+

Heahh Phy$ics Vol. 59, No. 5 (November), pp. 659-668, 1990 Printed in the U S A .

0017-9078/90 $3.00 .00 0 1990 Health Physics Society Pergamon Press pic

Analyses and Modeling for Internal Dose Estimates MODELS OF RADIOIODINE TRANSPORT TO POPULATIONS WITHIN THE CONTINENTAL U.S. Andre Bouville * Radiation Effects Branch, National Cancer Institute, Bethesda, MD 20892 and

Mona Dreicer Lawrence Livermore National Laboratory, Livermore, CA 94550 and

Harold L. Beck Department of Energy, Environmental Measurements Laboratory, New York, NY 100 14 and

Walter H. Hoecker Air Resources Laboratory, National Oceanic and Atmospheric Administration, Silver Spring, MD 209 10 and

Bruce W. Wachholz Radiation Effects Branch, National Cancer Institute, Bethesda, MD 20892 Abstract-A methodology is being developed to estimate the exposure of Americans to I3'I originating from atmospheric nuclear weapons tests carried out at the Nevada Test Site (NTS) during the 1950s and early 1960s. Since very few direct environmental measurements of I3'I were made at that time, the assessment must rely on estimates of 13'1 deposition based on meteorological modeling and on measurements of total B activity from the radioactive fallout deposited on gummed-film collectors that were located across the country. The most important source of human exposure from fallout I3'I was due to the ingestion of cows' milk. The overall methodology used to assess the "'I concentration in milk and the I3'I intake by people on a county basis for the most significant atmospheric tests is presented and discussed. Certain aspects of the methodology are discussed in a more detailed manner in companion papers also presented in this issue. This work is carried out within the framework of a task group established by the National Cancer Institute.

INTRODUCTION

portant tests, the I3'I exposures from fallout for representative individuals and for the populations of each county of the contiguous U.S. during the time of the tests. The most significant atmospheric weapons tests with respect to fallout occurred in the 1950s, during which time most of the monitoring of environmental radioactivity consisted of gross /3 or y measurements. Because the radioactive half-life of I3'I is about 8 d, the I3'I activity present in the samples collected more than 25 y ago has completely decayed and cannot be measured retrospectively. Therefore, the estimation of I3'I exposures dating back to the 1950s must be derived from the original measurements of gross ,k? or y activity, from current or past measurements of radionuclides other than I3'I, or from mathematical models.

ONE PART of Section 7 (a) of Public law 97-4 I4 directs the Secretary of Health and Human Services to "conduct scientific research and prepare analyses necessary to develop valid and credible assessments of the exposure to I3'I that the American people received from the Nevada atmospheric bomb tests." The National Cancer Institute (NCI) was requested to respond to this mandate. In doing so, a task group, established to assist the NCI in this effort, suggested that it might be possible to estimate, for each of the most im-

* Author to whom correspondence should be addressed. 659

660

Health Physics

November 1990, Volume 59, Number 5

In addition to the present study, two other efforts are concerned with the exposure of more specific populations to I3lI from fallout: The Off-Site Radiation Exposure Review Project of the Department of Energy (Church et al., this issue) is estimating exposures of downwind residents to fallout, and the University of Utah is conducting an epidemiological study of thyroid disease among populations of Utah (Wachholz, this issue). Transport models used in the three studies to estimate the extent to which individuals or populations were exposed to I3II are similar. There are some differences that distinguish this study from the other two, however, because of the larger geographic scope of this study. The models used here are of a more generic nature since the level of detail required in the other two studies is not practical for a continent-wide assessment. Moreover, because most of the fallout in the eastern part of the country was associated with precipitation (i.e., “wet” fallout), whereas “dry” deposition was predominant in the western part of the country (Beck et al., this issue), precipitation receives a greater emphasis in this study than is required for the other two. Once l 3 l I from fallout has been deposited on vegetation, the main pathway to man is, for most individuals, via the grass-cow-milk chain (Eisenbud and Wrenn 1963; Garner and Russell 1966). In the assessment of 1311 exposures on a continental scale, certain assumptions and methodologies enable estimates of the following parameters to be made for each of the nearly 3,100 counties in the contiguous United States:

testing at Nevada Test Site (NTS), show that 1 3 1 1 from weapons tests is partitioned among three physicochemical forms: gaseous organic, gaseous inorganic, and particulate (Perkins 1963; Perkins et al. 1965; Voilleque 1979). From measurements taken after a Chinese nuclear weapons test, the partitioning between these three forms was shown to vary with the time elapsed following the detonation (Voilleque 1979). Voilleque ( 1986) speculated that more than half the l3II would be associated with particular diameters of less than about 20 pm, with the remainder presumably being in organic and inorganic gaseous forms. In the absence of better data, it has been assumed that all I 3 l 1 was in particulate form.

the activities of I3II deposited on soil and vegeta-

Dispersion of the radioactive cloud The amount of I3’I produced in each explosion was derived from Hicks ( 1981). The I3lI activity per unit yield was found to be about 5 PBq kt-I of fission for the shots considered in the assessment. The apportionment of the I3II activity between the cloud top, the cloud stem, and the local deposition in the immediate vicinity of the test site was estimated as follows, according to the type of test:

0

tion, 0 the amount of l3’I consumed by dairy cows and the resulting I3II concentrations in cow’s milk, and 0 the I3II ingested by people.

The purpose of this paper is to present the overall methodology currently used in the assessment of the I 3 l I exposures from fallout resulting from the atmospheric nuclear weapons tests camed out at the Nevada Test Site during the 1950s and early 1960s. Although all aspects of the methodology are subject to revision before completion ofthe study, it is likely that changes will be minimal. Parts of the methodology are described in more detail in companion papers presented in this issue (Beck et al.; Dreicer et al.; Hoecker and Machta). ESTIMATION OF ACTIVITIES DEPOSITED ON THE GROUND

Meteorological modeling and reanalysis of historical monitoring data are two complementary methods used to estimate I3II deposited on the ground following each test. For both approaches, the assumption is made that the I3’I was in particulate form, as were the majority of radionuclides produced in the atmospheric nuclear weapons tests. Limited measurements, unrelated to weapons

METEOROLOGICAL MODELING

The radioactive cloud that is formed after an atmospheric detonation near the ground surface usually is in the shape of a mushroom, extending from the ground surface to the highest layers of the troposphere, and occasionally reaching into the stratosphere. It contains hundreds of different radionuclides, including I3’I. The meteorological prediction of I3II deposition, presented in detail in another paper (Hoecker and Machta, this issue) and discussed here briefly, involves two steps: a) dispersion of the radioactive cloud across the US., and b ) estimation of the amount of I3’I deposited on the ground.

Type of test

Surface or tower Balloon or airdrop

Apportionment of 13’1 activity Cloud top

Cloud stem

Local deposition

0.8

0.1

0.1

0.9

0.1

0

The 1311 activities in the cloud are assumed to be homogeneously distributed within the cloud top and cloud stem. The dispersion of the radioactive cloud has been analyzed for each important atmospheric test using routine weather maps that depict airflow at constant pressure levels. These maps, which were provided twice a day by weather services, were used to construct, at several altitudes ranging between 3 and 13 km, 6-h trajectories of air parcels originating at the Nevada Test Site and moving across the U.S. In general, trajectories at those various

Models of radioiodine transport to populations within the continental U.S. 0 A. BOUVILLEet al.

altitudes diverged in both direction and speed after leaving the detonation site. The radioactive cloud was often stretched by vertical wind shear to many hundreds of kilometers before it left the U.S. This large shear resulted in a great dilution of I3'I. Additional distribution was caused by lateral spreading of the cloud by eddy or turbulent diffusion, which Hoecker and Machta (this issue) assumed to occur at a rate of about 7 km h-I. The meteorological model predicts the spatial coverage of the radioactive cloud at each 6-h interval and the I3'I activities per unit area contained in the radioactive cloud at each county centroid of the continental U.S. Hoecker and Machta (this issue) assumed in their model that at any given time, the 13'1distribution is uniform within the boundaries of the cloud segments created by lateral spreading and vertical shearing between the altitudes at which the trajectories are determined. Deposition on the ground Deposition of "'I on the ground results from two processes: impaction of aerosols on the ground surface (dry) and precipitation (wet). In the western part of the country, most I3'I deposition was from dry processes, since weapons testing generally was not allowed under atmospheric conditions such that wet deposition was likely to occur within a few hundred kilometers from the NTS. That operational precaution, however, did not extend to the eastern part of the country, where most 13'1deposition occurred as a result of wet processes (Beck et al., this issue). Therefore, the amount of rainfall is an important parameter in attempting to estimate the extent to which wet deposition occurred. In order to approximate the amount of rain that occurred across the country during the time of interest, recorded daily rainfall amounts reported by the National Oceanic and Atmospheric Administration (NOAA) were averaged on a county basis. The endpoint of the meteorological model is the estimation of the amounts of 13'1deposited by precipitation. This involves not only the knowledge of daily rainfall amounts but also that of many other uncertain factors, among which are the efficiency of the rain-out process, the exact location of the radioactive cloud segment, the location and dimensions of the precipitating cloud, and the physicochemical form of I3'I. Because of the complexity of the problem, an empirical method was established based on the use of relationships between the column content of I3'I in the overhead cloud and the 13'1 deposition estimated from the monitoring data (gummedfilm) , which are discussed in the following section. REVIEW AND REANALYSIS OF HISTORICAL MONITORING DATA For counties near the NTS, monitoring consisted mainly of exposure-rate measurements using portable survey instruments (Beck and Anspaugh 1990; Thompson, this issue). This close-in monitoring network will not be discussed here. Over the remainder of the U.S., monitoring of fallout deposition in the 1950s was carried out primarily by the

66 1

Environmental Measurements Laboratory (EML) in cooperation with the U S . Weather Bureau (Beck 1984; Beck et al., this issue; Harley et al. 1960). The EML network effectively fulfilled its original purpose of indicating quickly where and when fallout occurred. The network was not designed to derive the amounts of specific radionuclides deposited on the ground; however, since it represents the only radiological monitoring data available on a daily basis over the entire U.S. during most of the atmospheric testing period, it was considered that, by careful reevaluation of the data, it might be possible to derive estimates of 13'1deposition at the location of the gummed-film stations. The EML deposition network across the US. evolved gradually from the use of trays of water at 10 locations in 1951, to the use of gummed-paper collectors at 93 locations in 1952, and finally to the use of gummed-film collectors at about 100 locations until the end of the decade. A "gummed-film collector" consisted of a 0.3 m X 0.3 m exposed area of gummed film that was positioned horizontally on a stand 0.9 m above the ground. Usually two films were exposed during a 24-h period beginning at 1230 GMT. The samples collected were ashed and counted for total p activity. Beck ( 1984) reviewed and reanalyzed the available gummed-film data that could be found in the HASL/ EML archives, together with other less extensive fallout data, in order to derive depositions of 137Cs,I3'I, and 1331. One of the difficulties in the reanalyses of monitoring data was that original data may have been mislabeled or not assigned to the appropriate nuclear weapons test. In an effort to alleviate this problem, the areas of observed deposition systematicallywere compared with the areas predicted by the meteorological model to be covered by the radioactive cloud. In case of a significant discrepancy, a more detailed trajectory analysis was carried out to determine whether the suspect gummed-film results could be explained from meteorological considerations (Hoecker and Machta, this issue). If not, the suspect results were discarded. The resulting data set includes daily depositions of 13'1at up to approximately 100 locations in the U.S. during most of the atmospheric testing period. Those 13'1depositions are associated with information on the precipitation amounts occurring during the same 24-h periods. Some of the gummed-film results are presented in another paper in this issue (Beck et al., this issue). ESTIMATION OF 1311 DEPOSITION IN ANY GIVEN COUNTY The daily deposition densities of I3'I need to be estimated for each of the nearly 3,100 counties in the contiguous U.S. For this purpose, both meteorological modeling and reanalysis of historical data have limitations. EML was at that time called the Health and Safety Laboratory ( HASL) .

662

Health Physics

The position of the radioactive cloud is not always in agreement with the areas of deposition derived from monitoring data, usually because of the simplifying assumptions used in the model. In particular, measured depositions often occurred over a longer time period than that predicted by the meteorological model. In addition, although the meteorological model has the potential of predicting 13'1 deposition by wet processes, it can only do so in a crude way for those areas where precipitation occurred during the predicted passage of the radioactive cloud. On the other hand, the reanalysis of historical monitoring data provides the best available estimates of I3'I deposition per unit area but, under the best conditions, only for up to about 100 locations. In order to estimate the daily 1311 deposition in any given county, the following procedure, in which preference is systematically given to the monitoring data, has been applied: 0 For the tests for which gummed-film data are available (from October 1951 to November 1958), the deposition densities were obtained in most cases by interpolating between the counties with measured data using a kriging procedure. 0 For tests in which gummed-film data are not available (before October 1951 or after November 1958), meteorological modeling was used to estimate deposition densities in the counties where precipitation occurred during the passage of the radioactive cloud. Counties where precipitation did not occur during the passage of the radioactive cloud were assigned a zero deposition.

ESTIMATION OF 1311 CONCENTRATIONS IN FRESH COWS' MILK The transfer of I3'I from deposition on the ground to fresh cows' milk is relatively well documented (e.g., Bergstrom 1967; Black and Barth 1976; Garner 1967; Kirchner et al. 1983; Wicker and Kirchner 1987). Figure 1 illustrates the parameters involved in that transfer. The time-integrated concentration of I3'I in milk (IC) corresponding to an estimated deposition density on the ground (DG) on a given day and in a given county was calculated as: IC

=

Ten X P I X F,, DG X F* X In 2

(1)

in which F* is the mass interception coefficient [ m 2 kg-' (dry weight)], Tenis the effective half-time of retention by the vegetation (d), PI is the pasture intake [ kg (dry weight) d-'1 , and F,,, is the intake-to-milk transfer coefficient (d L-'). Each of these parameters will be discussed.

MASS INTERCEPTION COEFFICIENT Iodine- 1 3 1 concentration in milk is directly proportional to that fraction of activity deposited that is intercepted by vegetation ( F ) ,or the interception coefficient, which depends, among other factors, on the meteorolog-

November 1990, Volume 59, Number 5

* interception * retention

by vegetation

on vegetation v Time-in tegrated concentration in veaetation

* pasture intake intake-to-milk

by dairy cows transfer coefficient

Fig. I . Iodine-I31 transfer from deposition on the ground to fresh cows' milk.

ical conditions and the type and density of vegetation. Values of interception coefficients obtained in laboratory or field experiments conducted under dry or light spray conditions with artificial radionuclides show a large range of variation between 0.02 and 0.82 (Miller 1980). A much narrower range of 1 to 4 m2 kg-' (dry) is obtained for the mass interception coefficient ( F * ), defined as the interception coefficient ( F ) divided by the standing crop biomass ( Y ) . Chamberlain ( 1970) proposed that the interception coefficient could be estimated as: F = 1 - e-ay (2) where the numerical value of a , the foliar interception constant, is 2.8 m kg-' (dry weight) for elemental iodine and small-size aerosols under dry or light spray conditions. The value of F* is then obtained as: 1 - e-aY F* = (3) Y If the product N X Y is much smaller than 1, which is often the case, eqn ( 2 ) can be approximated as:

F=aXY,

(4)

and the numerical value of the mass interception coefficient ( F * ) is equal to that of the foliar interception constant. There is evidence that the value of a decreases as the particle size increases (Romney et al. 1963; Anspaugh et al. 1986; Whicker and Kirchner 1987) and, therefore, that the interception coefficient decreases as the particle size increases. In the case of atmospheric nuclear weapons tests, large-size particles fall out near the detonation site and smaller particles are deposited as the radioactive cloud moves farther away. Simon (this issue) estimates that the

Models of radioiodine transport to populations within the continental U.S. 0 A. BOUVILLEet al.

'

variation of a (m kg-', dry weight) as a function of distance, D ( km ) ,can be expressed as: a(D) = 7.01

X D'.I3.

(5)

Using this expression, the value of a increases with distance from the NTS and is approximately equal to 2.8 m2 kg-' (dry) for D = 1600 km. Beyond that distance, the value of a is taken to remain constant at 2.8 m 2 kg-' in order to remain consistent with the value proposed by Chamberlain ( 1970) for elemental iodine and small-size aerosols. All of the laboratory and field experiments were conducted under dry or light spray conditions (Miller 1980) and do not, therefore, provide any information on the values to be expected in the case of moderate or heavy rainfall. In a limited number of cases, however, I3'I fallout was measured in rain and vegetation after atmospheric nuclear weapons tests. The interception coefficient values derived from those measurements show a large range of variation, from less than 0.09 to about 0.9, with a high scatter for any given rainfall levels but with a tendency to decrease as the rainfall amount increases (Voilleque 1986). Adapting an expression originally developed by Horton (1919) for the initial retention of rainwater by vegetation, Voilleque ( 1986) proposed that the variation of the mass interception coefficient as a function of the rainfall amount P (mm) can be estimated by:

S F*=E+-

P'

where E = 1.3 m kg (dry weight) is the in-storm evaporation fraction per unit areal density of vegetation, and S = 16 mm kg-' (dry weight) m-2 is the rainfall storage capacity per unit areal density of vegetation. According to this expression, the mass interception coefficient is inversely related to the rainfall amount. The application of eqn (6), however, yields values of the interception coefficient that are greater than one for low rainfall amounts associated with high-standing crop biomasses, which is physically impossible. In order to resolve this inconsistency, the inverse relationship between the mass interception coefficient and the rainfall amount proposed in eqn (6) was assumed only to apply to daily rainfall amounts in excess of 5 mm. The linkage between eqn ( 3 ) , used for dry deposition, and eqn (6), used for wet deposition, is done assuming that, for light rain (less than PI = 5 mm), the value of F* for a distance D and a daily rainfall amount P is obtained by linear interpolation:

P

F * ( D , P ) = F * ( D , O ) + [F*(Pt)-F*(D,O)] X - . PI (7) Given the importance of the interception coefficient in the assessment of I3'I exposures, and the limited information on its value under conditions of moderate or heavy rainfall, a research program was designed to inves-

663

tigate the dependence of the mass interception coefficient on the nature and physicochemical form of radionuclides, the rainfall amount and intensity, and the type and height of vegetation (Hoffman et al. 1989). The results of these experiments are in general agreement with those derived from the model. For 13'1 in soluble form, the experimental values of the mass interception coefficient are about 10 times lower than those predicted by the model. However, for the case in which I3'I is attached to particulates, which is the form likely to have been predominant in fallout, there is good agreement between experimental and predicted values of the mass interception coefficient, especially for amounts of rainfall in excess of 10 mm.

EFFECTIVE HALF-TIME OF RETENTION After 13'1 is deposited on vegetation, environmental removal processes combine with radioactive decay to reduce the initial amount on the vegetation surface (Miller and Hoffman 1979). The time necessary for one-half of the activity to be removed by environmental processes is referred to as the environmental half-time ( T,). This time value, together with the radioactive half-life ( T,) , determines the effective half-time ( T g ) :

Measured values of T, for particulates on vegetation range from 9 to 71 d, with most values between 10 and 20 d (Miller and Hoffman 1979). Values of T, may be expected to vary markedly as a function of the growth of vegetation and of meteorological conditions. Given the short radioactive half-life of 13'1,however, the effective half-time Tefis not particularly sensitive to large variations of the environmental halftime T,. The average value of T, was assumed to be 14 d, yielding an effective half-time, Tef, of slightly more than 5 d.

PASTURE INTAKE BY DAIRY COWS Significant concentrations of I3lI in milk can only arise from the occurrence of fresh fallout deposition followed by the consumption of fresh pasture by the cow reasonably soon thereafter. Because of the climatological, geographical, and agricultural diversity of the U.S., a large data base is necessary to estimate the amount of pasture eaten by cows in counties across the U.S. Such a large data base is provided by the Dairy Herd Improvement Association ( DHIA ) , which maintains extensive records on dairy cattle and milk production. The method by which the pasture intake by dairy cows is estimated is described in detail in another paper in this issue (Dreicer et al., this issue). The daily pasture intake PI,, in week wand state s is obtained as the product of the maximum daily intake of dry matter DM, by cows in state s and of the fraction of the maximum dry matter intake that is due to pasture FPw,s in that week and that state: PI,,, = DM, X FP,,. (9)

664

Health Physics

The maximum daily intake of dry matter, DM,, can be estimated from the cow's body weight and its milk and fat production (NRC 1978) . These data are available in the DHIA records for a large number of herds across the U.S. and for each year of the atmospheric testing period. For the purpose of this study, state averages for the entire testing period have been used in determining the daily intake of dry matter. The fraction of daily dry matter intake by cows that is obtained from pasture FP,,s in each state has been estimated on a weekly basis using the expert opinions of U.S. Department of Agriculture Extension Specialists and of other knowledgeable persons asked to help reconstruct pasture feeding practices during the 1950s. Although subjective, these estimates are the best obtainable information on the seasonal variation of pasture practices at that time. Table 1 presents the estimated pasture intakes obtained from eqn ( 9 ) , for states of the northeastern part of the country. More detailed results are given in Dreicer et al. (this issue).

November 1990, Volume 59, Number 5

The type of vegetation ingested by the cow has been shown to have an influence on the value of F,,, but there is not enough information to take this variation into account. d L-' used in this study is assumed The value of 4 X to be independent of any influencing parameter.

ESTIMATION OF

1311

INTAKES BY HUMANS

Assuming that the time-integrated 13'1 concentrations in fresh cow's milk produced in any county of the U.S. have been estimated, it is necessary to determine how much milk was produced and where it was consumed. Accordingly, information is needed on the milk production in each county, on the milk distribution pattern within each county and each state, on the delay between production and consumption of milk, and on the consumption of milk as a function of factors, such as race, age, and sex. The time-integrated I3II concentrations in the milk consumed by man and the corresponding I3'I intakes are assessed for each atmospheric nuclear test of significance.

INTAKE-TO-MILK TRANSFER COEFFICIENT The intake-to-milk transfer coefficient, F, (d L-'), is defined as the time-integrated concentration of l3II in milk per unit of I3II activity consumed by the cow. This transfer coefficient has been determined experimentally in a large number of studies, including tracer experiments with stable or radioactive iodine and field studies in which pasture was contaminated by I3'I resulting from releases from nuclear facilitiesor from fallout from nuclear weapons tests. Reported values range from 2 X 10-3 to 4 X 10-2 d L-I (Hoffman 1979; Ng et al. 1977), but it seems that fallout studies yielded values in the lower part of the range. In this study, it is assumed that the average value of F,,, for 13'1and for cows is 4 X lop3d L-I. There is conflicting evidence regarding the influence of milk yield on the value of the transfer coefficient, F,.

Time-integrated 311concentrations in consumed milk The time-integrated 13'1 concentrations in consumed milk are assessed in each county as follows: 0 The amount of milk consumed annually in each county is estimated as the product of the population of the county and of the per capita consumption rate in the state. The amount of milk consumed within the county is compared to the fluid milk available from within the county for consumption purposes. The difference between the quantity of available fluid milk and the amount of milk consumed represents the amount of fluid milk for consumption purposes that is imported into or exported out of the county. The available fluid milk for consumption purposes has two components:

Table 1. Estimated pasture intake by dairy cows in the 1950s in Northeastern US. Estimated pasture intake (kg(dry weight) d-l) ~

State

averaged over the pasture season

for the first week of June

Connecticut

5.6

8.8

Delaware

4.9

5.5

Maine

7.9

9.6

Maryland

6.5

7.6

Massachusetts

6.8

11.4

New Hampshire

7.6

9.8

New Jersey

6.1

7.6

New York

5.1

6.8

Pennsylvania

4.4

7.7

Rhode Island

8.3

9.7

Vermont

7.3

9.9

Models of radioiodine transport to populations within the continental U.S. 0 A. BOUVILLE et al.

( 1 ) The amount of milk consumed on farms, which is reported by year and by state and is apportioned by county within the state using the number of farms in each county as a guide. A delay of 1 d between production and consumption is assumed for this category of milk. ( 2 ) The amount of milk sold for fluid use (i.e., that which is either retailed directly from the farm or sold to a distributor). This amount is obtained in the manner estimated by Dreicer et al. (this issue), in which the total milk production in the county and the amount used for manufactured products (cheese, butter, yogurts, etc.) are also considered.

In order to model the local distribution of the milk sold for fluid use in the 195Os, each state is divided into regions determined by the state agricultural service reports, milk marketing order areas, major population areas, or state topography. The surpluses and deficits of milk in the counties of the established regions are balanced, to the extent possible, within each of the regions by pooling the milk in the surplus counties of the region and distributing it to the deficit counties of that region. If a milk surplus or deficit remains after intra-regional pooling, the milk can be shipped to, or from, another region. Milk flow is estimated on the basis of available marketing statistics and upon the advice of experts. Milk is assumed to have been consumed within 2 d of its production if it was produced in the same county, within 3 d if it was produced in the same region, and within 4 d if it was produced from outside the region. In summary, four categories of fluid milk for human consumption with different delay times are considered: Milk consumed on the farm (delay: 1 d); 0 Milk sold for fluid use: (a) produced in the same county (delay: 2 d); (b) produced in the same region (delay: 3 d); and (c) produced in another region (delay: 4 d). 0

Iodine-I 3 I activity intake from milk by humans Iodine-13 1 intake from milk by humans is the product of the time-integrated concentration of I3'I in the milk ingested and of the milk consumption rate. Individual I3'I intakes from milk vary widely from person to person because of variability in such factors as environmental parameters, patterns of milk production and distribution, and dietary habits. Therefore, realistic estimates of individual intakes can be made only if specific information is available on the individual considered (age, sex, place of residence, source of milk, delay between production and consumption, milk consumption rate). In the absence of personal data, only average intakes over large or homogeneous groups of people can be estimated with reasonable accuracy. For this reason, the I3'I intakes of milk by humans estimated in the NCI study for each county and for each nuclear test are averages over specified population groups deemed to be represen-

665

tative of a large spectrum of individuals. Information on average milk consumption rates according to age, sex, and region of the country can be found in Dreicer et al. (this issue). Although ingestion of cows' milk is generally the predominant contributor to I3'I intake, other exposure routes need to be considered for individuals who consume little or no cows' milk. These exposure routes, which include inhalation and ingestion of goats' milk, cottage cheese, leafy vegetables, and eggs, are considered in the NCI study but are not discussed in this paper. ILLUSTRATIVE RESULTS

The methodology previously discussed is to be applied to each county of the contiguous U.S. following each significant atmospheric nuclear test. It is too early at this phase of the study to provide final estimates, but a hypothetical example can demonstrate how the various models and procedures are used. Many factors influence 13'1intake through ingestion of cows' milk in a given county: the magnitude and type of deposition (dry or wet), the distribution of deposition in the total area that supplies milk in the county considered, and the pasture intake by cows in that area, which depends on the time of year in which the weapons test took place. The importance of deposition distribution is illustrated in the following examples, in which it is assumed that following a nuclear test, a total dry deposition of 10 TBq occurred in either one of two different regions of New York state (Fig. 2): 1) Deposition in the New York City region only and nowhere else, resulting in an I3'I deposition density of about 2,000 Bq m-' in that region ( 1954 population: 10 million people; area: 5,100 km2). Milk production for fluid use in the New York City region was approximately 2 X lo7 L in 1954, while the expected milk consumption was 2 X lo9 L. Most milk consumed in the region was therefore imported. 2) Deposition in the New York North region only, yielding an I3'I deposition density of about 700 Bq m-' in that region ( 1954 population: 0.2 million people; area: 14,000 km ') . In that region, the milk production for fluid use was about 3 X lo8 L in 1954, greatly exceeding the estimated 3 X lo7 L needed by the population at that time. Most milk produced for fluid use in the region was therefore exported, about 90% of the surplus being shipped to the New York City region. Although the deposition patterns are hypothetical and chosen to be very simple for the purposes of these examples, the deposition levels that are used are within the range of values observed in a northeastern state after an atmospheric nuclear test of moderate yield. The calculation of average individual I 3 'I intakes corresponding to the two examples considered has been carried out using the following parameter values:

Health Physics

666

November 1990, Volume 59, Number 5

New York City

Fig. 2. Hypothetical I3'I deposition in New York state.

0

Standing crop biomass Y

=

0.3 kg (dry weight)

m -2. Foliar interception constant LY = 2.8 m 2 kg-' (dry weight). 0 Effective half-time of retention by vegetation, T e , = 5.1 d. 0 Pasture intake: 6.8 kg (dry weight) d-I. 0 Milk distribution: as described in Dreicer et al. (this issue). Table 2 presents the time-integrated I3'I concentrations obtained for the four categories of milk considered. For the two distributions of deposition, there are large differences in the average concentrations in the various categories of milk. As expected, the highest concentrations are obtained in the milk produced and consumed locally when deposition occurs in the region. A maximum of 680 Bq d L-' is found in the milk hypothetically consumed on a farm in the New York City region for the assumed deposition of 10 TBq in that region. However, the volume-weighted average I3'I concentration in milk consumed in the New York City region is about four to five times lower when deposition occurs in that region (7 Bq d L-') compared to when it occurs in the New York North region (30 Bq d L-' ). This reflects milk transfer from the New York North region to the New York City region and shows the importance of assessing the movement and dilution of milk from its site of production to its site of consumption. Iodine- 131 intakes by representative individuals through ingestion of cows' milk can be calculated using

the time-integrated concentrations in milk presented in Table 2 and the milk consumption rates given in Dreicer et al. (this issue). For example, in the case of deposition in the New York City region, individual intakes of people living in the New York City region are found to range from 7 Bq d L-' X 0.19 L d-' = 1.3 Bq for a 70-y-old female drinking milk from a supermarket, to 680 Bq d L-' X 0.75 L d-'= 510 Bq for a male teenager drinking milk from a local farm. Similar calculations are being carried out for several population groups in each of the contiguous counties of the U.S. and for each atmospheric nuclear test of significance. It is to be noted that, as indicated by Wachholz (this issue), assigning thyroid doses per unit activity intake of 13'1 and assessing the risk of developing thyroid cancer per unit of thyroid dose from I3'I are topics considered by other task groups at NCI. SUMMARY AND CONCLUSIONS A methodology is being developed by an NCI task group to estimate I3'I exposures that Americans received from the NTS atmospheric weapons tests that were detonated in the 1950s and early 1960s. The most important contributing factor to those exposures is due to the intake of cows' milk. Various steps in assessing milk concentration and intake by humans on a county basis for the most significant atmospheric tests are discussed. The most important factors that influence 13'1intake from ingestion of cows' milk in a given county appear to be the atmospheric process by which 13'1is returned to the earth (i.e., dry or wet deposition), the magnitude and distribution

Models of radioiodine transport to populations within the continental U.S. 0 A. BOUVILLEet al.

667

Table 2. Time-integrated concentrations of 13’1in the four categories of milk (Bq d L-’) consumed in each region for the hypothetical cases of 10 TBq deposition in either the New York City region or the New York North region. 1311 in milk (Bq d L-l) Deposition in New York City region

Deposition in New York North

Category

NYC

NY North

NYC

NY North

Milk consumed on farm

680

0

0

250

Milk sold which originated from the same county

110

0

0

230

Milk from the region pool

-a

-a

-a

-a

Milk from other regions

0

-a

30

-a

Volume-weighted

7

0

30

240

a There is no volume of milk corresponding to this category.

of deposition in the total area that supplies milk to the county considered, the amount of fresh-pasture ingested by dairy cows, and the consumption rate of milk by man. Acknowledgments-The authors would like to thank the following people for their assistance: L. R. Anspaugh, Lawrence Livermore National Laboratory; R. 0. Gilbert, Battelle Pacific Northwest Laboratories; C. V.

Gogolak and A. Hutter, Environmental Measurements Laboratorv: F. 0. Hoffman,Oak Ridge National Laboratory; L. Machta and hi: Smith, National Oceanic and Atmospheric Administration; T. B. Kirchner and F. W. Whicker, Colorado State University: S. L. Simon. Office of Resident Scientist, Republic of the Marshall Islands;J. Till, Radiation Assessments Corporation; P. Voilleque, Science Applications International Corporation; D. Wheeler, Nevada Operations Office of the US. Department of Energy.

REFERENCES Anspaugh, L. R.; Koranda, J. J.; Ng, Y. C. Internal dose from ingestion. In: Anspaugh, L. R.; Koranda, J. J., eds. Assessment of radiation dose to sheep wintering in the vicinity of the Nevada Test Site in 1953. Las Vegas, NV: U.S. Department of Energy; DOE-239; 1986. Beck, H. L. Estimates of fallout from Nevada weapons testing in the western United States based on gummed-film monitoring data. New York, NY: U S . Department of Energy, Environmental Measurements Laboratory; EML-433; 1984. Beck. H. L.; Anspaugh, L. R. The county data base: Estimates of exposure rates and times of arrival in the off-site radiation exposure review project (ORERP) Phase I1 Area. Washington, D.C.: U.S. Department of Energy; U.S. DOE report NVO-320; 1990. Beck, H. L.; Helfer, I. K.; Bouville, A.; Dreicer, M. Estimates of fallout in the continental U.S. from Nevada weapons testing based on gummed-film monitoring data. Health Phys. 59:565-576; 1990. Bergstrom, S. 0. W. Transport of fallout 1-131 into milk. In: Environmental contamination by radioactive materials. New York: Pergamon Press; 1967: 159- 174. Black, S. C.; Barth, D. S. Radioiodine prediction model for nuclear tests. Washington, D.C.: U.S. Environmental Agency; Report EPA-60014-76-027; 1976. Chamberlain, A. C. Interception and retention of radioactive aerosols by vegetation. Atmos. Environ. 457-78; 1970. Church, B. W.; Wheeler, D. L.; Campbell, C. M.; Nutley, R. V.

Overview of the Department of Energy’s Off-Site Radiation Exposure Project (ORERP). Health Phys. 59503-5 10; 1990. Dreicer, M.; Bouville, A. C.; Wachholz, B. W. Pasture practices, milk distribution, and consumption in the continental U.S. in the 1950s. Health Phys. 59:627-636; 1990. Eisenbud, M.; Wrenn, M. E. Biological deposition of radioiodine-A review. Health Phys. 9:1133-1139; 1963. Garner, R. J. A mathematical analysis of the transfer of fission products to cows’ milk. Health Phys. 13:205-2 12; 1967. Garner, R. J.; Russell, R. S. Isotopes of iodine. In: Russell, R. S., ed. Radioactivity and human diet. New York: Pergamon Press; 1966: 297-3 15. Harley, J. H.; Hallden, N. A.; Ong, L. D. Y. Summary of gummed-film results through December 1959. New York, NY: U.S. Atomic Energy Commission, Health and Safety Laboratory; HASL-93; 1960. Hicks, H. G. Results of calculations of external y radiation exposure rates from fallout and the related radionuclide compositions. Livermore, CA: Lawrence Livermore National Laboratory; UCRL-53152; 1981: parts 1-8. Hoecker, W. H.; Machta, L. Meteorological modeling radioiodine transport and deposition within the continental United States. Health Phys. 59:603-6 17; 1990. Hoffman, F. 0. The coefficient for the transfer of radionuclides from animal intake to milk, F(m).In: A statistical analysis of selected parameters for predicting food chain transport and

668

Health Physics

internal dose of radionuclides. Oak Ridge, TN: Oak Ridge National Lab; ORNL/NUREG/TM-282; 1979: 64-79. Hoffman, F. 0.; Frank, M. L.; Blaylock, B. G.; von Bernuth, R. D.; Deming, E. J.; Graham, R. V.; Mohrbacher, D. A.; Waters, A. E. Pasture grass interception and retention of I3'I, 'Be, and insoluble microspheres deposited in rain. Oak Ridge, TN: Oak Ridge National Laboratory, Environmental Sciences Division; Report ORNL-6542; May 1989. Horton, R. E. Rainfall interception. Mon. Weather Rev. 49: 603; 1919. Kirchner, T. B.; Whicker, F. W.; Otis, M. D. PATHWAY: A simulation model of radionuclide transport through agricultural food chains. In: Analysis of ecological systems: Stateof-the-art in ecological modelling 5. New York Elsevier Applied Science Publishers; 1983: 959-968. Lloyd, R. D.; Gren, D. C.; Simon, S. L.; Wrenn, M. E.; Hawthorne, H. A.; Lotz, T. M.; Stevens, W.; Till, J. E. Individual external exposures from Nevada Test Site fallout for Utah leukemia cases and controls. Health Phys. 59:723-737; 1990. Miller, C. W. An analysis of measured values for the fraction of a radioactive aerosol intercepted by vegetation. Health Phys. 38:705-712; 1980. Miller, C. W.; Hoffman, F. 0. The environmental loss constant for radionuclides deposited on the surfaces of vegetation, A,. In: A statistical analysis of selected parameters for predicting food chain transport and internal dose of radionuclides. Oak Ridge, TN: Oak Ridge National Lab; EG/TM-282; 1979: 43-50. National Research Council. Nutrient requirements of dairy cattle. 5th revised ed. Washington, D.C.: National Academy Press; 1978. Ng, Y . C.; Colsher, C. S.; Quinn, D. J.; Thompson, S. E. Transfer coefficients for the prediction of the dose to man via the

October 1990, Volume 59, Number 4

forage-cow-milk pathway from radionuclides released to the biosphere. Livermore, CA: Lawrence Livermore National Laboratory; UCRL-5 1939; 1977. Perkins, R. W. Physical and chemical form of "'I in fallout. Health Phys. 9: 11 13; 1963. Perkins, R. W.; Thomas, C. W.; Nielsen, J. M. Measurements of airborne radionuclides and determination of their physical characteristics. In: Radioactive fallout symposium series 5. Springfield, VA: National Technical Information Service; CONF-765; 1965. Romney, E. M.; Lindberg, R. G.; Hawthorne, H. A.; Bystrom, B. G.; Larson, K. H. Contamination of plant foliage with radioactive fallout. Ecology 44:343-349; 1963. Simon, S. L. An analysis of vegetation interception data pertaining to close-in weapons' test fallout. Health Phys. 59: 619-626; 1990. Thompson, C. B. Estimates of exposure rates and fallout arrival times near the Nevada Test Site. Health Phys. 59:555-563; 1990. Voilleque, P. G . Iodine species in reactor effluents and in the environment. Palo Alto, CA: Electric Power Research Institute; EPRI Report NP- 1269; 1979. Voilleque, P. G. Initial retention by vegetation of "'I in wet depositions of fallout. Report prepared for the National Cancer Institute; 1986. (Available from Paul G. Voilleque, SAIC, 101 South Park Avenue, P.O. Box 50697, Idaho Falls, ID 83405-0697.) Wachholz, B. W. Overview of the National Cancer Institute's activities related to exposure of the public to fallout from the Nevada Test Site. Health Phys. 59:5 1 1-5 14; 1990. Whicker, F. W.; Kirchner, T. B. PATHWAY A dynamic foodchain model to predict radionuclide ingestion after fallout deposition. Health Phys. 52: 717-737; 1987.