Transfer of natural radionuclides from soils to plants in a wet marshland

Transfer of Natural Radionuclides from Soils to Plants in a Wet Marshland* A. MARTÍNEZ-AGUIRRE and M. GARCÍA-LEÓN Facultad de Física, Universidad de Sevilla, Apdo 1065, 41080 Sevilla, Spain Soils and two wild plant species (Spartina densiflora and Spartina marítima) have been collected from the Odie! river marsh and analysed for 21"Po and 238U. The radioactivity concentrations in plants and soils reflect the impact of the operation of two phosphoric acid production factories located close to the marsh. Concentration ratios (CR) of these radionuclides, between plant and underlying soil, have been studied. First results show that the concentration ratios (CR) seem to be dependent on radionuclide soil concentration. Introduction lt is well known that the use ofphosphate rock as raw material for phosphoric acid production provokes the redissemination into the environment of noticeable amounts of natural radioactivity. This is the case for the phosphate-based fertilizer production complex, formed by two factories located at the Odiel river (Southwestern Spain, see Fig. 1). We have previ ously shown that discharges from such industries clearly spike the Odiel river with easily measurable concentrations of natural radionuclides (Martínez Aguirre et al., 1994). Furthermore, the Odiel water tidal movements transport the radioactivity into the marsh area located on the right bank of the river, in front of the factories (see Fig. 1), a natural reservation where an important biological activity takes place. In this paper we present additional data on the radioactivity concentrations of 210Po and 238U in soils from the marsh. We also give first results on the presence of such radionuclides in two marsh plants, Spartina densiflora and Spartina marítima, which grow in the area. lt will be seen that discharges from the fertilizer industries also modify the radioactivity content of such plants. Preliminary results are pre sented on the behaviour of the concentration ratio (CR), defined as the ratio between the radionuclide concentration in the plant and in the underlying soil, for the plants studied. Samples and Radioanalytical Procedures The Odiel marsh is mainly composed of four small islands separated by four little channels. The area is affected by the tidal flow of water from the Odiel *Work supported under UE project FI3P-CT920035. river, where two phosphoric acid factories are located. They release part of their wastes directly in the river. The sampling stations are shown in Fig. l. Both soil and plant samples were collected during low tide in areas covered by water during high tide. Sorne 1 kg of surface soil (6 cm deep) was taken from each station and stored in plastic bags. Aliquots were dried and powdered in the laboratory. Density and organic matter were determined for each soil sample. Spartina densiflora and Spartina m'lritima plants were collected from the same areas. ilthough the latter was found only in nine ofthe stations explored. The roots of the plants were discarded and the lea ves and stems were rinsed to remove any trace of solid particles, dried, powdered and analysed as a whole. Either the soil or plant samples were spiked with knowri amounts of 208Po and 232U to estímate the recovery yields, digested with concentrated HN03 and aqua regia. The residue was then dissolved with 8 M HN03• A solvent extraction technique with tributylphosphate (TBP) was used for Po and U extraction. The final Po solution was self-deposited onto silver planchets while that containing U was electroplated onto stainless steel planchets. 210Po and U-isotope activities were measured by ion-implanted Si detector a-spectrometry. Details of the radio chemical and counting methods can be found in Martínez-Aguirre ( 1991 ). Results and Discussion The 210Po and 238U radioactivity concentrations in mBq g- 1 dry weight are presented in Table 1 for soils and Spartina densiflora and in Table 2 for soils and Spartina marítima. Values of concentration rations ( CR), defined as the ratio between the concentration (dry mass) of a radionuclide in the plant and the o 1.5 km HUELVA Foret ' \ ' ' ' ' ' ' -'� ' ' ' \ I ' ' \ r._/ ..... \ \ .,,- .... / 'i \ \ / .... \ 1 ' ' ' \ ' ' ', \ .... \ ',1 Fig. 1. Location of the sampling stations in the Odie! marsh area. The locations of the fertilizer factories are also given. concentration (dry mass) of the same radionuclide in the substrate, are given in the same tables. Soils Results for 21ºPo in soils, reported already (Martínez-Aguirre and García-León, 1996) seem to suggest clearly that the intrusion of waters from the Odiel river during high tides disseminates natural radionuclides into the marsh area. The 210Po pattern distribution presented in Table 1 indicates how the waters enter to the marsh. After an input of radio activity is produced, the contamination plume moves up the river during high tides. Part of this plume enters the marsh through the northern area of Bacuta Table l. Data of "ºPo and "'U in mBq g-' (dry weight) in so ils and Spartina densiflora sample s from the Odi e! mars h. Concentration ratio s in eac h station, defined as the ratio between the concentration of an element in the plant and that in its substrate, are also given Code ""Po so il "'U "ºPo plant '"U CR (""Po ) CR ("'U) MI 199 ± 10 268 ± 12 23.6 ± 1.6 16.6 ± 1.1 0.119 ± 0.010 0.062 ± 0.005 M2 778 ± 41 877 ± 19 21.2 ± 1.1 15.1 ± 1.1 0.027 ± 0.002 0.017 ± 0.001 M3 643 ± 28 795 ± 23 25.1 ± 1.3 26.2 ± 1.9 0.039 ± 0.003 0.033 ± 0.003 M23 210 ± 11 320 ± 10 17.3 ± 1.8 12.7 ± 1.1 0.082 ± 0.009 0.040 ± 0.004 M4 580 ± 26 625 ± 11 45.2 ± 4.5 42.6 ± 2.7 0.078 ± 0.008 0.068 ± 0.004 M13 222 ± 11 213 ± 10 8.60 ± 0.55 6.65 ± 0.47 0.039 ± 0.003 0.031 ± 0.003 M5 213 ± 10 267±9 5.58 ± 0.41 6.61 ± 0.58 0.026 ± 0.002 0.025 ± 0.002 M7 130± 7 178 ± 9 8.90 ± 0.77 11.1 ±0.8 0.069 ± 0.007 0.062 ± 0.005 M6 141 ± 7 161 ± 12 6.52 ± 0.43 4.07 ± 0.43 0.046 ± 0.004 0.025 ± 0.003 M9 188 ± 10 150± 9 8.97 ± 0.60 6.85 ± 0.56 0.048 ± 0.004 0.046 ± 0.005 M8 64.6± 4.0 95.5 ± 8.2 9.50 ± 0.76 11.2 ± 0.8 0.147 ± 0.015 0.117 ± 0.013 M20 233 ± 11 308 ± 14 30.6 ± 1.9 19.1 ± 1.1 0.131 ± 0.010 0.062 ± 0.004 MIS 565 ± 23 716 ± 18 50.1 ± 2.4 16.2 ± 1.2 0.089 ± 0.006 0.023 ± 0.002 Ml7 41.0 ± 3.2 31.3 ± 7.5 8.61 ± 0.58 4.74 ± 0.51 0.210 ± 0.021 0.151 ± 0.040 M16 441 ± 20 494 ± 14 15.0 ± 0.8 17.9 ± 0.9 0.034 ± 0.002 0.036 ± 0.002 Ml5 131 ± 7 109±7 9.92 ± 0.63 8.31 ± 0.66 0.076 ± 0.006 0.076 ± 0.008 Ml2 44.2 ± 2.5 61.7 ± 7.6 13.0 ± 0.7 9.92 ± 0.63 0.290 ± 0.023 0.161 ± 0.022 Mll 194± 9 105 ±9 6.82 ± 0.52 3.09 ± 0.26 0.035 ± 0.003 0.029 ± 0.003 MIO 27.9 ± 2.0 152±9 12.8 ± 0.8 11.5 ± 0.7 0.460 ± 0.044 0.076 ± 0.006 M19 54.4 ± 3.7 46.8 ± 7.4 8.36 ± 0.68 2.37 ± 0.20 0.154 ± 0.016 0.151 ± 0.009 Ml4 54.7 ± 3.9 75.6 ± 6.9 15.8 ± 1.2 2.16 ± 0.23 0.290 ± 0.030 0.029 ± 0.004 M21 16.2 ± 2.5 30.7 ± 7.3 5.74 ± 0.48 2.93 ± 0.36 0.354 ± 0.062 0.095 ± 0.025 M22 37.2 ± 2.5 58.9 ± 8.1 9.78 ± 0.69 4.20 ± 0.37 0.263 ± 0.026 0.071 ± O.Oll Table 2. Data of "ºPo and "'U in mBq g- 1 (dry weight) in so ils and Spartina marítima sample s from the Odi e! marsh. Concentration s ratio in each station are also given Code MI M2 M3 M6 MIS Ml6 Mll Ml9 Ml4 ""Po so il 199 ± 10 778 ± 41 643 ± 28 141 ±7 565 ± 23 441 ±20 194±9 54.4 ± 3.7 54.7 ± 3.9 1000 800 600 400 .s ::> 200 o o "'U 268 ± 12 877 ± 19 838 ± 18 189±8 716 ± 18 494± 14 105±9 46.8 ± 7.4 75.6 ± 6.9 '"'Po plant "'U 36.2 ± 1.9 36.5 ± 2.8 40.6 ± 1.1 49.6 ± 3.1 43.4± 2.1 48.8 ± 3.0 13.8 ± 1.0 12.4 ± 0.8 22.6 ± 1.1 17.5 ± l.! 9.92 ± 0.45 6.20 ± 0.37 14.7 ± 0.9 8.42 ± 0.58 5.05 ± 0.41 3.51 ± 0.26 5.86 ± 0.45 3.01 ± 0.42 Soil Samples :a: :a: 200 400 Po (mBq/g) I 600 CR (''"Po ) 0.182 ± 0.013 0.052 ± 0.003 0.068 ± 0.004 0.098 ± 0.009 0.040 ± 0.003 0.023 ± 0.001 0.076 ± 0.006 0.093 ± 0.010 0.107 ± 0.011 800 CR (238U) 0.136 ± 0.012 0.057 ± 0.004 0.058 ± 0.004 0.066 ± 0.005 0.024 ± 0.002 0.013 ± 0.001 0.080 ± 0.009 0.075 ± 0.013 0.040 ± 0.007 Fig. 2. 238U activity concentrations vs 21ºPo activity concentrations in soil samples. U= (8.12 ± 15.8) +(l.143 ± O.OSI) Po with r = 0.980. 50 Spartina Densiflora 40 I 30 I .§_ 20 I :::> 1:I 10 o o 10 20 30 40 50 Po (mBq/g) Fig. 3. 238U activity concentrations vs 210Po activity concentrations in Spartina densif[ora samples. U= ( - 1.57 ± 1.57) + (0.885 ± 0.091) Po with r = 0.908. island. There the radioactivity is distributed, with the major amount passing into the Mojarrera channel. When the waters move downriver, during low tides the plume would move to the east of Saltés island. Thus concentrations of 210Po up to 780 mBq g- 1 were found, which are clearly above typical levels in uncontaminated soils. However, the farther part of the marsh (Punta Umbría river) seems to be unaffected by the fertilizer production, with activity concentrations similar to uncontamined soils (UNSCEAR, 1988). The same conclusions can be derived by examin ing the 238U results, with concentrations up to 880 mBq g- 1 in the same areas as those with high 210Po concentrations. If we compare 210Po with 238U concentrations, as in Fig. 2, both radionuclides are linearly related with a regression coefficient of 0.980. This relationship reveals the same origin for both radionuclides in the soils. In general, the 238U activity concentrations are slightly above the 210Po activity concentrations, with a ratio (U/Po) of 1.143 ± 0.051, given by the slope of the line. 1t is interesting to note that both for 210Po and 238U the levels in the northem marsh (samples MI, M2 and M3) are higher than in the rest of the samples. This suggests these locations are the sink for, at least, those radionuclides most likely related to tidal interactions. Plants The levels of 210Po and 238U in both Spartina species are very similar, ranging from 6 to 50 mBq g- 1 for 21ºPo and from 2 to 50 mBq g- 1 for 238U. The case of samples located at the northem marsh is interest ing since again the highest levels are found there. Furthermore, the radioactivity concentrations found for Spartina marítima are higher than those for Spartina densiflora. lt can be related to the fact that Spartina maritima samples in such stations are covered by water for a longer period than those of Spartina densiflora. The distribution pattern of 210Po and 238U concentrations found in soils is reflected also in the plants, thus showing that releases from the factories enter into the biosphere. Furthermore, as was found in soils, a linear relationship (Fig. 3) is found for both radionuclides, with a regression coefficient of 0.980. Moreover, the slope of the line (0.885 ± 0.091) indicates that, contrary to what was found in soils, the 21ºPo concentrations in plants are slightly greater than the 238U concentrations. Concentrations ratios ( CR) Toe transfer of 21ºPo and 238U from soil to plants was also studied by using the concept of concen tration ratio ( CR). Theoretically it should give information on the amount of radionuclide which enters the plant from its substrate. It is certainly important for understanding the dissemination of radioactivity in nature as a whole. Tables 1 and 2 give the CR values obtained for Spartina densiflora and Spartina marítima, for 21oPo and 238U. The CR values range from 0.026 to 0.460 and from 0.017 to 0.161 for 210Po and 238U, respectively. In general, the CR is higher for 210Po than for 238U, reflecting a greater transfer of 21ºPo to the plant than that of 238U. In the case of Spartina maritima, the CR values range from 0.023 to 0.182 for 210Po and from 0.013 to 0.136 for 238U. Again, the CR for 21oPo is higher than for 238U, but the differences are much less significant than in the case of Spartina densiflora. Comparing the CR values for both plant 0,5 0,4 ' 0,3 o:: 0,2 f 0,1 o.o o II .---;¡ ____ _ :e: :e: ---- *L° 200 Spartina Densiflora CR = 2.456 Cs-0·663 -�----------�------,.,------- 400 600 800 Cs (Po mBq/g) Fig. 4. "ºPo concentration ratio (CR) in Spartina densiflora vs 210Po concentration in the underlying soil in mBq g-1• CR = 2.456c,-0·663• species, there seems to be a tendency for a lower CR in the case of Spartina densiflora than in Spartina maritima; this is clearer in the case of 238U. lt can be observed that CR depends on soil concentrations. At this stage of the study it is not possible to identify clearly any trend for the case of Spartina maritima samples. However, it seems that for Spartina densiflora the CR decreases when the concentration in the soil increases; this is clear for 210Po, but not so clear for 238U. This behaviour can be seen in Figs 4 and 5, where the CR values for 210Po and 238U for Spartina densiflora vs the concentration of the same radionuclides in the underlying soil are plotted, respectively. The continuous lines represent the best fit of the curve 0,2 o:: 0,1 o I I 1 CR =aC� (1) to the experimental data where C, is the radioactivity concentration in the soil and b :;;; 1. The parameters of these fits are given in Table 3 for both plant species. Spartina Densiflora CR = 0.343 Cs -o.374 r�}----�: ___ - :t:I I I :e: __ :a:, _______________ ](_ :e: 200 400 600 800 1000 Cs (U mBq/g} Fig. 5. 238U concentration ratio (CR) in Spartina densiflora vs 238U concentration in the underlying soil in mBq g-1• CR = 0.343C,-0·374• Table 3. Relationship between concentration ratio ( CR) of ''°Po and "'U in soils and Spartina species of the type et. r is the correlation coefficient of the lines log CR vs log C,. s, is the error in the y-axis intercept and sb the error in the slope Radionuclide r Relationship s, sb Spartina densiflora 21op0 -0.817 CR = 2.456C,-0·'" 0.549 0.105 "'U 0.612 CR = 0.343C,-0·"' 0.082 0.106 Spartina maritima 21op0 -0.603 CR = 0.504C,-0·"º 0.218 0.182 23su -0.354 CR = 0.177C,-0·"' 0.097 0.227 A first approach should consider the CR to be independent of C., being constant over severa! orders of magnitude. However, according to experimental evidence such a situation seems to be only valid for non-essential elements at non-toxic substrate levels (Mengel and Kirkby, 1979; Timperley et al., 1970). In fact, Sheppard and Sheppard (1985) suggest a non-linear relationship between CR and C, for essential elements or when an element mimics an essential element. Moreover, Tracy et al. (1983) studied the uptake of 226Ra, 210Pb and U from soils to garden plants. They observed a clear trend towards increasing concentration in plants with increasing concentration in soils. Such trends were modelled by functions of the type (2) CP being the element concentration in soil and b' ¿: 1. Equation (2) is identical with equation (1) since CR = CP/C, and b = b' -1, so that our results seem to follow the same trend as that found by Tracy et al. (1983) for other plant species. A final comment can be made regarding the results obtained. Figure 6 presents a hypothetical relation ship of CR vs C, for essential and non-essential elements, taken from Sheppard and Sheppard (1985). According to this, plants readily take up elements essential for growth when C, is low, whereas CR is generally constant for non-essential elements at this C, range (Mengel and Kirkby, 1979; Timperley et al., 1970). When C, increases, CR can either remain Deficient .. .. Toleran! Toxic Toxicity Threshold Non-toxic /Lethal Soil Concentrotion - Fig. 6. Hypothetical relationship of CR vs soil concentration for both essential and non-essential elements through the deficient to toxic soil-concentration range. From Sheppard and Sheppard, 1985. constant (no toxicity) or can decrease, leading to toxicity or death. Toxicity begins at a toxicity threshold of C., and from then CR tends to decrease. Lethal C, values of essential or non-essential elements may not exist. In that case CR may be constant over severa! orders of magnitude of C,. The preliminary nature of our results makes it diflicult to provide definitive conclusions. Never theless, it is relatively clear that 21ºPo follows the above mentioned pattern, with a behaviour typical of essential elements. This is not so evident in the case of 238U. However, as stated by Sheppard and Sheppard (1985), U behaves as an essential element at low C,. Such behaviour is even clearer for 210Po for the plant species studied, although for Spartina maritima this is not very clear, but at least 210Po seems to follow the same pattern. Conclusions 21ºPo and 238U radioactivity concentrations have been studied in samples of soils and two wild plants, Spartina densifl.ora and Spartina marítima, collected from the Odiel river marsh area. Results have shown that the operation of the two phosphoric acid production factories, located close to the marsh, have an impact on it by enhancing the radioactivity levels in both soils and plants. The distribution pattern of radioactivity along the marsh indicates that the Odie! river waters intrude into such area by tidal movements. Concentration ratios ( CR) have been studied for both Spartina species. lt has been found that the CR is not constant, but depends on soil radioactivity concentrations, C,. In fact it is higher for low C, than for high C,. This finding has been obtained previously for other plant species and radionuclides, and opens an interesting line of research. References Martínez-Aguirre, A. (1991) Radioactividad natural en diversos compartimentos naturales de Andalucía, Ph.D. Tessis, University of Seville, Seville, Spain (in Spanish). Martínez-Aguirre A. and García-León M. (1996) Radio active impact of the phosphate ore processing in a wet marshland in Southwestem Spain. J. Envir. Radioactiv. (in press). Martínez-Aguirre A., García-León M. and Ivanovich M. (1994) The distribution of U, Th and 226Ra derived from the phosphate fertilizer industries on an estuarine system in Southwest Spain. J. Envir: Radioactiv. 22, 155. Mengel, K. and Kirkby, E.A. (1979) Principies of Plant Nutrition (2nd Edn). International Potash Institute, Bern. Sheppard M. I. and Sheppard S. C. (1985) The plant concentration ratio concept as applied to natural U. Health Phys. 48, 494. Timperley M. H., Brooks R. R. and Peterson P. I. (1970) The significance of essential and non-essential trace elements in plants in relation to biogeochemical prospecting. J. Appl. Ecology 7, 429. Tracy B. L., Prantl F. A. and Quinn J. M. (1983) Transfer 226Ra, 210 Pb and U from soil to garden produce: assessment of risk. Health Phys. 44, 469.