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Wash-off of Sr-90 and Cs-137 from two experimental plots. Model testing using Chernobyl data

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Wash-off of Sr-90 and Cs-137 from two experimental plots. Model testing using Chernobyl data
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  Abstract-The Wash-ofP' scenario is designed to test models concerned with the movement of trace contaminants fromterrestrial sources to bodies of water, specifically the contam. ination of surface water by wash-off of radionuclides initially deposited onto soils. Particular emphasis is placed on chemicalspeciation and on the geochemical and geophysical processes affecting transfer of contaminants from soil to water. Thescenario gives descriptions of two experimental plots near the Chernobyl power plant, one using heavy rain and one usingsnow melt, together with characteristics of the initial aerialdeposition of the radionuclides and data on topography, soiltype and characteristics, and time-varying precipitation. Pre- dictions are requested for (1) the vertical distribution of concentrations of exchangeable and nonexchangeable forms of 137Cs and eosr in the soil of the experimental plots, (2) concentrations of 137Cs and eosr in runoff water from the experimental plots, and (3) total amounts of 137Cs and eosr removed by runoff from the experimental plots. Test data (field measurements) are available for all endpoints. Health Phys. 70(1):8-12; 1996 Key words: Chernobyl; environmental transport; contamina- tion, environmental; 137Cs INTRODUCTION Ttre WASg-oFF' scenario is the first of a set of three model-testing scenarios developed by the Post Chernobyl Data Working Group of 'BIOMOVS II (Biospheric Model Validation Study, Phase II). Information on par- ticipating in the test exercises or obtaining the test data for individual test scenarios is provided in an accompa- nying article (Hoffman et al. 1996). This scenario offers an excellent opportunity to test models concerned with the movement of trace contami-ƒ“ ‘d“Dive,Oak Ridge,TN 37830Mƒ¿•´Crƒ^Œmƒ^‹ƒ^á15‚ê,1994;•ƒbT—[•ƒ¿•sc/ƒ^eCƒ^‹ƒ^31“øÐ1995,ƒ¿c?›ƒ_A•=ƒpƒ`1995)0017-9078/96—•.00/0Copyright E1996 Hcalth Physics SOcietyAleks‚ªrƒÆttƒÆØ‚¿,scPI•EJcc,Prof•œYáŒÜ‚ÇrrFe oFD,v‚ÜmttF2F“z‚Ç2C‚ÇK7FƒÒygS,—ÍI4ŒûWTƒŒØÆ° 7 Kanayagawa, Fukushima, 9601296, JapanPhone : + 81 - 24 - 504 - 2848Mobile : + 81 - 80 - 5844 - 0962 h ttp : //w w. i e r.f u k u s hi ma- u, ac j p,/ e-mail : 1701 @ipc.fu k us hi ma-u,a cjp al exeikonoplev @ gma il,com EXPERIMENTAL E. Popov,x O. F. Popov,T A. V. Scherbak,tand F. O. Hoffman+ nants from terrestrial sources to water bodies. The particular objective is to take account of chemical spe- ciation, its effects on the transfer of contamination from soil to water, and the geochemical and geophysical processes that affect such transfer. Although the present scenario deals with the contamination of water subse- quent to aerial deposition of radionuclides following the Chernobyl accident, the processes underlying the sce- nario may be adapted to many sources of contanrination of surface and ground water.Surface water runoff from contaminated land is one of the main processes responsible for the contamination of water bodies. Therefore it is very important to have a reliable model for prediction of the contamination of surface water by natural runoff. The large area of landcontaminated by the Chernobyl accident has become a continuing source of radionuclides entering natural wa- ters and the aquatic ecosystem. This has provided a unique opportunity to develop a data base suitable for testing the predictive capability of mathematical modelsto assess radionuclide wash-off into rivers and lakes from soils contaminated by the deposition of fuel particles. Overall, the scenario will provide the following oppoftunities: (1) to evaluate the movement of contami- nants from soil to water; (2) to calculate the attenuationand migration of contaminants in soil over different time scales; and (3) to increase understanding of contaminanttransport at the process level. The scenario examines the wash-off of eosr and tttcs from two experimental plots established in the vicinity of the Chernobyl reactor. Later phases of the scenario development will extend the calculations to account for wash-off on larger spatial scales, including the flood plain of the Pripyat River, which provides input to the Kiev reservoir, and then the whole Dnieper River basin, starting with the watershed processes and includ- ing consequent transpolt processes in the rirrer itself. A distinguishing feature of radioactive contamina- tion due to a nuclear accident is the chemical speciation of the radionuclides present in atmospheric fallout and deposited on the soil. When the Chernobyl accident occurred, nuclear fuel debris (UO2 + UO.), including 8PaperˆêŽÞODEL TESTING USING C]I.WASH•]OFF OF9•‹Sr AND 137cs FROŽÞŽ¿TWOPLOTS ESTABLISHED IN THE VICINITY OFTHE CHERNOBYL REACTOR  fission products, was released into the environment.Some of the radionuclides were also deposited onto the ground as aerosol particles that had been formed in theatmosphere due to condensation. These parlicles varied in size, chemical composition, and behavior in the environment.Fuel particles have a radionuclide composition sim-ilar to that of inadiated fuel, but with varying proportions of highly mobile and volatile decay products. The fuelparticles are of very low solubility in water and hence do not participate in exchange processes between soil and soil-water solutions or in plant contamination via rootuptake from soil. However, as the fuel particles slowly degrade, mobile fission products may be released. Thedegradation rate of the particles is dependent on the size and chemical composition of the particles; these are a function of the distance from the Chernobyl Nuclear Power Plant (NPP). The time scale of the particle degradation varies from 1 to 10 y for different points in the 30-km zone around the nuclear power plant (Kono- plev and Bulgakov 1992; Konoplev et al. 1993), In terms of predicting the migration of 137Cs and ' Sr in soil-water systems, it is reasonable to identify the following chemical forms: (1) dissolved cations (Az); Q) exchangeable forms (Ar.), including radionuclides ab- sorbed onto soil or bottom sediments by ion-exchangemechanisms; and (3) non-exchangeable forms, includingradionuclides of nuclear fuel particles (Ar) and fixedradionuclides on mineral or organic components of soil or bottom sediments (A.). Over the course of time, Fig. 1. Schematic representation of the transformation processes of radionuclide chemical species. The chemical forms are represented as follows: Ar, radionuclides of nuclear fuel particles (non-exchangeable); Ar, dissolved cations; Ar., exchangeable forms,including radionuclides absorbed onto soil or bottom sediments by ion exchange mechanisms; and A., non-exchangeable radionu-clides fixed on mineral or organic components of soil or bottom sediments. K is the constant for the ion-exchange equilibrium, and \ is the rate constant for the process at issue (ij). The chemical forms may be considered in units of either mass or activity. The units of the ion-exchange equilibrium constant K will depend on the choice of concentration units for A, and Ar . Radioactive decay has been omitted from the diagram because the time scale of the wash-off process being studied here is considerably smallerthan the halfJives for the radionuclides in question. Table 1. Soil characteristics of the experimental plots. CharacteristicPIot HRPlot SMGeneral description Effective water-holding capacity of the 0-10 cm layer of sotl, Vo Hydraulic conductivity' (velocityof downward percolation of water in the soil profile), mm mtn - Cation exchange capacity (CEC) of the upper 1 cm of soil, meq g-r dry weightAlluvial acid sod Cultlvatcd sodpodzollc soll45•}6 30•}3034•}007 031 •}003015•}002 007•}001 'Hydraulic conductivity was calculated as the difference between rainfallintensity (mm min 1) and runoff intensity (mm min-r) during specialexperiments using artificial rain on I-m2 plots. radionuclide chemical species undergo transformation processes (Fig. 1). The rate of migration processes is sensitive to the chemical speciation of the radionuclides in soil and other environmental media. A fraction A, moves into the dissolved state with retardation occurring through ion-exchange interaction with the solid phase. The solid-phase species .A.1, A2 , and A. move only with the particles in which they are incorporated. EXPERIMENTAL DETAILSTwo experimental plots were constructed in areas contaminated by dry deposition resulting from the Cher- nobyl accident. Deposition (>957o) occurred between 26 April and 10 May 1986. The contamination of the plotswas determined by taking randomly selected soil sam- ples; each sample was 180 cm' in arca and 10 cm in depth. Runoff samples were collected in plastic bottlesand filtered with a 0.45 p,m filter. The 'Cs content insoil samples, filters, and water samples was determined through gamma spectrometry without preconcentrations using a Canberras gamma counter with a semiconductor detector (Makhonkb et al. 1985). The eosr content was determined with a radiochemical analysis that used carbonates of precipitation (Sereda and Shulepko 1966). Al1 samples were processed at the Institute of Experi-mental Meteorology, SPA Typhoon, in Obninsk, Rus- sia. INPUT INFORMATION Experimental plot for runoff following heavy rain (plot HR) ^ The 625-m experimental plot is located 7 km from the Chernobyl Nuclear Power Plant, 0.6 km from the village of Benevka on the oxbow of the Pripyat River. It is defined on three sides by dikes and on the lower part by a collection chute. The plot size is 25 m X 25 m; the mean grade of the slope is 5Vo. The total aerial contam-'uanberra lndustfles,Connecticut 06450. Inc.. 800 Research Parkway, Meriden.  10 Health physics ination in the experim.e^ntal plot was determined to be 1.4 t 0.1 MBq m 'of ' Cs and 1.8 -f 0.4 MBq m-2 of  Sr 1* standard deviation (SD)1. Descriptions oithe soil and plot HR are given in Tables I, 2, and 3. The vegetation on the experimental plot consisted primarily of mixed grasses. The predominant wild plant species included Achillea millefolium, Ae godpodium pada[raria, Carex vulgaris, C. nigra, C. flacca, C. pilosa, Comarum p alustre, D e schamp sia caespito sa, F ilip endula ulmaria, Leuc anthemum v ul g ar e, M e lampy rum ne mo r o s um, M en - tha arvensis, Myosotis palustris, Pedicularis palustris, P oly g onum li s to rta, P olytric hum c ommLlne, Ranunculus repens, Trifulium repens, Veronica chamaedrus, and Vaccinium myrtillus. Table 2. Soil descriptions for the experimental plots. Soil layer Depth (cm)pHJanuary 1996, Volume 70, Number 1 Intense artificial rain was used to simulate the formation of natural surface runoff on the two experi- mental plots. This was done for two main reasons. First, there was an urgent need at the time to predict the contamination of water bodies due to runoff caused by rainfall, flooding, and snow melt from areas of contam- inated soil. However, there was a lack of rainfall durins the period immediately after the accident, and it was no-t possible to wait until there was natural surface runoff. Second, in the 30-km zone there is poor surface runoff formation; the mean annual runoff coefficient is -ISVo. The observations were carried out during an exper- iment conducted on 14 October 1986. The aftificial heavy rain was applied by pointing a fire hose up in the air and letting the water come down. A second artificial rainfall application was used to test the dependence of radionuclide wash-off on initial conditions (i.e.. beforethe first rainfall application, the soil was dry; before the second application it was wet). A hydrograph of the runoff dynamics is given in Fig. 2. Chnacteristics of the artificial rain applications are given in Table 4. The chemical composition of the artificial rain is not repre- sentative of the actual rain in the region, in that the arlificial rain contained more dissolved salts than actual rain would have contained. The intensitv of the artificial rain was representative of actual rain that can cause surface runoff. The applicability of data obtained withartificial rain to actual environmental conditions has been discussed by Bulgakov and Konoplev (1992) and Bulga- kov et al. (1992). E-xperimental plot for runoff from snow melt (plot SM) The 1,000-m2 experimental plot is located on a north-facing slope near the town of Chernobyl. It is defined on three sides by dikes and on the lowei part by a collection chute. The plot size is 50 m X 20 m; themean grade of the slope is 47o. The total aerial contam- ination in the experimental plot was determined to be 4g0+- 155 kBq m-' of 'Cs and 270 'r 48 kBq m-2 of eoSr (* SD). The observations were carried out in winter and Humuscontent (Vo) Soil typePlot HRRoot zoneAl HorizonB Homzon0-8 51 8-12 sandy loam8 to 15-20 46-48 4-6 sandy loalll15-20 to 40-50 46 1-2 sandy loal13Plot SMIRoot zonePlowld layŒ¤B Holttzonˆ•@ˆ•@ˆ 5.8 4-5 sandy loam 5.8 1-1.5 sandy loam 5.0-5.2 0.3-0.8 loamy sand-sand Table 3. Characteristics of plot HR. CharacteristicValueWater table depth (m)Standing water depth during artificial rain appl icatior.(mm)Measured natural plecipitation at runoff plot during 1986 (mm) May June JulyAugustSeptemberOctoberBiomass density wet weight (g - ') dry weight (g m-') Vertical distribution of soil density in experimental plot HRLayer (cm) Voiumetric density (g cm-3 dry weight) Porosity (Eo, cm3/cm3)Chemical forms of radionuclides in the soil of plo{ HR prior to artificial rain applicationRadionuclide Mobile fomu (7o) Non-exchangeable form (7o) 0.5-5(yearly average, 1.5) 5-1020.5(cstmatcd)39.519560144.6264510•}200130•}400-5 5-10‚‚ƈ‚‚‚‚’Œ‚”•1004959•‹Sr149426137cs0 10 20 00 40 50 60 70 80 90Hme,lnninFig.2.Hydrograph of thc runoff dyna•¡lics For plot i=R.‚‚Œ• u Amount in solution and exfacted in 1 N ammonium aeetate solution.  Table 4. Characteristics of the artificial rain applied to plot HR. CharacteristicValucWash•\ofC of 9•‹Sr alld 137cs•œA V KoNOPLEV ET AL Table 5. Characteristics of plot SM. CharacteristicValue pHIonic composition of water (mg L-t) c*+ K+ Na+ Mg HCO3- Start of rain application Duration (nin) Rain amount (mm) Rainfall intensity (mm min-') Soil moisture content before rain (Vo, gH2O g-r soil wet weight)72555171217016:29 17:22Soil density (0*10 cm) Soil porosity (0-10 cm)Snow storage in the snow melt periodDepth of freezing (January and February) pH of snow waterCation composition of snow water(mg L-t) c** K+Na*NHo* Mg'*155 g cm~3ˆÊ•Aeight375%(cm3 pcr Cm3)2351111n60-100 cm53151.319144.416218137 spring in the snow melt period of 1988 (March). A hydrograph of the runoff dynamics is given in Fig. 3, Descriptions of the soil and of the snow water are given in Tables I,2, 5, 6,7, 8, and 9. ASSESSMENT TASKSPredictions for experimental plot HR Midpoint. Calculate the vertical distribution of the total amounts (Bq g-' dry weight) qld the percentage of exchangeable forms of 'Cs and Sr in soil immedi- ately prior to rain application on 14 October 1986. Include the 957o confidence intervals about the best estimates of the mean concentrations. Suggested soillayers are 0-0.5 cm, 0.5-1.0 cm, 1.0-2.0 cm,2.0-3.0 cm. 3.0-5.0 cm. and 5.0-10.0 cm. Endpoints. 1. Calculate the total concentrations (Bq L*1; of 137Cs and eosr in surface runoff during the experiment,18 19 20 21 22 23 24 25 26 27date and iime,March 1988Fig•B3.Hydl•Egraph of thc mnoff dynamics for plot SM.Table 6. Chemical forms of the radionuclides deposited with atmospheric fallout.^ RadionuclideWIobilc folib%Non-exchangeable fonn 7o9•‹Sr137csa Samplcs wctt talcen d‚¨ly ttom fallout collcctors begl•¡ning on 26 April1986,cons‚°vcd by a spccial lncthod and storcd Tllc chcmical foms•Averedeterlmned in the laboratory duttng 1988 Sal•¡lplcs•Avere dded and storcdin scalcd contalners to e•¡sure dlat the chellllcal forms measured•Averc thcsamc as•Avtte depOsited Thc valucs arc intcgrated estmates bascd on thetotal actvity rcccivcd by tlc plot over the entre tmc of initial depositionb AInount in soluion and exttactcd•Avith ammonium acetateTable 7.Chelnical radionuclidc forms in thc soll of plot SM at thccnd ofthc cxpcimcnt(5 ApŒÜ11988).Radlo•¡uclldcMoblle forma% Non-exchangeable forlr' Vo9•‹Sr137cs ' Water-soiuble * exchangeable, together with the percentages in dissolved and partic-ulate (i.e., associated with suspended particles) forms. Include the 95Vo confidence intervals about the bestestimates of the mean concentrations. 2. Calculate the amounts (Bq) of t37Cs and eosr lost from the HR experimental plot in the 24 h since application began. Include the 957o confidence inter-vals about the best estimates of the mean values. Predictions for experimental plot SM Midpoint. Calculate the verlical distribution of totalamounts (Bq g-' dry weight) and the percentage of exchangeable forms of 'Cs and St in soil as of 15 March 1988. Include Ihe 957o confidence intervals aboutthe best estimates of the mean concentrations. Suggested soil layers are 0-0.5 cm, 0.5-1.0 cm, 1.0-2.0 cm, 2.0-3.0 cm, 3.0-5.0 cm, and 5.0*10.0 cm.‚’‚ˆê•AŸ•¬‚Ì‚b‚n’†kˆê—r‚n‚b‚Á•A  12 Health Physics Table 8. Air and soil temperature for plot SM during the snow melt period of 1988. Air temperature, 'C Soil temperature (0-2 cm), 'C•¡me•¡meMarch19889:0015:00 21:00 9,00 15,00 21:00 January 1996, Volume 70, Number 1 Acknowledgmenrs-This scenario was developed by A. V. Konoplev (Institute of Experimental Meteorology, SPA Typhoon, Obninsk, Rus- sia) and F. O. Hoffman (SENES Oak Ridge, U.S.A.) on the basis of experiments carried out in the 30-km zone of the Chernobyl Nuclear PowerPlant by scientists from the Institute of Experimental Meteorology, SPA  Typhoon, Obninsk, Russia, and the Ukainian Hydrometeorological Institute, Kiev, Ukraine. We are grateful to SENES Oak Ridge, Inc., and personally to K. M. Thiessen and J. S. Hammonds for very helpful comments and valuable help in preparation of this manuscript for publi- cation. REFERENCES Bulgakov, A. A.; Konoplev, A. V. The role of chemicalspeciations of radionuclides in soils in their transfer to surface runoff. In: Proceedings of the Interxational Sympo- sium on Radioecology, Chemical Speciation-Hot Parti- cles. Znojmo: Society of Czechoslovak Radioecologists 12-16 October; 1992: 17 20. Bulgakov. A. A.;_Konoplev, A. V.; Popov, V. E. Prediction of ' Sr and 'Cs behavior in soil-water system after the Chernobyl accident. In: Ecological and geophysical aspects of nuclear accidents. Moscow: Gidrometeoizdat', 1992: 2l-42 (in Russian). Hoffman, F. O.; Thiessen, K. M.;Watkins, B. Opportunities for the testing of environmental transport models using data obtained following the Chernobyl accident. Health Phys. 70:5-7; 1996.Konoplev, A. V.; Bulgakov, A. A. Behavior of the Chernobyl- srcin hot particles in the environment. In: Proceedings of the International Symposium on Radioecology, Chemical Speciation-Hot Particles. Znojmo: Society of Czechoslo- vak Radioecologists 12-16 October, 1992: 56-60. Konoplev, A. V.; Viktorova, N. V.; Virchenko, E. P., Popov, V. E.; Bulgakov, A. A.; Desmet, G. M. Influence of agricultural countefineasures on the ratio of different chem- ical forms ofradionuclides in soil and soil solution. Science Total Environ. 137:147-162; 1993. Makhonko, K. P.; Silantiev, A. N.; Shkuratova, L G. Monitor-ing of environmental radioactivity in the vicinity of nuclear power plants. Moscow: Gidrometeoizdat; 1985 (in Rus-sian).Sereda, G. A.; Shulepko, Z. S. Methods of determination ofradioactivity of the environment. Vol. 2. Moscow: Gidrom-eteoizdat; 1966 (in Russian).•¡¡Date1516171819202122232425a-65-36372.3-25-25-4.610192.10813041184115294.425454.475-1,7051.0-08-20-0.5-050311162.5-06 -373.9 -2.869 130.5 0.3-14 -0714 -10-2.1 -2.214 028 1139 146 0a Rtlnoff ceased after 25 MIarch 1988.Table 9.Variations in attnosphcdc prccipitaton at plot SM duringtllc s•¡o•¡7 1nelt poƒrod of MIttch 1988.Date 15•¡21 22 23 24 25 26 27 28-31 Precipitation, mm Endpoints 1. Calculate the total concentrations (Bq L-i; of 137Cs and eosr in surface runoff at the following time periods, together with the percentages in dissolvedand particulate (i.e., associated with suspended parti-cles) forms: a) at peak flow for 18 March 1988,20 March 1988, and24 March 1988; and b) at the minimum flow on the night of 20 - 2I March 1988. Include Ihe 957o confidence intervals about the bestestimates of the mean concentrations. 2. Calculate the total amounts (Bq) of t3tcs and eosr lost from the experimental plot as of 31 March 1988. Include the 957o confidence intervals about the bestestimates of the mean values.
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