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OCR for page 857
VRadioactivity In
Drinking Water
Since it was discovered that ionizing radiation produces detrimental
biological effects, many national and international groups have studied
the sources and levels of radiation to which the human population is
exposed, and have estimated the corresponding biological ejects. Some
of these groups have also been responsible for establishing permissible
levels of exposure. Consequently, there is a large body of information on
the biological effects of ionizing radiation. The Subcommittee on
Radioactivity in Drinking Water has relied heavily on the reports of those
other groups and has abstracted and summarized pertinent sections. In
some cases it was possible to take new published and unpublished
information into account in this assessment of the probable ejects of the
radioactivity in drinking water on the population of the United States.
Among the groups whose reports were used were: the National
Academy of Sciences Advisory Committee on the Biological Ejects of
Ionizing Radiation (BEIR), the United Nations Scientific Committee on
the Ejects of Atomic Radiation (UNSCEAR), the International Com-
mission on Radiological Protection (ICRP), and the National Council on
Radiation Protection and Measurements (NCRP).
BACKGROUND RADIATION
The natural ionizing radiation, to which all people are exposed, includes
cosmic rays and products of the decay of radioactive elements in the
857
OCR for page 858
858 DRINKING WATER AND H"LTH
TABLE VII-1 Estimated Total Annual Whole-Body Doses from
Natural Radiation in the United States (from BEIR Committee, 1972)
Source
Annual Doses, mrem
Cosmic rays
Terrestrial radiation
External
Internal
Total
44
40
18
102
earth's crust and atmosphere. Part of the terrestrial radiation dose is from
sources external to the body, and part is due to the inhalation and
ingestion of radioactive elements in air, food, and water. In the United
States, this unavoidable background radiation gives, on the average, an
annual dose of about 100 mrem to the population (Table VII-1~. There is,
however, great variability in the amount of background radiation, which
depends on regional geological characteristics and altitude. It has been
found, for example, that the annual background dose in Colorado is 100
mrem (or more) higher than that in Louisiana (BEIR Committee, 1972~.
Mankind has always lived with such radiation, to which, however, the
radionuclides in drinking water contribute but a small share.
ABUNDANCE OF RADIONUCLIDES IN WATER
Minute traces of radioactivity are normally found in all drinking water.
The concentration and composition of these radioactive constituents vary
from place to place, depending principally on the radiochemical
composition of the soil and rock strata through which the raw water may
have passed.
Many natural and artificial radionuclides have been found in water,
but most of the radioactivity is due to a relatively small number of
nuclides and their decay products. Among these are the following
emitters of radiation of low linear energy transfer (LET): potassium-40
(40K), tritium (3H), carbon-14 (TIC), and rubidium-87 (87Rb). In addition,
high-LET, alpha-emitting radionuclides, such as radium-226 (226Ra), the
daughters of radium-228 (228Ra), polonium-210 (tempo), uranium (U),
thorium (Th), radon-220 (220Rn), and radon-222 (222Rn), may also be
present in varying amounts.
OCR for page 859
Radioactivity in Drinking Water 859
Natural Radionuclides
SOURCES OF LOW-LET RADIATION
Some of the radionuclides that are responsible for the natural radioactivi-
ty in drinking water come from radioactive elements, and their decay
products, that were incorporated in the earth at its formation, and others
are produced continuously by cosmic ray bombardment. Tritium is
produced by cosmic ray interactions with atmospheric oxygen and
nitrogen. It is then oxidized to tritiated water, which mixes into the
hydrosphere. Tritium concentrations in water supplies vary from about
10 to 25 psi/liter (Jacobs, 1968~.
In similar fashion, carbon-14, produced by cosmic ray [~4N(n,p)~4C]
interactions with atmospheric nitrogen (UNSCEAR, 1972, p. 29), is
oxidized to SCOW, which is generally found at a concentration corre-
sponding to about 6 psi )4C per gram of carbon. In water containing
about 1 mg of carbon per liter, a concentration of 0.006 psi/liter might
be expected. In ocean water, the concentration might be about 0.1
psi/liter (NCRP, 1975, p. 351.
Of all the natural radionuclides that occur in water and emit low-LET
radiation, potassium-40 is likely to be the most significant. This
primordial radionuclide occurs as a constant percentage (0.0118~o) of
total potassium. Adults in the United States ingest about 2,300 psi of
potassium-40 per day, but almost all of it is derived from foodstuffs. Since
potassium concentrations in man seem to be under homeostatic control,
wide fluctuations in drinking-water potassium would have negligible
ejects on internal concentrations. Assuming that there is 0.2% potassium
in soft tissue, a dose rate of 19 mrad per year has been estimated; of this,
17 mrad are due to beta radiation (UNSCEAR, 1972, p. 30~. In 1970,
some California drinking water, for example, contained up to 4 psi/liter
of potassium-40. Consumption of 2 liters per day of such water might
contribute as much as 8 psi per day, but this is a negligible fraction of the
total daily intake of 2,300 psi of a nuclide that is the largest natural
contributor to total body somatic and genetic dose.
SOURCES OF HIGH-LET RADIATION
Radionuclides that are produced by the decay of uranium-238 and
thorium-232 are widely distributed throughout the earth's crust. The
majority of them are alpha-emitters and include isotopes of polonium,
radon, and radium (UNSCEAR, 1972, p.31~. Concentrations of uranium
in drinking water are extremely variable, apparently ranging from 0.02 to
OCR for page 860
860 DRINKING WATER AND H"LTH
200 ~g/liter in fresh waters. The thorium content of drinking water has
not been extensively measured, but its concentration in the human
skeleton is about 1 fCi/g of ash; the corresponding abundance of
uranium in the skeleton is about 10 times greater.
The natural alpha-emitters that occur in drinking water appear to be
bone seekers. Of these' radium-226 and its daughters and the daughters of
radium-228 probably have the greatest potential for producing radiation
doses of some consequence to man. The radium-226 content of fresh
surface water is variable, ranging from 0.01 to about 0.1 psi/liter. Some
groundwater may contain up to 100 psi/liter. Drinking water obtained
from surface supplies generally does not contain significant amounts of
radium, and treatment processes, such as flocculation and water-soften-
ing, can remove the bulk of radium from water.
In the Midwest of the United States there is an area where groundwa-
ters contain significant levels of radium-226 and radium-228. This area
primarily in Iowa, Illinois, Wisconsin, and Missouri includes an
estimated population (1960 census) of approximately 1 million persons.
The weighted mean concentration of radium-226 has been estimated to
be approximately 5 psi/liter (Peterson et al., 1966~. Rowland, Lucas, and
Stehney (1975) have reported that approximately 500,000 people in
Illinois and Iowa have drinking water supplies whose radium-226 content
is 3 6 psi/liter; about 300,000 people, 6-9 psi/liter; and about 120,000
people live in areas where well water contains 9-80 psi/liter of radium-
226. A personal communication (Rowland, Lucas, and Stehney, 1976)
from the same investigators stated that, of the last group, 113,000 people
drink water that contains less than 20 psi/liter, and 5,700, 20-25
psi/liter. The one community (1,200 persons) that had a well in which 80
psi/liter of radium-226 was found, now uses water from a well
containing only 3 psi/liter.
In addition, a survey in 1966 that was designed to locate water supplies
with high concentrations of radium found water supplies with more than
3 psi of radium-226 per liter in areas other than those of the northern
Midwest described above (Hickey and Campbell, 1968~. These supplies
served approximately 145,000 people. Thus, it appears that in the entire
United States approximately 1.1 million people consume water that
contains more than 3 psi/liter of radium-226.
The major additional contribution to the alpha-emissions in drinking
water is due to the decay of radium-228; although other alpha-emitting
natural radionuclides have been found in drinking water, they occur in
exceedingly small concentrations. For example, one analysis of water
containing 5 psi of radium-228 per liter was found to contain less than
OCR for page 861
Radioactivity in Drinking Water 861
0.02 psi/liter of thorium isotopes and only 0.03 psi/liter of uranium
(Stehney, 1960~.
Two other radium isotopes may be present in drinking water, but
although both radium-223 and radium-224 may contribute to the gross
alpha activity of water measured soon after drawing from the tap, their
contributions to the long-term dose deposited in the skeleton are
negligible because they have short half-lives. However, radium-228,
which decays by beta emission, and therefore does not contribute to gross
alpha activity in drinking water, will, as a result of its subsequent decay
scheme, give rise to a series of alpha-emitting daughter products. It is
these radium-228 daughter products, and radium-226 and its daughters,
that produce, in our opinion, the major alpha-particle dose to the tissues
of the body, particularly to the skeleton. Thus, when discussing radium in
drinking water, it is essential to distinguish between the isotopic mixture
measured in freshly drawn drinking water and the long-term alpha dose
that might be accumulated in tissue.
Because of the different decay schemes for radium-226 and radium-
228, different alpha doses are received under equilibrium conditions from
each of these two radium isotopes. In waters of low alpha-particle
radioactivity, the activity concentration of radium-228 is generally equal
to that of radium-226, whereas at high radioactivity concentrations it is
only half that of radium-226 (Lucas and Krause, 1960~.
The abundance of the radioactive gas radon-222, which is formed by
the decay of radium-226, is not highly correlated with the radium
concentration in fresh water. Radon concentration is generally 1
psi/liter in surface water, but activity concentrations in groundwater are
typically a few thousand times greater. Some mineral or spa waters,
however, may contain 500,000 psi/liter.
Consumption of water containing 1 ,uCi of radon-222 will result in a
stomach dose of about 20 mrads, but the doses to other organs will be
lower by at least a factor of 10 (UNSCEAR 1975, p. 35~. Furthermore,
consumption of 2 liters per day of water containing 1 nCi/liter of radon-
222 would deliver an annual stomach dose of about 12 mrad.
In three large American cities, the total daily intake of uranium,
radium-226, radium-228, and lead-210 in water have all been quoted to
range approximately between 0.01 and 0.05 psi/day (NCRP, 1975, p.
92~. When compared with other components of the diet, drinking water
usually contributes less than about 2% of these alpha-emitting radionu-
clides to the daily dietary intake (NAS-NRC, 1973~. The greatest dose
potential from alpha-radiation from naturally occurring radionuclides in
drinking water will be related to the ingestion of radium-226 in areas
where its concentration is high.
OCR for page 862
862 DRINKING WATER AND H"LTH
Artificial Radionuclides
To some extent, all drinking water obtained from surface sources will
reflect contamination from atmospheric testing of nuclear weapons.
Extensive measurements have been made of the contribution of airborne
fission products to drinking water contamination and in particular to the
levels that were produced by testing weapons before the Nuclear Test
Ban Treaty of 1963. The sharp decrease in radioactive fallout since that
date has been followed by a corresponding decrease in the radioactivity
of surface water. Although the analyses are not very extensive, the
temporal characteristics provide some information that is useful in
predicting the transport and fate of radionuclides in water. Some of the
longer-lived radionuclides still persist from early tests, together with
smaller quantities of fission products injected irregularly into the
atmosphere from the testing of weapons by nontreaty nations.
Many of the states conduct periodic surveys of the radioactivity of
drinking water. Unfortunately, these consist, for the most part, in
counting only the gross beta and gross alpha activity in the water. In
addition, there is a considerable body of data on the temporal patterns
and regional concentrations of the fission products strontium-90 and
cesium- 137, the physical half-lives of which are about 30 yr.
There appears to be a fairly good correlation between the measurement
of solids in finished water and radioactivity content measured as beta
activity (Figure VII-1~. It is likely that potassium-40 in soil suspensions
might account for such an observation. Because they account for a major
part of the potential dose from nuclear fission and activation products,
and because of their biological significance, considerable attention has
been devoted to strontium-90, cesium-137, iodine-131, tritium, and carbon-
14 as potential water contaminants. These, however, are not necessarily
correlated with the solids content of drinking water.
Sources of man-made radionuclides, in addition to atmospheric
weapons tests, include local discharge of radiopharmaceuticals and the
possible entry of radioactivity into watersheds from the use and
processing of nuclear fuel to produce electric power.
RADIOPHARMACEUTICALS
The release of radioactive materials in the exhaust air and liquid wastes
from medical institutions has been studied many times in different
locales. No evidence yet suggests a drinking-water hazard from medical
effluents. This conclusion is based on data collected in many surveys
OCR for page 863
Radioactivity in Drinking Water 863
1000
Q 800
In
o
In
> 600
o
n
In
~ 400
o
200
o
o
O /
/ O
/ O
o
- 0/
// 8o8
O / 1 1 1 1 1
0 10 20
RADIOACTIVITY (psi/Liter)
30 40 50
FIGURE Vll-l Relationship between total dissolved solids and radioactivity of
California domestic water. Goldberg ( 1976).
(Soda et al., 1975; Gesell et al., 1975; Klement et al., 1972; Kaul and
Loose, 1975~. The agents to which particular attention was given in these
surveys were radioactive iodine and technetium-99m. Both are widely
used in medical practice, and there is special concern over the iodine
isotopes, because of their potential effects on the thyroid gland.
Since 1950, eight groups have reported on the extent of release of
radioisotopes in areas of the United States where there were active
clinical nuclear medicine programs. Because of recent rapid increases in
OCR for page 864
864 DRINKING WATER AND H"LTH
the numbers and kinds of procedures being conducted, Sodd et al. (1975)
studied the use and discharge of iodine-125, iodine-131, and technetium-
99m in the Cincinnati area. They measured the radioactivity from these
nuclides in the influent, effluent, and sludge at the sewage-treatment
plant, as well as the activity in the Ohio River 10 miles above and 5 miles
below the plant. Gesell et al. (1975) conducted a similar survey of medical
usage and concentrations in sewage of iodine-131 and technetium-99m in
the Houston area. The general conclusions reached by both groups
indicated that the eject on levels of radioactivity in drinking water of the
medical usage of radioisotopes that they studied appears to be of
negligible importance.
The Cincinnati study was centered about the largest sewage treatment
plant serving that city. This plant receives the effluent from 10 hospitals
that use radionuclides in clinical nuclear medicine. Approximately 60%
of the patients were outpatients, so control of biological wastes was not
attempted. Radioactivity in the sludge accumulated at the plant exceeded
that in the water. Sludge concentrations of iodine-131 and technetium-
99m were measurable, but that of iodine-125 was below the limit of
detectability (10 psi/liter).
It was estimated that between logo and 30~o of the total amount of
technetium-99m given to patients in Cincinnati hospitals was discharged
in sewage effluent into the Ohio River. Typically, about 300 mCi/week of
this nuclide were estimated to reach the river, where dilution with river
water was calculated to give concentrations downstream of about 1
psi/liter. In fact, analysis of river water showed identical values
upstream and downstream of 3-4 + 3 psi/liter. These are lower, by a
factor of about a million, than the current maximum permissible
concentration (6 ,uCi/liter; NRC, 1976) of technetium-99m in water for
the general population. Comparable results were obtained for iodine-131.
Smaller amounts were used, and the concentrations in sludge and water
were lower than those of technetium. No differences between upstream
and downstream levels were detected. Under the assumption that the
same dilution had occurred, the medical uses of iodine-131 in the area
were calculated to produce a maximal increase in concentration in the
river of about 0.3 psi/liter. This value is about one thousandth of the
current maximum permissible concentration of iodine-131 in water (300
psi/liter; NRC, 1976) for the general population.
Thus, at present, given current rates of use, patterns of disposal, and
radiation protection guidelines, many orders of magnitude separate the
concentrations of radioactivity in drinking water due to medical uses of
radioisotopes from conceivably hazardous levels. Projections of the rate
OCR for page 865
Radioactivity i'
n Drinking Water 865
of increase in use of radiopharmaceuticals have been made by the
Environmental Protection Agency (Klement et al., 1972~. They estimate
that there may be a 12-fold increase in the medical use of these agents by
the year 2000, on the basis of the annual increments in whole-body
radiation dose from the use of these agents in medicine. This represents a
very small incursion, and probably will not be measurable.
NUCLEAR FUEL CYCLE ACTIVITIES
Among the major effluents from the use and processing of nuclear fuel
are tritium, plutonium, and krypton. Of these, only tritium, which is
released as a gas, and plutonium can possibly enter water supplies. The
predominant form of plutonium release from nuclear power and
processing plants is as an aerosol that will have little or no impact on
drinking water. Although a single incident has occurred in which as much
as 18,750 Ci of plutonium were released from liquid storage on a local
basis, none apparently reached opposite water supplies (AEC, 1974, pp. 49-
50~. The usual rate of release from liquid storage at controlled sites is
about 1 mCi/yr. Continuing improvement in methods of storage should
further reduce this rate. Nevertheless, the adequacy of monitoring water
supplies in the vicinity of nuclear facilities should be reviewed periodical-
ly.
Because of its exceedingly long half-life (1.7 x 107 yr), the possible
consequences of the release of iodine-129 during nuclear fuel reprocess-
ing were considered. This radionuclide has a specific activity of about 173
,uCi/g. In a recent review, Soldat (1976) calculated that the maximal
isotope ratios of i29I:~27I would be about 10~ in water near nuclear
facilities. His calculations indicate that consumption of 2 liters/day of
water containing iodine-129 at 1 psi/liter deliver an annual thyroid dose
of about 5 mrem to an adult and about 10 mrem to an infant. Peak
activities in water have been reported to be about 0.01 psi/liter, which
would correspond to an annual thyroid dose of about 0.05 mrem to an
adult.
RADIATION DOSE CALCULATIONS
Estimates of the radiation doses expected to be produced by radionu-
clides ingested in water were calculated by means of the methods and
parameters given in NCRP Report 22 (NBS Handbook 69, 1963 revision)
and ICRP Publication 2 (ICRP, 1959~. To approximate the equilibrium
OCR for page 866
866 DRINKING WATER AND HEALTH
levels that take into account build-up, retention, decay, and elimination
of various radionuclides, annual doses were computed for the fiftieth year
of constant intake of 1 psi/year, and 2 liters per day of water containing
1 psi/liter. These doses are presented in Table VII-2. At earlier times, the
annual doses may be lower than those shown, and for a few long-lived
radionuclides (e.g. 90Sr, 226Ra), they may never reach equilibrium, but the
values in Table VII-2 are within 204Yo of the theoretical equilibrium levels.
These values were obtained by using the NCRP and ICRP metabolic and
dosimetric models for all radionuclides, except for the isotopes of the
alkaline earth elements radium and strontium, which are discussed
below.
ISOTOPES OF ALKALINE EARTH ELEMENTS
For the alkaline earth elements, the recent metabolic model of ICRP
Publication 20 (1973) was used.
In its 1959 report on permissible doses (ICRP, 1959*), Committee II of
the ICRP used an exponential model of retention for all radionuclides to
calculate maximum permissible concentrations in water. The committee
pointed out, however, that there was good evidence that retention of
radium-226 and other bone-seeking radionuclides is best represented by a
power function model (Norris et al., 1958~. In the case of radium-226, the
calculated body burden from intake at constant daily rate for 50 yr is
about a factor of 10 smaller by the power function model than by the
ICRP exponential model. This may be shown by use of the equations and
the values for metabolic parameters that are given in the ICRP report.
Ingestion of 1 psi of radium-226 per day in water is assumed in the
sample calculations given below.
According to the ICRP exponential model, the amount of a radionu-
clide, ~f2, that accumulates in an organ from constant ingestion rate, a, is
given by:
qf2 afw o 693 (1-e--0 693 Tic)
where q= total amount in the body,f2 = fraction of q in the organ of
reference (0.99 for bone), fw = fraction of radionuclide ingested in water
*The maximum permissible concentrations of radionuclides in ICRP 1959 and NCRP 1963
are identical.
OCR for page 867
Radioactivity in Drinking Water 867
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OCR for page 894
894 DRINKING WATER AND H"LTH
induced by the radiation: developmental and teratogenic risks, genetic
risks, and somatic risks.
Developmental and Teratogenic Risks
Although the developing fetus is sensitive to radiation, the total low-dose-
rate doses that would be delivered during the sensitive periods of
gestation are so small that no measurable ejects of the radiation from
drinking water will be found. The lowest dose level at which any eject
has been reported is 3 mrem/day or 1,100 mrem/yr in contrast to the
0.244 mrem per year described above.
Genetic Risks
For the general population, the maximum permissible dose of man-made
radiation is 170 mrem/yr, excluding medical uses of radiation. This
amounts to a 5 rem genetic dose in each 30-yr generation. This dose
would increase the current incidence of genetic diseases, which is about
94,400 per million live births, by about 200 per million in the first
generation. The estimate of 200, however, is so uncertain that there are
very large limits about the value. The gonadal dose of 0.244 mrem/yr
calculated for the hypothetical drinking water is expected to increase the
genetic diseases from the 94,000/106 live births by 200x0.244
mrem/30x 170 mrem = 0.0098 additional genetic diseases per million
live births per year. Since there are approximately 3.6 million live births
in the United States each year, this is an increase of 0.035 total genetic
diseases in the United States per year. If one takes the unlikely extreme
limits of the estimated genetic hazards of radiation (about 4,000) instead
of the value 200, the increase is 0.7 cases per year.
Somatic Risks
The natural background of radiation can be estimated to cause 4.5 to 45
fatal cases of cancer per year per million people, depending on the risk
model used to make the calculation (Table VII-l 1~. Less than 1% of this
will be contributed by the radionuclides in drinking water.
Variations in the radium content of drinking water, however, may
cause appreciable differences in the radiation dose to the skeleton and, in
turn, in the risks of associated carcinogenic ejects. Under average
conditions, the annual dose to bone from radium amounts to approxi-
mately 6.4 mrem/yr, which represents about 6% of the total dose to the
skeleton from all sources of natural background radiation (roughly 100
OCR for page 895
Radioactivity in Drinking Water 895
mrem annually). The highest radium levels in drinking water (25 psi/liter
of 226Ra and an additional 12.5 psi/liter of 228Ra), however, may be
expected to deliver a dose to the skeleton of about 600 mrem/yr, which
would represent a sixfold increase in the total dose to bone from all
natural sources combined. If the carcinogenic risks associated with
skeletal irradiation are assumed to be 0.2 fatal cases of bone cancer per
million persons per year per rem (Table VII-9), then for a period up to 30
yr, in a population with a typical distribution of ages, the risks
attributable to natural background radiation can be estimated to range
up to about 0.6 per million persons per year under average conditions,*
and to 4.2 per million per year under conditions of maximal intake of
radium in the drinking water (about 600 mrem/yr from the radium).
In addition to these risks, the possibility of carcinogenic ejects from
radium on cells adjacent to bone, such as those in epithelia lining cranial
sinuses and those in the bone marrow, should also be mentioned.
However, the risks of such effects are likely to be appreciably smaller and
cannot be estimated precisely from existing data. In comparison with the
overall risks of cancers of all sites combined, of which 4.5 to 45 (i.e.
9000/200) fatal cases per million per year can be attributed to natural
background radiation at average levels (Table VII-11), the additional 3.6
fatal bone malignancies per million per year ascribable to maximal
intakes of radium in drinking water constitute a significant increment. It
should be noted that only about 120,000 people drink water estimated to
contain between 9 and 25 psi/liter. Thus the excess bone cancers in this
group would be between 0.16 and 0.43 per year; that is to say, one excess
bone cancer every 2 to 6 yr. Since about 113,000 of the 120,000 people
drink water containing less than 20 psi/liter, the true number of excess
bone cancers will lie somewhere towards the lower end of the range.
When interpreting the above estimates, it must be remembered that
they depend on dose-response models that remain highly uncertain. For
example, the value given for the combined frequency of deaths from all
types of cancer attributable to natural background radiation namely,
45 deaths per million per year is higher by a factor of three or more
than estimates derived with any of the other risk models cited (Table VII-
l l). Likewise, the corresponding risk estimates for skeletal cancer could
vary widely, depending on the postulated dose-response relationship.
Although the value yielded by the BEIR Committee's absolute risk model
(0.2 fatal cancers per million per year per rem) is not greatly different
from the value yielded by the BEIR Committee's relative risk model
(since 9 out of the 1,704 fatal cancers per million per year are bone
*(0.2 fatal cases x 0.1 rem/yr x 30 yr)/(106 persons per yr per rem)
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896 DRINKING WATER AND H"LTH
cancers, this would be approximately 0.09 fatal bone cancers per million
per year per rem), both models, in postulating a linear nonthreshold
dose-response relationship, give substantially higher estimates than do
models postulating dose-dependent and dose-rate-dependent variations
in the risk per rem. Given the uncertainties in present knowledge, the
BEIR Committee's absolute risk model as used in the foregoing would
seem to provide an acceptably conservative approach for the purposes at
hand.
SUMMARY RADIOACTIVITY IN DRINKING WATER
Everyone is exposed to some natural radiation that comes from both
cosmic rays and terrestrial sources. Although there are large geographic
variations in the amount of natural background radiation, the average
background dose in the United States is about 100 mrem/yr. A small
proportion of this unavoidable background radiation comes from
drinking water that contains radionuclides.
By far the largest contribution to the radioactivity in drinking water
comes from potassium-40, which is present as a constant percentage of
total potassium. Only a small percentage of the total potassium40 body
burden, however, comes from drinking water. The total body dose from
other possible radioactive contaminants of water constitutes a small
percentage of the background radiation to which the population is
exposed. Although the amounts of individual radioactive contaminants
fluctuate from place to place, calculations made for a hypothetical water
supply that might be typical for the United States have shown that a total
soft-tissue dose of only 0.24 mrem/yr would be contributed by all the
radionuclides found in the water. Even with rather wide fluctuations in
the concentrations, the total contribution of the radionuclides will remain
veer small.
However, bone-seeking radionuclides such as strontium-90, radium-
226, and radium-228 account for a somewhat larger proportion of the
total bone dose. This is particularly true for the two isotopes of radium
because they, or their daughters, emit high-linear-energy-transfer (LET)
radiation, and because certain restricted localities have been found to
have rather high concentrations of radium in drinking water. Neverthe-
less, in the hypothetical typical water supply, less than lOgo of the annual
background dose comes from such radiation. It has also been estimated
that the total population exposed to levels of radium greater than 3
psi/liter is about a million people. About 120,000 people are exposed to
radium at levels greater than 9 psi/liter.
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Radioactivity in Drinking Water 897
Risk estimates were made of three kinds of adverse health ejects that
radiation could produce: developmental and teratogenic effects, genetic
effects, and somatic (chiefly carcinogenic) effects.
Developmental and Teratogenic Effects
The developing fetus is exposed to radiation from radionuclides in
drinking water for nine months. Thus, the total dose accumulated by the
fetus will be very small. Furthermore, although the fetus is sensitive to the
effects of radiation in some stages of development, these periods are
sharply limited and extremely short. For this reason, too, the total dose
administered that could possibly have developmental and teratogenic
effects would be extremely small. Current concentrations of radionu-
clides in drinking water lead to doses of about one five-thousandth of the
lowest dose at which a developmental effect has been found in animals.
Therefore, the developmental and teratogenic effects of radionuclides
would not be measurable.
Genetic Effects
It has been estimated that there are about 94,400 genetic diseases per
million live births in the United States. The maximum permissible dose of
man-made radiation for the general population (170 mrem/yr) has been
estimated to increase this number in the first generation by 17~215, with
an unlikely upper limit of 4,250. On the basis of a 30-yr generation and
3.6 million live births per year in the United States, we would expect the
0.24 mrem soft-tissue dose, or gonad dose, to lead to 0.0098 additional
cases of genetic disease per million live births per year or 0.035 additional
cases of genetic disease in the United States per year. Even at the unlikely
extreme upper limit of possible genetic effects of radiation of around
4,000 extra cases in the first generation, there would still be less than one
additional case per year in the 94,400x3.6 = 340,000 live births with
genetic defects. The wide fluctuation in bone dose caused by fluctuations
in the radium concentration of drinking water would not have any
sensible effect on the genetically significant dose, because radium is
predominantly a bone seeker and will deliver very little radiation to the
gonads.
Somatic and Carcinogenic Effects
The natural background of radiation can be estimated to cause 4.5 to 45
cases of cancer per million people, depending on the risk model used. The
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898 DRINKING WATER AND H"LTH
per year amount of whole-body radiation from radionuclides in typical
drinking water contributes less than 1% of this amount, and thus, for
cancers other than those in bone, may cause a negligible increase in the
total. Radium, however, can contribute somewhat less than 7% of the
total bone dose received from background radiation in areas of "normal"
radium concentration. The average carcinogenic risk associated with
skeletal irradiation by radium in a population with a typical distribution
of ages is estimated to approximate 0.2 fatal cases of bone cancer per
million persons per year per rem. Therefore, over a period from 10 to 40
yr after the beginning of skeletal irradiation, the average risk attributable
to natural background radiation is estimated to range from 0.6 per
million persons per year, under typical conditions, to as much as 4.2 per
million per year, in regions where 25 psi/liter of radium-226 are found in
the drinking water. It has been noted that in the United States 120,000
people are estimated to drink water containing between 9 and 25
psi/liter of radium-226, and only a small number lie near the upper end
of this range. The number of excess cancers in this group would therefore
lie between 0.16 and 0.43 per year. Since not all the 120,000 people drink
water containing 25 psi/liter of radium-226, the latter number is
inordinately high.
CONCLUSIONS
The radiation associated with most water supplies
proportion of the normal background to which all human beings are
exposed, that it is difficult, if not impossible, to measure any adverse
health ejects with certainty. In a few water supplies, however, radium
can reach concentrations that pose a higher risk of bone cancer for the
people exposed.
FUTURE NEEDS
is such a small
The precision of estimation of the health risks associated with radioactivi-
ty in drinking water could be enhanced if several water systems were
analyzed to determine the complete distributions of beta and alpha
radiation that constitute the gross counting measurements.
Because the precise ratio of radium-228 to radium-226 in water has not
been measured extensively, an attempt should be made to determine the
ratio in several ground and surface waters whose content of radium-226 is
known. Activity concentrations of the waters to be analyzed should range
OCR for page 899
Radioactivity in Drinking Water 899
from about 0.1-50 psi/liter. The percentage of the daughter radionu-
clides present should be determined.
Because radon is a noble gas that is quickly released from water, it is
possible that, in some areas of high radon content, water vapor
containing radon might constitute an inhalation hazard when such water
is used, for example, in humidifiers or for showers. A determination
should be made whether or not radon emanations from water do indeed
constitute an inhalation hazard.
The models used in this report do not take into account the possibility
that the finely divided solid particles that occur in water may alter the
uptake of radionuclides. The elects of the solids in drinking water on the
metabolism and uptake of radionuclides merit investigation.
GLOSSARY
Absolute risk. Excess or incremental risk due to exposure to a toxic or
injurious agent (e.g., to radiation). Difference between the risk (or
incidence) of disease or death in the exposed population, and the risk in
the unexposed population. Usually expressed as number of excess
cases in a population of a given size, per unit time, per unit dose (e.g.,
cases/106 exposed population/year/rem).
Curie (Ci). Unit of radioactivity. 1 Curie = 3.7 x 10~° nuclear
transformations per second. Some fractions are: millicurie (1 mCi =
1O-3 Ci), microcurie (1 ,uCi = 10-6 Ci), nanocurie (1 nCi = 10~ Ci),
picocurie (1 psi = 10-~2 Ci), femtocurie (1 fCi = 1O-~5 Ci).
Latent period. Period between time of exposure to a toxic or injurious
agent and appearance of a biological response.
LET. Linear energy transfer. Average amount of energy lost by an
ionizing particle or photon per unit length of track in matter.
Plateau period. Period of above-normal, relatively uniform, incidence of
disease or death in response to a toxic or injurious agent.
Rad. Unit of dose or radiation (energy) absorbed in any medium, except
air. 1 Rad = 100 erg/g.
Relative risk. Ratio of the risk in the exposed population to that in the
unexposed population. Usually given as a multiple of the natural risk.
Rem. Unit of radiation dose equivalence. Numerically equal to absorbed
dose in red multiplied by a quality factor that expresses the biological
effectiveness of the radiation of interest, and other factors. Equal doses
expressed in rem produce the same biological effects, independently of
the type of radiation involved.
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900 DRINKING WATER AND H"LTH
Roentgen (R). Unit of radiation (energy) absorbed in air. 1 R = 2.58 x
1O-4 coulomb/kg of air.
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Representative terms from entire chapter:
beir committee
