210Pb enhancement in rivers affected by the phosphate rock processing in southwestern Spain

21ºPb enhancement in rivers affected by the phosphate rock processing in southwestern Spain A. Martínez-Aguirre and M. García-León Facultad de Física, Universidad de Sevilla, Apdo. 1065, 41080 Sevilla, Spain ABSTRACT The activity levels of 21ºPb in solution and bottom sedimenta were measured in the estuary formed by the Odiel and Tinto rivera in southwest Spain. The levels in both water and sediment samples indicate that the discharges from two fertilizer factories in the area enhance the radioactivity of such samples. l. INTRODUCTION Recently, high radioactivity levels of U, Th and 226Ra isotopes were found in solution, suspended matter and bottom sediments of the Odiel and Tinto rivers located in southwestem Spain [l-3]. The anomaly was attributed to two fertil izer factories located in an industrial complex surrounded by these two rivers (Fig. 1). These factories release their wastes directly to the Odiel river or store them in uncovered piles near the Tinto river channel. In this paper, we have extended previous work to the study of210Pb in solution and bottom sediments. 2. SAMPLES AND EXPERIMENTAL METHODS Seven points on the Odiel river, four on the Tinto river and one at the conflu ence of both rivers (Fig. 1) were sampled for water and bottom sediments in July 1990 (dry season) and in February 1991 (wet season) to evaluate seasonal variations in 21ºPb activities. The pH of the waters was measured at the time of sampling. The suspended matter was separated by filtration through Nu cleopore filters of 0.45 µm pore size. Immediately, the filtrate was acidified to pH -2 with concentrated HNOa, The small size fraction (d < 63 µm) was separated from sediment samples in order to determine the 21ºPb content. Determinations of percent organic matter and percent of water were carried out on each bottom sediment sample. N t � PHOSPHOGYPSUM PILES 0 FERTILIZER INDUSTRIES o 3km Fig. l. Map ofthe Odiel and Tinto rivers which shows the sampling stations along both river channels. The fertilizer factories and the phosphogypsum storage area are also shown. 21ºPb was determined through the measurement of its daughter 21ºPo by cx-spectrometry, afler secular equilibrium was achieved. The samples were spiked with known activities of208Po to determine the chemical recovery ofthe total procedure. Samples were first acid digested with concentrated HN08 followed by aqua regia and, finally dissolved in 8 M HN08• Po was separated using a solvent extraction method with TBP as the organic phase and 8 M HN03 as the inorganic phase. After vigorous shaking Po remains in the inorganic phase whereas other radionuclides join the organic phase. The solution containing the Po was evaporated to dryness and self deposition of Po onto silver planchets was encouraged by shaking gently at 60ºC. The planchet was finally measured by cx-spectrometry with ion implanted detectors. Further details on the radiochemical procedure are presented in Morón et al. [ 4). 3. RESULTS AND DISCUSSION To simplify the subsequent discussion we will first present the results for water samples from both rivers for all sampling campaign.s, followed by those ob tained for bottom sediments. Samples O represent those talcen in the Odiel river, T in the Tinto river and OT in the confluence of both rivers. Sediment samples are identified as SO, ST and SOT for the Odiel, Tinto and the conflu ence respectively. The index represents the sampling station along each river (Fig. 1). 3.1. Waters The results obtained for the analysis of 210Pb in water samples talcen during the low tide in 1990 (dry season) and 1991 (wet season) are presented in Table l. The levels of 21ºPb in the Odiel river area under study range from 3. 7 ± 0.5 to 289 ± 22 mBq 1-1 with the maxima, in summer and in winter, in front of one of the fertilizer outfalls (Fig. 2). The levels decrease quickly downstream from this point, due, probably, to precipitation ofFe and Mn during mixing downstream [5]. This activity pattern is similar to those obtained previously for other radionuclides [1,2) reflecting the presence of a local source of radioactivity (the fertilizer industries) along the river channel. The minimum levels found in sum mer are clearly higher than those in winter, probably due to a lower water stream flow of the river in the dry season. In the case of the Tinto river the levels along the channel are quite constant, ranging from 17.0 ± 1.5 to 26.6 ± 3.0 mBq 1-1 in summer 1991, with a slight decrease downstream of the channel. In 1991, the levels range from 15.3 ± 3.1 to 45.6 ± 2.7 mBq 1-1• In both campaigns the TABLE I Data of21ºPb in mBq 1-1 in water samples (low tide) taken from the Odie! and Tinto river basins in 1990 (dry season) and 1991 (wet season). The pH ofthe waters at the time ofsampling is also given. N.M means not measured 1990 1991 Code pH 210pb pH 210pb 01 2.86 21.1±2.5 3.90 N.M. 02 6.37 7.9±1.0 5.52 7.8±0.7 03 6.42 10.2±3.4 5.90 3.7±0.5 04 6.35 289±22 5.96 36.4±2.6 05 6.32 34.4±2.2 6.03 8.4±0.8 06 6.48 12.2±1.4 6.06 7.4±1.1 07 6.40 14.0±1.0 6.02 50.6±3.9 Tl 2.41 41.7±3.1 2.88 N.M. T2 5.54 26.6±3.0 3.22 45.6±2.7 T3 5.82 19.5±1.5 5.56 15.3±3.1 T4 6.10 17.0±1.5 6.40 30.7±5.7 OT 6.56 11.7±1.0 6.23 10.5±1.2 º' Odlel and Tinto woters 02 03 04 05 06 07 ----1990 - - -- --1991 OT T4 T3 T2 T1 Fig. 2. 210pb in mBq 1-1 in water samples taken from the Odiel and Tinto rivers in 1990 and 1991. maximum level in the Tinto river was found in station T2. This station is located close to an artificial stream coming from the · phosphogypsum storage area. In fact, the pH of the water at this station is lower than at the rest of the stations, at 3.22 in 1991 (Table 1), which reflects the wastes released at this point. We also observe that the level of 210Pb found at station T4 of the Tinto river is quite similar to that found at station 07 of the Odiel river. This fact could be explained by the influence that contaminated waters from the Odiel river have on the Tinto river channel. Thus, contaminated water from the Odiel river can flow upstream into the Tinto river when the tide changes from low to high. In the Netherlands, Koster et al. [6] obtained levels of210Pb of0.9 mBq 1-1 in waters of the Oosterchelde system while in the area around a fertilizer factory in the Nieuwe Waterweg system they found 10 mBq 1-1. In general, the authors assumed levels below 1 mBq 1-1 as a background level, those higher than 3 mBq 1-1 as probably enhanced and finally, those higher than 5 mBq 1-1 as clearly enhanced. Lewis [5] reported dissolved 21ºPb in normal river waters to be less than 0.2 mBq 1-1• By contrast in acid river waters, dissolved 210Pb was found to reach 230 mBq 1-1 in the Colorado system and 2.8 mBq i-1 in the west branch of the Susquehannah river [7]. Comparing the levels of 21ºPb in the Odiel and Tinto rivers for both sampling campaigns we should consider both rivers to be clearly enhanced in 210Pb activity by the fertilizer industries, particularly in the area close to the outfalls. The background level away from the outfalls appears to be around 7 mBq 1-1• We will consider separately the case of samples 01 and Tl, which were collected at the source of both rivers, far from the fertilizer industries (20 km upstream in both cases) and unaffected by their wastes. Both sampling stations present levels of 210Pb much higher than the usual values in fresh river waters. This fact must be related with the acid pH of the waters (Table 1) at both stations. These acid pHs produce a dissolution of 210Pb from soil particles and consequently a higher 21ºPb activity in solution, as was previously found in the Colorado river and in the Susquehannah river (6, 7]. This effect of dissolution from bottom sediments by the acid pH of the waters was also found for other radionuclides in the same sampling stations (1-3). 3.2. Sediments The small size fraction (d::;; 63 µm) of 7 bottom sediments from the Odiel river, 4 from the Tinto and 1 from the confluence ofboth rivers sampled in 1990 and 1991 were separated for 210Pb determinations. The results obtained together with percent of water and percent of organics for each sediment sample are presented in Table 2. Levels of 21ºPb are quite high in the area close to the fertilizer industries in the Odiel river, with a maximum level of 799 ± 48 mBq g-1 in 1990 and 534 ± 25 mBq g-1 in 1991. These levels ofradioactivity at stations 04 to 07 are much higher than the background level, ::;;100 mBq g-1 reported elsewhere [6]. Only in two stations (Fig. 3), upstream from the industries, in both sampling campaigns, does the level of 210Pb fall in the non enhanced range (::;;100 mBq g-1). 210Pb activity levels at station 07 were much lower (::;;100 mBq g-1) in the 1990 TABLE2 Data of210Pb in mBq g-1 in the small size fractions ofbottom sedimenta taken from the Odie! and Tinto river basins in 1990 (dry season) and 1991 (wet season). Percent of water and percent of organice in the sediment samples are siso given. N.M. means not measured Code %Water % Organics 210pb %Water % Organice 210pb S01 52.58 5.73 N.M. 61.18 15.40 N.M. S02 26.64 11.76 13.9±1.3 24.53 3.09 8.0±0.7 S03 60.58 9.51 27.2±1.5 57.21 8.25 40.9±2.7 S04 52.53 10.91 212±10 39.13 11.57 284±14 S05 65.51 5.26 438±21 59.83 9.74 420±31 S06 58.18 9.47 799±49 67.93 12.95 534±25 S07 65.35 9.47 78.6±3.7 55.64 9.58 411±24 STl 79.63 8.38 N.M. 39.80 2.25 N.M. ST2 38.78 4.34 96.7±6.1 51.37 8.96 33.1±3.3 ST3 59.29 9.64 39.8±1.9 56.8 11.72 233±9 ST4 61.46 8.64 74.4±4.8 56.03 10.93 47.9±2.5 OT 64.98 8.69 149±11 66.77 11.49 204±16 Odlel and Tinta 1edlment1 502 503 504 S011 508 507 ___ 1990 ------1991 set St4 5t3 St2 Fig. 3. 21°Fb in mBq g-1 in bottom sediment samples taken from the Odiel and Tinto rivers in 1990 and 1991. sampling campaign than in that of 1991. This must be related to sorne drainage activities carried out in the area in 1990. In the Tinto river the levels range from 33.1 ± 3.3 to 233 ± 9 mBq g-1 in the fine fraction for both years. 21ºPb levels in the Tinto river sediments, for the same sampling campaigns, are much lower than in the Odiel river. Moreover, there is a clear peak of activity in station T3 in the sampling campaign of 1991 (Fig. 3), with levels much higher than the typical background level. This station is located at the confluence with a small natural stream, Estero del Rincón, (Fig. 1) which crosses the phosphogypsum storage area, and could transport sorne gypsum particles with high activity levels. These particles will eventually be deposited on the Tinto river bed at the confluence. High activity levels of other natural radionuclides were also found at the same station in 1988 and 1989 [1,2]. The stations upstream from the industries at the Odiel river and also stations ST2 and ST4 of the Tinto river, however, present levels of 21ºPb much lower and similar to the background level (SlOO mBq g-1). In general, no correlations between 210Pb content and organic or granular composition were found in either river, and the activity pattems obtained are similar to those found for other radionuclides in previous years [1,2]. 4. ACKNOWLEDGEMENTS The authors wish to thank J.M. Abril, G. Manjon and J. Diaz for helping in the sampling. The work was partially supported under contract DGICYT PB890621 and EEC contract FI3P-CT920035. 5. REFERENCES l. Martínez-Aguirre, A. and M. García-León, 1991. Natural radioactivity enhance ment by human activities in rivers ofthe southwest of Spain. J. Radioanal. Nucl. Chem. Lett., 155: 97-106. 2. Martínez-Aguirre, A., M. García-León and M. lvanovich, 1994. The distribution of U, Th and 226Ra derived from the phosphate fertilizer industries on an estuarine system in southwest Spaín. J. Environ. Radioactivity, 22: 155-177. 3. Martínez-Aguirre, A., M. García-León and M. Ivanovich, 1994. U and Th distribu tion in solution and suspended matter from rivers affected by the phosphate rock processing in southwestern Spain. Nucl. Instrum. Meth. Phys. Res. A, 339: 287293. 4. Morón, M.C., A. Martínez-Aguirre and M. García-León, 1988. Determination and levels of U, Th and Po from different environmental samples. In: International Conference on Environmental Radioactivity in the Mediterranean Area. SNE ENS, Barcelona, p. 111. 5. Lewis, D.M., 1977. The use of 210Po as a heavy metal tracer in the Susquehanna river system. Geo. Cosmochim. Acta, 41: 1557. 6. Koster, H.W., P.A. Marwitz, G.W. Berger, A.W. Weers, P. Van Hegel and J. Niewuenwize, 1991. 210Po, 210Pb and 226Ra in aquatic ecosystems and polders, anthropogenic sources, distribution and enhanced radiation doses in the Nether lands. In: V International Symposium on the Natural Radiation Environment, Salzburg, Austria. 7. Scott, M.C., 1982. The chemistry of U and Th series nuclides in rivers. In: M. lvanovich and R.S. Harmon (eds.), Uranium Series Disequilibrium: Applications to Environmental Problems. Oxford Science Publications, Chapter 8.