unwanted runoff from the road, associated erosion, and possibly
create conditions where vehicles could track mud elsewhere, creating
more dust.
Dust palliatives are an alternative to water.
They can be used to suppress dust on a driving surface, keeping moisture
in the road, and actually absorb humidity from the ambient air, further
suppressing dust.
Chemical road treatments or palliatives work to
keep dust in control. There are too many to list, but generic examples
of these chemical palliatives include anionic asphalt emulsions, latex
emulsions, resin-water emulsions, and old familiars like calcium
chloride. When considering chemical palliatives for dust suppression,
the agency should ascertain whether the chemical is biodegradable or
water-soluble, and what effect its application could have on the
surrounding environment, including water bodies and wildlife.
Calcium chloride and other palliatives
A familiar, hygroscopic dust-control agent is
liquid calcium chloride. CaCl absorbs water vapor from the air and
moisture extant in the road structure. At 77 degrees F and 75% humidity
— common conditions during the summer in the Midwest and South — it
absorbs more than twice its weight in water. In addition, calcium
chloride solutions attract more moisture to the road than they give up
in evaporation. Thus a treated road surface can retain moisture even
during the heat of summer.
Calcium chloride is available dry or as slurry,
and often is placed as a 35% solution using a tank truck with a
rear-mounted distribution bar. Once on the road, CaCl is attracted to
negatively charged soil particles, such as clays, which help resist
leaching. “Calcium chloride may move deeper into the base during wet
weather, but will rise toward the surface during dry spells,” advises
the Rhode Island Technology Transfer Center. But calcium chloride’s
effectiveness in dry regions will be limited due to the low relative
humidity.
Related products include magnesium chloride and
sodium chloride.
Another dust palliative, lignin sulfonate, is a
byproduct of the wood, pulp, and paper processing industry. It has been
used for decades to control dust and stabilize gravel on unpaved roads.
For dust control, this tree sap, as it’s called, is spread on a road’s
surface, but for stabilization and dust control it can be mixed into the
top layer of the road. Periodic retreatment will be needed throughout
the year.
Lignin sulfonate (and its derivative cousin
products) is moderately hygroscopic and the wood sugars act as a binding
agent for the particles. It boosts the dry strength of the road soil and
is not as dependent on relative humidity as calcium chloride. It’s been
found not to work well on materials such as decomposed granite, which
remain coarse upon weathering.
Its applications can be effective for about four
months. It is also sprayed on dust to facilitate blading into windrows
for pickup. It is produced in the form of a 50% solids liquid, and is
generally diluted with two to four parts water.
Another palliative, Molex, is a byproduct of the
processing of sugar beets and also works to suppress dust. A
concentrated liquid extract of beet molasses, Molex has a strong
affinity for moisture, contains high levels of potassium chloride
(replacing calcium chloride), is noncorrosive, and won’t freeze under
normal conditions.
Crusts cure dusts
While not hygroscopic in action, road surface
crusts created by a variety of liquid chemicals can keep dust down and
PM10 counts in check.
Bituminous products, including tars and resins,
are proven dust fighters. Fuel oils have been commonly used, have low
solubility in water, and actually waterproof the surface of your unpaved
road, but their use is declining for environmental reasons.
Alternatively, asphalt emulsions can be used to suppress dust.
One bituminous product, Lion Prime, is used by
some states for stabilization of road shoulders and is also used in
spray application on copper mine tailings to control very fine powdery
dust. It will effectively penetrate 2 to 3 inches and act as a binding
agent, but will decompose after several years.
Coherex is used by lumber companies and the
Forest Service as a dust palliative on forest roads. The Washington
State Department of Transportation has used it to control piles of
volcanic ash from Mt. St. Helens and other sources.
And a new generation of acrylic polymers is
making an appearance. One such line: the Pave Cryl acrylic polymers used
for sealing/coating, crack fillers, and dust control, from manufacturer
Rohm and Haas.
As a road manager, it pays to make sure you know
what you are getting when you contract for dust control. In the early to
mid-70s, streets and parking lots in Times Beach, Missouri, a river town
of 2,200 southwest of St. Louis, were sprayed with inexpensive waste oil
(at 6 cents a gallon) to control dust. The waste oil recycler who
undertook the work used reclaimed oil mixed with oil laced with dioxin,
PCBs, and other chemicals. In 1982, residents learned of the
contamination and national headlines ensued.
The EPA began a $32 million buyout of all
residents in 1983, homes were torn down, soil remediated, and the
$110-million Superfund site cleanup ended in 1997. The site is now a
409-acre state park commemorating old U.S. 66. An article about the town
and the ensuing chaos by the last mayor of Times Beach may be read at
www.greens.org/s-r/078/07-09.html.
Dust control best practice
Best practices for unpaved road maintenance
includes dust control, because when fines are lost from a gravel
surface, remaining aggregates and fines stay loose, with corrugation or
washboarding and other ills resulting (see Erosion Assaults the Unpaved
Road, February 2005, or visit www.betterroads.com/articles/feb05d.htm.).
With dust control, the gravel stays in place and will not be lost due to
traffic. Reduced maintenance with motor graders will be an added bonus.
The California Storm Water Quality Association
urges that oil or oil-treated subgrade should not be used for dust
control because the oil may migrate into drainageways, or seep into the
soil.
Effectiveness of dust control treatments depends
on soil, temperature, humidity, and wind velocity, the CSWQA says.
Chemically treated subgrades may make the soil water-repellent,
promoting dust and fines retention, but interfering with long-term water
infiltration and the vegetation of the site.
As a mulch tack or chemical mulch, asphalt
treatments require a 24-hour curing time to avoid adherence to equipment
and workers’ shoes, the CSWQA says. “Application should be limited
because asphalt surfacing may eventually migrate into the drainage
system,” the association says.
Best construction practices include provision of
wind breaks, which can reduce wind velocity, thus limiting the
possibility of suspended particles. Wind breaks can be trees or shrubs
left in place during road construction or maintenance, or constructed
barriers such as a wind fence, snow fence, tarp curtain, hay bale, crate
wall, or sediment wall. Stone placed on dirt areas is an effective dust
deterrent as well.
If water is used exclusively, in California it
prevents dust only for a short period and should be applied daily (or
more often) to be effective, they state. “Furthermore, in compacted
areas, watering and other liquid dust control measures may wash sediment
or other constituents into the drainage system. Care must be taken not
to overuse water.”
|
Click here for copyright permissions!
