Sustainable agricultural use of natural water sources containing elevated radium activity

Chemosphere xxx (2013) xxx–xxx

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Chemosphere

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Sustainable agricultural use of natural water sources containingelevated radium activity

0045-6535/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2013.11.020

Abbreviations: Ra, Radium; BTC, breakthrough curve; ETp, potential evapotrans-piration; MCL, Maximum Contaminant Level; TF, transfer factor; REW, RadiumEnriched Water; HRW, High Radium Water; LRW, Low Radium Water.⇑ Corresponding author. Tel.: +972 54 9799182.

E-mail address: [email protected] (E. Tripler).

Please cite this article in press as: Tripler, E., et al. Sustainable agricultural use of natural water sources containing elevated radium activity. Chemo(2013), http://dx.doi.org/10.1016/j.chemosphere.2013.11.020

Effi Tripler a,⇑, Gustavo Haquin b, Jean Koch b, Zehava Yehuda a,c, Uri Shani c

a Southern Arava Research and Development, Hevel-Eilot 88820, Israelb Radiation Safety Division, Soreq Nuclear Research Center, Yavne 81800, Israelc Department of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel

h i g h l i g h t s

� The environmental implications of using water containing Ra for irrigation were investigated.� Radium was found to accumulate in crops leaves following the evapotranspiration current.� Sorption of 226Ra to soil particles hinders its matrix mobility.� Crops can be irrigated with the activity of 226Ra of 0.6–1.6 Bq L�1.

a r t i c l e i n f o

Article history:Received 30 June 2013Received in revised form 11 November 2013Accepted 12 November 2013Available online xxxx

Keywords:RadiumIrrigation waterLysimeter studyRadiological hazard

a b s t r a c t

Relatively elevated concentrations of naturally occurring radium isotopes (226Ra, 228Ra and 224Ra) arefound in two main aquifers in the arid southern part of Israel, in activity concentrations frequentlyexceeding the limits set in the drinking water quality regulations.

We aimed to explore the environmental implications of using water containing Ra for irrigation.Several crops (cucumbers, melons, radish, lettuce, alfalfa and wheat), grown in weighing lysimeters wereirrigated at 3 levels of 226Ra activity concentration: Low Radium Water (LRW) < 0.04 Bq L�1; High RadiumWater (HRW) at 1.8 Bq L�1 and (3) Radium Enriched Water (REW) at 50 times the concentration in HRW.The HYDRUS 1-D software package was used to simulate the long-term 226Ra distribution in a soilirrigated with HRW for 15 years. Radium uptake by plants was found to be controlled by its activity inthe irrigation water and in the soil solution, the physical properties of the soil and the potential evapo-transpiration. The 226Ra apeared to accumulate mainly in the leaves of crops following the evapotranspi-ration current, while its accumulation in the edible parts (fruits and roots) was minimal. The simulationof 15 years of crop irrigation by HYDERUS 1-D, showed a low Ra activity concentration in the soil solutionof the root zone and a limited downward mobility. It was therefore concluded that the crops investigatedin this study can be irrigated with the natural occurring activity concentration of 226Ra of 0.6–1.6 Bq L�1.This should be accompanied by a continuous monitoring of radium in the edible parts of the crops.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Relatively elevated concentrations of natural radium isotopesare found in groundwater in the southern part of Israel in thetwo main aquifers of the Negev Desert and the Arava Valley: theNubian Sandstone aquifer (Kurnub Group) and the Lower Creta-ceous aquifer (Judea Group). Radium is transferred from the hostrock into the aquifer by geochemical processes and is commonly

found in the groundwater as four isotopes: 226Ra (T1/2 = 1600 y)from the 238U decay series, 228Ra (T1/2 = 5.75 y), and 224Ra(T1/2 = 3.66 d) from the 232Th decay series and to a lesser exten223Ra (T1/2 = 11.435 d) from the 235U decay series (Vengosh et al.,2007). The water in some of the wells in the Southern Aravacontains radium isotopes at concentrations of 1–2 Bq L�1. Theseactivity concentrations exceed the Maximum Contaminant Level(MCL) for drinking water in Israel set at 0.5 Bq L�1 and 0.2 Bq L�1

for 226Ra and 228Ra, respectively (Koch and Haquin, 2008).With regard to uptake by plants, most of the information is ex-

pressed as a transfer factor (TF), which is calculated as the ratio ofthe element concentration in plants to its concentration in soil. Acomprehensive compilation of soil to plant transfer factors was

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Fig. 1. A schematic representation of a single weighing-drainage lysimeter.

2 E. Tripler et al. / Chemosphere xxx (2013) xxx–xxx

conducted by the International Atomic Energy Agency (IAEA,2010), in which they are reported separately for temperate envi-ronments and for tropical and subtropical environments. Ra trans-fer factors are reported for up to 12 crop categories, for differentplant tissues in part of the crop categories, and for up to 4 soiltypes. The largest amount of Ra data relates to temperate environ-ments, far fewer data are available for tropical ecosystems andnone are reported for subtropical environments (to which theArava Valley belongs). The TF depends on soil characteristics, planttype, tissue type, climate conditions and the physico-chemicalform of the radionuclide (Vandenhove et al., 2005; Vandenhoveet al., 2009; IAEA, 2010).

Field and laboratory measurements can be combined withmathematical models to yield better predictions of the long termdistribution pattern of radionuclides in the soil. Particularly, incases where environmental and health hazards can occur underconditions of regularly fertigation with radionuclides. Mecha-nism-base numerical models have become a frequently used toolfor simulations and pre dictions related to water and ions in the va-doze zone (Van Genuchten and Šimunek, 2004). Long-term(<200 years) simulation of 226Ra fate in agricultural ecosystems,where phosphogypsum fertilizer was applied for 27 years, wasstudied by Coelho et al. (2013), using the HP-1 multicomponenttransport model. The software combines HYDRUS 1D (Šimuneket al., 1998), a model simulating one-dimensional variablysaturated water flow and ion transport in soils, with PHREEQC(Parkhurst and Appelo, 1999), a model that calculates geochemicalreactions and ions distribution in soils.

Elevated radium concentrations may be a dominant factor inthe potential utilization of groundwater as drinking water, as wellas for agricultural purposes. Natural occurring radionuclides,including radium, are present in foodstuffs at varying activity con-centrations. In prone areas, where radium concentrations in theground water are significantly higher than the limits in the drink-ing water standards, it is important to investigate the effect ofusing such water sources for agricultural purposes. Irrigation withwater containing elevated radium concentrations may contami-nate the soil and the radium may consequently find its way intothe food chain.

The overall objective of the current study was to develop crite-ria for the sustainable use of water containing high level of radiumas irrigation water. Specifically, we aimed to explore chemicalreactions of the added radium to soil particles and its movementin the soil, to investigate radium uptake and transport in plantsand to develop a comprehensive soil-plant model for predictingRa uptake in plants.

2. Materials and methods

2.1. Lysimeter studies

To study the effect of drip irrigation on radium accumulation inthe soil and plant uptake an experimental setup consisting of 9weighing-drainage lysimeters was installed at Yotveta in the AravaValley in southern Israel. The lysimeter system, illustrated in Fig. 1,allowed us to carry out controlled experiments in which water andRa fluxes were measured with high accuracy, and hence, to com-pute water and ions balances. Each lysimeter consisted of a PVCcontainer filled with soil, a bottom layer of highly conductive por-ous media (rockwool) in tight contact with the soil, and a drainagepipe filled with the same material extending downward from thelysimeter bottom. The height and the diameter of the growth con-tainers were 0.85 m and 1.5 m, respectively, corresponding to avolume of 1.5 m3. The rockwool drainage extension prevented sat-uration at the lower soil boundary while permitting water to flow

Please cite this article in press as: Tripler, E., et al. Sustainable agricultural use o(2013), http://dx.doi.org/10.1016/j.chemosphere.2013.11.020

out of the soil and be collected. The lysimeter system includedautomatic water and fertilizer preparation and delivery. Ion andwater boundary conditions of the lysimeters are as in Tripleret al. (2012). Individual lysimeters were positioned on squareweighing platforms with load cells situated in each corner.

All lysimeters were filled with local soil, either an Arava loamy-sand soil, or an Arava sandy soil). The physical and mechanicalparameters of the soils are described in Table 1. Both soils haveaverage activity concentrations of 226Ra of 21 ± 3 Bq kg�1 and232Th of 12 ± 2 Bq kg�1.

Several controlled experiments were carried out at three ra-dium concentration levels of the irrigation water: (1) lysimeters1–3 with Radium Enriched Water (REW) – 50–100 Bq L�1; (2) lysi-meters 4–6 with High Radium Water (HRW) at 1.8 Bq L�1; and (3)lysimeters 7–9 with Low Radium Water (LRW) – <0.04 Bq L�1. Thepurpose of the REW treatment was to simulate an accelerated pro-cess of accumulation over a long period of time, i.e. to study theeffect of many years of irrigation with water containing high con-centration of radium. Along the course of the entire study, the dailyirrigation quantity was set so that the drainage is 1/4–1/3 of the to-tal irrigation water amount. This criterion afforded optimized stea-dy state conditions of water content and soil solution ionsconcentration. Each treatment was equipped by an irrigation valveand a water meter, which gave a signal pulse, for every 5 � 10�4 m3

of delivered water, to an attached automated controller. The dailywater balance was calculated from:Z L

0

@h@t

dZ ¼ IrðtÞ � DrðwÞ � ETðw; ETpÞ ð1Þ

where h (m3 m�3) is the water content, t is the time, z is a specificdepth of interest, L is the total depth of the lysimeter, Ir is theirrigation water amount, Dr is the collected drainage water at thebottom of the lysimeter, ET is the evapotranspiration, whereas wand ETp represents the soil’s matric head and the potential evapo-transpiration, respectively. The right hand side of Eq. (1) expressesthe measured difference in the weight of the lysimeter between00:30 and 23:30 of the same day.

f natural water sources containing elevated radium activity. Chemosphere


https://www.researchgate.net/publication/259645781_User's_Guide_to_PHREEQC_(Version_2)A_Compute_Program_for_Speciation_Batch-Reaction_One-Dimensional_Transport_and_Inverse_Geochemical_Calculations?el=1_x_8&enrichId=rgreq-a51afc7b-f9a0-465a-8673-ab99e5d38797&enrichSource=Y292ZXJQYWdlOzI1OTM1MTMxMDtBUzoxMDE2NTUzMTYwMDg5NjVAMTQwMTI0NzkyNDgzMA==

Table 1Physical characterization of Southern Arava soils.

Soil parameter Arava loamy sand Arava fine sand Units

Particle size distributiona Sand 83.0 93.0 %Silt 8.0 4.0 %Clay 9.0 3.0 %

Bulk densityb 1.30 1.60 Mg m�3

Organic matter content 1.30 <0.50 %Sat. water contenta hs 0.36 0.30 m3 m�3

Residual water contenta hr 0.030 0.005 m3 m�3

Sat. hyd. conductivityc Ks 0.15 0.21 m h�1

Air entry pressure headc wa �0.20 0.05 MPore size distribution indexc b 0.55 0.39 –

a Shani et al. (1987).b Ben-Gal and Shani (2002).c Shani et al. (2007).

E. Tripler et al. / Chemosphere xxx (2013) xxx–xxx 3

226Ra uptake was studied in various crops. Cucumbers, melons,lettuce and radish were grown on an Arava loamy-sand soil inlysimeters. Wheat grass, tomatoes and alfalfa were grown on anArava sandy soil. Alfalfa was grown continuously for more than6 months. The accumulated amount of 226Ra applied through irri-gation, for each lysimeter, during all the experiments, is shown inTable 2. Plant materials sampled from each lysimeter were oven-dried over night at 105 �C, ground and quantitatively ashed at450 �C. Ash samples were placed in standard cylindrical sealedcontainers of 55 ml volume. The ash samples were then placedin standard cylindrical 55-mL containers, in which they were keptsealed during 21 d to achieve secular equilibrium of the radonwith its decay products. They were then measured by low back-ground high resolution gamma spectrometry during at least24 h. The 226Ra activity concentration in the ash was determinedusing the gamma peaks count rate of 214Bi at 609.3, 1120.3 and1764.5 keV and of 214Pb at 295.2 and 351.9 keV. The counting effi-ciency of the measuring geometry was calculated using MCNPsimulations and a measured standard solution (QCY48 E&Z).The accuracy of the measurements is periodically demonstratedthrough the Mixed Analyte Performance Evaluation Program(MAPEP, 2013). Additionally, core samples of the same volumewere also taken from the soil of the lysimeters and measuredby high resolution gamma ray spectrometry (Haquin et al.,2008). The 226Ra minimum detectable activity concentrationsfor 24-h measurements are 0.05 and 0.17 Bq kg�1 for vegetationand soil samples, respectively.

Batch experiments and miscible-displacement study were car-ried in the laboratory, in order to determine the partitioned coeffi-cient of 226Ra (Kd) and the breakthrough curve (BTC). The ambienttemperature, throughout the experiments, was maintained at25 ± 1 �C.

Table 2The accumulated 226Ra activity in the irrigation water for each lysimeter (1–9), in the dvariance, which is the sum of the variance contributions of each water quantity applied a

Lysimeter Accumulated 226Ra activity in irrigation per experiment (Bq 1�1

1 2 3Cucumbers Melons Lettuce/Radis

1 52.80 ± 0.6176 55.30 ± 0.6411 55.32 ± 0.6462 52.80 ± 0.6176 55.30 ± 0.6479 56.12 ± 0.6463 52.80 ± 0.6176 87.30 ± 1.0208 88.12 ± 1.0304 0.86 ± 0.0102 2.11 ± 0.0241 2.92 ± 0.0345 0.86 ± 0.0102 2.11 ± 0.0241 2.92 ± 0.0346 0.86 ± 0.0102 2.11 ± 0.0241 2.92 ± 0.0347 0.02 ± 0.0002 0.05 ± 0.0005 0.06 ± 0.0008 0.02 ± 0.0002 0.05 ± 0.0005 0.06 ± 0.0009 0.02 ± 0.0002 0.05 ± 0.0005 0.06 ± 0.000


2.2. 226Ra adsorption isotherms

Increasing concentrations of 226Ra (2 Bq L�1 to 2 � 105 Bq L�1)were added to 30 g of oven-dried Arava sandy loam soil in a totalvolume of 100 ml. Solutions were adjusted to a pH of 7.5 using0.1 M NaHCO3. The test tubes were shaken at 25 �C for 24 h orfor 30 d. They were then centrifuged at 3000 rpm for 1/2 h andthe supernatant was filtered. The equilibrium concentrations ofRa in the solution and in the solid phase were determined by directgamma ray spectrometry (Haquin et al., 2008). The adsorption iso-therms were constructed by plotting the sorbed 226Ra activity(Bq Kg�1) against its activity in the equilibrium solution (Bq L�1).

2.3. 226Ra breakthrough curve

Oven-dried Arava sandy loam soil at a weight of 1038.4 g waspacked in a PVC column diameter and length of 5 and 20 cm,respectively). The bulk density was measured and the porositywas calculated accordingly, yielding values of 1.71 g cm�3 and0.35, respectively. Before the displacement experiment, the soilcolumn was slowly saturated by feeding from the bottom a back-ground solution of 0.1 M NaHCO3 buffered to pH 7.5. After an ini-tial equilibration period with more than 10 pore volumes (PV), acontinuous pulse of 226Ra solution having an activity concentrationof 4545 Bq L�1 was injected at the top of the soil column. The aver-age pore-water velocity was 6.1 cm h�1. Effluent samples were col-lected from the bottom of the column. The radon gas was purgedfrom the samples with aged nitrogen and the radium was thenmeasured using a Quantulus 1220 instrument (Perkin Elmer Ltd.)by alpha pulse height analysis for alpha/beta separation in the Li-quid Scintillation Counting (LSC) technique. The BTC was then con-structed by plotting relative concentration (effluent concentration

ifferent experiments (1–6). The confidence intervals were calculated from the totalt every irrigation event.

103)

4 5 6h Wheat grass Tomatoes Alfalfa

1 55.24 ± 0.6569 60.65 ± 0.70944 55.24 ± 0.6569 60.65 ± 0.7094818 4.66 ± 0.0545 10.07 ± 0.117818 4.66 ± 0.0545 10.07 ± 0.117818 11.88 ± 0.13917 0.10 ± 0.0011 0.22 ± 0.00257 0.10 ± 0.0011 0.22 ± 0.00257 0.13 ± 0.0014




divided by influent concentration) against dimensionless time rep-resented by the PV.

2.4. 226Ra transport modeling

The measured BTC was used to calculate transport parametersof 226Ra in the Arava soils, by means of a linear sorption non-equi-librium transport model (Šimunek et al., 1998), expressed as:

hR@C@t¼ hD

@2C@x2 � hv @C

@x� qb

@S@t

ð2Þ

@S@t¼ a½KdC � S� ð3Þ

R ¼ 1þ qbKd

hð4Þ

where R is the retardation factor (dimensionless), C is the activityconcentration of the solution (Bq cm�3), D is the hydrodynamic dis-persion coefficient (cm2 h�1), x is the distance (cm), v is the averagepore-water velocity (cm h�1), qb is the bulk density (g cm�3), S isthe sorbed concentration (Bq Kg�1) and a is a first-order rate con-stant describing the kinetics of the sorption process (h�1). The rateof Ra sorption (@S/@t) was calculated assuming a linear isotherm(e.g. Eq. (3)).

Data of the BTC and the initial and boundary conditions of themiscible displacement experiment were used as an input for STAN-MOD software (van Genuchten et al., 2012), in order to calculate226Ra transport parameters of D, R and a. STANMOD is a win-dows-based computer software package for evaluating solutetransport in porous media using analytical solutions of the convec-tion–dispersion solute transport equation.

The patterns of Ra transport and distribution in the soil and inthe drainage water were modeled by means of the HYDRUS 1-Dsoftware package (Šimunek et al., 1998). This software simulatesthe one-dimensional water flow and solute transport involved inconsecutive first-order decay reactions in variably saturated soils.It uses the Richards equation for simulating variably saturated flowand the advection–dispersion equations for solute transport intro-duced in Eq. (2). Spring and autumn growth of tomatoes was sim-ulated for 15 years, under the southern Arava typical climaticconditions and irrigation water quality (1–2 Bq L�1 226Ra) and withthe fitted sorption and transport parameters. The soil hydraulicparameters, i.e. k(w) and w(h), were taken from Table 1. The irriga-tion water quantity and potential evaporation in the autumnamount to 610 and 530 mm, respectively. Accordingly, the time-variable boundary conditions in the spring were set to 720 and625 mm, respectively. The soil profile was leached with 200 mmof water prior to every growing season. Although HYDRUS 1-D iscapable to account for radioactive processes, 226Ra decay was not

Table 3The 226Ra activity in fruits and leaves of irrigated crops grown in experiments 1–5, growsquare brackets represent one standard deviation around the mean.

Lysimeter (226Ra concentration in water) 226Ra# concentration (Bq kg�1)

Cucumber (3a) Melon (3)

Fruit Leaves Fruit Leaves

1–3 (50–100 Bq L�1) 6.6 177 2.6 66[1.52] [43.68] [0.65] [17.28

4–6 (2 Bq L�1) <0.6 1.7 <0.15 0.90[0.41] 0.23]

7–9 (<0.04 Bq L�1) <0.6 1 <0.12 0.30[0.26] [0.07]

a Number of replicates or lysimeters.


considered, since its half-life (1600 years) is much greater thanthe time period of the simulation.

3. Results and discussion

226Ra uptake in plants irrigated with water containing elevatedconcentrations of the radionuclide was measured for cucumbers,melons, lettuce, radish, wheat grass, tomatoes and alfalfa. Averageactivity concentrations in the leaves and fruits where relevant, arepresented in Table 3. The 226Ra activity concentrations in the leavesof plants were up to 37 times higher than its concentrations in thefruits (Table 3). The range of 226Ra activity concentrations in edibleparts of crops irrigated with HRW (2 Bq L�1) was narrow, regard-less of the soil type. The influence of the accumulated monthlyevapotranspiration on the 226Ra translocation in alfalfa leaves isshown in Table 4. 226Ra uptake in alfalfa plants increased withincreasing evapotranspiration. A paired-sample t-test was used toreject the hypothesis that 226Ra activity concentrations in leavesof alfalfa, irrigated from May to Nov. 2008 with LRW, are similarto those in leaves of plants irrigated with HRW (p = 0.112).

A linear relationship was found between the accumulated 226Raactivity in the soil and the normalized (to ET) 226Ra activity con-centration in leaves (Fig. 2), without dependence on the type ofcrop grown. Two linear regression curves were obtained, one foreach soil type, indicating different levels of 226Ra uptake by crops.226Ra accumulation in the leaves of crops grown on the Arava loa-my-sand soil is higher than in those of crops grown on the Aravasandy soil. The measured depth profile of the total 226Ra activityconcentration for the lysimeters irrigated with REW is presentedin Fig. 3. It is evident that 226Ra accumulation in the soil upperlayer is greater in the Arava loamy-sand soil than in the sandy soil.Downward 226Ra transport is more pronounced in the sandy soil,due to its low clay content and high hydraulic conductivity.

Our results indicate that 226Ra uptake by crops grown in Aravaloamy-sand soil, characterized by a clay content of 9%, is higherthan its uptake in the sandy soil (clay content of 3%). However,previous studies showed a negative correlation between CEC and226Ra activity in plants (Vandenhove and Van Hees, 2007), andbetween clay content and availability of 226Ra to plants (BlancoRodríguez et al., 2008). This contradiction can be explained bythe 1-D distribution of 226Ra in both soils as shown in Fig. 3. Thetotal 226Ra activity concentration in the topsoil is higher in theArava loamy-sand soil than in the sandy soil. It is noteworthy thatthe total 226Ra activity added to the loamy-sand soil and the sandysoil is 55315 and 60646 Bq, respectively (Table 2). Similarly, thetotal calculated radium in the profile in the corresponding soilswas 2360 and 2225 Bq kg�1 cm�1. Therefore, a higher uptakeintensity of 226Ra is expected for the loamy-sand soil, since underregulated drip irrigation regime, the greatest root density is likelyto be found in the topsoil.

n on Arava loamy-sand soil and Arava fine-sand soil. The numbers appear inside the

Lettuce (3) Radish (3) Wheat grass (2) Tomatoes (2)

Leaves Fruit Leaves Leaves Fruit Leaves

7.9 3.3 13.2 14.2 0.6 20.1] [2.07] [0.86] [3.19] [3.34] [0.15] [5.12]

1.3 <0.16 0.9 0.6 0.1 3.7[0.31] [0.24] 0.14] [0.02] [0.98]

1.1 <0.12 <0.17 0.3 <0.05 2.3[0.26] [0.07] [0.58]



Table 4Concentrations of 226Ra in leaves of alfalfa, irrigated with HRW and LRW (lysimeters 6and 9, respectively), and cumulative evapotranspiration, during the growth season.The confidence intervals for the ET were calculated from the characteristic error of thewater meter, and the intervals for the 226Ra content in the leaves were calculatedfrom the Mixed Analyte Performance Evaluation Program (MAPEP, 2013).

Month Accumulated ET(mm)

Lysimeter6 226Ra (Bq kg�1)

Lysimeter9 226Ra (Bq kg�1)

May 407 ± 44.21 <0.85 <1.0Jun. 304 ± 32.46 0.45 ± 0.13 0.55 ± 0.15Jul. 318 ± 38.69 1.05 ± 0.22 0.41 ± 0.17Aug. 449 ± 44.21 1.40 ± 0.55 0.63 ± 0.16Oct. 524 ± 60.62 1.76 ± 0.45 1.49 ± 0.31Nov. 280 ± 29.49 1.07 ± 0.24 0.73 ± 0.23

Accumulated 226Ra activity in irrigation (Bq)

0 20000 40000 60000 80000 100000

Nor

mal

ized

226 R

a ac

tivi

ty c

once

ntra

tion

in

leav

es (

Bq

*kg

-1*m

m-1)

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

6

2

1.67 10 0.0087

0.96; 0.0001

Y X

R p

−= ⋅ += <

7

2

6.43 10 0.00012

0.94; 0.0001

Y X

R p

−= ⋅ −= <

Exp. 1-3

Exp. 4-6

Linear regression exp. 1-3

Linear regression exp. 4-6

Fig. 2. Normalized⁄ 226Ra activity in leaves as a function of accumulated 226Raapplied to the soil by means of drip irrigation.

Total 226Ra in soil (Bq kg-1)

0 50 100 150 200 250

Dep

th(c

m)

0

10

20

30

40

50

60 Exp 4-5Exp 1-3

Fig. 3. Depth profile of 226Radium in two lysimeters irrigated with water enrichedwith 226Ra, measured at the end of two experiment sets. The loamy-sandy soil isrepresented by filled squared symbols, while the fine-sandy soil is symbolized withnon-filled circles. The error bars represent one standard deviation around the mean.

226Ra activity concentration in equilibrium solution(Bq x l-1)

0 1000 2000 3000

Sorb

ed 22

6 Ra

activ

ity c

once

ntra

tion

(Bq

x kg-1

)

0

10000

20000

30000

40000

50000

60000

70000

2

22.4

0.98

S C

R

= ⋅=

2

8.37

0.99

S C

R

= ⋅=

30 d1 d

Fig. 4. Sorption isotherms of 226Ra onto Arava loamy-sand soil. 226Radium wasequilibrated with the soil for 1 or 30 d.

Pore volumes0 2 4 6 8 10

C/C

0

0.0

0.1

0.2

0.3

0.4Measured Fitted

Fig. 5. The 226Ra miscible displacement data and optimized simulation (filled andunfilled dotted curve, respectively), for 226Ra transport in Arava loamy-sand soil, atan input concentration (C0) of 4545 Bq L�1.


The temporal pattern of the sorption isotherms, obtained whenequilibrating the soil with elevated 226Ra solutions, is illustrated inFig. 4. A linear sorption model was assumed, since the range of226Ra in the equilibrium solution was in the magnitude of 10�13–10�10 M. The Kd value (24.2 l Kg�1) obtained by equilibrating thesoil with 226Ra solution for 30 d is significantly higher than its va-lue when equilibrated for only 1 d (8.3. 7 l Kg�1), indicating that Ra


sorption to soil particles is time dependent. Tachi et al. (2001) re-ported that the Kd of 226Ra sorbed on bentonite and smectite is inthe range of 10–103 l kg�1 and its sorption mechanism is domi-nated by ion exchange phenomena. The clay content of the soilused in the current work (9%) is much lower than in the study ofTachi et al. and therefore a lower Kd is expected.

The 226Ra breakthrough curve is shown in Fig. 5. Radium activ-ity was detected in the effluent approximately after half a PV wascollected. The relative activity of the effluent (C/C0) rose to 5% at1 PV and from there on Ra activity increased at a constant rate. Arelative activity of 32% was reached after 9.5 PV. Measured 226Rawas compared to predicted steady-state values calculated STAN-MOD software using the following equation:

RMSE ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1n

Xn

i¼1

ðRaOi � RaPiÞ2vuut ð5Þ

where RaOi and RaPi are the measured and predicted 226Ra activities,respectively, and n is the total number of measurements indexed byi. The calculated RMSE for the data given in Fig. 5 was 0.0166.Therefore, the fitted curve that was calculated with the STANMODsoftware, assuming linear sorption non-equilibrium miscible dis-placement, is in good agreement with the measured results.



Table 5Parameters estimation, for Arava sandy soil, obtained by the application of the linearnon-equilibrium model, to the 226Ra miscible displacement experiment, at an inputconcentration of 4545 Bq L�1.

Parameter Units Value Lower Upper

D cm2 h�1 2.09 �4.39 8.57R – 20.2 13.3 27.1Kd L kg�1 3.61 2.12 5a h�1 0.05 0.015 0.085

0 2 4 6 8 10 12 14

226R

a in

soi

l sol

utio

n (B

q * l-1

)

0.0

0.5

1.0

1.5

2.0

2.5

B

C

A Root-zone Radrainage Ra activity

Years

Fig. 6. HYDRUS 1-D simulation of 226Ra activity concentration in the rhizospheresolution (Balck) and in the deep drainage at a depth of 100 cm (gray) as a function ofconsecutive days. Circled letters indicates a rise in the activity in the root-zone andin drainage water (A and B, respectively), and a temporal reductions in the root zoneactivity (C).


The values of the 226Ra BTC parameters calculated with STAN-MOD are presented in Table 5. The calculated Kd is smaller thanthe value obtained from the two batch experiments. This can beexplained by the short time period of the BTC measurements(approximately 6 h); compared with the two batch experiments.Evidently, the partition coefficient is a function of the contact timeof Ra in the solution with the adsorbing matrix. The relativeadsorption equilibrium time of radium to soils was previouslyfound to be reached within hours to days; Wang et al. (1993) foundthat adsorption approaches equilibrium after an equilibration timeof about 11 h. Similar findings were found by Laili et al. (2010) onorganic matter. The fitted value of D is small, suggesting that bothRa hydrodynamic dispersion and diffusion are hindered bysorption to soil particles. The fitted values of D, R and a are alsoin good agreement with results obtained by Tsang et al. (2007),for Cd sorption and miscible displacement measurements: 2.7–5.71 cm2 h�1, 21–33 and 0.015–0.048 h�1, respectively.

Simulation of 15 years of tomatoes irrigated with HRW quality(1.8 Bq L�1), under the Southern Arava climatic conditions is pre-sented in Fig. 6. The average root zone 226Ra activity was weightedby the modeled 1-D root density pattern. The 226Ra activity con-centration in the root zone reaches a steady-state level of1.95 Bq L�1 after 6 years, while its activity in the drainage water(below 100 cm) reaches a value of 1.24 Bq L�1 after 15 years. Irriga-tion with saline water entails salt leaching, before each growingperiod. As a result, a temporal rise in Ra activity, both in theroot-zone and in its deep-drainage (A and B, respectively), andcounter-wisely, a short-term reduction after steady-state conditionhas been achieved (C), was calculated by HYDRUS 1-D.

4. Conclusions

The effect of irrigation with water containing elevated 226Raconcentration on soil and crops has been experimentally studied,on well irrigated crops grown in lysimeters. The results revealed


that the 226Ra is mainly distributed in the upper 10–20 cm of thesoil. Radium is then transported into the crop leaves and fruits.

It was found that Ra uptake in plants is mainly controlled byenvironmental conditions: soil solution activity, soil texture andpotential evapotranspiration. 226Ra accumulates in the leaves fol-lowing the evapotranspiration current independently of the croptype, while its accumulation in fruits and roots is minimal. Forthe sake of comparison, 226Ra activity concentration in the edibleparts (excluding leaves), is well under the activity concentrationof the a-emitting radionuclides recommended in the Codex Ali-mentarius for radionuclides of anthropogenic origin.

The 226Ra mobility in a typical Arava loamy-sand soil is hin-dered by its sorption to soil particles and its sorption in this soilwas found to be time-dependent Importantly, radium activity inthe edible parts is a function of its soil solution activity.

Simulation of 15 years of crop irrigation with relatively high226Ra activity concentration using a 1-D transport model predictslow Ra activity in the root zone, which may cause a minor, if notnegligible, accumulation in edible tissues other than leaves.

Irrigation method has a potentially great impact on Ra distribu-tion in agricultural ecosystems. The crops in this study were irri-gated with drippers, installed at each lysimeter. Therefore, itshould be noted that Ra accumulation in the leaves can be higher,in case of sprinkler irrigation regime, since foliage uptake of Ra islikely to occur.

Further research should be focused on the kinetics of Ra adsorp-tion to soil particles, on Ra–Ca competitive adsorption, Ra sorptionunder high ionic strength and lysimeter miscible displacementstudies under unsaturated water content conditions.

Acknowledgement

This project was funded by Grant No. 4500121077 from IsraelWater Authority, Department of Water Quality.

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Sustainable agricultural use of natural water sources containing elevated radium activity