Natural radioactivity in groundwaters around a fertilizer factory complex in South of Spain

Natural radioactivity in groundwaters around a fertilizer factory complex in South of Spain J.M. Alcaraz Pelegrina, A. Martı´ nez-Aguirre* Departamento de Fı´sica Aplicada, EUITA, Universidad de Sevilla, Ctra. Utrera km. 1, 41013 Sevilla, Spain Abstract A study ofthe concentration ofthe U isotopes, 226Ra and 230Th in a groundwater system has been carried out. This aquifer, located in the provinces of Sevilla and Huelva, is the most important in the south of Spain having a surface area of 2500 km2. The proximity of a fertilizer factory complex to this aquifer system is of particular concern given that it releases a significant part ofits waste directly into the estuary ofthe Odiel and Tinto rivers, also storing a further part on the right bank ofthe Tinto river. Investigation has been made ofthe environmental impact ofthe fertilizer f actory, either as a result ofleaching ofradionuclides from the phosphogypsum piles or otherwise as a result ofintrusion of the Odiel and Tinto waters, both ofwhich are in close contact with the aquifer. Results show U concentration in waters of this system around the phosphogypsum piles to be significantly higher than those in other areas ofthe same aquif er. The low 226Ra concentrations f ound in the same locations add support to the origin of part of the U in these samples. K eywords: Natural radioactivity; Fertilizer plant; Groundwater; Spain 1. Introduction In southwest Spain, the presence of elevated levels of U, Th and 226Ra in rivers around a fertilizer factory complex has been established in several previous studies (Martı´nez-Aguirre and Garcı´a-Leo ´n, 1991; Martı´nezAguirre et al., 1994). In particular, elevated concentrations of several natural radionuclides were found in water, suspended particles and sediments collected along the course of the Odiel and Tinto rivers. Contamination along the Odiel river was mainly attributed to solid and liquid waste discharges which were made directly into the course of this river at its estuary. This area is highly affected by tidal movement, with contamination migrating both upand downstream. Tidal movements of the waters also lead to contamination of the Tinto river in areas close to the Odiel, these rivers having a common confluence into the Atlantic ocean. A smaller degree of contamination of the Tinto river has been observed to be due to storage of solid wastes (phosphogypsum piles). Concentrations as high as 950 and 2000 mBq=gofU and 226Ra were obtained in samples of phosphogypsum collected in 1988 (Martı´nez-Aguirre, 1991). These waste piles, connected by several small tributaries to the Tinto river, allow for the possibility of leachate entering the riverine system. However, the phosphogypsum piles are mainly calcium suphates and, complexed in this form, Ra is insoluble. The groundwater system of interest is the most important in southwestern Spain, covering a surface area of 2500 km2. This aquifer system, used both for consumption and irrigation water, may be affected either through intrusion by the elevated concentration levels of contaminants in the Odiel and Tinto rivers or by leachate from the phosphogypsum piles. Consequently, contamination of this aquifer system would affect a large region of Spain. Investigating the possible influence of the fertilizer manufacture, either by direct leaching of *Corresponding author. Tel.: +34-95-423-36-69; fax: +3495-423-2644. E-mail address: [email protected] (A. Martı ´nez-Aguirre). radionuclides from the phosphogypsum piles or intrusion of Odiel and Tinto river waters, is the main objective of this paper. 2. Samples and experimental methods Twenty-two monitoring stations (see Fig. 1) have been selected across the Almonte-Marismas groundwater system in order to determine what influence the contamination produced by the fertilizer factory located in the Huelva estuary may have on the system. Some of the samples have been collected near the city of Sevilla in order to compare these with water samples collected around the fertilizer complex. Activity concentrations of the U-isotopes, 230Th and 226Ra have been measured in the samples. In January 1997, a 5 liter quantity of water was sampled at each monitoring station, pH and temperature being measured at the time of sampling. Suspended matter was separated as soon as possible by filtration through 0:45 mm pore size Millipore filters and the solution acidified to pH 2 with HNO3to avoid the growth of microorganisms and to minimize water–wall interactions before the analysis. From the original sample, 1–3 liters of each of these aqueous media was spiked with known activities of 232U and 229Th for U and Th determinations, respectively. After homogenization, the solution was evaporated to approximately 1 liter and iron hydroxides precipitation with concentrated ammonia was carried out. The anionexchange resin method has been used for U and Th separation and purification (Martı´nez-Aguirre, 1991). Final solutions were electroplated onto stainless-steel planchets and U and Th activities measured by aspectrometry with surface barrier or ion-implantated Si detectors. 226Ra activity concentrations were measured in most water samples by using an LB770 acounter. Samples, each of 1 liter, were first neutralized, followed by addition of 5 mg of Ba carrier. Precipitation of Ra–BaSO4, by adding H2SO41 M, was then carried out Fig. 1. Map of Almonte-Marismas groundwater system. In the figure the locations of the sampling stations are shown. and collected following filtration using 0:45 mm pore size Millipore filters. After 20 days (to allow secular equilibrium of 226Ra daughters) the ingrown alpha activity of the samples was counted (Martı´nez-Aguirre and Perian ˜ez, 1998). 3. Results and discussion In Table 1, activity concentrations for U-isotopes, 226Ra and 230Th are presented together with the pH and temperature of the waters at the time of sampling. As seen, most samples have close to neutral pH, in the range 6.7–8. Exceptions are samples A11 and A14, with pHs of 8.8 and 8.5, respectively. The water temperatures ranged from 128Cto188C with no apparent trend between stations. Samples 7–22 were collected around the Tinto river channel and the phosphogypsum piles (see Fig. 1). U-isotope activity concentrations ranged from 0.35 to 208 mBq=lfor238U and 0.22 to 181 mBq=lfor234U, revealing two orders of magnitude difference between samples. The samples can be grouped in three different sets of concentration, low, medium and high. In the first set, 238U concentration ranged from 0.35 to 17:4 mBq=l, in the second 30.3 to 45:7 mBq=l and in the third 71 to 93 mBq=l (also including one outlier with 208 mBq=lin sample A8). From data in Table 1 and Fig. 1, most samples with U concentrations within the third set (71–208 mBq=l) have been found in areas close to the course of the Tinto river, particularly around San Juan del Puerto village and the upper part of the Tinto river channel. An exception is sample A2 collected close to the city of Sevilla, with a concentration well above those in the same area (A1 and A3). The maximun U concentration has been found in sample A8, collected upstream of the phosphogypsum piles and close to Bonares village. Two samples, A12 and A13, located close to Palos de la Frontera village, at the right bank of the Tinto river, have U concentrations in the high range. However, samples collected in Moguer village, close to Palos de la Frontera have much lower U concentrations than samples A12 and A13. In particular, sample A14 yields a minimum concentration, 0:35 mBq=lof238U. It is well known that precipitation of iron oxyhydroxides takes place at pH 8, resulting in a decrease of activity concentration in the aqueous phase. The high pH of sample A14 is strongly supportive of a low U concentration. The same reasoning can be applied to sample A11, with high pH and U concentration lower than that of samples A12 and A13. Table 1 U-isotopes, 226Ra and 230Th activity concentrations (mBq=l) in water samples collected across the Almonte-Marismas groundwater system a Sample code pH Tð8CÞ238U234U226Ra 230Th A1 7.7 16.7 7.93 0.70 13.0 1.0 NM ND A2 7.7 17.2 82.0 4.7 120.3 6.3 9.76 0.29 0.32 0.06 A3 7.6 16.1 6.58 0.50 7.07 0.52 6.94 0.17 2.38 0.17 A4 7.6 13.9 10.6 0.6 12.0 0.7 NM 2.01 0.21 A5 7.6 12.8 90.7 5.3 103.7 6.0 8.30 0.18 ND A6 7.3 15.2 44.3 3.7 46.3 3.7 NM 0.23 0.06 A7 6.7 12.8 3.41 0.28 3.55 0.28 NM 4.0 0.3 A8 7.8 16.7 208 10 181 9 4.91 0.14 ND A9 7.7 11.7 43.0 2.7 44.7 2.7 NM 0.14 0.04 A10 7.1 13.7 13.4 0.7 13.2 0.7 NM 0.70 0.09 A11 8.8 13.5 34.6 1.6 38.9 1.8 NM 0.17 0.04 A12 7.3 17.8 93.6 3.8 119.3 4.8 5.49 0.15 0.10 0.03 A13 7.3 15.9 91.8 4.5 91.2 4.4 14.3 0.3 0.04 0.02 A14 8.5 13.8 0.35 0.08 0.22 0.06 ND 0.04 0.02 A15 7.5 16.2 16.8 0.9 18.7 1.0 2.15 0.10 ND A16 7.3 15.4 17.4 0.9 23.1 1.1 2.62 0.09 0.12 0.04 A17 7.5 16.7 30.3 1.6 27.3 1.4 1.80 0.07 0.11 0.04 A18 7.7 14.2 90.3 3.8 109 4.5 1.93 0.08 0.17 0.05 A19 7.5 15.6 93 4 120 5 2.65 0.11 NM A20 7.9 16.7 71 3 112 4 1.89 0.08 NM A21 8.0 15.6 90.3 4.5 93.7 4.7 2.84 0.10 0.10 0.03 A22 7.2 15.8 45.7 2.1 53.0 2.5 12.6 0.2 NM a The pH and temperature of the waters at the time of sampling are also given. ND means not detected and NM not measured. The presently reported U concentrations ranging between 0.02 and 16:6mg=l (see Table 2) are generally higher than those obtained by Andrews and Kay, 1982; Kronfeld, 1974; Kraemer, 1981 for other aquifers but are in good agreement with those obtained by Osmond and Cowart, 1976, for some aquifers in the USA. It is possible that some of the water samples and thus, some locations in the aquifer may be affected by releases from the fertilizer factories. 234U=238U activity ratios are shown in Table 2. As observed in most samples, these isotopes are in approximate secular equilibrium, 234U being slightly in excess of 238U. Only one sample, A8, shows a slightly lower value of 234U than 238U. No relationships have been found between the activity ratio, U concentration and=or sample location. All activity ratios are in agreement with other results found in the current literature for groundwaters (Osmond and Cowart, 1976). The activity concentration of 226Ra in water samples ranges from 1.8 to 14:3 mBq=l (Table 1). As for U isotopes three sets of activity concentration range may be suggested: the first showing concentrations ranging from 1 to 3 mBq=l which are located around San Juan del Puerto (samples A15–A21), while the second set which ranges from 5 to 10 mBq=l, were obtained in samples collected far from the Tinto river (A2, A3, A5 and A8) an exception being sample A12. The highest 226Ra concentrations were found in samples A13 and A22. In contrast to the case for U, the lowest 226Ra concentrations were found in the area around the phosphogypsum piles. If we consider that phosphogypsum is composed of calcium sulfate a highly insoluble complex, this result is to be expected. In all cases, other than sample A3, the 226Ra activity concentration is much lower than the activity of U isotopes, with the 226Ra=234U activity ratios being below 0.2. In sample A3, both radionuclides are practically in secular equilibrium. The difference in comparison with other samples cannot be explained at this time, although the relatively low U concentration for the particular area is noted. 230Th has been also measured in the samples. Generally, the activity concentrations are much lower than U and 226Ra, ranging from 0.04 to 4 mBq=l, being for most samples in a range below 0:3 mBq=l. Exceptions are sample A10 with 0:7 mBq=l and samples A3, A4 and A7 with concentrations above 1 mBq=l. The higher 230Th activity concentrations are not related with the vicinity of the samples to the phosphogypsum piles, the concentration in this area being in fact very low. The 230Th=234U activity ratio is well below unity, reflecting the higher solubility of 234U relative to 230Th. Table 2 U concentrations and associated activity ratios in water samples collected from the Almonte-Marismas groundwater system Sample code [U] (mg=l) 234U=238U226Ra=234U230Th=234U A1 0.642 0.057 1:64 0:16 A2 6.60 0.38 1:47 0:11 0:081 0:005 40:003 A3 0.53 0.04 1:08 0:09 0:986 0:076 0:337 0:034 A4 0.85 0.05 1:14 0:06 0:166 0:019 A5 7.30 0.43 1:14 0:09 0:080 0:005 A6 3.56 0.30 1:05 0:12 40:005 A7 0.27 0.02 1:04 0:11 1:127 0:123 A8 16.6 0.8 0:87 0:02 0:027 0:002 A9 3.4 0.2 1:07 0:05 40:003 A10 1.08 0.06 0:98 0:05 0:053 0:007 A11 2.78 0.13 1:13 0:04 40:004 A12 7.53 0.31 1:28 0:02 0:046 0:002 40:001 A13 7.39 0.36 0:99 0:03 0:157 0:008 40:001 A14 0.018 0.005 1:58 0:58 0:182 0:104 A15 1.36 0.08 1:12 0:06 0:115 0:008 A16 1.40 0.07 1:33 0:06 0:113 0:007 40:005 A17 2.44 0.13 1:11 0:05 0:066 0:004 40:004 A18 7.26 0.31 1:21 0:03 0:018 0:001 40:002 A19 7.48 0.32 1:29 0:03 0:022 0:001 A20 5.71 0.24 1:58 0:03 0:017 0:001 A21 7.32 0.37 1:04 0:03 0:030 0:002 40:001 A22 3.77 0.17 1:16 0:04 0:238 0:012 Contamination from the fertilizer production would probably also give activity ratios below unity, albeit higher than the normal values as found in the Odiel and Tinto river waters (Martı´nez-Aguirre and Garcı´a-Leo ´n, 1991, Martı´nez-Aguirre et al., 1994). 4. Conclusion U concentration in samples of water from the Almonte-Marismas groundwater system, collected in the vicinity of solid waste (phosphogypsum) piles of the Huelva fertilizer factory, are significantly higher than those in other areas of the same aquifer. This could either result from U leaching from the phosphogypsum piles or intrusion of Odiel and Tinto river waters into the aquifer system. The low 226Ra concentrations found in the same locations are related to the high insolubility of calcium sulfates. Acknowledgements We would like to thank Dr. R. Perian ˜ez from the University of Seville and Dr. M. Jurado from the University of Badajoz for their help in the sampling works. References Andrews, J.N., Kay, R.L.F., 1982. 234U=238U activity ratios of dissolved uranium in groundwaters from a Jurassic limestone aquifer in England. Earth Planet. Sci. Lett. 57, 139–151. 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