Chemistry Reference
In-Depth Information
high concentrations of dissolved organic matter, the
uranium is in solution as the soluble organic complex
(Brunskill and Wilkinson, 1987).
Typical sources of elevated uranium water con-
centrations include well water, spring water, and
effl uent or leachate from various types of mines and
mills. Well water tends to contain higher concentra-
tions than surface water in the same area because of
the increased surface area of and contact time with the
rock and soil particles through which aquifer water
fl ows. This difference may not be recognized in areas
where regulators require distributors but not individ-
ual well owners to analyze drinking water. The pop-
ulation-weighted average for U.S. surface water was
reported as 0.8
population-weighted average of 33 Bq/kg (UNSCEAR,
2000). Concentrations are not uniformly distributed,
tending to be lower in basic rocks (e.g., basalt) and
higher in acidic rocks (e.g., salic) and granite. Areas
suitable for mining uranium, silver, and phosphate can
exceed a mass fraction of 0.1. Canada is the world's
largest producer of uranium. As of 2005, its McArthur
River mine was considered to have the largest high-
grade ore deposit at 2 × 10
11
g U
3
O
8
with an average ore
grade of 25% (Cameco, 2005).
4.1.4 Air
The source of atmospheric uranium is resuspended
surface soil on the basis of air particles and soil from
the same area having similar uranium concentrations.
Uranium in ambient air is typically in the attogram/m
3
(nBq/m
3
) range, with
234
U/
238
U activity ratios ranging
from 1 to 7, differing signifi cantly from the equal pro-
portions found in crystal rock (EPA, 1999).
Levels measured in 37 U.S. cities for 1993 gave results
ranging from 13-265 aCi/m
3
(4 × 10
−7
-1 × 10
−5
Bq/m
3
;
1.7 × 10
−11
-2.9 × 10
−10
g/m
3
) for Honolulu, Hawaii, and Las
Vegas, Nevada, respectively. This last value was partially
attributed to enrichment in
234
U based on a
234
U/
238
U
ratio of 1.64. This survey was repeated in 52 U.S. cities for
2003, and values ranged from 7.6-202 aCi/m
3
(2.8 × 10
−7
-
7.5 × 10
−6
Bq/m
3
; 1.0 × 10
−11
-2.9 × 10
−10
g/m
3
) for Barnwell,
South Carolina, and El Paso, Texas, respectively, with an
average of 42.5 aCi/m
3
(1.6 × 10
−6
Bq/m
3
; 5.6 × 10
-11
g/m
3
)
(EPA, 2004). The
234
U/
238
U ratio was lower for Las Vegas
than in the earlier survey. Care should be taken in
assessing the presence of either depleted or enriched
uranium in the sample on the basis of isotopic uranium
ratios because of their variability in nature. Examples of
areas expected to be unaffected but having ratios differ-
ent from unity include Fairbanks, Alaska (ratio, 1.63),
Lansing, Michigan (ratio, 2.02), Hartford, Connecticut
(ratio, 0.79), and Albany, New York (ratio, 0.73). These
mass values were calculated on the basis of reported
concentrations for the three natural isotopes and their
half-lives.
Two estimates of annual intake were 0.3 pCi/y
(0.01 Bq/y, ~0.4
g/L (Lowry
et al
., 1987). This can be compared with higher levels
reported for a European bottled water (~1000 pCi/
L [~37 Bq/L]) and for Finland (up to 150 Bq/L). An
example of an area where differences occur between
surface and groundwater is the state of South Carolina.
Most drinking water in that state derives from surface
sources for which levels are below the EPA MCL, but
sampling of wells in the city of Simpsonville revealed
such elevated levels that a health study of residents
was conducted (ATSDR, 2002). Several months after
consumption from these wells ended, urine from 105
residents was found to be elevated (avg, 0.481
µ
g/L, with a range of 0.08-7
µ
µ
gU/g
creatinine; range, ND-2.659
gU/g creatinine) and 85%
exceeded the 95th percentile concentration of a national
reference population. Six months later, the average
concentration had decreased, but 87% remained above
the national 95th percentile, indicating that retention
of uranium from long-term exposure is detectable in
urine long after exposure ends (Orloff
et al
., 2004). Dif-
ferences between potable surface and ground water
can be magnifi ed when the well water derives from
aquifers fl owing through uranium-rich deposits. Well
water concentrations in some Canadian provinces were
so high as to prompt Health Canada to develop public
health recommendations that individual provinces can
adopt. Mining areas throughout the world are sources
of elevated uranium that could lead to high drinking
water concentrations. Examples include the Uravan site
in Canada (1500-16,000 pCi/L [~56-590 Bq/L]) and the
tailings pond of the United Nuclear site in the state of
New Mexico (as high as 140 Bq/L). A break in the United
Nuclear tailings pond dam in 1979 sent 352 million liters
of tailings liquid into the Rio Puerco (EPA, 1988).
µ
µ
g/y) (UNSCEAR, 1988) and 3 pCi/y
(0.1 Bq/y, ~4
g/y) (Cothern, 1987), showing that ura-
nium intake varies with location. Levels incrementally
increase downwind of industries (coal power plants,
heavy metal mines or mills) or natural events (erupt-
ing volcanoes) that release uranium. For example,
those near a Canadian refi nery were found to range
from 2-200 ng/m
3
(Tracy and Meyerhof, 1987).
Reports of uranium concentrations downwind of
a nuclear detonation were not found, but even well-
designed bombs may only be 20% effi cient, leaving at
least 80% of the original uranium (Alt
et al
., 1989) to
µ
4.1.3 Soil and Rock
Uranium is present in most rocks and soil as a min-
eral with an average mass fraction of approximately
2 × 10
−6
(0.05 Bq/g). UNSCEAR has determined that the
median world concentration is 35 Bq
238
U/kg, with a
