Risk
Assessment Calculations
Exposure Reconstruction Case Study
Beach Reclamation (Lead)
By: Henry P. Shotwell,
Ph.D., CIH, Senior Vice-President, Atlantic Environmental,
Inc.
Facts: A 6-month old infant and her mother
took residence in a house located 60 feet away
from a beach reclamation project that had been
contaminated with metallic lead from industrial
activity. The house was in the path of continuous
on-shore breezes that blew across the contaminated
beach and left a thin layer of sandy dust on surfaces
both inside and outside the house. The condition
lasted for at least 16 months.
The toddler was observed standing in her crib
and “chewing” on the crib’s side rails. Because
of colic and some delays in reaching development
benchmarks, she was evaluated and found to have
an elevated blood lead level. Soil analysis showed
the maximum level of lead to be 320 mcg/kg. Was
the lead content of the construction dust sufficient
to elevate the child’s blood lead level?
Approach: From soil data, estimate the amount
of lead per unit mass of dust, given that dry,
summertime conditions produce an average of about
mg dust (from soil) per cubic meter of air. Then,
given the distance between the house and the construction
site, calculate the mass of lead likely to plate
out on the baby’s crib from the amount in the
air, using Stoke’s equation and a continuous,
24-hour/day deposition rate. Compare to threshold
of 10 mcg Pb/dL blood for infantile lead poisoning.
Calculations
- Soil to Air
The highest concentration of lead in beach soil
(sand) was reported to be (5,200) micrograms
per kilogram of soil. This is equivalent to
micrograms of lead per milligram of soil. Given
that the average concentration of dust in the
are was 3 mg/M3 air, the worst-case
airborne lead level would be 0.0156 micrograms
Pb/M3 air.
- Deposition from Air
According to Stoke’s Law, (Vs = 0.003SGd2,
where Vs = settling velocity in cm/sec.,
SG = specific gravity and d is the average particle
diameter), particles of sand (average SG = 1.6;
average diameter = 5 microns). Lead-contaminated
sand particles, carried from the beach would
settle out at a rate of 2.8 inches per minute.
Given a ceiling height of 7.5 feet and an average
height from the floor to the top of the crib
side-bar of 4 feet, a settling distance of 3.5
feet would require 15 minutes to form a layer
of dust on the crib surface. The fact that
the contaminated sand generates an average of
0.0156 mcg Pb/M3 air and the fact
that lead was being continuously deposited was
sufficient to convince a jury that the child’s
lead poisoning was directly attributable to
the reclamation activity on the beach.
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