U and Th distribution in solution and suspended matter from rivers affected by the phosphate rock processing in southwestern Spain

U and Th distribution in solution and suspended matter from rivers affected by the phosphate rock processing in southwestern Spain 1 A Martínez-Aguirre ª·*, M_ García-León 3, M. Ivanovich b " Facultad de Ffoca, Univer.l'idad de Sed/la, Apdo. 1065, 41080 Sevilla, Spain h Af:A Technology, Harwe/1 Laboratory, Oxon OXl l ORA, UK A study is presented on the distribution of U and Th isotopes in thc solid and liquid fractions of waters collected along the Odie! and Tinto rivers (southwest Spain). Such rivers are affected by the operation of phosphate fertilizer factories. The analysis of activity concentrations and activity ratios in both fractions provides interesting inforrnation on the way in which the factories disturb their clase environment. l. Introduction Recently (1,2] high natural radionuclidc activity lcv cls wcre found in dissolution and bottom sediments collected from the Odiel and Tinto rivers in southwest crn Spain (Fig. 1). The anomaly was attributed to an industrial complex surrounded by these rivers in which two phosphatc fertilizer factories are located. These factories release their wastes directly to the Odiel rivcr or storc thcm in uncovered piles ncar thc Tinto rivcr channcl (Fig. 1 ). In this work the distribution of U and Th isotopcs in solution and suspended mattcr of water samples collected from such rivcrs are rcported. The study of thc activity conccntrations in both fractions confirms that thc operation of thesc factories is responsiblc for thc radioactivity present in the local cnvironment. The additional analysis of sorne rclevant activity ratios pro vides information on the sourcc tcrm. 2. Samples and methods Six water samples from the Tinto river, scven from thc ()diel river and two from the confluence of both rivers (sce Fig. 1) wcre taken during J uly 1989. 10 litcr water samples werc collcctcd along thc Odiel and Tinto river basins during July 1989 (sce Fig. I ). The suspended matter was separated as soon as 1 Work partially supported under contract DGICYT PB89- 0621 and the AMA of Junta de Andalucía, and partially supported by AEA Technology Corporate Research and Devclopment programme. *Corresponding author. possible by filtration through Nuclcporc filters of 0.45 µm pore size. The remaining water was immediately acidified to pH - 2 with concentrated HN03• Sampling stations werc mainly distributed along thc lower reaches of thc rivers close to the industrial area of Huelva (Odie) river) or to the phosphogypsum piles arca (Tinto rivcr ). The exccptions were samplcs O 1 and TI, col lected far upstream of thesc areas, close to the sources of the rivcrs. Samples OT were collected at the conflu encc of both rivcrs, station 1 still being close to thc industrial arca and station 2 on the Atlantic Ocean coast. U and Th isotopcs werc cxtracted using anion ex changc resins (Dowex AG1-X8 hydrochloric form) and electroplatcd onto a stainless steel planche!. 236 U and 229 Th were used for yield determinations. Isotopic ac tivities were measurcd with Si ion-implantcd detector a-spectrometry. The radiochemical methods can be found in detail in refs. (2,3]. 3. Results and discussion To simplify the discussion we will first present the results obtained for the Odiel river, into which the wastes from the fertilizcr factorics are directly rc leascd, and after that those obtained for the Tinto river, close to thc area where the phosphogypsum piles are locatcd (sce Fig. 1 ). 3.1. Odie! river Results on U and Th isotopic activities in the dis solved and particulatc fractions are given in Table 1. Fig. l. Map of the Odiel and Tinto rivers. The sampling stations along the river basins, as well as the fertilizer factories (Foret, Fosfórico) and phosphogypsum piles emplacemcnts are shown. Table 1 U and Th isotopic activities in mBq/1 in the dissolved (d) and particulate (s) fractions of water samples taken from the Odiel river. Stations OT1 and OT2 are located at the confluence of the Odie] and Tinto rivers (see Fig. 1 ). NO= not detected and NM = not measured Station 2)HLJ 234u 2.12Th 2.10Th 228Th 01 d 198 ± 7 316 -t 11 13.4 ±0.6 49.7 ± 1.7 22.9 ±0.9 s 0.115+ 0.013 0.215 ± 0.019 0.048 + 0.013 0.219 -t 0.025 0.169 ± 0.054 02 d 34.2 ± 1.1 36.3 ± 1.2 0.58 :::0.30 7.0 ± 1.3 2.7 ±0.8 s 1.91 ± 0.07 2.02 ± 0.07 0.265 ± 0.023 1.64 ±0.06 0.228 ± o.cm 03 d 33.0 ± 2.0 35.0 ± 2.1 1.8 ±0.1 12.0 ±0.5 NM s 2.24 ± 0.08 2.42 :t O.Cl9 0.59 ±0.04 3.7 ±0.1 0.63 ±0.04 04 d 54.1 ± 3.4 58.7 ± 3.7 2.2 ±0.2 14.3 ±0.7 NM s 2.17 ± 0.08 2.17 ± 0.08 0.52 ±0.03 2.67 ±0.09 0.50 :t-0.03 05 d 52.7 ± 3.6 54.3 ± 3.7 2.5 ±0.03 18.9 ± 1.3 7.5 + 1.2 s 25.3 + 0.5 25.4 ± 0.5 2.13 ±0.08 18.3 ±0.4 2.03 ±0.08 06 d 131 ± 11 136 ±12 4.7 ±0.7 34.2 ±3.3 8.8 ±2.8 12.1 ± cu 12.4 + cu 1.21 ±0.05 9.6 +0.2 1.19 ±0.05 07 d 35.8 ± 0.9 39.2 ± 0.9 1.8 ±0.1 11.2 -t 0.4 2.4 ±0.2 11 5 + cu 12.2 t 0.3 2.3 ±0.1 15.9 :t 0.4 2.2 ±0.1 OT1 d 49.3 ± 1.3 53.7 ± 1.4 0.55 ±0.07 3.2 ±0.2 O.SO ±0.08 1.19 ± 0.07 1.17 ± 0.07 0.13 ±0.02 1.13 +O.Cl6 ND OT2 d 34.5 + 2.0 38.2 + 2.2 0.08 ±0.03 0.61 ±0.08 NM 1.93 ± 0.12 2.2 ± 0.1 0.48 ±O.OS 3.6 ±0.2 0.52 ±0.05 The data on the dissolvcd fraction have been widely discusscd in ref. [2]. Thcy rcflcct thc cxistcncc of a local sourcc of activity between stations 04 and 06 rclatcd to the operation of thc phosphatc fcrtilizer complex. In fact, thc general levcls of activity are clcarly highcr than those found for unpcrturbcd rivcr systcms [4-8]. The case of station 01 was rclatcd to thc low pH ( - 3) of thc watcrs, which promotcs thc dissolution of U and Th isotopes from the solid phasc to thc water. Such an effect causes the lowering of U and Th isotope activitics in thc particulatc fraction at this location, as can be sccn in Table 1. Thc samc conclusions can be derived from a general review of the suspended matter results. lt has bccn found, for instance, that concentrations bclow 1.36, 1.30 and 1.49 mBq/1 of 232Th, 23ºTh and 228Th, rcspcc tivcly, are prcscnt in suspended matter from sorne Japancsc rivcrs [4]. Comparing with the data in Table 1, the concentrations of 2.12Th and 228Th in thc Odie! river suspended matter are very similar to thosc ob tained in the above refcrcncc. Only samples takcn around thc industrial arca, stations 05, 06 and 07, present concentrations highcr than thosc in thc Japanese rivers. In the case of 23ºTh, the trend is different. In fact, the conccntrations are, in general, much higher than those given in ref. [4]. Thus, it sccms obvious that a local sourcc of activity is present be twccn stations 05 and 07. In fact, the U conccntra tions at thesc stations are about ten times highcr than in the samples upstream, with the maxima at station 05 whcrc a phosphatc factory is locatcd. Thc distribution of activity betwcen the dissolved and particulatc fractions can also be found from thc data in Table l. Less than 10% of the total U isotope activity is associatcd with thc solid fraction along the Odie! river basin, with a mínimum of lcss than 1 % at station 01 (thc naturally low pH in these waters could cxplain the absence of activity in thc solids, scc rcf. [3]). Thc cxccptions are stations 05 and 07 wherc 25-35% of the total U isotopc activitics are incorpo ratcd into the suspended matter (sec Fig. 2a). Betwccn 20 and 30% of 232Th and 23ºTh total activitics appears in the solid fraction, with thc cxcep tion again of stations 05 and 07. There, the contcnt of 232Th in suspended matter ranges from 46 to 56% and that of 2· 1ºTh from 48 to 49%. Consequently, an impor tant part of thc radioactivity released by thc fertilizcr factories to thc Odie! river is associated with solid matter. The distribution of 228Th between the fractions is differcnt than thosc of the other Th isotopcs. In general, its prcsence in thc dissolvcd fraction is higher than that for 232Th or 230Th. This should be explained in tcrms of the solubility of 228Ra, the parent of 228Th. Thc case of station OT2 is intcresting. Thcre, more than 80% of Th isotopc activities are containcd in thc particulatc fraction. This is not strangc, sincc this sample must be considered as a seawatcr sample; and it is known that in such systcms Th isotopes are quickly adsorbed onto the surfacc of solid particles. In Table 2 sorne relevan! activity ratios for both dissolved and suspended matter are prcscnted. The Th/U mass ratio is also includcd in the table. As commented in rcf. [2] the activity and mass ratios in dissolved mattcr hclp to support thc hypothc sis of inputs of radioactive material into the Odie! rivcr. Indced, except for the case of station O 1, the values obtained for thc 234U / 238U activity ratio are compatible with the cxistcncc of secular cquilibrium. This condition is not, of course, accomplished in thc majority of rivcr systcms [9]. On the contrary, thc valuc (a) 35 30 25 20 15 10 5 80 80 40 ·-··· 1 01 02 03 04 05 08 07 OT1 OT2 ··�, 01 02 03 04 05 08 07 0T1 0T2 Fig. 2. Contcnt in % of the total activity of 238U (a) and 2'ºTh (b) in thc suspended fraction along the Odiel river. Tahlc 2 Activity ratios as well as thc Th/C mass ratio in thc dissolved (d) and particulatc (s) fractions of water samples taken from the Odicl river Station 214U/ 2.lHU 2.10Th/ 2_14U 01 d 1.60+0.02 0.157 + 0.008 1.88±0.27 1.01 ±0.14 02 d 1.06 ± ().OJ 0.71 ±0.21 s 1.05 +0.05 0.81 +0.04 m d l.07t0.02 0.34 ±0.03 l.08±(J.05 1.52 ±0.07 04 d 1.08±0.10 0.24 :: 0.02 0.99+0.0S 1.23 ±0.06 05 d 1.02±0.09 0.37 ±0.04 1.00±0.01 0.72 +0.02 06 d 1.04+0.04 0.25 ±0.03 1.03+0.02 0.77 ±0.02 07 d 1.10±0.02 0.29 +O.O! 1.06± 0.02 uo +0.04 OT1 d 1.09 ± 0.02 0.060 + 0.004 0.98+0.08 0.97 ±0.08 ()'(, d 1.11 ± 0.04 O.O 16 + 0.002 1.15 -0.08 1.63 ±0.12 found is typical of thc primary mincrals uscd for fertil izcr production. As cxpcctcd, both thc Th/U and 2·1ºTh/ 214U mass and activity ratios in thc dissolvcd fraction are much bclow unity for ali thc stations, but still highcr than in othcr fluvial systcms [ 10]. Thus, a general contamina tion with Th isotopes along ali the Odiel river channel is confirmed. Finally, valucs much above unity for the 2.1ºTh/ 2.12Th activity ratio could also revea( thc high contamination of thc rivcr basin by membcrs of thc 218U radioactivc dccay chain. Thc study of such ratios for suspended mattcr givcs the samc conclusions. Thc 2·14U / 2.ixu activity along the channcl is again closc to unity, which is not typical for natural systcms [ 11 ]. This suggcsts an externa( origin of such particlcs, Tahk 3 Same as Tahk I hut for the Tinto riwr Station 21xu 2.14u TI d 115 :+ 4 211 ±7 s 0.252:: 0.027 0.317 + 0.029 T2 d 40 �2 42.6 .i 2.0 s 1.08 :: 0.08 0.98 t0.07 n d 47.6 ± 2.2 51.7 ± 2.4 1.96 ±0.09 2.04 +().()9 T4 d 48.1 + 2.4 53.4 +2.6 s 2.08 +().()9 2.30 +O.JO TS d 42.6 ± 1.0 46.l ± 1.1 1.05 ±0.08 1.27 ±0.10 T6 d 45.8 ±1..'.i 49.3 ± u, 0.218+0.019 0.261 ± 0.032 210Th/232 Th 3.IO-:t-0.14 4.6 ± 1.4 11.9 -6.0 6.2 .i 0.6 6.6 ±0.6 6.2 +0.4 6.5 +0.7 5.1 -+:0.4 8.5 +0.6 8.6 t 0.3 7.2 ±0.9 8.0 ±<U 6.1 :t-0.4 6.8 .i 0.3 5.9 +0.7 8.8 ± 1.4 7.4 ± 2.8 7.4 :r 0.8 22xTh/ 2.12Th 1.70±0.07 3.6 :: 1.5 4.5 + 2.5 0.86±0.14 1.07+0.09 0.97-t 0.()9 3.00+0.25 0.95 .i 0.05 1.9 ±0.7 0.99±0.06 1.3 ±0.1 0.96±0.06 0.91 ± 0.18 1.06+0.15 Th/U 0.208 ± O.O 12 1.33 ±037 0.053 ± 0.027 0.43 -:t-0.04 0.167±0.017 0.806 ± 0.059 0.134 + 0.017 0.74 ±0.06 0.151 ±0.018 0.258 ± O.O 11 0.110 ± 0.018 0.307 ± 0.015 0.157 ±0.011 0.62 .t_0.03 0.034 ± 0.004 0.34 ±0.06 0.008 ± 0.003 0.77 ±0.09 probably from the fertilizer industries. The case of station O 1 is interesting. At this station an excess of the daughtcr, similar to that found in the solution, is obtained. This should be rclated to the large interac tion betwecn phases due to the low pH of the waters [3]. Values slightly higher or close to unity are found for the 2'ºTh/ z.14u activity ratio, except for samples 05 and 06. There, a clear enrichment of 2·14U appears, which confirms the presence around such stations of a local source of activity. However, this can be deduced more clearly from thc Th/U mass ratio. In general, values much above unity are found for the Th/U mass ratio is suspended matter in unperturbed rivers [ 12.13]. In our case the ratio is clcarly less than 1. Thc enrichment of the particles in 2·18U and deseen- 2.12Th rn1Th 22xTh 19.6 ± 0.9 52.2 :: 2.2 71 ±3 0.092:t: 0015 0.215 ± 0.021 0.125 :t:0.0:,8 48 +14 91 +27 13 .i 4 0.073 t 0.015 0.89 ± 0.06 0.058±0.019 0.78 ± 0.10 5.65 + 0.35 1.00 ±0.13 0.170+ 0.020 1.40 + 0.06 0.153 ± 0.022 21.6 + 4.9 41.1 ± 9.0 2.25 ±0.78 0.133+ 0.018 1.33 ± 0.06 0.223 -t-0.025 0.55 + 0.10 3.92 ± 0.28 :-.¡o 0.134± 0.032 U2 + 0.08 0.116 ± 0.032 0.63 + 0.07 3.80 + 0.17 0.68 ±0.08 ().()4( ± 0.0()<) 0.258-+: O.OICJ 0.061 ± 0.029 dants can also be deduced from the 23ºTh/ 232 Th activ ity ratios. Although being bclow unity, they are still higher than expccted for unperturbed rivers [4,12]. Finally, as found in the curren! literature (4,12] thc 228Th/ 232Th activity ratio is closc to unity, cxccpt for station 01. There, thcrc cxists an excess of 228Th as occurs for dissolvcd matter. The effcct is similar to that obtaincd for the 234U/ 238U activity ratio and should be explained in thc samc way. 3.2. Tinto ricer Rcsults for U and Th isotopic activities are given in Table 3. As in thc case of the Odie! river thc dissolved fraction data havc alrcady bcen discussed in rcf. [2]. Wc only mention herc thc more relevan! conclusions. From stations T2 to OT2 thc activities of U isotopes are only slightly higher than those found for other rivers (7,9, 10], presenting a quite flat pattern. Thus, it is difficult to deduce from thcm any influence of the phosphogypsum piles arca in the Tinto rivcr. For the case of Th isotopes, the situation is clcarly differcnt. Two wcll defined peaks of Th isotopc activities appear at stations T2 and T4, with values highcr than those found in thc Odicl river. Excluding thc two maxima, Th isotope activities are lowcr than thosc found in the Odie! river and similar to those reported for othcr world rivers (4,5,8, 10]. Thc maxima were explaincd as thc rcsult of radioaetivity transported from thc phosph ogypsum piles to the Tinto river, by natural or artificial rivulets which cross it. The high activitics obtaincd for station TI is, as in the case of station 01 of thc Odie! river, a conscquence of thc low natural pH ( - 2) of thc waters. On the contrary, from analysis of thc activities in suspended mattcr one cannot deduce any contamina tion of the Tinto river coming from thc phosphogyp sum piles arca. In fact, only the 230Th activities around the arca are slightly higher than thosc found in thc curren! literature [4]. As will be secn later thc study of the activity ratios will clarify this point. As for thc distribution of the activity between solid and liquid fractions, sorne comments can be madc in thc case of the Tinto river. Lcss than 5% of the U isotopes total activities is associatcd with thc solid fraction along thc Tinto river, with a minimum in sample Tl, which should be ex plained as in thc case of samplc O 1 of the Odie! river (see Fig. 3a). The case of Th isotopes is different. Again, a mínimum of activity incorporated into the sol id fraction is found in sample TI. Toe reason is as given bcfore. However, minima Oess than 5%) are also found for samples T2 and T4 whcrc maxima in solution appcarcd (see Fig. 3b). In the other samples the con tcnt of Th isotopc activitics in suspended matter rangcs from 20 to 30%. This clearly shows that the Tinto river (d) 6, 5� 4 3 o T1 ( b) 100 80 60 40 T2 T3 20- 11 0"_._ _. T4 T5 T6 0T1 OT2 T1 T2 T3 T4 T5 T6 OT1 0T2 Fig. 3. Same as Fig. 2 but for the Tinto river. is recciving radioactive materials from the phosphogyp sum piles area through the points T2 and T4. The case of 228Th is diffcrcnt. In ali cases, 90% or more is incorporated into the dissolved fraction. The reason is as explained before, whcn commcnting on the same effect in the Odie! river. As already stated, thc study of activity ratios and thc Th/U mass ratio clarifies the problem a little (Table 4). In thc dissolvcd fraction [2] the 234U / 238U activity ratio approaches unity, exccpt for station Tl, reflecting the typical equilibrium of primary minerals. Thc Th/U mass ratio and the 2·1ºTh/ 234U activity ratio are always below unity, as in unpcrturbcd systcms (4,6], exccpt in the case of stations T2 and T4, whcrc thcy are higher Table 4 Samc as Tahlc 2 hui for the Tinto river Station 21-1u¡2.1xu 2'ºTh/ n•u TI d 1.83 + 0.03 0.25 ±0.01 1.26 t 0.17 0.68 ±0.09 T2 d 1.07 ±0.()4 2.14 ±0.64 s 0.91 ±0.09 0.91 ±0.09 T3 d 1.09±0.04 0.11 ± ().()! 1.()4 ± 0.(ló 0.69 ±0.04 T4 d 1.11 ± 0.04 0.77 +0.17 1.11 ± 0.06 0.58 .i 0.04 T5 d 1.08±().()2 0.085 + 0.()()7 1.22 ± 0.13 1.()4 ±O.JO T6 d 1.08±0.03 0.077 + 0.004 s 1.20 ± 0.15 0.98 ±O.JI than 1. This confirms the role of contamination of sampling points T2 and T4 in the Tinto rivcr. Both ratios decreasc quickly along thc Tinto channel, which rcllccts fast Th association with thc solid matter. Thc 2'ºTh/ 212Th activity ratio is always higher than thosc found in normal aqueous systems. This is due to the presencc of 234U in solution. Howcvcr, thc value of this activity ratio at stations T2 and T4 rcveals that sorne material, rich in 2·'2Th, is being incorporated into thc river. The origin of such material is clearly thc phosphogypsum piles. Indced, the 228Th/ mTh activ ity ratio is very close to I, except for stations T2 and T4, where an important defect of 228Th appears. The rcason for such a defect is thc absence of 228Ra in solution. This is due to the fact that Ra in phosphogyp sum is very insoluble. Therefore, the material trans ported into the river by rivulcts at stations T2 and T4 must be very poor in Ra isotopes. Again the ratios obtained for the suspended frac tion confirm the already commented conclusions. As in the Odiel river, thc 234U / 238U is closc to unity along the Tinto river channel. The Th/U mass ratio for suspended particles reveals lJ enrichment of the partí eles ali along the channel, since it is on average below unity, very different to the data in the curren! litera ture [ 12, 13]. The exccss of 23ºTh observed from the 23ºTh/ 232Th activity ratio, not usual in other fluvial systems (4,15], supports such a conclusion. Finally, val ucs much below unity for thc 2:11,Th/ 234U activity ratio ncar stations T3 and T4, as found at stations 05 and 06 of the Odiel rivcr, also suggest that U-enriched material is being incorporated into thc river. The rest of the stations havc values of this activity ratio similar to those found in the current literature (14]. The 228Th/ mTh activity ratio is, in general, compatible with thc cxistcncc of secular c4uilibrium betwcen both isotopes. They, in fact, agree wcll with thc valucs in the curren! litcraturc (4,15]. 2.11,Th/ n2Th 2.67 :t 0.09 2.4 ±0.4 1.90 ± 0.21 12.I i 2.7 7.2 +0.8 8.3 + 1.0 1.90 ± 0.21 10.0 + 1.4 7.1 .i 1.3 9.4 ±2.U 6.03±0.61 6.4 + 1.4 22HTh/ 2'2Th 3.60±0.12 1.4 ±0.5 0.27±0.05 0.79± U.JO 1.27 ±0.22 0.91 ±0.16 0.10±0.03 1.67 + 0.30 0.81 ±0.28 1.09 ± 0.17 1.50 ± 0.76 Th/U 0.52 ±0.03 1.15 ±0.23 3.7 ± 1.1 0.209 ± 0.049 0.050 ± 0.006 0.266 ± 0.034 1.38 ±0.32 0.198 ± 0.031 0.040 ± 0.()()7 0.405 ± 0.090 0.()42 t 0.()()4 0.56 ±0.13 We do not mcntion here thc case of station TI. sincc the comments are thc samc as thosc madc for station 01 of the Odicl rivcr. Acknowledgements We are very gratcful to Drs. J.M. Abril, G. Manjcín and Mr. Díaz for hclping with the sampling and Mrs. Brcnda Boardman of AEA Technology for producing thc final manuscript. References [ 11 A. Martínez-Aguirrc and M. García-León, J. Radioanal. Nucl. Chem., Lett. 155 (1991) 97. [2] A. Martíncz-Aguirrc, M. García-León and M. lvanovich, to be published in J. Environ. Radioact. [JJ A. Martínez-Aguirre, M. García-León and M. Ivanovich. to be puhlished in J. Environ. Radioact. [4] Y. Miyake, Y. Sugimura and T. Yasujima, Meteoro!. Geophys. 24 (1972) 67. [5] P. Santschi, Y.H. Li and J. Bcll, Earth Planct. Sci. Lctt. 45 (1979) 201. [6] A. Martíncz-Aguirrc. Ph.D. Thcsis. Univcrsity of Sevilla, Spain. 1991 (in Spanish). [7] A. Martínez-Aguirre and M. García-León, Radial. Pro tect. Dosim. 45 (1992) 249. [8] A.J. Plater, M. Ivanovich and R.E. Dugdale. Appl. Geochem. 7 (1992) 101. [9] M.R. Scott, in: Uranium Series Disequilibrium: Applica· tion to Environmental Problems, eds. Ivanovich and Har mon (Clarcndon Press, Oxford, 1982) chap. 8. [ IOJ A. Martíncz-Aguirre and M. García-León, to be pub lished in J. Radioanal. Nucl. Chem. Art. [ 11 J J .K. Osmond and J .B. Cowart, Geol. Soc. Am. Abstr. Prog. 8 (1976) 823. [12] J.M. Martin and J.M. Whitficld, in: Trace Mctals in Sea Water, eds. Wong, Boyle, Bruland, Burton and Goldberg. NATO Conf. Ser., Series IV, Marine Sciences 9 (Plenum, New York, 1983) p. 265. [13] R. wfvendahl, unpublished data, 1991. [14) M. Ivanovich and R.S. Harmond, (eds.), Uranium Series Disequilibrium: Applications to Environmental Problcms (Clarendon Press, Oxford. 1982). [15) J.M. Martin, V.N. Nijampurkar and F. Salvadori, in: Biogeochemistry of Estuarine Sediments, ed. Goldberg (UNESCO, Paris, 1978) p. 111.