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Journal of Environmental Radioactivity 99 (2008) 1247–1254

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate/ jenvrad

Influence of soil texture on the distribution and availability of 238U,230Th, and 226Ra in soils

P. Blanco Rodrıguez a, F. Vera Tome b,*, J.C. Lozano c, M.A. Perez-Fernandez d

a Departamento de Fısica, Facultad de Ciencias, Universidad de Extremadura, Avd. Elvas s/n, 06071 Badajoz, Spainb Departamento de Fısica Aplicada, Facultad de Ciencias, Universidad de Extremadura, Avd. Elvas s/n, 06071 Badajoz, Spainc Laboratorio de Radiactividad Ambiental, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spaind Area de Ecologıa, Universidad Pablo Olavide, Carretera de Utrera km. 1, 41013 Sevilla, Spain

a r t i c l e i n f o

Article history:Received 17 October 2007Received in revised form 21 February 2008Accepted 1 March 2008Available online 22 April 2008

Keywords:UraniumThoriumRadiumSoil textureLabile activity

* Corresponding author. Tel.: þ34 924289524; fax:E-mail address: fvt@unex.es (F. Vera Tome).

0265-931X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.jenvrad.2008.03.004

a b s t r a c t

The influence of soil texture on the distribution and availability of 238U, 230Th, and 226Ra in soils wasstudied in soil samples collected at a rehabilitated uranium mine located in the Extremadura region insouth-west Spain. The activity concentration (Bq kg�1) in the soils ranged from 60 to 750 for 238U, from 60to 260 for 230Th, and from 70 to 330 for 226Ra. The radionuclide distribution was determined in three soilfractions: coarse sand (0.5–2 mm), medium-fine sand (0.067–0.5 mm), and silt and clay (<0.067 mm). Therelative mobility of the natural radionuclides in the different fractions was studied by comparison of theactivity ratios between radionuclides belonging to the same radioactive series. The lability of these ra-dionuclides in each fraction was also studied through selective extraction from the soils using a one-stepsequential extraction scheme. Significant correlations were found for 238U, 230Th, and 226Ra between theactivity concentration per fraction and the total activity concentration in the bulk soil. Thus, from thedetermination of the activity concentration in the bulk soil, one could estimate the activity concentrationin each fraction. Correlations were also found for 238U and 226Ra between the labile activity concentrationin each fraction and the total activity concentration in bulk soil. Assuming that there is some particle-sizefraction that predominates in the process of soil-to-plant transfer, the parameters obtained in this studyshould be used as correction factors for the transfer factors determined from the bulk soil in previousstudies.

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1. Introduction

Naturally occurring radionuclides are present in many naturalresources. Human activities that exploit these resources may leadto enhanced concentrations of radionuclides or enhanced potentialexposure. Such activities include, for example, the mining andprocessing of ores and the combustion of fossil fuels. Failure tosuitably manage these wastes can lead to the spread of radioactivecontaminants over large areas of soils. In the case of the uraniumseries, among the most interesting radionuclides are 238U as thelong-lived parent of the series, and 226Ra as the long-lived parent of222Rn.

In the assessment of the impact caused by a given contaminantin soils, it is very important to identify the factors that govern itsmobility and bioavailability. These factors strongly depend on thephysico-chemical characteristics of the contaminant. Echevarriaet al. (2001), working with spiked soils, studied the behaviour ofnatural uranium in soil solution, and concluded that pH is the most

þ34 924289651.

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important individual parameter in explaining the variations ob-served in the soil solution uranium concentration. This is partlybecause pH has the greatest influence on the speciation of the el-ements in a given environment. Also working with spiked soils,Vandenhove et al. (2007a) concluded that though pH plays animportant role in determining U concentration in soil solution,factors such as amorphous Fe, organic matter and clay content, andcation exchange capacity (CEC) can influence the soil solutionuranium concentration in the same pH range. For radium, Van-denhove and Van Hees (2007), working with spiked soils, foundthat the radium concentration in soil solution is related to the or-ganic matter content or the CEC of soils. However, they found norelation with the clay content.

In all these results, the soil solution concentration was consid-ered as a measure of the mobility of radionuclides in soils. But it isnot clear that this concentration is related to the bioavailability, i.e.,to the transfer to plants. In this sense, Vandenhove et al. (2007b),working with natural soils rather than spiked ones, concluded(through correlations) that extraction fractions for the exchange-able phases of the soils can provide valid information for the as-sessment of the bioavailable fraction and therefore for the transfer

Fig. 1. Map of the study zone in which the mine is located. The major topographic features and the sampling points are marked.

P. Blanco Rodrıguez et al. / Journal of Environmental Radioactivity 99 (2008) 1247–12541248

to plants. This is particularly the case for the NIST sequential pro-cedure (Schultz et al., 1998) as well as for others such as the BCRmethod (Quevauviller et al., 1993) or DGT (DGT Research Ltd.,2001).

In previous work (Vera Tome et al., 2002), we studied the mo-bility and distribution of various radionuclides and estimated thetransfer factors to plants (Vera Tome et al., 2003) using the totalactivity of the soils. It is now recognized that the activity con-centration of the bulk soil does not represent the amount of

radionuclide available for plants, and that other definitions arenecessary to represent the transfer of these elements to plants(Baeza and Guillen, 2006; Blanco Rodrıguez et al., 2006; Vanden-hove et al., 2007b).

We here study the influence of soil texture on the distributionand availability of 238U, 230Th, and 226Ra, in soils. The study wasconducted in an area that presents high uniformity in its soil ty-pology (basically granitic) from which four soil samples were takenalong the soil toposequence in the direction N / S. The distribution

Table 1Concentration of major elements (g kg�1) in bulk soils for the four sampling points,together with pH, LOI (loss on ignition), conductivity, and texture

SU1 SU2 SU3 SU4

Si 344 346 324 376Al 29.1 31.7 34.6 21.6Fe 14.8 8.7 11.1 2.7Mn 0.31 0.23 0.39 0.01Mg 4.2 3.2 7.1 0.42Ca 1.43 2.93 10 0.29Na 3.41 6.23 6.97 6.34K 10.4 16.7 10.9 19.3Ti 4.38 2.52 3.54 1.38P 0.52 0.55 0.5 0.41pH 6.2 6.5 6.7 4.4LOI (%) 4.2 5.0 2.8 6.4Conductivity (mS cm�1) 184 185 232 244Coarse sand F1 (%) 40.3 43.9 41.4 39.6Fine sand F2 (%) 42.0 44.8 49.6 51.2

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of 238U, 230Th, and 226Ra in different granulometric fractions wasdetermined for each soil. The measurement of other variables of thesoil samples such as the major elements (essential and non-essential), pH, conductivity, and weight loss on ignition (LOI),together with the granulometric distribution, could help one tounderstand the association of natural radionuclides with each soil.We also consider the distribution paths for these radionuclides bymeans of the activity ratios between radionuclides belonging to thesame natural radioactive series. Local breaking of radioactiveequilibrium (activity ratio equal to one) for members of the sameradioactive chain may indicate differentiated mobility. Finally, thedistribution of the labile activity concentration in each fraction wasalso studied. Because the same extraction step was followed toevaluate the availability of the radionuclides for U, Th, and Ra, theresults could shed some light on the differentiation in the absorp-tion of those radionuclides by soils.

Silt and clay F3 (%) 17.7 11.4 9.1 9.2

Fig. 2. Activity concentration of 238U, 230Th, and 226Ra in the four sampling points.

2. Materials and methods

2.1. Study area, sampling, and sample preparation

The study was performed at the rehabilitated uranium mine ‘‘Los Ratones’’(Fig. 1) in the Extremadura region (south-west of Spain). The mine covers an area ofapproximately 2.3 km2. It was in exploitation from 1960 to 1974, and restorationwork was completed between 1998 and 1999. The zone is characterized by a warmcontinental climate, with moderate winters and hot summers (annual averagetemperature of 15 �C), and moderate rainfall (typically 500 mm per year). Geo-logically, the area is classified as basically granitic.

Fig. 1 shows some key characteristics of the zone, including the affected zone,rubble piles from the mine, and the infiltration area for underground courses ordischarge areas to surface streams.

The terrain can be considered smooth, without any notable topography. How-ever, the slopes permit the existence of numerous flows for runoff water, which mayplay an important role in the local distribution of the radionuclides. The naturaltopography was slightly modified with inert material from the mining activities.

The radiological characterization of the zone for uranium, thorium, and radiumisotopes in the soil samples allowed us to distinguish one importantly affected zonenear the mine, mainly due to the preferential direction of the runoff and streamwaters from the installation, and an unaffected zone in which the soils present muchlower concentrations of these radionuclides (Vera Tome et al., 2002). The two zonesdiffer greatly in the 238U, 230Th, and 226Ra activity concentrations in the soils. Theirmean values (range) in Bq kg�1 were 10 924 (4352–18 717), 10 075 (3571–18 779),and 5289 (1151–13 420) in the affected zone, and 184 (93–328), 234 (108–512), and251 (191–492) in the unaffected zone, for 238U, 230Th, and 226Ra, respectively.

In other studies (ENRESA, 1993), the A horizons were determined to be between5 and 10 cm. Therefore a layer of 10 cm deep was sampled in the present work. Soilsampling was done with an EIJKELKAMP split-tube sampler.

The four sampling sites were chosen along the soil toposequence initiated nearthe main shaft at the north. The slope gradient decreases in the orderSU3> SU1> SU2> SU4, and the relative heights in the order SU1> SU2>SU3> SU4.

A prior study of the variability associated with the sampling had already beencarried out. For this, a 5 m� 5 m quadrate was considered per sampling point, di-vided into nine equal cells. In each cell, three soil cores were randomly sampled andthen mixed. The activity concentrations of 238U, 230Th, and 226Ra were determined ineach of the nine cells. A statistical analysis of the results obtained at each samplingpoint gave an estimate of the uncertainty associated with the soil sampling (i.e., theinhomogeneity of the soil). The resulting coefficients of variation for the concen-tration of each radionuclide were 27% for 238U, 30% for 230Th, and 28% for 226Ra.

After that, another sampling campaign was performed in order to carry out thestudy described here. Sampling was performed during spring to collect plant sam-ples as well as soil samples. At each sampling point, 10 soil cores were randomlycollected and then mixed. Soil samples were homogenized and quartered in situ,with about 3 kg taken per sampling point. They were oven-dried at 80 �C to constantweight, and sieved at a pore size of 2 mm. Representative aliquots were carefullyselected from the original bulk soil samples (Riddle, 1993). For this, a manual sampledivider was used to divide the sample down to 150–200 g. The remainder was di-vided by sieving into three subsamples: coarse sand F1 (size 0.5–2 mm), fine sand F2(0.067–0.5 mm), and silt and clay F3 (<0.067 mm). Thus, for each sampling pointfour samples were finally obtained: the three subsamples of different particle-size,and the bulk soil. All the samples, except those with the smallest size particle(<0.067 mm) were milled to a maximum grain size of 200 mm, using a Herzog mill.

Three aliquots were taken of each subsample. One was used for radiochemicalassays, a second was used to determine pH, conductivity, and weight loss on ignition(LOI), and the third to determine the concentrations of major elements. Material for

sampling and pretreatment of the samples was always cleaned before initiating theprocedure for each sample in order to minimize cross-contamination.

Sample treatment prior to the radiochemical assay was performed by acid di-gestion under pressure in a microwave oven (Millestone Mod. ETHOS 900). A firstattack was performed using HF and HNO3 (3:6 mL) as reagents (Kingston andHaswell, 1997). After digestion, 2 mL of HClO4 were added to the solution, which wasthen evaporated to fuming in order to eliminate the excess fluoride and thus avoidthe formation of the very stable and insoluble thorium fluoride. Two further attackswere then performed using 8 mL of aqua regia in order to completely digest theundissolved particles (Lozano et al., 2001).

The analysis of major elements for the digested samples was carried out by ICP-OES (Yobin-Ivon Mod. Ultima II). These analyses were carried out by the Servicio deAnalisis Quımico of the Universidad de Salamanca, Spain. Sample digestion prior toassay was performed with a microwave oven (Millestone Mod. ETHOS Plus Micro-wave Labstation), using the standard method USEPA Method 3052 validated forsiliceous and organic based matrices. This uses a combination of HF and HNO3

(1:3 mL), and temperatures of 180 �C for 20 min.The labile fraction per sample was obtained by Schultz’s method (Schultz et al.,

1998).

2.2. Radiochemical methods and measurement techniques

The activity concentrations of the uranium, thorium, and radium isotopes in thedifferent soil fractions were determined by alpha-spectrometry with PIPS semi-conductor detectors of 450 mm2 active area, housed in NIM spectrometers (Can-berra, Mod. 7401VR), coupled to low-noise preamplifiers, amplifiers, anda multichannel analyser. For the uranium and thorium isotopes, chemical separationusing tri-n-butyl phosphate (TBP), and further thorium purification by anionic ex-change resin (Jiang et al., 1986), was followed by electrodeposition to form the high-

Fig. 3. Activity concentration of 238U, 230Th, and 226Ra in the three granulometric fractions considered and in the bulk soil for the four sampling points. Note the different scales.

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resolution alpha sources (Hallstadius, 1984; Vera Tome et al., 1994). The methodused for the determination of radium was based on chemical purification by pre-cipitation of PbSO4/BaSO4, and the subsequent source preparation by micro-precipitation of Ba(Ra)SO4 (Blanco Rodrıguez et al., 2002). The radiotracers used forthe determination of uranium, thorium, and radium were 232U, 229Th, and 225Ra,respectively.

3. Results and discussion

Table 1 lists the main chemical and physical characteristics ofthe bulk soils. The upstream soils (SU1, SU2, and SU3) were slightlyacidic, while the downstream soil (SU4) was notably acidic (NRCS,2007). This lower pH may reflect the arrival of subterranean flowsto the surface near the point SU4. These flows pass through un-derground areas with ore rich in pyrite. The water’s oxidative actionon the mineral increases the acidity of the water (Carretero et al.,2001).

The concentrations of the major elements were statistically in-distinguishable at the first three sampling sites, but there weresome differences in the SU4 soil. The concentrations of six of the 10major elements at point SU4 were lower than that at the otherthree points (Table 1), the exceptions being Si, Na, K, and P. That thepH of this soil was by far the lowest could have led to greater sol-ubility of these major elements, and their consequent leaching faraway from point SU4.

The highest value of LOI (loss on ignition) again corresponded topoint SU4. This soil has the highest moisture content with greaterherbaceous species richness than the other points. Moreover, oc-casional livestock activity could have caused an additional sup-plement of organic compounds (Perez-Fernandez et al., submittedfor publication).

3.1. Soil texture

Table 1 also presents the granulometric data (in percentages) forthe soil samples.

A one-way ANOVA was done to check for differences in soiltexture. In the case of the coarse sand fraction (F1) significant dif-ferences were detected. The post hoc analyses (Student’s t-test)indicated that the differences were due to soil SU2, which hada significantly higher coarse sand content.

Significant differences were also found for the fine sand fraction(F2), with the content being in the order SU1< SU2< SU3< SU4.The fine sand content of the soils thus seems to be correlated withthe toposequence used in selecting the sampling points. Bearing inmind that the rainfall regime in the study area is sporadic withbouts of intense rainfall, particles of these sizes can be easilytransported by runoff after heavy rains.

As for the finest fraction (silt and clays, F3), the results showedthat the lowest contents correspond to the soils SU4 and SU3,which are statistically indistinguishable, followed by soil SU2, andfinally soil SU1. Again, the slope gradient seems to be correlatedwith the content in silt and clays.

The results for the distribution of fine sand and of silt-clays seemsomewhat contradictory. Under normal conditions, resuspensionmainly involves the smallest particles, but in this case the fractionmost affected by stream waters is the fine sand. A possible expla-nation may be that infiltration processes at the upstream points leadto deeper penetration of the smallest particles (F3), thus hinderingtheir transport by surface waters compared to heavier particles (F2).

In spite of the differences found in the granulometry of thesampled soils, they are all fairly similar and can be classified asloamy sand.

Fig. 4. Regressions between the activity concentration in each granulometric fractionand the total activity concentration in soils for 238U, 230Th, and 226Ra. Note the differentscales.

P. Blanco Rodrıguez et al. / Journal of Environmental Radioactivity 99 (2008) 1247–1254 1251

3.2. Distribution of radionuclides in the granulometric fractions

Fig. 2 shows the total activity concentrations of 238U, 230Th, and226Ra in the four sampled soils. The activity concentrations of theseuranium series nuclides are in agreement with the values found inother uranium mineralized areas (Radhakrishna et al., 1996).

In soils SU1, SU2, and SU3, the three radionuclides presentsimilar patterns. As will be discussed below, in these soils the threeisotopes are close to radioactive equilibrium, i.e., they presentsimilar activity concentrations. In SU4, however, the concentrationof 238U was much greater than that found at the other three points,while the other two radionuclides (230Th and 226Ra) had loweractivity concentrations than 238U, and similar to those observed atthe other three points.

Fig. 3 shows, for the four soil sampling points, the activityconcentrations for each granulometric fraction and the bulk soil.In all cases, the greatest activity concentrations corresponded tothe finest particle-size fraction (F3), followed by the intermediatesize fraction (F2), and finally the coarsest fraction (F1). The onlyexception to this behaviour was the case of 226Ra at SU3, wherethe fractions F1 and F2 have comparable activity concentrations.The activity concentration decreased as the particle-size in-creased, as expected (Frindik and Vollmer, 1999; Livens andBaxter, 1988).

This result is especially significant in the case of the uraniumisotope at point SU4. As mentioned above, at this point for the bulksoil, there was enrichment in the total concentration of uranium.We observed that the enrichment was much greater in the finestparticle fraction, i.e., despite the possible contribution of particletransport, the uranium that was leached mainly by vertical in-filtration in the highest areas was fixed at this point preferentiallyon the finest particles. For all three radionuclides considered, theactivity concentrations observed in the finest particle fraction (F3)were greater than those found in the bulk soil in all the soils, but inthe case of the soil SU4 the difference was particularly notable, from698� 21 Bq kg�1 d.w. in the bulk soil to 1879�72 Bq kg�1 d.w. inthe finest particles.

Systematic correlations were found between the activity con-centration in each fraction and the total activity concentration inthe bulk soil (Fig. 4).

In the case of 238U, the strongest correlations were for the twofiner fractions (r¼ 0.991 and r¼ 0.995 for F2 and F3, respectively).The coarsest particles (F1) were more poorly correlated (r¼ 0.560),and within a confidence level of 95% the corresponding regressionwas not statistically significant. In all cases, the intercept at theorigin was statistically indistinguishable from zero. The significantregressions were CU,Fi¼ aCU,soil (i¼ 2, 3), with a¼ 0.90� 0.08 fori¼ 2, and a¼ 2.9� 0.2 for i¼ 3.

For 230Th, again the strongest correlations between the activityconcentration per fraction and the total activity concentration werefor the two finer fractions, with correlation coefficients of 0.960(F2) and 0.931 (F3). The correlation coefficient for the coarsestfraction was r¼ 0.867. Again as in the uranium case, the regressioncorresponding to fraction F1 was not statistically significant, and inall cases the intercept was statistically indistinguishable from zero.The significant regressions for 230Th were CTh,Fi¼ bCTh,soil (i¼ 2, 3),with b¼ 1.26� 0.26 for i¼ 2, and b¼ 2.3� 0.6 for i¼ 3.

For 226Ra, there were strong correlations between the activityconcentration per fraction and the total activity concentration in allthe fractions. The correlation coefficients (r) for F1, F2, and F3 were0.960, 0.985, and 0.974, respectively. Again the intercepts werestatistically indistinguishable from zero. In this case, the re-gressions were CRa,Fi¼ dCRa,soil (i¼ 1, 2, 3), with d¼ 0.75� 0.15 fori¼ 1, d¼ 1.4� 0.2 for i¼ 2, and d¼ 2.2� 0.3 for i¼ 3.

One can conclude that there is a strong correlation between theactivity concentration in the finest fractions and the activity

concentration in the bulk soil, so that from the determination of thebulk soil activity concentration, one could calculate the activityconcentration in each fraction. This result could be very useful if themobility of radionuclides in the soils is preferentially associatedwith a certain granulometric fraction. Assuming that there is someparticle-size fraction that plays the principal role in the soil-to-plant transfer process, one could use the relationships found todetermine the transfer factor from the determination of the bulkactivity concentration. In this sense, the parameters obtained alsoshould be used as correction factors for this type of soil to recal-culate the transfer factors determined in previous studies.

Table 2Activity ratios in the three granulometric fractions and in the bulk soil for the four sampling points

Soil Activity ratio F1 (0.5–2 mm) F2 (0.067–0.5 mm) F3 (<0.067 mm) Bulk soil

SU1 230Th/238U 0.94� 0.05 0.91� 0.05 0.89� 0.05 0.97� 0.06226Ra/230Th 0.87� 0.07 0.95� 0.05 1.17� 0.08 0.93� 0.06

SU2 230Th/238U 0.86� 0.04 0.97� 0.06 0.87� 0.05 0.90� 0.04226Ra/230Th 1.02� 0.05 0.99� 0.08 0.92� 0.05 0.98� 0.05

SU3 230Th/238U 1.10� 0.07 1.17� 0.09 0.93� 0.07 0.98� 0.07226Ra/230Th 1.11� 0.07 0.84� 0.06 1.17� 0.09 1.21� 0.08

SU4 230Th/238U 0.35� 0.02 0.21� 0.01 0.18� 0.01 0.22� 0.01226Ra/230Th 1.16� 0.07 1.11� 0.06 0.75� 0.04 0.83� 0.05

F1: coarse sand, F2: fine sand, and F3: silt and clay.

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3.3. Activity ratios

In a closed system, radioactive equilibrium is expected betweenradionuclides belonging to the same radioactive series. This meansthat the activities of radionuclides in equilibrium must be equal.Therefore, the activity ratios between 238U, 230Th, and 226Ra, allmembers of the 4nþ 2 series, can provide information on the dif-ferences in their distribution pathways.

Table 2 lists the 230Th/238U and 226Ra/230Th activity ratios in thethree fractions and in the bulk soil for the four soil samples. SoilsSU1, SU2, and SU3 can be considered as the source, in the sense thatthe radionuclides were not significantly differentiated in theirconcentrations. For these three soils, the 230Th/238U and 226Ra/230Th activity ratios were close to the radioactive equilibrium valuein all the fractions and in the bulk soil (Table 2). This indicates that,at these three sampling points, the three radionuclides are dis-tributed in the same way among the three granulometric fractions.

In soil SU4, there was no radioactive equilibrium (Table 2). In thebulk soil, the activity ratios showed enrichment of uranium withrespect to thorium and radium. This enrichment was found in allthree soil fractions, although there were differences between them:the ratios 230Th/238U and 226Ra/230Th decreased as the particle-sizedecreased, but while the 230Th/238U ratio was statistically in-distinguishable in fractions F2 and F3, the difference was somewhatmore marked for the 226Ra/238U ratio. These findings suggest thatthe uranium mobilized was associated mainly with the finestparticles.

3.4. Availability of uranium, thorium, and radium

The pattern of the labile activity concentration in each gran-ulometric fraction for 238U was similar in SU1 and SU3 (Fig. 5). Inthese soils, the highest percentage was found in the coarsest frac-tion and the lowest in the finest fraction. However, the pattern wasdifferent in soils SU2 and SU4, especially in the latter. In these soilsthe highest percentage of labile activity concentration was found inF2, corresponding to fine sand. The foregoing results of the totalactivity found in each soil indicate that soil SU4 seems to be froma collecting point where there is enrichment in uranium isotopeswith respect to their daughters. Moreover, from the study of theactivity ratios, it is clear that this enrichment was associated mainlywith fractions F2 and F3. Now one sees that the lability of 238U inthis soil was associated with fraction F2.

The 230Th behaviour is different in each soil (Fig. 5). In soils SU2and SU4 there was no especially available fraction. In soil SU1, thehighest percentage was found in the finest fraction, and the op-posite was the case in soil SU3.

In the case of 226Ra, the distribution pattern was similar in all thesoils, with the highest percentages in the finest fractions (Fig. 5).This expected result indicates that the sorption/desorption process

that governs the lability of radium in soils usually takes place in thefinest particles.

The activity ratios for labile fractions showed important differ-ences from the ratios for the absolute activities. The ratio230Th/238U, calculated taking into account the labile activity for thebulk soil, was systematically lower than the values of the absoluteactivity ratio, indicating that U had a higher availability than Th:0.891 vs 0.919 (3%), 0.373 vs 0.911 (59%), 0.758 vs 1.12 (32%), and0.076 vs 0.263 (71%) in SU1, SU2, SU3, and SU4, respectively. It canbe seen that SU2 and SU4 presented the greatest differences, co-herent with their lower slope gradients.

The 226Ra/230Th ratio in every fraction (F1, F2, and F3) and also inthe bulk samples, as obtained from the labile activities, clearlybroke the almost-secular equilibrium observed with the absoluteactivity ratios. In general, 226Ra was more easily removed from thebulk soil and from any of the fractions than 230Th. It is not sur-prising that the highest ratios corresponded to the F3 fraction,which would be expected to have more ion exchange sites. Withthe method used here, Mg can occupy sites thus displacing Ra onavailable portions of the particles. This effect would be less no-ticeable in coarser particles which would have a greater percentageof 226Ra associated with the most resistant phases of the particlecore. Previous studies performed in the same area (Blanco Rodrı-guez et al., 2005) demonstrated, by application of a sequentialmethod, that 226Ra is distributed bimodally, with an importantpercentage in the most labile parts and the rest in the most re-sistant (oxides and residual) fractions. Thorium was found prefer-entially in the most resistant phases.

In general, from the ratios of labile activities for 226Ra and 238U,there were lower values of 226Ra/238U compared to 226Ra/230Th,with values not far from unity in soils SU1, SU2, and SU3, and thecoarser particles. The highest values again corresponded to F3 ineach soil.

The search for some type of systematic behaviour underlyingthe data from the four soils studied led us to also consider linearcorrelations between the labile activity concentrations in each soilfraction and the total activity concentration in each soil (Fig. 6). Thestrongest correlations were obtained for 238U, with correlationcoefficients (r) 0.996 (F1), 0.966 (F2), and 0.973 (F3). In all cases, theintercepts at the origin were statistically indistinguishable fromzero. A strong correlation was also found between the total labileactivity concentration and the total activity concentration(r¼ 0.945). 230Th did not present a clear trend. Only the fraction F2showed a strong correlation (r¼ 0.998). For 226Ra, the strongestcorrelations were obtained for the two finest fractions, with cor-relation coefficients (r) of 0.861 (F1), 0.999 (F2), and 0.968 (F3).Only the last two were statistically significant.

In future work we will study the possible relationship betweenthe available activity concentrations in each granulometric fractionand the transfer to plants. In this sense, if the bioavailability is re-lated to the available activity concentration in some granulometric

Fig. 5. Percentage of 238U, 230Th, and 226Ra availability in each granulometric fractionconsidered for the four sampling points. Note the different scales. Fig. 6. Regressions between the labile activity concentration in each granulometric

fraction and the total activity concentration in soils for 238U, 230Th, and 226Ra. Note thedifferent scales.

P. Blanco Rodrıguez et al. / Journal of Environmental Radioactivity 99 (2008) 1247–1254 1253

fraction, the correlations found could be used as correction factorsfor the TF calculated from the bulk soil concentrations.

4. Conclusion

From the study of the distribution of natural uranium, 230Th, and226Ra in three granulometric fractions, we found that the activityconcentration decreased as the particle-size increased, as expected.The activity concentration of the finer fractions presented a strongcorrelation with the activity concentration in the bulk soil. This

result could be useful if the mobility of radionuclides in the soil ispreferentially associated with a certain granulometric fraction.

The objective of the present work was to evaluate the capacity ofeach granulometric fraction to liberate radionuclides in order tomake estimates of their availability. This capacity was found to bedependent on both the fraction and the radionuclide. For uranium,the greater percentage of the available fraction was in the coarserfractions (sand or fine sand, depending on the soil), but for radiumit was in the finest fraction. Clearly, strong correlations were found

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for both the uranium and the radium availabilities with the totalcontent of radionuclides in each soil.

Acknowledgements

Thanks are due to the Ministerio de Educacion y Ciencia, PlanNacional de IþDþI (2004–2007) (CTM2005-02910/TECNO project)and the Fondo Social Europeo de Desarrollo Regional (FEDER)for financial support. We also acknowledge financial support fromthe Empresa Nacional de Resıduos Radiactivos (ENRESA), theSpanish National Agency for Radioactive Management (project0078000102).

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