Reconstruction of Long-lived Radionuclide Intakes for Techa Riverside Residents

transcript

RECONSTRUCTION OF LONG-LIVED RADIONUCLIDEINTAKES FOR TECHA RIVERSIDE

RESIDENTS: STRONTIUM-90

E. I. Tolstykh,* M. O. Degteva,* L. M. Peremyslova,* N. B. Shagina,*E. A. Shishkina,* V. A. Krivoshchapov,† L. R. Anspaugh,‡ and B. A. Napier§

Abstract—Releases of radioactive materials from the MayakProduction Association in 1949–1956 resulted in contamina-tion of the Techa River; a nuclide of major interest was 90Sr,which downstream residents consumed with water from theriver and with milk contaminated by cows’ consumption ofriver water and contaminated pasture. Over the years, severalreconstructions of dose have been performed for the approx-imately 30,000 persons who make up the Extended TechaRiver Cohort. The purpose of the study described here was toderive a revised reference-90Sr-intake function for themembers of this cohort. The revision was necessary becauserecently discovered data have provided a more accuratedescription of the time course of the releases, and more is nowknown about the importance of the pasture grass-cow-milkpathway for the members of this cohort. The fundamentalbasis for the derivation of the reference-90Sr-intake functionremains the same: thousands of measurements of 90Sr contentin bone with a special whole-body counter, thousands ofmeasurements of beta-activity of front teeth with a specialtooth-beta counter, and a variety of other measurements,including post mortem measurements of 90Sr in bone, measure-ments of 90Sr in cow’s milk, and measurements of beta activityin human excreta. Results of the new analyses are that themajor intake started in September 1950 and peaked somewhatlater than originally postulated. However, the total intake foradult residents has not changed significantly. For children ofsome birth years, the intake and incorporation of 90Sr in bonetissue have changed substantially.Health Phys. 101(1):28–47; 2011

Key words: contamination, environmental; dose, internal;food chain; metabolism

INTRODUCTION

RADIOACTIVE CONTAMINATION of the Techa River (SouthernUrals, Russia) occurred in 1949–1956 due to releases ofliquid radioactive wastes from the Mayak ProductionAssociation (MPA), Russia’s first facility for the manu-facture of plutonium. The main long-lived radionuclidesin the releases were 90Sr and 137Cs. Contamination ofthe components of the Techa River system (water,bottom sediments, floodplain soils, and floodplaingrasses) resulted in chronic external and internalexposure of about 30,000 residents of riverside com-munities. The 90Sr-intake function is a fundamentalcomponent of the dose-reconstruction system used forassessment of radiogenic health effects in the TechaRiverside residents (Degteva et al. 2000a, 2006;Kossenko et al. 2002; Krestinina et al. 2005, 2007).

Several investigations have been devoted to thereconstruction of radionuclide intake for the Techa Riv-erside population, and these have differed in terms ofmethodological approaches. The first estimates of theintake function were based on measurements of radionu-clide contents in different foodstuffs (Marey et al. 1952,1966). However, the problem of intake reconstruction onthe basis of foodstuff contamination and 90Sr concentra-tion in river water was initially complicated by thefollowing factors:

1. Data are not available for measurements of radioac-tivity (even total-beta activity) in river-water samplesand foodstuffs in the early period of massive releases(up to mid 1951);

2. Because adequate methods of radiochemical analyseswere not available, only data on total-beta-activitymeasurements are available for the period from mid-1951 up to 1959 (the first results for 90Sr contents infish samples are dated 1958);

3. Since 1953 the use of the Techa River as drinkingwater was reduced because wells were drilled tosupply water, and prohibitive administrative sanctions

* Urals Research Center for Radiation Medicine, Vorovskogo68 a, 454076 Chelyabinsk, Russian Federation; † Southern Urals StateUniversity, Chelyabinsk, Russian Federation; ‡ Division of Radiobi-ology, Department of Radiology, University of Utah, Salt Lake City,UT; § Pacific Northwest National Laboratory, Richland, WA 99352.

For correspondence contact: Evgenia I. Tolstykh, Urals Re-search Center for Radiation Medicine, Vorovskogo 68 a, 454076Chelyabinsk, Russian Federation, or email at evgenia@urcrm.ru.

DOI: 10.1097/HP.0b013e318206d0ff

28 www.health-physics.com

were taken.** However, it is impossible to estimatequantitatively the amount of river water replaced bywell water in the daily diet of residents; and

4. Extremely irregular contamination of vegetable gar-dens was observed in the different settlements. Theextent of contamination was determined by the sourceof irrigation water and by whether or not the vegetablegarden was in the floodplain.

Subsequent estimates were based on measurements of90Sr in human tissues and on the retention function forstrontium in human bones (Rasin 1970; Kozheurov 1994).Practically all known methods of 90Sr estimation in humanswere used starting in 1951 in the study of the Techa Riverpopulation. Among them were in vivo measurements ofsurface-beta activity in first permanent incisors with use oftooth-beta counters (TBC); this information has been re-viewed by Tolstykh et al. (2003). Rasin (1970) expressedthe pioneering idea of using TBC data for purposes ofreconstructing the intake of 90Sr based on the followingfeatures of 90Sr retention in tooth enamel:

1. Strontium accumulation occurs during the period ofenamel mineralization of permanent teeth; this allowsfor subsequent measurements of beta activity in frontteeth with TBCs (if permanent teeth were not ex-tracted for medical or trauma reasons);

2. The period of mineralization is relatively short. Forfirst permanent incisors, it is a period of 0.5–4 y, andthe majority of enamel is formed during 1 y; and

3. The rate of strontium removal from enamel is veryslow; it is insignificantly greater than the rate ofradioactive decay.

A basic equation was formulated that relates (bymeans of additional parameters) the age dependence inTBC measurements and the 90Sr-intake function for adultresidents of Muslyumovo, a settlement on the TechaRiver that is used as a reference location. The additionalparameters were the following: age-dependent strontiumtransfer from the gastrointestinal tract to tooth enamel;the strontium-retention function in tooth enamel; and theratio of 90Sr intake by children of different ages to that byadults. Therefore, the reconstruction of relative 90Srdietary intake was reduced to solution of the basicequation. This task belongs to the class of mathematical

inverse problems, and its partial solutions were describedearlier (Kozheurov and Degteva 1994). These partialsolutions were obtained on the basis of simplifying andrather arbitrary assumptions; nevertheless, the solutionproposed by Kozheurov (1994) was very important forrough evaluation of 90Sr intake.

The rough solution allowed the reconstruction of arelative 90Sr-intake function for the period 1950–1959.The absolute values of 90Sr intakes were derived from (1)numerous in vivo 90Sr measurements of Techa Riversideresidents with use of the SICH-9.1 whole body counter(WBC) (Kozheurov 1994; Kozheurov et al. 2002), and(2) a semi-empirical function of strontium retention in thehuman bone (Degteva and Kozheurov 1994). The resulting90Sr-intake function was used in assessment of internaldoses from 90Sr for the Techa Riverside residents. Further-more, the 90Sr-intake function was used as a basis forreconstruction of the intake of other (non-strontium) radio-nuclides from the releases by taking into account the ratio ofother radionuclides to 90Sr in the liquid radioactive wastes.These data were implemented for the calculation of esti-mates of internal dose in the Techa River DosimetrySystem-2000 (TRDS-2000) (Degteva et al. 2000a and b),which was elaborated for the purpose of estimating radio-genic risk for residents on the Techa River.

The purpose of the current study is revision of thereference-90Sr-intake function for the Techa Riverside resi-dents with the use of essentially similar methodologicalapproaches based on (1) improved data from databasesestablished at the Urals Research Center for RadiationMedicine (URCRM) on residence histories of the exposedpopulation; (2) detailed analysis of archival data on envi-ronmental contamination; (3) extended sets of TBC andWBC measurements; (4) new mathematical approaches tosolution of the inverse problem (Zalyapin et al. 2004);and (5) additional information (Degteva et al. 2008) fromthe Mayak Production Association concerning the begin-ning of the radioactive releases into the Techa River, thetime-dependence and the radionuclide composition of thereleases.

The revised 90Sr-intake function described in thispaper has been used in the latest version of the TechaRiver Dosimetry System, which is TRDS-2009D(Degteva et al. 2009). Where appropriate, contrasts aregiven between values used in TRDS-2009D and TRDS-2000. The symbol “D” denotes deterministic; a stochasticversion of this system is under development.

METHODS AND RESULTS

The reconstruction of 90Sr intake for Techa River-side residents includes the following steps:

** Decree of the USSR Council of Ministers of 6 January 1953 onprohibition of use of river water along the entire length of the TechaRiver; three Decrees of the USSR Council of Ministers and RussianFederation Council of Ministers in 1953–1954 on well drilling inriverside settlements; Decree of USSR Council of Ministers of 11 June1954 on exclusion of any household utilization of Techa River water,and establishment of prohibited area; Decree of Chelyabinsk RegionalExecutive Committee 760 of 9 July 1954 on establishment ofprohibited areas within the limits of riverside settlements and nearbridges.

29Reconstruction of long-lived radionuclide intakes ● E. I. TOLSTYKH ET AL.

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1. Reconstruction of relative time-dependent 90Sr intakeduring 1950–1955 for residents of Muslyumovo fromin vivo TBC measurements. Muslyumovo is thereference settlement for dosimetric Techa River stud-ies because the most complete dosimetric informationis available for this village: measurements of 90Srconcentration in water, soil and foodstuffs, and invivo and post mortem measurements of 90Sr contentsin bone tissues and teeth. The location of Muslyu-movo is shown in Fig. 1;

2. Reconstruction of a 90Sr-intake function for the period1954–1980 obtained from measurements of 90Sr con-centration in cow’s milk collected from individualhouseholds in Muslyumovo;

3. Assessment of 90Sr-intake functions for Muslyumovoresidents of different ages with use of WBC measure-ments of 90Sr-body burden and a semi-empiricalfunction of strontium retention in bone (Degteva andKozheurov 1994); and

4. Estimation of applicability of 90Sr-intake functions forMuslyumovo residents as reference for other TechaRiverside settlements.

The problem of 90Sr-intake reconstruction requiresthe use of different mathematical and statistical ap-proaches. Some approaches are described in correspond-ing subsections: the analysis of TBC measurements forprecise evaluation of the beginning of massive radioac-tive releases into the Techa River, and the statisticalapproach to uncertainty analysis of the 90Sr-intake func-tion. The mathematical approach to reconstruction ofrelative 90Sr intake in 1950–1955 for Muslyumovo resi-dents on the basis of in vivo TBC measurements was

described earlier as a solution of an inverse biophysicsproblem (Zalyapin et al. 2004). Linear regression analy-sis and non-parametric correlation analysis (Spearmanrank order correlation) were used for analysis of TBC,WBC, and radiochemical measurements of environmen-tal samples. The calculations were performed with use ofthe STATISTICA computer code (StatSoft, Inc., Tulsa,OK) or the KaleidaGraph Data Analysis and GraphicPresentation code (Synergy Software, Denver, CO).

Step 1. Reconstruction of relative 90Sr intake in1950–1955 for Muslyumovo residents on the basisof in vivo TBC measurements

The age-dependent content of 90Sr in teeth, Y(T, tu),for persons born at time T is related to the 90Sr-intakefunction in the following basic equation:

Y(T, tu) � ��tinit

�(t � T,t) X(t)k(t � T) R(tu � t)dt,

(1)where

� �a scaling factor (number of counts per unittime that correspond to a unit of activity inteeth);

�(�, t) � the ratio of 90Sr intake for children of age� at time t relative to intake for adults;

x(t) � the rate of 90Sr intake at time t;k(�) � the 90Sr-transfer coefficient from the gas-

trointestinal (GI) tract to enamel of thefirst permanent incisors at age �;

Figure 1. Scheme of the Techa River and positions of riverside settlements included in analysis of age-dependent TBCdata. Metlino Village is the closest to the site of releases.

30 Health Physics July 2011, Volume 101, Number 1

R(�t) � retention function (i.e., fraction of 90Srremaining in enamel of permanent incisorsat time �t after intake at age �); theretention function is equal to

� e��t, �t � 00, �t � 0 ,

where � is the rate of 90Sr elimination from tooth enamel,for which the value of 0.043 y�1 is used as describedbelow;

tu � 1967, weighted average year of TBC measure-ments; and

tinit � date of the beginning of 90Sr intake thatcoincides with the beginning of massive re-leases into the Techa River.

Some values and functions in eqn (1) require expla-nation. Eqn (1) contains two unknown functions: x(t), therate of 90Sr intake at time t; and k(�), the 90Sr-transfercoefficient from the gastrointestinal (GI) tract to enamelof the first permanent incisors at age �. Other functionscan be described numerically on the basis of independentdata sets. Zalyapin et al. (2004) found a singular andstable solution of eqn (1) that allows quantification of thetwo unknown functions. The following subsections pro-vide detailed descriptions of each parameter, its evalua-tion, and the limitations of the evaluations.

Tooth-beta-counter (TBC) data. Gas-flow Geiger-Muller detectors (Ivanov et al. 1962; Kozheurov et al.2000) were used for in vivo measurements of surface-beta activity in front teeth. The diameter of the TBCdetector window was 1 cm, and only the upper and lowerfirst incisors were measured. The TBC measurementswere performed only if all anterior teeth were present.The unit of TBC measurement is counts per minute(cpm); i.e., the TBC measurements give relative valuesof 90Sr content in teeth. The decision level of the TBCmethod has been evaluated as 9 cpm (Kozheurov et al.2000); the relative error for TBC measurements is 10%.

The cohort of Muslyumovo residents drawn from theURCRM Register must meet the requirements that (1) theypermanently lived in Muslyumovo during the main periodof radionuclide intake, defined as 1949–1959, and (2) theywere measured with the same type of TBC.

To ensure that data selected agree with the criteria,the cohort of Muslyumovo residents investigated withthe TBC was fully researched for each measurementresult with use of information from registries and medi-cal records. Analyses comparing the TBC distributions

within each age group showed that measurements ob-tained during the period 1963–1971 do not differ statis-tically, and these data were analyzed together. Theweighted average year of measurements is 1967. TBCdata can be used only for a narrow age group of persons(born in 1940–1957), because for older persons themeasured values are lower than the TBC-decision level(Kozheurov et al. 2000; Tolstykh et al. 2003). Hence, theTBC data selected for the current investigation include1,268 measurements obtained for 690 persons.

Analysis of TBC measurements for precise eval-uation of the beginning of massive radioactive re-leases into the Techa River and age dependence inTBC data, Y(T,tu). Analysis of eqn (1) shows that itssolution depends on an assumption of a zero time point(tu) that corresponds to the beginning of 90Sr intake. Theinitial assumption of the zero-time point (January 1950)used by Kozheurov (1994) was determined by availableinformation on the beginning of operation of the firstnuclear reactor at the MPA and the start of releases ofradioactive wastes into the Techa River. Two approacheswere used in the current study for a more accurateevaluation of the time when intensive radioactive re-leases into the Techa River commenced:

1. Analysis of archival information on the time of thelaunch of the reactors at MPA; increase in output;features of the technological process of weapons-grade plutonium production, including analysis ofwastes formed at each stage of the process; andmethods for storage and reprocessing of the radioac-tive wastes. Such information was obtained fromMPA archives by personnel of the Mayak CentralPlant Laboratory (CPL). This approach was describedin Degteva et al. (2008) with a detailed description ofmethods and assumptions used in the analysis ofarchival information. According to this report, releaseof radionuclides (including 90Sr) into the Techa Riverwas not significant until September 1950; and

2. Detailed statistical analysis of the TBC data forevaluation of the beginning of intensive 90Sr accumu-lation in the enamel of permanent anterior teeth.

Results of the TBC measurements were averagedwith use of a rolling average in the following way:Average of TBC data, Y�(t), as a function of birth date,was determined by averaging individual TBC data [Yi]obtained for persons in a defined interval of dates of birtht(interval); the interval was sequentially moved along theaxis of dates of birth by a step, and individual TBC datawere averaged in respective intervals:

Y�(t)�{[Yi]3 t(stepi, intervali)}, (2)where step � interval.

The width of interval was a multiple of a number ofmonths and changed from 3 to 12 mo. The width of stepwas also a multiple of a number of months and changedfrom 3 to 6 mo; the width of step was below or equal tothat of interval. Variation in step and interval parametersallowed a set of average TBC data as a function of dateof birth. Each set was described by an analytical functionwith use of the maximum likelihood method. Analysis ofthe extremes of the analytical function allowed determi-nation of a steady location of the peak in TBC for personsborn in March 1950 (Fig. 2), which is independent of thecharacteristics of averaging.

Formation of the crown enamel of permanent ante-rior teeth begins at the age of 0.5 y (6 mo); the enamelgrows at a maximum rate during the first year of life(Massler et al. 1941; Novik 1971). This means thataccumulation of 90Sr in the enamel of permanent incisorsbegins six months after birth. Thus, from interpretationof the data shown in Fig. 2, it can be concluded that thebeginning of intensive 90Sr intake should have occurredsince September 1950. Therefore, two independent ap-proaches indicated that massive releases of radioactivewastes into the Techa River commenced from September

1950. The zero-time point, corresponding to the begin-ning of 90Sr intake, is equal to September 1950 (1950.67expressed as fractional years).

Strontium retention in tooth enamel, R(�t). The90Sr-retention function is determined by the 90Sr-diminutionrate from tooth enamel and the adjacent layer of dentine.The analyses of repeated TBC measurements show thatan exponential function well describes the individual90Sr-retention function (Tolstykh et al. 2000). The aver-age rate of 90Sr diminution �, was estimated, and theaverage value taken for the present study is equal to0.043 y�1 (Tolstykh et al. 2000). This value is the sum oftwo values: the rate of 90Sr radioactive decay (0.0238y�1) and the biological elimination rate (0.019 y�1)related to the processes of ion exchange, transport ofliquid, mechanical attrition, etc.

Ratios of 90Sr intake for children to intake foradults, �(�, t). The ratio �(�, t) is the intake of 90Sr forchildren aged � years relative to adults in calendar year t.As described above, the enamel of anterior teeth startsforming 6 mo after birth; therefore, measurements ofsurface-beta activity of the permanent front teeth reflect90Sr intake from this age; i.e., from 6 mo. For conve-nience of the solution of the inverse problem, thefollowing assumptions were made:

1. The date of mineralization of the first incisors wassubstituted for the date of birth. Therefore, the date ofbirth is not used in calculations; instead, the “dateof tooth birth” is used, which is calculated as the date ofbirth plus 6 mo. Correspondingly, the age of a person isnot used in calculations; instead, the age of the firstincisors since the beginning of mineralization is used,which is calculated as the age of a person minus 6 mo;

2. The zero-time point is September 1950. The age of aperson at the zero-time point is the age in 1950.67; and

3. Coefficients �(�, t) are calculated from the zero ageof tooth.

Because the water pathway of radionuclide migra-tion was determinative for the Techa River situation, thecontamination of dietary components other than riverwater is determined by corresponding transfer coeffi-cients. Table 1 summarizes data on the contribution ofdifferent foodstuffs to the dietary intake of 90Sr forMuslyumovo residents in 1950–1951. As can be seen,the main sources of 90Sr in diet were river water andcow’s milk. It is known that children typically consumemore milk than adults. The relative 90Sr-intake coeffi-cient for children aged � during calendar year t, �(�, t), isdetermined on the basis of the following equation:

Figure 2. Averaged TBC data used for solution of the inverseproblem; the line represents the rolling average.

��, t� �Cw(t) � Mwc(�) � Cm(t) � Mmc(�)

Cw(t) � Mwa � Cm(t) � Mma,

(3)where

�(�, t) � relative 90Sr intake at age � in calendaryear t (equal to 1 for adults);

Cw(t) �concentration of 90Sr in river water duringcalendar year t;

Cm(t) �concentration of 90Sr in cow’s milk duringcalendar year t;

Mwc(�) �mass rate of consumption of river waterfor children aged �;

Mmc(�) �mass rate of consumption of cow’s milkfor children aged �;

Mwa �mass rate of consumption of river waterfor adults; and

Mma �mass rate of consumption of cow’s milkfor adults.

If the ratio of 90Sr concentration in milk to water,kmw(t), is known, eqn (3) can be presented as

��, t� �Cw(t) � Mwc(�) � kmw(t) � Cw(t) � Mmc�

Cw(t) � Mwa � kmw(t) � Cw(t) � Mma

�Mwc(�) � kmw(t) � Mmc(�)

Mwa � kmw(t) � Mma. (4)

As can be seen from eqn (4), the data necessary forcalculation of relative 90Sr intake for different age groupsare the age-dependence of water- and milk-consumptionrates and the ratio of 90Sr-concentration in milk-to-riverwater.

The daily milk consumption by Urals residents ofdifferent ages was repeatedly investigated by use of anumber of methods.

Data obtained for adults during periods close to theradionuclide releases are presented in Table 2. Theresults of Skryabin (1971) provide the most comprehen-sive data because they include data on seasonalconsumption and analyses of duplicate provisions. Skry-abin’s investigations were not performed in the TechaRiverside villages but did include neighboring settle-ments with similar lifestyle and housekeeping. There-fore, the value of 500 mL of daily milk consumption isassumed for adults in the present study.

For children of other ages, there are rough estimates(Skryabin 1971) for these age groups: 3 through 7 y(preschool children)—490 mL d�1; and 11 through 16 y(school children)—500 mL d�1. Hence, for these agegroups the daily milk consumption was considered thesame as for adults (500 mL d�1).

For children from birth to 1 y old, the age depen-dences of milk consumption were examined in detail in1966 (Peremyslova 1967). For children aged 0.5–1 y, the

Table 1. Contribution of different foodstuffs to the dietary intake of 90Sr for adult residents of Muslyumovo in1950–1951.a

FoodstuffDaily

consumption (kg)

90Sr concentration,relative to river water

Contribution to dailydietary intake (%)

Drinking water 1.0−1.6 1.0 92−96Milk and dairy products 0.5−0.6 0.04−0.1 2.0−3.5Meat 0.075−0.1 0.05−0.1Fish 0.02−0.03 1.0−2.0 2.0−4.5Vegetables 0.2−0.4 0.002−0.004

aComment: In 1950–1951 there were only two wells supplying about 4% of Muslyumovo residents. The data on daily diet andcoefficients of 90Sr transfer from river water to foodstuffs were taken from technical reports of the Biophysics Institute (Moscow) andfrom URCRM investigations of Techa River residents. Grain products were not contaminated because they were not cultivated on theTecha River floodplain.

Table 2. Daily consumption rates of milk and milk products for the rural population of Chelyabinsk Oblast, whichincludes the villages on the Techa River.

Period ofinvestigation

Institute or organizationperforming research

(reference)Method of

investigation Place of investigationMilk, milk

products (kg d�1)

1957−1958 Biophysics Institute, Moscow(Borovinskikh et al. 1958)

— Settlements in the upperTecha

1961 Biophysics Institute, Moscow(Marey et al. 1961, 1966)

Questioning of15 families

Muslyumovo 1.1

1959−1962 URCRM, Chelyabinsk(Borovinskikh et al. 1963)

Analyses of duplicateddaily diet

Muslyumovo, VerkhnyayaTecha, Brodokalmak

1961−1963 URCRM, Chelyabinsk(Skryabin 1971)

Analyses of duplicateddaily diet

Rural population inChelyabinsk Oblast

average daily consumption of cow’s milk was estimatedto be 350 mL.

In general, daily water consumption includes thefollowing components: (1) water excreted as metabolicwater due to oxidizing processes; (2) unboiled water; (3)water contained in milk and milk products; (4) otherbeverages including tea; (5) water used for cooking(water in soup, etc.), excluding liquid from foodstuffs;and (6) mass of liquid from foodstuffs before cooking.Only items (2), (4), and (5) are presumed to be derivedfrom the Techa River.

The approach used by ICRP experts (ICRP 1975) toestimate total liquid consumption is based on the theo-retical assumption that one expended kilocalorie requires1 mL of water. As noted in ICRP (1975), the dataobtained by this method (3,000 mL for men and 2,100mL for women) exceed available data on direct estima-tion of liquid consumption. For example, the maleresidents of the outskirts of London consume only about1,980 mL of liquid per day (beer and cider constituteabout 230 mL d�1) (ICRP 1975). According to data froma generalized analysis of water balance in humans, thephysiologically-required water intake (except for meta-bolic water) is about 2,100 mL d�1 for adults (Schmidtand Thews 1989). It is necessary to exclude from thisvalue the consumption of milk and water in otherfoodstuffs before cooking. For the Urals region, thesevalues were evaluated as 450–500 mL and 720 mL,respectively (Skryabin 1971). Therefore, the daily waterconsumption, which can be assumed to be river water, isequal to 880–930 mL. It should be noted that accordingto U.S. investigators (Ershow et al. 1991), the actualdaily liquid consumption for healthy women was 1,900mL, and tap-water (analogue of Techa River water)consumption was about 1,000 mL.

For the present investigation, the mass of river-waterconsumption for children was derived from data onphysiological liquid requirements for age groups takinginto account the contribution of milk, water in foodstuffsbefore cooking, and metabolic water (Table 3).

The ratio of 90Sr concentration in cow’s milk to that inriver water should be evaluated for the period from Sep-tember 1950 to December 1952. In 1953, a campaign of

well drilling was started, and a sufficient number of wellswere constructed by autumn 1953. It is assumed that since1954 the consumption of river water ceased, and the mainsource of 90Sr in diet became cow’s milk. The amount ofriver water used as drinking water in 1953 is unknown, andthe ratios for 1953 can only be obtained via interpolation.

Sources of 90Sr in cow’s milk in the period fromSeptember 1950 to December 1952 were (1) river waterand (2) floodplain grass contaminated due to 90Sr transferfrom soil. Floodplain grass became an additional sourceof 90Sr in cow’s milk after a major flood occurred inApril 1951.

In Muslyumovo there were no watering places forcows other than the Techa River. Daily water consump-tion for cows depends on the type of food (dry fodder orsucculent forage) and is about 35–50 L (Williams 1937).The concentration of 90Sr in milk due to river-waterconsumption is determined by a transfer coefficient,which is defined as the concentration of 90Sr in milkdivided by the daily intake of 90Sr by the cow. Thistransfer coefficient has been evaluated as 0.0013 d L�1

(Annenkov et al. 1973). Therefore for the period beforefloodplain contamination (October 1950–April 1951),the ratio of 90Sr concentration in cow’s milk to that inriver water is equal to 0.040–0.065.

It is difficult to estimate the 90Sr content in milk dueto forage-grass contamination because the floodplainswere not the only place for pasture. However, the floodof 1951 resulted in considerable contamination of flood-plain soil and grasses, and the ratio of 90Sr concentrationin cow’s milk to that in river water could vary signifi-cantly from 0.1 (minimum usage of floodplain-foragegrass) to about 3.5 (maximum, exclusive use offloodplain-forage grass in the period of decreased re-leases after 1951) (Degteva et al. 2008).

Thus, different versions of �(�, t) ratios were calcu-lated corresponding to different boundary assumptionson the ratio of 90Sr concentration in cow’s milk to that inriver water [kmw(t)] (Fig. 3). Eight versions were consid-ered that imitate different time-dependent changes in theconcentration ratio between milk and water. Suchchanges were mainly due to the massive flood of April–May 1951 and a sharp increase in 90Sr releases (up to 100times in comparison with earlier periods) that occurred ina short period during October 1951 (Degteva et al. 2008).The values of kmw(t) were used as minima and maxima ofexpert estimates.

For the calculation of ratios of 90Sr intake forchildren to intake for adults, �(�, t), the followingprocedures were performed: (1) calculation of �(�, t) fordifferent ages and times with expert point estimates ofwater- and milk-consumption rates and the ratio of 90Srconcentration in cow’s milk to that in river water; and (2)

Table 3. Values of river-water-consumption rate used for evalua-tion of �(�, t). The calculated values are based on Skryabin (1971),Mazurin and Vorontcov (1985), and Schmidt and Thews (1989).

Age (y)River-water

consumption (g d�1)

0.5 1001−3 3504−6 800�10 1,000

construction of the surface of �(�, t) values on the basisof these reference points with use of multivariate cubicspline interpolation.

After 1954, the �(�, t) values are unchanged becausethe main source of 90Sr in diet became cow’s milk.

Solution of the inverse problem. Solutions of eqn(1) were obtained for the eight versions of �(�, t) ratios;the solutions provide the relative 90Sr-intake functions foradults and rates of strontium transfer from the GI tract toenamel of permanent anterior teeth. For each �(�, t) ratio,a singular and stable solution was found.

Relative 90Sr-intake functions, x(t), for adult residentsof Muslyumovo obtained for the eight versions of �(�, t) areshown in Fig. 4. Dispersion in the values of relative90Sr-intake does not exceed 12%. This means that changesin the �(�, t) ratio in the range from the minimum to themaximum of expert estimates do not have a significantinfluence on the solution of eqn (1); the solution is stable.The accuracy of the solution is mostly influenced by theaccuracy of experimental data (TBC measurements):The standard error of averages of TBC data determinesthe width of dispersion of 90Sr-intake functions.

Fig. 5 shows relative values of the transfer coeffi-cient from the GI tract to the enamel of permanent firstincisors obtained for different values of the �(�, t) ratio.Analysis of estimates from Fig. 5 shows that the maininfluence of different versions of �(�, t) occurs at the age

of 1–2 y. Dispersion of values in strontium transfer inthis age period is about 25%. This means that thecontribution of milk to the total-90Sr intake at these agesinfluences the evaluated function of strontium transferfrom GI tract to enamel.

The main characteristic of a solution for eqn (1) iseither a misalignment or an agreement between originaldata (TBC measurements) and the results of eqn (1) with

Figure 3. Assumed ratios of 90Sr concentration in cow’s milk to that in river water in different calendar periods inMuslyumovo. Panels a, b, c, and d show different assumptions of changes in milk-to-water 90Sr concentration dependenton time after the beginning of the releases. Numbers correspond to the versions used in calculations of �(�, t).

Figure 4. Relative 90Sr intake functions for adult residents ofMuslyumovo obtained for different versions of �(�, t).

obtained values of unknown functions. Fig. 6 shows agree-ment between calculated and original TBC data. Fig. 6shows that all solutions satisfactorily describe the originalTBC data (i.e., are within the corridor of dispersion in TBCmeasurements characterized by a standard error).

Selection of �(�, t) ratio for estimation ofreference-90Sr-intake function for Muslyumovoresidents. Estimation of the reference-90Sr-intake func-tion requires selection of a solution corresponding to oneversion of the �(�, t) ratio; i.e., requires selection of areference �(�, t) ratio. As described in the previous

section, versions of the �(�, t) ratio were defined asboundary values (minimum and maximum) to evaluatestability of the solution. For selection of the reference�(�, t) ratio, the function of strontium transfer from GItract to enamel is considered in more detail.

Strontium transfer from the GI tract to enamel is amultiplication of two parameters: strontium absorption fromthe GI tract to blood (so called f1 value) and transfer fromblood to the enamel of permanent incisors that correspondsto the volumetric growth rate of the incisor crown. Thesefunctions are age-dependent. Strontium absorption from theGI tract was evaluated in a number of experiments that usedstable or radioactive strontium and calcium (Abrams et al.1997; ICRP 2003). The volumetric growth rate of the crownof anterior teeth can, at the present time, be qualitativelyevaluated only from published data, including radiologicaland histological studies of growth lines (Retzius lines) ontooth cuts as well as morphometric measurements ofenamel thickness along the crown. Analysis of publisheddata (Massler et al. 1941; Novik 1971) shows that intensivemineralization occurs during the first months of life andthen declines continuously.

The obtained solutions of strontium transfer fromthe GI tract to the enamel and available data on f1 valuesallow a rough evaluation of the relative growth (miner-alization) rate of the crown on anterior teeth. For this,versions of the �(�, t) ratio, corresponding to minimumand maximum values of strontium transfer from the GItract to the enamel at ages 0–2 y, were selected (versions3 and 6). Table 4 presents f1 values and solutions of eqn(1) for respective ages. A rough evaluation of the relativevolumetric growth rate of the incisor crown is shown inFig. 7. It is seen from Fig. 7 that the relative volumetricgrowth rate of the enamel is described by a steeperdecline if version 3 of the �(�, t) ratio is used in eqn (1).This rate is closer to available qualitative data on thevolumetric growth rate of the enamel than the rateobtained if version 6 of the �(�, t) ratio is used. For thisreason, version 3 of the �(�, t) ratio should be recognizedas closer to reality. However, it should be noted that theassumed �(�, t) ratios were considered as extreme ver-sions. As a result, the reference �(�, t) ratio (Table 5)corresponds to assumptions that are intermediate be-tween versions 3 and 6. Table 6 presents the resultingrelative 90Sr-intake-function values for Muslyumovo.

Step 2. Derivation of 90Sr-intake function forresidents of Muslyumovo in 1954–1980

After 1953, cow’s milk became the main source of90Sr in the diet of Muslyumovo residents. There are twodata sets related to 90Sr contents in milk samples:published data on total-beta-activity measurements(1956–1962) (Borovinskikh et al. 1963) and data on

Figure 5. Relative values of the transfer coefficient from the GItract to the enamel of permanent first incisors obtained fordifferent values of the �(�, t) ratio.

Figure 6. Agreement between calculated and measured TBC data.

radiochemical measurements performed at URCRM be-tween 1968 and 1980 (Table 7). The daily intakes of 90Srfor the period 1954–1980 were reconstructed from mea-surements of cow’s milk. For this purpose, the experi-mental data were described by a smoothed line with useof multivariate interpolation. It was assumed that thedaily consumption of cow’s milk by adults is equal to 0.5L as described above.

Step 3. Assessment of 90Sr intake function forresidents of Muslyumovo in 1950–1980

For assessment of reference-90Sr intake for adultresidents of Muslyumovo from 1950–1980, the follow-ing procedures were performed: (1) assessment of 90Srcontents in the skeleton (body) after unit intake of 90Srwith use of the semi-empirical function of strontiumretention in bone for adults (Degteva and Kozheurov1994) based on the relative 90Sr-intake function, and (2)normalizing (scaling) of the derived curve to the WBCdata on 90Sr-body burden in permanent residents ofMuslyumovo using the least squares method.

The method and results of WBC measurements ofTecha Riverside residents were described earlier

(Kozheurov 1994; Kozheurov et al. 2002). For thepurposes of the present study, the original set of WBCmeasurements was analyzed in detail. WBC measure-ments obtained for women who were pregnant and/orbreastfeeding in the period of maximum releases (1950–1951) were excluded from the analysis. These studiesshowed that accumulation of 90Sr in the skeleton ofpregnant and/or breastfeeding women was 1.5 timeshigher compared to non-pregnant and non-lactatingwomen due to increased diet and hormonal changes inmineral metabolism (Tolstykh et al. 2009). The set ofWBC measurements for permanent residents of Muslyu-movo born in 1920–1926 (adults at intake) comprises189 measurements for 50 persons (women and men). Fig.8 illustrates the principle of such reconstruction.

According to analysis of MPA archival data(Degteva et al. 2008), the contamination of the TechaRiver before September 1950 was insignificant; how-ever, some minor amounts of radioactive liquid did reachthe river. It is impossible to estimate the water contam-ination up to September 1950 exactly. Therefore, it wasassumed that the daily intake of 90Sr from January 1950until September 1950 was equal to 0.02 of the intakefrom September 1950 until March 1951.

The resulting values of 90Sr intake for adult Mus-lyumovo residents are given in Table 8. As follows fromthe description of eqn (1), TBC data allow reconstruction ofthe 90Sr-intake function for persons older than six months ofage. For children in the first six months of life, reconstruc-tion of 90Sr intake requires separate approaches.

The main source of 90Sr in the diet of newborns isbreast milk. Evaluation of the intake of this radionuclidewith breast milk should take into account the volume ofbreast milk consumed, 90Sr concentration in breast milk,and the period of breastfeeding. Assessment of theseparameters and development of the biokinetic model forstrontium transfer to breast milk were implemented under aEuropean Union-funded project and are described inShagina et al. (2007). One of the parameters required forestimation of 90Sr intake with breast milk in the TechaRiver settlements is the intake with maternal diet; i.e., thereference 90Sr-intake function evaluated in this study.

Figure 7. Relative volumetric growth rate of enamel of anteriorteeth evaluated from data outlined in Table 4 for two functions ofstrontium transfer from GI tract to the enamel; the two functionscharacterize minimum and maximum values.

Table 4. Data used for estimation of relative volumetric growth rate of incisor crown.

Age ofchild (y)

Age oftooth (y)

Sr absorption inGI-tract (f1 value)

f1 value, relative to unityat zero age of tooth

Sr transfer from GItract to enamel for

different versions of the�(�, t) ratio:

Version 3 Version 6

0.5 0.0 0.60 1.00 1.00 1.001.0 0.5 0.55 0.92 0.67 0.761.5 1.0 0.45 0.75 0.46 0.622.5 2.0 0.35 0.58 0.22 0.33

Further details on the methods developed for quantitativeevaluation of the intake of 90Sr during the first six monthsof life are provided in Shagina et al. (2007) and Tolstykhet al. (2008).

Step 4. Applicability of 90Sr-intake functions forMuslyumovo residents as reference for other TechaRiverside settlements

Age-dependent average TBC values obtained forpermanent Muslyumovo residents are the left part of

basic eqn (1) that connects the experimental TBC datawith an unknown 90Sr-intake function; therefore, thesolution (reflecting relative 90Sr-intake rates for theperiod 1950–1954) is very sensitive to the age depen-dence of the TBC measurements. This makes it possiblesimply to compare the age dependences of TBC mea-surements obtained for permanent residents of differentsettlements with data from residents of Muslyumovo inorder to investigate whether the schedules of intake weredifferent.

There are three factors that make it difficult toperform such comparisons for each village along theTecha River: The majority of villages had small popula-tions, the fraction of measured residents (relative to thetotal number of persons in the corresponding range ofbirth years 1940–1959) are lower than this fraction(67%) for Muslyumovo, and the levels of 90Sr measuredby the TBC in the majority of the Techa River settle-ments are lower than in Muslyumovo (where almost all

Figure 8. Normalization procedure for derivation of absolutevalues of 90Sr intakes in Muslyumovo with use of WBC data(averaged according to year of measurements) and semi-empiricfunction of 90Sr retention in bone corresponding to the recon-structed schedule of 90Sr intake.

Table 5. Reference values of the �(�, t) ratio for Muslyumovo.

Age oftooth (y)

Values of �(�, t) for different times

Sep1950

Mar1951

Sep1951

Mar1952

Sep1952

Mar1953

Jan1954

Jun1955

Jun1956

0 0.12 0.20 0.19 0.30 0.32 0.46 0.70 0.70 0.700.5 0.20 0.28 0.27 0.38 0.41 0.56 0.80 0.80 0.801 0.28 0.37 0.36 0.47 0.50 0.65 0.90 0.90 0.901.5 0.37 0.46 0.44 0.56 0.59 0.74 1 1 12 0.44 0.53 0.52 0.61 0.64 0.79 1 1 13 0.59 0.67 0.66 0.71 0.74 0.89 1 1 14 0.73 0.78 0.77 0.81 0.84 0.92 1 1 15 0.82 0.85 0.85 0.88 0.88 0.92 1 1 16 0.86 0.88 0.89 0.91 0.91 0.92 1 1 17 0.90 0.91 0.93 0.93 0.93 0.93 1 1 18 0.94 0.94 0.96 0.96 0.96 0.96 1 1 19 0.98 0.98 0.99 0.99 0.99 0.99 1 1 110 1 1 1 1 1 1 1 1 1

Table 6. Relative 90Sr-intake functions for adult residents ofMuslyumovo from September 1950 until September 1954.

Calendar year fraction Relative 90Sr intake

1950.67 1.001951.17 0.921951.67 0.721952.17 0.511952.67 0.361953.17 0.251953.67 0.161954.17 0.0481954.67 0.043

Table 7. Strontium-90 concentration in cow’s milk from individ-ual households in Muslyumovo.

Calendaryear

90Sr concentration (Bq L�1)Number of

measurementsMean SEa Median

1956 8 2 — 18b

1957 62 33 — 7b

1958 23 7 — 21b

1960 13 5 — 31b

1961 10 2 — 51b

1962 12 2 — 22b

1968 18 2.3 8 74c

1971 11 1 4 291c

1976 5 0.5 2 166c

1979 3 0.3 2.5 45c

aStandard error of the mean.bData on total �-activity; 90Sr concentrations were calculated on the basisof a single radiochemical measurement (Borovinskikh et al. 1963).cRadiochemical data (Panteleev et al. 1971; URCRM data base “Environment”).

residents used river water for drinking). Thus, it wasdecided to analyze the following three groups of personsbased on historical records:

1. Permanent residents of Metlino, who were evacuatedin 1956; Metlino was the closest village (7 km) to therelease site. A prohibition on using the Techa Riverfor drinking water was put into effect during themiddle of August 1951 (that is about 2 y earlier incomparison to villages further downstream); sinceAugust 1951, a supply of clean water was arranged forthe approximately 1,500 persons in Metlino. There-fore, the relative time dependence in Metlino shouldbe different from that in Muslyumovo. TBC data forMetlino residents born in the period corresponding topeak 90Sr content in tooth enamel in Muslyumovo(born in the first half of 1950) are fewer, whichrenders it impossible to use eqn (1) for derivation ofan intake function specific for Metlino. As a result,another approach, which is described in the Appendix,was used to reconstruct reference levels of 90Sr intakefor Metlino;

2. Permanent residents of eight small villages located onthe upper Techa River at distances of 32–65 km fromthe release site (Fig. 1); all of these small settlementswere evacuated in 1955–1956. These villages hadsimilar conditions of water supply (river water andwells) during the period of massive releases and wereconsidered as a single cluster characterizing a refer-ence for the upper Techa River region. According toWBC data (Degteva et al. 2000b), the residents ofthese settlements had accumulated similar amounts of

90Sr in their bodies, which indicated comparablelevels of total 90Sr intakes; and

3. Permanent residents of six non-evacuated settle-ments located 105 km downstream relative to thesite of releases (Fig. 1). These persons had twosources of water supply (wells and river water)during the period of massive releases. According toWBC data (Degteva et al. 2000b), these settlementsare similar in terms of 90Sr-body burdens of theirresidents. Therefore, this data set was considered asa cluster for settlements on the lower parts of theTecha River.

The age dependences in TBC-measured levels forthe two groups of persons living in the Upper and LowerTecha Riverside are shown in Fig. 9, where the data arecompared with similar data for Muslyumovo. As can beseen, persons from the “Upper-Techa cluster” and resi-dents of Muslyumovo have similar age dependences inTBC levels (Fig. 9a). That is in agreement with WBCdata: The average level of 90Sr-body burden for theUpper-Techa cluster was found to be equal to 0.8–1.5relative to the values obtained for Muslyumovo residents(Degteva et al. 2000b). The age dependences in TBClevels for the “Lower-Techa cluster” (Fig. 9b) are signif-icantly (by a factor of three) lower than for Muslyumovo;that also is in agreement with data on 90Sr-body burdens.However, the trends in age dependences in TBC levelsfor both the Upper- and Lower-Techa clusters are simi-lar. An especially important fact is that there is notendency for a shift in the peak position. This implies thatthe residents of these settlements had relative timedependences in 90Sr intake in 1950–1956 that weresimilar to Muslyumovo. Therefore the data from Mus-lyumovo can be used as a 90Sr-reference intake functionfor all riverside residents, except for the inhabitants ofMetlino.

DISCUSSION

A special approach was used for the reconstructionof 90Sr-intake functions for Techa Riverside residents.Only data on 90Sr in humans (skeleton and teeth) and toa lesser extent data on 90Sr measurements in environmen-tal and milk samples (obtained after major intake in1950–1952) were applied for the reconstruction. Itshould be noted that contradictory data on the TechaRiver source-term were not used in assessment of the90Sr-intake function with an exception for the date of thebeginning of massive radioactive releases. Archival dataand in-depth analysis of TBC measurements allowedestimation of the starting point of massive releases asSeptember 1950; that led to significant changes in the

Table 8. Reference 90Sr-intake function for adult residentsof Muslyumovo.

Calendarperiod

Intake of90Sr (kBq)

Calendarperiod

Intake of90Sr (kBq)

Jan 1950 25 1964 1.7Sep 1950 868 1965 1.5Mar 1951 740 1966 1.3Sep 1951 553 1967 1.2Mar 1952 392 1968 1.2Sep 1952 275 1969 0.94Mar 1953 183 1970 0.86Sep 1953 92 1971 0.78Mar 1954 39 1972 0.72Jan 1955a 8.1 1973 0.671956 6.6 1974 0.621957 5.4 1975 0.581958 4.5 1976 0.551959 3.8 1977 0.531960 3.2 1978 0.511961 2.7 1979 0.491962 2.3 1980 0.481963 2.0

aHere and thereafter the annual period from January to January of the nextyear is considered.

schedule of 90Sr intake in comparison with referencevalues previously used in the TRDS-2000.

Fig. 10a shows current estimates of the reference-90Sr intake in 1950–1956 for adult residents of Muslyu-movo compared with the TRDS-2000 estimates (Degtevaet al. 2000b). It is seen from Fig. 10a that the new

90Sr-intake function is “shifted” to the right in terms ofcalendar year. There is a significant increase in total 90Srintake in 1953–1954 (by a factor of 5–7) in Muslyumovocompared to previous estimates. The total-90Sr intake toadult Muslyumovo residents constitutes 3,185 kBq in1950–1956 and 3,220 kBq for the entire period ofinterest (1950–1980). The corresponding TRDS-2000values were equal to 3,140 kBq and 3,190 kBq, respec-tively, and are very close to current estimates.

Fig. 10b exemplifies changes in the 90Sr-intakefunction for children born in 1947, for whom the intakeof 90Sr with milk is most significant. For this reason, incontrast to adults, their intake of 90Sr is similar in theperiod from September 1950 until October 1951; fromMarch to October 1951 the contribution of 90Sr intakewith milk increased due to contamination of the flood-plain in 1951. As a whole, for children born in 1947, thetotal intake obtained in the current investigation is higherthan values in TRDS-2000 by about 35%: 2,390 kBqcompared with 1,770 kBq in 1950–1956 and 2,430 kBqcompared with 1,820 kBq in 1950–1980.

For purposes of radiation-risk assessment, individ-ual internal doses can be calculated on the basis ofindividual measurements of 90Sr (or measurements ofpersons living in the same household) and with use of anage-dependent 90Sr-reference intake function and an age-and gender-dependent strontium biokinetic model (Sha-gina et al. 2003). However, this process is possible foronly about 30% of the Techa River cohort. For a personwho has neither been personally investigated for 90Srbody (or tissue) contents nor have his/her relatives, the

Figure 9. Age dependent average TBC values obtained for permanent residents of (a) Upper-Techa settlements (n �101; 160 measurements) and (b) Lower-Techa settlements (n � 552; 951 measurements) in comparison with agedependent curve for Muslyumovo (the same as in Fig. 3). The bars represent standard errors.

Figure 10. Reconstructed 90Sr intakes in Muslyumovo in 1950 –1956 for (a) adult residents and (b) children born in 1947 incomparison with estimates of 90Sr intakes derived fromTRDS-2000.

averaged village-specific values of 90Sr intake must beused.

Verification of applicability of TBC data to90Sr-intake reconstruction

The use of 90Sr measurements in teeth is a uniquemethod elaborated for the specific situation on the TechaRiver. However, results of measurements obtained atlong periods after 90Sr intake indicate different age-dependences of 90Sr contents in teeth and the skeleton(Tolstykh et al. 2000, 2003). The explanation is thatfollowing incorporation in teeth and the skeleton, 90Sr iseliminated at different rates depending on various regu-latory processes in skeletal and dental tissues. In order toresolve a seeming contradiction, an analysis of correla-tion between TBC and WBC data was performed forpermanent Techa Riverside residents born in 1946–1949.(It should be noted that persons born in 1943–1945 arefew in number because fertility in the period of WorldWar II sharply decreased.) The TBC and WBC measure-ments performed at the same time for specific personswere selected. A short period of time (1982–1983) wasconsidered because the maximum number of TBC andWBC measurements was performed then.

Table 9 shows the correlations between individualWBC and TBC measurements in the considered agegroups. The analysis is for individual birth years, as 90Srcontent in teeth increases and 90Sr in the skeletondecreases with the increase in the year of birth (Tolstykhet al. 2000, 2003). As can be seen, significant correlation(p 0.05) was found for all narrow age-groups with theexception of persons born in 1948 (p � 0.06). SignificantSpearmen rank correlation was found in the combinedgroup of 1946–1949 birth years. Fig. 11 illustrates therelationship between individual whole body and teethmeasurements. As can be seen, for persons born in 1949(Fig. 11a) a clear relationship is observed. There is atendency for correlation between WBC and TBC in thecombined group (1946–1949), which is confirmed bysignificant rank correlation. Linear dependence, as ex-pected, is not significant (Fig. 11b). These resultsconfirm the appropriateness of the TBC data for recon-struction of 90Sr intakes.

Approaches to evaluation of uncertainties in thereference 90Sr-intake function

An important contribution to uncertainty in doses frominternal exposure is typically due to uncertainties in dietaryintake functions. Therefore, assessment of the uncertainty in90Sr dietary intake for the Techa River residents is animportant task in developing a system for dose assessment.Uncertainty in estimates of 90Sr dietary intake in the TechaRiver villages is determined by uncertainties in the follow-ing intake and retention functions:

1. Time-dependent 90Sr intake in 1950–1954 derivedfrom the solution of eqn (1). Uncertainty is mostly

Table 9. Correlation between 90Sr contents in anterior teeth and whole body (skeleton) obtained with TBC and WBCfor permanent Techa Riverside residents. Measurements were performed in 1982–1983.

Birthyear

Number ofpersons

TBC data, mean(min−max) (cpm)

WBC data, mean(min−max) (kBq)

Spearmen rankcorrelation p-value

1946 22 132 (0−304) 7.4 (2−20) 0.5 0.0011947 29 122 (1−320) 4.3 (0−12) 0.6 0.00031948 20 186 (7−476) 2.6 (0−11) 0.4 0.061949 40 387 (48−940) 2.2 (0−9) 0.5 0.0011946−1949 111 221 (1−940) 3.9 (0−20) 0.22 0.02

Figure 11. The relationship between individual WBC and TBCdata for (a) persons born in 1949 and (b) combined group of1946–1949 birth years.

due to variability of TBC measurements of 90Srcontent in teeth. Uncertainty in evaluated �(�, t) ratioswas shown not to influence the solution of eqn (1);

2. Time-dependent 90Sr intake in 1954–1980; the uncer-tainty is due to variability of estimated/measuredvalues of 90Sr concentration in milk; and

3. Retention function of strontium in bone; the uncer-tainty is due to variability of individual parameters ofmineral metabolism.

The above sources of uncertainty can be quantifiedbased on available published data and measurements.The uncertainty in each parameter can be described by astandard statistical distribution. Further, more detailedanalysis of uncertainty in the doses being calculated forthe TRDS is the subject of continuing work; someapproaches were described in Degteva et al. (2007).

The intake function is a series of annual intakeestimates for a reference person, derived through math-ematical modeling based upon a large collection oftooth-beta counts, whole-body counts, autopsy evalua-tions, and other observations, along with some assump-tions about the time-history of the releases into the TechaRiver. Recent significant revisions to the intake functionthat accounted for a large number of recent discoveriesand other modifications resulted in year-by-year changesfrom earlier estimates of about 25% for most years, withthe overall intake (the area under the curve) changing byless than this amount. It is believed that future revisionswill, if anything, be smaller than these. Therefore, theuncertainty in the reference intakes represented by thegeneric curve is subjectively estimated to be about 25%.

CONCLUSION

1. Detailed statistical analysis of TBC measurements ofsurface-beta activity of permanent front teeth, as wellas in-depth analysis of archival information from theMPA, showed that the releases of radioactive wasteswere not significant until September 1950; thus, it hasbeen assumed that the beginning of massive releasesto the Techa River was September 1950. A number ofparameters that are used for evaluation of 90Sr intakefrom measurements of teeth were verified, includingWBC measurements of 90Sr body burden, ratio of 90Srintake for children relative to adults, etc.;

2. Stability of the solution of the basic equation wasexplored with use of different boundary estimates ofthe �(�, t) ratio. Solutions obtained with differentassumptions on the ratio of 90Sr concentration incow’s milk and river water showed that variability of90Sr-intake functions lies within 12%; i.e., the solutionis stable;

3. Solution of the inverse problem allowed quantifica-tion of reference levels of 90Sr dietary intake. The totalintake for adults in the period of releases into the

Techa River (1950–1956) did not change signifi-cantly (higher by about 1.5% compared to the esti-mates obtained in TRDS-2000). However, the shift inthe beginning of intensive releases into the TechaRiver resulted in significant changes in the levels of90Sr intake in particular calendar periods. For childrenthe changes were more pronounced and amounted toabout 35%; and

4. The reference 90Sr-intake function allows adjustmentof individual intake for people who have measure-ments of 90Sr in body and tissues and evaluation ofaverage 90Sr intake in particular villages and house-holds. Implementation of these tasks will result inimprovement of estimates of individual internal dosesfor members of the Techa River cohorts.

Acknowledgments—This work has been funded by the U.S. Department ofEnergy’s Office of International Health Studies, the U.S. EnvironmentalProtection Agency’s Office of Radiation and Indoor Air, and the FederalMedical-Biological Agency of the Russian Federation. The authors alsoacknowledge the useful contributions that have been performed byRussian–European investigators working within the SOUL Project fundedby the European Union.

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Kozheurov VP, Degteva MO. Dietary intake evaluation anddosimetric modeling for the Techa River residents based onin vivo measurements of strontium-90 in teeth and skeleton.Sci Total Environ 142:63–72; 1994.

Kozheurov VP, Zalyapin VI, Shagina NB, Tokareva EE,Degteva MO, Tolstykh EI, Anspaugh LR, Napier BA.Statistical analysis of individual dosimetric data and theevaluation of uncertainties in instrumental techniques usedfor 90Sr-body-burden evaluation (whole-body count andtooth-beta count). Chelyabinsk and Salt Lake City: UralsResearch Center for Radiation Medicine and University ofUtah; Final Report for Milestone 1; 2000.

Kozheurov VP, Zalyapin VI, Shagina NB, Tokareva EE,Degteva MO, Tolstykh EI, Anspaugh LR, Napier BA.Evaluation of uncertainties in the 90Sr-body-burdens ob-tained by whole-body count: application of Bayes’ rule to

derive detection limits by analysis of a posteriori data. ApplRadiat Isot 57:525–535; 2002.

Krestinina LYu, Preston DL, Ostroumova EV, Degteva MO,Ron E, Vyushkova OV, Startsev NV, Kossenko MM,Akleyev AV. Protracted radiation exposure and cancermortality in the Techa River Cohort. Radiat Res 164:602–611; 2005.

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APPENDIX

Assessment of the 90Sr-intake function for residentsof Metlino

Two calendar periods were considered for assess-ment of the 90Sr-intake function for Metlino: (1) FromSeptember 1950 to July 1951, for which there are noappropriate measurements of human and environmentalsamples; and (2) from July 1951 to December 1956, forwhich measurements of the total-beta activity in excretafor Metlino residents are available.

September 1950–July 1951For this period, the relative intake function evalu-

ated for Muslyumovo can be used with allowances madefor features of 90Sr intake in Metlino. The features are asfollows: (1) The main source of water supply wasMetlinsky Pond on the Techa River, wells, and artesianwells; (2) the use of floodplain grass for fodder wasinsignificant; and (3) the influence of the flood in spring1951 that resulted in dilution of contaminated river waterwas less in Metlino than in Muslyumovo.

In order to project Muslyumovo data for an assess-ment of 90Sr intake in Metlino, it was necessary toevaluate the water pathway of dietary contamination foradult residents of Muslyumovo in April–August 1951.For this purpose the contribution of cow’s milk tothe total 90Sr dietary intake was estimated and then

subtracted from the total intake. Cow’s milk was con-taminated due to (a) the consumption by cows of con-taminated river water and (b) consumption by cows ofgrass from the contaminated floodplain. If the mainsource of 90Sr in human diet is the drinking of river water,the contribution of milk contaminated as a result ofconsumption of river water by cows is not significant andconstitutes 2–5% (Table 1 in main text). Contaminationof cow’s milk from consumption of grass from thecontaminated floodplain can be estimated from dataobtained in an experimental study in 1969–1970(Panteleev et al. 1971). In this study, cows were exclu-sively pastured in the floodplain near Muslyumovo andmeasurements of 90Sr concentration in milk, floodplainsoil and grass samples were performed (Table A1).

As can be seen in Table A1, the 90Sr concentration inmilk in 1968–1970 due to exclusive pasture in thefloodplain was 128 Bq L�1. With knowledge of the90Sr-concentration in river water, 96 Bq L�1 (Table A1),the daily water consumption by cows of 35–50 L(Williams 1937) and the 90Sr transfer from water per literof milk, 0.0013 d L�1 (Annenkov et al. 1973), it can beestimated that 90Sr concentration in cow’s milk fromconsumption of the river water by cows was about 5 BqL�1. As a result, 90Sr-concentration in milk due tofloodplain contamination in 1970 was considered as 123Bq L�1. In this study the coefficient of 90Sr transfer fromsoil to grass had been measured at the time correspondingto 20 y after the floodplain contamination had occurred.

Table A1. Results of an experimental study performed in theTecha River floodplain near Muslyumovo in 1968–1970 (accord-ing to Panteleev et al. 1971; Belova et al. 1978; URCRM DataBase “Environment”).

ParameterResult of

measurements

90Sr concentration in cow’s milk (BqL�1)

128 33a

90Sr concentration in Techa River water(Bq L�1)

Calculated 90Sr concentration in cow’smilk due to Techa River watercontamination (Bq L�1)

Calculated 90Sr concentration in cow’smilk due to floodplain contamination(Bq L�1)

Actual 90Sr-transfer coefficient from soilto grass, Ksoil-grass (Y20) (Bq kg�1 perkBq m�2)

3.5 (1.7−7.4)b

90Sr-transfer coefficient from soil to grassafter experimental floodplaincontamination,Ksoil-grass (Y1) (Bq kg�1 per kBq m�2)

50 (19−135)b

aStandard error according to Panteleev et al. (1971).b90% confidence interval of the 90Sr-transfer coefficient. The values wereobtained on the basis of primary data on 90Sr concentration in grass and soilsamples in the Muslyumovo flood plain (Panteleev et al. 1971; Belovaet al. 1978; URCRM Data Base “Environment”).

In addition the 90Sr transfer coefficient from soil to grassin the first year after floodplain contamination wasmeasured. For this purpose, a part of the floodplain soilwas artificially covered by river water with known 90Sractivity before vegetation had begun to grow. Samples ofgrass and soil were collected subsequently, and thetransfer coefficient was assessed; the results are summa-rized in Table A1. These results can be used to calculatethe concentration of 90Sr in cow’s milk due to pasture inthe floodplain in Muslyumovo in 1951:

Cm(Y1)�Cm(Y20)Ksoil3grass(Y1)

Ksoil3grass(Y20)Rd, (A1)

Cm(Y1) � 90Sr concentration in milk, con-taminated via the soil-grass-milkpathway during the first year afterfloodplain contamination;

Cm(Y20) � 90Sr concentration in milk, con-taminated via the soil-grass-milkpathway 20 y after floodplaincontamination;

Ksoil3grass(Y20) � 90Sr transfer coefficient fromfloodplain soil to grass in 1970(20 years after floodplaincontamination);

Ksoil3grass(Y1) � 90Sr transfer coefficient fromfloodplain soil to grass in 1951(the first year after floodplaincontamination); and

Rd �Correction factor for 90Sr radio-active decay � 1.6.

The use of eqn (A1) depends upon the followingthree assumptions. First, changes in 90Sr concentration inmilk in 1951–1970 were determined by two processes:(a) diminution of 90Sr biological accessibility (fixation insoil, solubility, etc.) that resulted in a decrease in the90Sr-transfer coefficient from soil to grass; and (b) 90Srradioactive decay. Second, the 90Sr transfer from grass tocow’s milk was constant following fresh contaminationand would have been the same in 1951 and 1970 (i.e., thefloodplain vegetation was the same). Third, cows wereexclusively pastured on the floodplain.

The concentration of 90Sr in cow’s milk in 1951 inMuslyumovo estimated with eqn (A1) is 2,800 Bq L�1

[123 � (50 � 3.5) � 1.6]. However, there were otherplaces for pasture, and the floodplain vegetation likelycontributed about 25–30% of the total cow’s diet. As aresult, the total intake of 90Sr with cow’s milk contami-nated due to consumption of floodplain grasses was

about 60–75 kBq in the period from March to September1951. The dietary 90Sr intake by adults in Muslyumovo inthis period was 740 kBq (Table 8); therefore, the contri-bution of the water pathway to the total dietary contam-ination was about 740–60 � 680 kBq. As a result, theratio of 90Sr intake in March–September 1951 to that inSeptember 1950–March 1951 was 680 � 868 � 0.78.

In spring 1951, a significant dilution of river wateractivity took place below Metlinsky Pond (in particularin Muslyumovo) due to the inflow of floodwater. Thus,the concentration of 90Sr in water decreased more sharplyas a function of distance from the site of releasescompared to other calendar periods (September 1950–March 1951). Therefore, in Metlino the ratio between90Sr intake with river water in the two periods (September1950–March 1951 and March–September 1951) should bedifferent (higher) from that in Muslyumovo. Thus, acorrection factor of 1.9 was used for Metlino, whichreflects the increased water-flow rate in Muslyumovoin March–September 1951. It should be noted that asharp increase in the water-flow rate near Muslyumovowas observed in a very short period of time (mostly inApril–May 1951), and the factor of 1.9 indicates anaverage increase in the flow rate in the period of interest.Therefore, the relative 90Sr intake in Metlino with con-taminated water in the two calendar periods (September1950–March 1951 and March–September 1951) wasequal to 1 and 1.5, respectively.

July 1951–December 1956Measurements of total-beta activity in excreta for

Metlino residents obtained in 1951–1956 were used forrelative 90Sr-intake reconstruction from September 1951 toDecember 1956. These data were summarized in twopublications and represent the results of measurements inexcreta (urine and feces separately) obtained during expe-ditions of the Institute of Biophysics in 1951–1952 (Mareyet al. 1952) and total-beta activity in excreta (Khokhryakovet al. 1968) for the period of 1951–1956 (Table A2).

It has been assumed that the change in total betaactivity in feces reflects the time dependence in intake foradults, so the contribution of excretion of endogenous90Sr is neglected, and 90Sr to 89Sr ratios did not changesignificantly during the period of interest. To obtain theamount of 90Sr excreted in feces during the period1952–1956, values of total-beta activity in excreta(Khokhryakov et al. 1968) were corrected for thefeces-to-excreta ratio as derived from the data of Mareyet al. (1952) (Table A2). An average value for thefeces-to-excreta ratio in 1952 was estimated to be 0.75.Therefore, urine excretion could be excluded from thesummarized data on total beta activity in excreta by useof this factor.

Table A3 presents the relative 90Sr intake in Metlinofor the entire period 1950–1956. The evaluation of absolutevalues of 90Sr intake for residents of Metlino was made with(1) the relative 90Sr-intake function, (2) WBC data for adultresidents of Metlino (97 measurements were obtained for 39residents born in 1920–1926), and (3) an age-dependentfunction that describes the retention of strontium in bone(Degteva and Kozheurov 1994). The resulting reference90Sr intake function for adult residents of Metlino is alsoshown in Table A3. As was done for Muslyumovo, it wasassumed that the daily intake of 90Sr from January untilSeptember 1950 is equal to 0.02 of the intake fromSeptember 1950 until March 1951.

Because the evaluation of 90Sr dietary intake inMetlino was made with data obtained in Muslyumovo andsome expert assumptions, it was very important to validatethe intake estimates. For validation purposes, post mortemmeasurements of the total-beta activity in bone samplesfrom Metlino residents since February 1952 (n � 25persons) and in vivo WBC measurements for Metlino

residents of similar years of birth (n � 159 persons) wereused. These measurements were not used for evaluation ofthe absolute 90Sr intake values. It is seen from Fig. A1 thatmodeled 90Sr content in the skeleton derived from thereference 90Sr-intake function for Metlino (Table A3) andthe semi-empirical function of strontium retention in bone(Degteva and Kozheurov 1994) is in good agreement withthe results of measurements, which is most important for the

Figure A1. Modeled and measured 90Sr content in the skeletonfor adult residents of Metlino. Measurements include 25 postmortem radiometry/radiochemical measurements and 159 WBCmeasurements for adults born in 1900–1910.

Figure A2. Reconstructed 90Sr intakes in Metlino in 1950–1956for (a) adult residents and (b) children born in 1947 in comparisonwith estimates of 90Sr intakes derived from TRDS-2000.

Table A2. Summary of results of measurements of total-betaactivity in excreta (arithmetic average, kBq d�1) for Metlinoresidents in 1951–1956. According to Marey et al. (1952) theratios of maximal-to-minimal measured values were estimated asabout 20.

Calendarperiod, year

Number ofmeasurements Urine Feces

Excreta,total

July 1951a 16 7.7 305 313September 1951a 37 2.3 11 13December 1951a 9 1.0 3.3 4.41952a 198 0.5 1.5 2.01952b 409 — — 1.91953b 46 — — 1.81954b 568 — — 0.71955b 1, 390 — — 0.31956b 17 — — 0.1

aAccording to Marey et al. (1952).bAccording to Khokhryakov et al. (1968).

Table A3. 90Sr intake-function for adult residents of Metlino.

Calendar period Relative intake Intake of 90Sr (kBq)

Jan 1950 0.02 24Sept 1950 1 880Mar 1951 1.46 1, 280Sept 1951 0.033 28Mar 1952 0.011 9.3Sept 1952 0.008 7.1Mar 1953 0.008 6.7Sept 1953 0.006 5.1Mar 1954 0.004 3.2Sept 1954 0.002 2.1Mar 1955 0.001 1.2Sept 1955 0.001 0.78Mar 1956 0.0005 0.42Sept 1956 0.0001 0.08

period 1951–1956. It should be noted that monitoring forradionuclide content in the bone tissues of the exposedpopulation commenced in other Techa Riverside villages,including Muslyumovo, only in 1955.

Comparison of the new 90Sr-intake function with thefunction considered in TRDS-2000 (Fig. A2) shows thatmaximal 90Sr intake occurred in 1951; i.e., later com-pared to previous estimates. However, the total intake of

90Sr by adult residents of Metlino in 1950–1956 esti-mated in the present study is the same as in TRDS-2000(2,250 kBq). For children born in 1947 (considered as anexample), the schedule of intake changed significantly.The total intake in 1950–1956 estimated in the presentstudy is equal to 1,500 kBq and is higher than wasestimated in TRDS-2000 (1,270 kBq).

Reconstruction of Long-lived Radionuclide Intakes for Techa Riverside Residents

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