Risk
Assessment Calculations
Exposure Reconstruction Case Study
Brownfields Project (Chromium)
By: Henry P. Shotwell, Ph.D., CIH
Senior Vice-President
Atlantic Environmental, Inc.
Facts: A 54-year-old male, heavy equipment operator
was assigned to remove soils from a Brownfields
project which had been contaminated with chromium
wastes from tanning operations along a riverbank.
The machinery was a clam-shell bucket suspend
from the end of a 75-foot long boom at a 60°
angle. The soils to be excavated were wet. Previous
soil analyses showed a maximum chromium concentration
of 1000 mcg/kg of soil (ppb).
The operator weighed 312 pounds and stood 5 feet,
6 inches in height. He had 2 previous myocardial
infarcts and was a poorly controlled diabetic.
Five weeks after the start of work on this project,
the operator suffered a third heart attack which
rendered him disabled. He consulted an attorney
and a lawsuit was filed, alleging his exposure
to chromium in the soils he was excavating caused
his third heart attack.
Approach: The Plaintiff’s attorney was
advised that: a) metallic chromium has essentially
no vapor pressure and does not evaporate into
the air, b) the wet soils would prevent the generation
of inhalable chromium-contaminated dust and c)
the medical literature does not associate exposure
by inhalation to metallic chromium with heart
attacks.
As an exercise, it was decided to see what possible,
worst-case exposure might be.
Assumptions: We assumed excavation activity ran
for 8 continuous hours per day, 5 days per week
for 5 weeks. We further assumed that it was possible
to aerosolize all 1000 ppb of chromium in the
soil, and that this aerosol was available for
inhalation.
Calculations: Using trigonometry, we calculated
the distance from the operator’s cab to
the point where the clam-shell touched the ground.
An hypotenuse of 75 feet and an angle of 60 degrees
calculates to a distance of 37.5 feet (75cos 60°).
Using 37.5 feet as the radius of a hemisphere
with the operator at the center, we calculated
the volume of the dome of air over the operation.
The volume of a hemisphere is 0.67 pr3 and calculated
out to be 110,447 cubic feet (3,128 M3).
If 1,000 mcg of chromium was present in every
kilogram of excavated soil, and we estimate the
clam-shell volume to be 5 cubic yards (135 cubic
feet; 3.82 cubic meters) and if the mass of each
scoop of soil is 3,000 pounds per cubic yard,
then each scoop will contain 1,361 kg of soil
and 1,361 mg of chromium.
If all the chromium in each scoop were made airborne,
there would be 1,361 mg Cr. per 3,128 M3 or 0.435
mg chromium per M3 of air, which is less than
the current OSHA PEL of 0.5 mg/M3 for chromium.
Discussion: This reconstruction assumes that
with every new scoop of soil, the chromium content
of each previous scoop has “plated out:
is no longer available for inhalation, so that
a steady state is reached whereby the air concentration
remains at about 0.435 mg/M3.
Under normal circumstances, there would be periods
of time when no excavation took place: lunch,
toilet breaks, etc. Thus, in reality the actual
exposure to a contaminant which could be aerosolized
would be much less than this.
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