different lead sources in an abandoned uranium mine ......rui m.p. santos1,* & colombo c. g....

Geochemistry: Exploration, Environment, Analysis, Vol. 12, 2012, pp. 241 –252 DOI: 10.1144/1467-7873/11-RA-076

1467-7873/12/$15.00 © 2012 AAG/Geological Society of London

Lead is considered one of the most common contaminants of ecosystems. However, the metal concentration by itself does not provide enough evidence of its origin, since natural processes can equally induce the emergence of local high Pb concentra-tions. Correct identification of Pb sources and paths in the envi-ronment is a requirement for designing any corrective action for land recovery and rehabilitation. The high potential of Pb iso-topes to act as fingerprints has been recognized for some dec-ades (Chow & Johnstone 1965). In fact, Pb shows isotopic ratios significantly different according to its natural or anthropogenic provenance since the abundance of different Pb isotopes results from the radioactive decay of isotopic 238U, 235U and 232Th to 206Pb, 207Pb and 208Pb, respectively, throughout geological time. Non-radiogenic 204Pb has remained constant since the Earth’s formation. Lead isotopic ratios vary according to the geological location, so they can be used as tracers, a tool more valuable than the bulk concentration of this element, since Pb isotopic ratios are not affected by any physical or chemical process in the terrestrial environment (Helland et al. 2002).

Lead isotopic signatures have been applied with undeniable success to the identification of Pb sources in diverse areas. Countless studies using Pb isotopic ratios have been published with many applications, such as atmospheric aerosols (Faure

1986; Sturges & Barrie 1987; Erel et al. 1997; Monna et al. 1997; Mukai et al. 2001; Simonetti et al. 2004), gasoline (Lord III 1994; Hurst 2000; Åberg 2001), natural and saline waters (Miyazaki & Reimer 1993; Halicz et al. 1994; Barrett 1999; Murphy & Hall 2000; Roy & Négrel 2001; Benkhedda et al. 2004; Cheng & Foland 2005), soils (Bjørlykke et al. 1993; Munksgaard & Parry 1998a; Monna et al. 2000a; Probaska et al. 2000;Teutsch et al. 2001; Emmanuel & Erel 2002; Kaste et al. 2003; Ettler et al. 2004; Haack et al. 2004; Moura et al. 2004), sediments (Chiaradia et al. 1997a; Munksgaard et al. 1998b; Kawamura et al. 1999; Monna et al. 1999; Monna et al 2000b; Bindler et al. 2001; Erel et al. 2001; Munksgaard et al. 2003; Ettler et al. 2006; Gioia et al. 2006), digested samples (Cumming & Richards 1975; Wiedenbeck et al. 1995) and inks (Chiaradia et al. (1997a, b); Gulson et al. 1997). These studies are mere examples of the different applications and evidence of the use of Pb isotopic ratios as tracers for environmental purposes. Isotopic studies in abandoned mines areas have been used extensively as an indicator of anthropogenic contribution in many ecosystems, and more particularly, to investigate the impact of mining in the surrounding environment.

The Portuguese National Uranium Enterprise (ENU) has been exploiting uranium at the Urgeiriça mine since 1912 up

Different lead sources in an abandoned uranium mine (Urgeiriça - Central Portugal) and its environment impact – isotopic evidence

Rui M.P. Santos1,* & Colombo C. G. Tassinari21UCTM-Lab LNEG, Rua da Amieiria – Apartado 1089, S. Mamede de Infesta, Portugal

2CPGeo- Instituto de Geociências da USP, Rua do Lago, 562 – Cidade Universitária, São Paulo, Brazil*Corresponding author (e-mail: [email protected])

AbSTRACT: Different lead sources were identified in a large uranium tailings deposit (5Mton) in the Central Region of Portugal using lead isotopic ratios obtained by ICP-QMS. These ratios helped to clarify the different sources of Pb within the tailings deposit and the impact of the tailings on the surroundings. Ten depth profiles were used for isotopic characterization of the tailings deposit; the lead background signa-ture was evaluated in seven regional rocks (granites) and was defined as being 28 ± 1 mg kg-1 for Pb bulk concentration and with isotopic ratios of 1.264(2) for 206Pb/207Pb and 1.962(7) for 208Pb/206Pb. In order to understand Pb isotope distribution within the tailings deposit, simple mixing/mass balance models were used to fit experimen-tal data, involving: (1) the background component; (2) uranium ores (pitchblende) characterized by the ratios 206Pb/207Pb of 1.914(3) and 208Pb/206Pb of 1.235(2); and (3) an unknown Pb source (named ‘Fonte 5’) characterized by the ratios 206Pb/207Pb of 3.079(7) and 208Pb/206Pb of 0.715(1). This unknown source showed high radio-genic ratios found in the water of some tailings depth profiles located in a very spe-cific position in the dump. In terms of isotopic characterization, 69% of the deposit material resulted from the background source, 25% from uranium minerals and only 6% from other uranium mines in the region. Finally, the environment impact revealed that the pollution was focused only in the beginning of the stream and not in the sur-roundings, nor in the groundwater system. The lead in the water was found only in colloidal form with a clear pitchblende signature. Those data revealed possible remo-bilization phenomena along the bedside and margins of the watercourse.

KeywoRDS: lead, isotope, source, uranium mine

research-articleResearch article12X10.1144/1467-7873/11-RA-076R. M. P. Santos & C. C. G. TassinariLead isotope ratios by ICP-MS in abandoned uranium mine2012 by Michael David Campbell on December 4, 2018http://geea.lyellcollection.org/Downloaded from

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R. M. P. Santos & C. C. G. Tassinari242

until the late nineties together with other mines in the region, such as Quinta do Bispo and Cunha Baixa. All chemical treat-ments were processed in the Urgeiriça Chemical Plant. As a result of the mining activities, about five Mton of residues are currently deposited in tailings deposits next to the old mines. These geological waste materials frequently have significant contents of dangerous chemical products, as well as radio-active elements, which are exposed to external geodynamic processes that promote the transfer of these elements to dif-ferent environmental compartments. Therefore, this area is considered an important pollution focus where we believe that new analytical strategies such as Pb isotope characterization will be able to provide new, accurate and useful information to complement earlier studies. Lead isotopes will be used as trac-ers to investigate the different Pb sources in the tailings deposit, but also to clarify the impact of the tailings deposit on the surrounding environment.

GeoloGiCAl SeTTinGThe abandoned Urgeiriça mine is located in the central part of mainland Portugal (Fig. 1), close to the city of Nelas. The region comprises granites and other rocks and is part of the so-called ‘Beiras Uraniferous Region’ which involves an area of c. 10 000 km2 in the Central Iberian Zone. This flattened region with an average height of 400 m rises slightly to the east and is sur-rounded by the Mondego and Dão river valleys and belongs to the Iberian Meseta. The uranium-bearing ore of Urgeiriça is of vein-type, striking N60ºE with variable dip, between 75º and 90º; besides the major mineralogy, composed of siliceous min-erals, it contains also uraninite, pyrite, fluorite, sphalerite, native Pb sulphide, chalcopyrite, calcite and several secondary ura-nium minerals (Neiva 1968). It cuts a Variscan porphyritic medium to coarse-grained biotite granite which is the dominant rock type observed in the area, and was emplaced during the last ductile deformation phase D3 (320–310 Ma; Azevedo & Aguado 2006). The granite outcropping in the region of Urgeiriça presents average contents of U varying between 8 and 10 mg kg-1. All granites are intersected by a set of faults that control the drainage pattern included in the Mondego

watershed; the main watercourse that drains the mining area has the local designation of ‘Ribª. da Pantanha’, and the corre-sponding watershed can be delimited as illustrated in Figure 2.

MeThoDSinstrumentationThe main data of this study were obtained with an ICP-QMS instrument PQExCell (VG Elemental) from INETI - Geosciences Laboratory at Oporto, Portugal. TIMS data were obtained using a MAT262 (Thermo Finnigan) at the CPGeo - University of São Paulo, Brasil and it was only used to validate the ICP-QMS Pb isotope methodology previously developed (Santos et al. 2007). The ICP-QMS equipment is installed in a clean (Class 100-1000) and temperature-controlled room. The solutions were introduced via a peristaltic pump (Spetec Perimax 12) with an auto-sampler (50/60 Cetac ASX500). A standard sample introduction system, consisting of a cross-flow nebulizer, a Scott-type double path spray chamber cooled at 4ºC and a Fassel quartz torch, was used. The plasma was ignited 2–3 hours before measurements to ensure system sta-bility according to factory specifications. Tuning conditions were properly adjusted in order to obtain the maximum stabil-ity (%RSD ≤ 0.5) and sensitivity for the four Pb isotopes. Instrumental conditions were kept untouched during the entire analytical run. A mass calibration was performed in every daily run, in order to ensure an error better than ≤ 0.010 amu for all Pb isotopes (Santos et al. 2007).

Study area and samplingFigure 2 shows a map of the mining site where the tailing deposit is quite evident (detail c). Ores exploited in Urgeiriça, or transported from other mines of the region, were ground to fine-sized particles in order to allow the leaching processes for uranium recovery. The tailings dump is therefore composed of very fine sand (<1 mm) rejected as a result of that treatment. Historical records also indicate that a few tons of high-grade uranium ore, not yet milled, are still stored nearby the mine facility (Pereira et al. 2004).

Fig. 1. Location of Urgeiriça Mine in the Iberian geological context according to Julivert et al. (1974), modified by Farias et al. (1987) and Pereira (1987).

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Lead isotope ratios by ICP-MS in abandoned uranium mine 243

Figure 2c shows the location of the 10 tailings cores drilled by the Enterprise (ENU) up to the bedrock, for water moni-toring purpose in the past. Our team was allowed to collect solid samples (between 0 and 25 m depth) on drill remains and water samples from each core hole.

For background characterization (‘geogenic’ Pb) 3 kg of seven regional rocks (granites) were sampled (Fig. 2b). For ura-nium minerals (pitchblende, autunite, torbernite and other sec-ondary minerals) and galena characterization, c. 500 mg of sample were obtained from the LNEG Laboratory Museum in São Mamede de Infesta, where the Urgeiriça Mine is well rep-resented. All the solid samples were finely ground in an agate mortar to <200 mesh.

For environmental purposes, 16 well water samples with depths of between 5 and 100 m were collected in the ground-water system around the tailings deposit and along the main watercourse (Rib.ª da Pantanha) (Fig. 2b).

Samples and standards preparationSolid sample treatmentA tri-acid attack in a Teflon reactor was used to dissolve the solid samples. Five ml of HNO3 (Merck, 60%, ultrapure) and 2 ml of concentrated HF (Riedel-de-Haën, 48% p.a. plus) were added to each 100 mg of sample (previously dried at 105ºC). The Teflon reactor was closed and then heated at 170ºC during 24 hours in a sand bath. After this period, the acid solution was evaporated to dryness at 80ºC. Then, 3 ml of HCl (Merck, 30%, ultrapure) were added; again, the Teflon vessel was closed and placed during a second period of 24 hours in a sand bath at 150ºC. After this second period of time, it was evaporated to dryness at 80ºC. The residue was taken up in 1 ml of HCl (Merck, 30%, ultrapure) and 1 ml of HNO3 (Merck, 60%, ultrapure) and quantitatively transferred

with a polypropylene pipette and made up to 50.0 ml with ultrapure water.

Water samples treatmentAll water samples were collected in 1.5 l PET bottles for metal concentration analyses and Pb isotope determination. These samples were filtered through a membrane (hydrophilic poly-ether sulfone) of 0.1 µm porosity (Acrodisc filters from Pall Corporation) and preserved with HNO3 (Merck, 60%, ultrapure) at 2% (v/v). The filtered water cores deposits (col-loidal phases) were preserved for subsequent Pb metal and iso-topic ratios analyses.

Lead isotope standard preparationA 25 mg l-1 standard solution of certified Pb isotopic ratios was also prepared from NIST NBS 981 isotope Pb standard using ultrapure HNO3 obtained by double sub-boiling distilla-tion in a Duopor system (Milestone) and deionized water pro-duced by a Milli-Q Elemental system (Millipore), with resistivity better than 18 MΩ cm. A uranium mono-element standard 1000 mg l-1 from Alfa Aesar was used as stock solu-tion to keep a constant U concentration in samples and Pb isotope standards.

All the solutions were volumetrically prepared using Eppendorf fixed volume micropipettes. These micropipettes were calibrated every 6 months and internally verified between calibrations. The uncertainty of preparation (taking in account NBS 981 purity, balance uncertainty, pipettes uncertainty and volumetric flasks tolerance) is 0.044 µg l-1 for 25 µg l-1 standard Pb solutions. As an example, the 50 µl micropipette that has been used presents a repeatability deviation of 0.4%. So, the highest variability expected for the 25 µg l-1 lead Pb concentra-tion is 0.1 µg l-1. All the chemical analyses were carried out in a

Fig. 2. Location of the study area: (a) Urgeiriça Mine from Central Region of Portugal; (b) Detail of the study area showing all collected samples; (c) Detail of 10 core drills in the tailings deposit.For background characterization (Pb geogenic) seven regional rocks (granites) were geologically surveyed and sampled for 3 kg sample each. For U minerals (pitchblende, autunite, torbernite and other secondary minerals) and galena characterization, about 500 mg of sample were collected from the LNEG Laboratory Museum in São Mamede de Infesta, where Urgeiriça Mine is well represented. All samples were finely ground in an agate mortar to c. 100% < 200-mesh.




class 100-1000 clean laboratory according to the following analytical procedures.

Analytical proceduresLead and U concentration and Pb isotopic composition were determined in all the collected samples. For Pb isotope deter-mination, all of the solid samples were diluted in PFA volu-metric flasks to reach a final Pb concentration of 25 mg l-1, all the other samples below 25 mg l-1 were analyzed without fur-ther dilution. Before filling the flasks to the mark with HNO3 2% (v/v), appropriate volumes of U stock solution were intro-duced in order to keep a constant U concentration between samples and Pb isotope standard. For this purpose, samples and standards were grouped into three different batches according to the samples’ initial U concentrations (0.5, 5 and 27 mg l-1).

Uranium matrix effect, or uranium mass bias, was calcu-lated for each pair of isotopes 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 206Pb/207Pb and 208Pb/206Pb and the interpolated measured ratios were compared with the certified values. Uranium mass bias corrections were individually carried out for each Pb isotopic ratio through the specific mass bias inter-polated factor, defined according to the procedure previously developed (Santos et al. 2007). The mass discrimination factor was evaluated at regular intervals in the working day and par-ticularly before, during and after the measurement of real sam-ples, according to the exemplified isotopic analyses sequence: blank; NBS 981(1); Sample 1; NBS 981(2); Sample 2; NBS 981(3);…etc. In the previous work (Santos et al. 2007), we found mass bias deviation (MB(%) = 100*((Rcert./Rmeas) -1)/∆m) close to 2% for 206Pb/204Pb and 207Pb/204Pb, 0.9% for 208Pb/204Pb, -2.3% for 206Pb/207Pb and -0.3% for 208Pb/206Pb for the NIST SRM 981 isotope Pb standard with 5000 µg l-1 of U added.

Blocks of 10 measurements have been used to obtain data for all standards, blanks and samples. The isobaric interference of 204Hg on 204Pb was corrected for by monitoring 202Hg and correcting the Hg contribution using a 204Hg/202Hg ratio of 0.23. The blank Pb content was always subtracted in all ana-lytical solutions. The mass bias correction was accomplished by external standardization, whereas the concerned isotopic ratios were measured in NIST NBS 981 standard solutions matched according to sample matrices and analysed just before and after the sample that was being corrected.

ReSUlTSlead isotope composition of regional rocksThe natural Pb sources in the study area were the geological background, reported as the average of the seven granite sam-ples collected in the area (Table 1), and different kinds of natu-ral uranium ores: (1) pitchblende or uraninite (UO2 - the main uranium ore); (2) autunite (Ca(UO2)2(PO4)2,10-12(H2O)) and torbernite (Cu(UO2)2(PO4)2,8-12(H2O)); (3) other minerals designated as secondary that belong to the Urgeiriça paragen-esis; and, finally, (4) galena (PbS).

According to the values found in the background gran-ites, we can conclude that Pb isotopic ratios are very simi-lar; however, two granites (GR1 and GR5) were slightly more radiogenic. The reported Pb isotopic ratios for the background are 206Pb/204Pb = 19.95(6); 207Pb/204Pb = 15.77(5); 208Pb/204Pb = 39.06(17); 206Pb/207Pb = 1.264(2) and 208Pb/206Pb = 1.962(7).

As expected, extremely radiogenic ratios for autunite and tor-bernite with 206Pb/204Pb = 9266(268) and 206Pb/204Pb = 10372

(182), respectively, were obtained. These uranium minerals, together with pitchblende, were recovered by chemical process-ing during the industrial activity. Although the industrial plant met a requirement of maximum recovery of these ores, some losses, inevitably left in the residues, could contribute to pollu-tion of the surrounding environment (Fig. 3).

Tailings deposit profilesFigure 3 illustrates Pb concentrations versus 206Pb/207Pb for several layers of the 10 depth profiles (cores-Pz) in the tailings deposit, according to the values obtained in Table 1. The ratio 206Pb/207Pb was chosen since it reveals more information about the uragenic Pb present in this type of deposit.Data illustrated in Figure 3 show three dif-ferent situations between Pb concentrations and values of 206Pb/207Pb: (1) a homogeneous behaviour in cores Pz2, Pz6 and Pz8; (2) an inverse relationship in cores Pz1, Pz3, Pz7 and Pz10; and (3) a direct relationship in cores Pz4, Pz5 and Pz9. In this last case, whenever Pb concentration increases (reaching values of 682 to 1069 mg l-1 for cores Pz5 and Pz4, respectively) the 206Pb/207Pb ratio always fol-lows this increase, reaching radiogenic values, particularly in cores Pz4 and Pz5, between 15 and 20 m height (like a ’sandwich section’). These observations suggest the involvement of another radiogenic source.

In all cores Pb concentrations and the corresponding 206Pb/207Pb ratios were very different from those found in the background (average of seven granites), a fact that supports the existence of possible mixtures with other radiogenic sources in the region.

As reported, in some of the profiles (Pz1, Pz3, Pz4, Pz5, Pz7 and Pz10) it was possible to collect water inside the cores for Pb isotope determination (Table 2), including the corre-sponding suspended material (SM) (Table 3). In the isotope diagram (Fig. 4), the different materials of these profiles reveal valuable information. It is true that profiles Pz1, Pz3, Pz7 and Pz10 show a small dispersion between water, suspension and sections’ materials (Fig.4C). However, two exceptions occur in the Pz7 and Pz10 waters since they have more radiogenic ratios than the solid material. Such facts are not surprising since it was likely that pluvial waters could percolate through different circuits on distinct zones of the respective profiles and could incorporate different fingerprints or even different source mixtures.

Other profiles, such as Pz4 and Pz5, show a huge disper-sion between their section materials (Fig.4B) but mainly between their waters and suspension material (Fig.4A). These particular cases support the fact that another Pb source could have been deposited on the tailings deposit.

In almost all the profiles, their water and the respective sus-pension materials reflect isotopic ratios similar to those found in other profiles sections (e.g. Pz1, Pz3, Pz7 and Pz10). By contrast, in Pz4 and Pz5, the corresponding suspended mate-rial in water results from another layer with greater solubility (e.g.: Pz4) or from another source through which water had circulated (e.g.: Pz5).

There is no doubt that Pb isotopic ratio information of water profiles are an important tool to identify the presence of other Pb sources in the tailings deposit, since it percolates through different layers. Without those data it is impossible to identify other radiogenic material existing in the tailings deposit. Accordingly, the presence of another unknown radio-genic material is certain. This new Pb source was designated as ‘’Fonte 5 and has the most radiogenic ratios found in the tailing deposit (Pz5 Water).




Table 1. Concentration and isotopic composition of Pb in solid samples

SOLID SAMPLES Depth/m [Pb]a/mg kg-1 [U]b/mg kg-1 206Pb/204Pbc 207Pb/204Pbc 208Pb/204Pbc 206Pb/207Pbc 208Pb/206Pbc

GranitesGR1 – 24.5 <51 21,59(7) 15,89(6) 39,54(19) 1,359(3) 1,834(7)GR2 – 24.7 <51 19,26(5) 15,75(7) 39,47(21) 1,223(3) 2,051(11)GR3 – 24.0 <51 19,16(3) 15,70(4) 39,06(14) 1,219(2) 2,040(7)GR4 – 35.2 <51 19,44(7) 15,79(5) 38,62(17) 1,231(2) 1,988(7)GR5 – 28.9 <51 20,81(5) 15,84(3) 38,79(14) 1,314(2) 1,863(6)GR6 – 27.6 <51 19,53(6) 15,70(5) 38,84(21) 1,243(2) 1,987(7)GR7 – 30.3 <51 19,84(6) 15,75(6) 39,14(13) 1,259(3) 1,973(3)Mean (Granites) – 27.9 <51 19,95(6) 15,77(5) 39,06(17) 1,264(2) 1,962(7)

MineralsSecondary – 3.8E+03 7.8E+04 107,07(22) 20,18(6) 38,53(10) 5,304(10) 0,360(1)Torbernite – 1.4E+02 1.9E+05 10372(182) 523(9) 41(1) 19,816(64) 0,0040(1)Autunite – 1.3E+03 3.3E+05 9266(268) 493(14) 56(2) 18,773(26) 0,0060(1)Pitchblende – 1.1E+04 1.3E+05 31,12(6) 16,26(4) 38,43(10) 1,914(3) 1,235(2)Galena – 2.0E+05 7.6E+03 18,41(4) 15,65(4) 38,36(8) 1,176(3) 2,084(5)

Tailing cores - ProfilePz1 - 1/1 18.1 110 214 23,53(8) 15,87(5) 38,66(12) 1,482(2) 1,643(4)Pz1 - 1/2 9.6 127 257 22,43(8) 15,86(4) 38,66(14) 1,414(4) 1,722(5)Pz1 - 1/3 5 141 167 21,53(11) 15,77(5) 38,61(21) 1,365(5) 1,795(5)Pz2 - 2/1 19.9 76.5 124 21,38(8) 15,80(3) 38,72(17) 1,353(5) 1,813(6)Pz2 - 2/2 11.1 84.6 92.6 21,26(7) 15,83(3) 38,77(14) 1,342(4) 1,826(6)Pz2 - 2/3 3.2 65.6 76.1 21,16(5) 15,82(5) 38,86(15) 1,338(4) 1,838(4)Pz3 - 3/1 12.6 168 266 22,70(10) 15,79(7) 38,50(19) 1,437(5) 1,695(7)Pz3 - 3/2 7.4 120 329 23,71(5) 15,98(3) 38,87(15) 1,484(2) 1,639(6)Pz3 - 3/3 2.6 139 235 22,71(7) 15,80(2) 38,61(22) 1,437(4) 1,700(8)Pz4 - 4/1 25.7 133 143 23,70(9) 15,84(6) 38,88(24) 1,496(4) 1,640(5)Pz4 - 4/2 20.9 129 213 25,98(6) 16,04(4) 39,28(9) 1,619(4) 1,512(3)Pz4 - 4/3 15.7 138 177 37,51(17) 16,66(8) 39,37(24) 2,251(6) 1,049(3)Pz4 - 4/4 11.7 270 212 33,42(8) 16,42(4) 38,76(8) 2,034(3) 1,160(2)Pz4 - 4/5 6.9 292 181 39,45(13) 16,79(6) 39,03(19) 2,350(8) 0,991(4)Pz4 - 4/6 2.1 1.1E+03 209 24,86(8) 15,92(3) 38,37(30) 1,561(4) 1,545(12)Pz 5 - 5/1 25.5 183 106 22,91(11) 15,89(7) 39,12(22) 1,442(3) 1,706(7)Pz 5 - 5/2 20.7 197 172 23,22(7) 15,91(4) 39,22(20) 1,458(5) 1,687(8)Pz 5 - 5/3 16.9 142 137 34,72(20) 16,49(8) 39,21(25) 2,105(4) 1,130(4)Pz 5 - 5/4 12.3 216 150 32,90(13) 16,42(7) 39,04(16) 2,003(5) 1,186(4)Pz 5 - 5/5 9.1 281 189 30,78(8) 16,29(5) 38,92(10) 1,888(4) 1,264(3)Pz 5 - 5/6 4.3 682 232 26,33(12) 16,03(4) 38,38(40) 1,640(7) 1,458(10)Pz6 - 6/1 18.2 66.3 139 23,15(8) 15,87(5) 38,65(13) 1,459(2) 1,671(3)Pz6 - 6/2 13.4 80.5 113 22,02(7) 15,85(4) 38,78(15) 1,390(4) 1,762(5)Pz6 - 6/3 7.9 83.4 77.9 22,56(7) 15,90(4) 38,81(12) 1,419(2) 1,723(5)Pz6 - 6/4 1.6 89.3 80.9 23,13(9) 15,85(5) 38,60(13) 1,459(4) 1,669(4)Pz7 - 7/1 18.5 95.4 86.4 21,91(4) 15,77(4) 38,62(10) 1,388(2) 1,763(3)Pz7 - 7/2 13.7 109 102 21,56(7) 15,81(4) 38,71(17) 1,363(3) 1,791(5)Pz7 - 7/3 8.9 93.7 101 21,53(6) 15,83(4) 39,02(17) 1,359(3) 1,810(6)Pz7 - 7/4 4.1 543 111 20,08(9) 15,67(6) 38,27(26) 1,280(5) 1,905(7)Pz8 - 8/1 21 229 241 22,01(6) 15,78(6) 38,58(15) 1,394(3) 1,754(7)Pz8 - 8/2 15.4 165 208 22,52(9) 15,83(7) 38,57(27) 1,423(3) 1,713(8)Pz8 - 8/3 11.1 150 202 23,10(9) 15,83(6) 38,68(19) 1,459(3) 1,674(6)Pz8 - 8/4 7.9 153 176 22,21(7) 15,76(4) 38,61(23) 1,408(4) 1,737(12)Pz8 - 8/5 1.7 161 172 23,19(10) 15,88(4) 38,81(18) 1,460(4) 1,672(7)Pz9 - 9/1 7.2 143 154 22,74(5) 15,76(4) 38,35(15) 1,442(3) 1,685(6)Pz9 - 9/2 4.2 49.9 1336 21,71(5) 15,75(4) 38,60(9) 1,378(3) 1,778(3)Pz10 - 10/1 3.2 142 261 23,61(11) 15,84(5) 38,57(24) 1,489(5) 1,634(6)Pz10 - 10/2 2.2 39.7 208 25,22(11) 16,01(5) 38,74(13) 1,575(3) 1,536(4)

aLead concentration was determined by ICP-MS with analytical uncertainties (1σ) ≈ 0.6%bUranium concentration was determined by ICP-MS with analytical uncertainties (1σ) ≈ 3.4%cUncertainties in the last digit(s), given in parentheses, are one standard deviation of the mean for in-run statistics (n=10)

DiSCUSSionlead sources identification in the tailings depositAccording to historical records, the mine dump has residues from different uranium mines of the region, the ranges in

isotopic ratios of the different layers provide clear evidence of the different Pb sources deposited there.

The isotope diagram (Fig. 5) illustrates all the identified Pb sources and all the samples collected in the tailings deposit. As depicted in the diagram, three Pb sources (background,




Fig. 3. Illustration of the several tailings profiles according to Pb concentration (mg kg-1) and 206Pb/207Pb vs height.




pitchblende and Fonte 5) are not only necessary but sufficient to explain all the Pb isotopic measurements obtained. The other two sources (secondary and galena) were rejected. In

fact, galena is a Pb mineral that stops its isotopic evolution at the moment that is formed and so its Pb isotopic ratios are less radiogenic than regional granites. The secondary minerals’

Fig. 4. Pb isotope diagram for tailings cores with respective waters and suspension material. (a) Most radiogenic ratios found in water of Pz5, considered as unknown Pb source (Fonte 5); (b) Heterogeneous isotope ratios in Pz4 and Pz5 layers; (c) Homogeneous isotope ratio in Pz1, Pz3, Pz7 and Pz10, except for the Pz7 water, and Pz10 water.

Table 2. Concentration and isotopic composition of Pb in water samples

WATER SAMPLES [Pb]/µg l-1 [U]/µg l-1 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 206Pb/207Pb 208Pb/206Pb

Regional watersB1 0.13 1.0 18,36(5) 15,61(7) 38,27(15) 1,175(3) 2,085(5)B2 0.12 0.30 18,32(2) 15,56(1) 37,74(3) 1,1770(1) 2,0605(1)B3 <0,1 <0,4 18,56(7) 15,66(9) 37,79(21) 1,186(4) 2,038(9)BP24 0.08 1.4 n.d.

wells < 15 metersM1 <1,2 250 n.d.R1 <0,1 91.1 n.d.R2 <0,2 124 n.d.R3 <0,1 82.1 n.d.P2 0.05 5.1 n.d. 1,293(5) 1,816(1)P3 0.06 38.9 n.d. P4 <0,05 0.6 n.d. P5 0.10 3.6 17,85(9) 15,45(8) 37,52(19) 1,1543(5) 2,1025(7)P6 <0,05 <0,2 n.d. P7 0.15 2.0 18,64(7) 15,69(6) 38,09(14) 1,1880(4) 2,0439(4)P8 <0,05 0.29 n.d. P9 0.06 0.48 n.d.

wells > 15 metersF2 0.84 422 20,69(6) 15,76(4) 38,63(9) 1,312(3) 1,867(4)F3 <0,05 5.8 n.d. F10 0.73 7.3 17,59(1) 15,55(1) 37,29(3) 1,1309(1) 2,1204(2)F21 0.36 5.4 18,28(1) 15,56(1) 37,87(2) 1,1744(1) 2,0715(8)

water coresPz1 Water 17.2 2.1E+04 22,07(14) 16,01(19) 38,99(45) 1,377(13) 1,769(22)Pz 3 Water 31.3 2.7E+04 23,32(7) 15,83(6) 38,52(23) 1,472(4) 1,652(8)Pz4 Water 46.5 536 46,04(21) 16,96(6) 38,70(19) 2,714(6) 0,841(2)Pz5 Water 61.4 1.2E+03 53,83(18) 17,48(7) 38,52(16) 3,079(7) 0,715(1)Pz7 Water 5.6 4.3E+03 30,70(42) 17,05(43) 41,41(66) 1,762(36) 1,365(26)Pz10 Water 5.5 31.8 34(2) 17,48(83) 42(2) 1,968(59) 1,192(25)

n.d.: Not Determined; Pb concentration is too low or below the quantification limit




Fig. 5. 206Pb/207Pb vs 1/[Pb] for all the tailings samples including all the Pb sources: (1) background; (2) secondary; (3) pitchblende; (4) galena and (5) Fonte 5 (Pz5 water).

Table 3. Concentration and isotopic composition of Pb in suspension material

SUSPENSION MATERIAL (SM) [Pb]/mg kg-1 [U]/mg kg-1 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 206Pb/207Pb 208Pb/206Pb

Regional watersB1 12.1 30.3 19,47(10) 15,69(7) 38,27(16) 1,241(4) 1,965(6)B2 5.7 20.5 20,50(10) 15,89(6) 38,60(16) 1,291(5) 1,879(3)B3 1.4 1.8 18,871(3) 15,602(3) 38,10(1) 1,20950(2) 2,01866(3)BP24 24.3 25.4 19,36(7) 15,71(5) 38,31(9) 1,232(3) 1,978(6)

wells < 15 MetersM1 1.4 231.3 19,48(3) 15,64(2) 38,11(6) 1,2450(1) 1,9561(2)R1 5.6 9.2E+03 20,27(9) 15,93(8) 38,77(26) 1,271(5) 1,915(10)R2 31.5 1.2E+04 20,98(8) 15,73(5) 38,20(18) 1,333(4) 1,821(5)R3 39.5 2.8E+03 21,97(10) 15,82(5) 38,35(20) 1,389(4) 1,745(6)P2 34.2 378 21,60(8) 15,87(4) 38,56(21) 1,361(4) 1,784(7)P3 <0,4 32.9 19,11(3) 15,64(2) 38,09(5) 1,2224(1) 1,9928(2)P4 2.7 3.3 19,19(10) 15,88(6) 38,70(14) 1,207(4) 2,021(5)P5 50.9 138 17,79(6) 15,53(7) 37,63(13) 1,145(2) 2,115(4)P6 122 8.9 18,14(7) 15,66(5) 38,18(16) 1,158(5) 2,106(8)P7 22.1 25.9 18,58(6) 15,70(3) 38,20(12) 1,183(4) 2,056(5)P8 3.2 5.0 19,28(7) 15,88(8) 38,67(18) 1,217(6) 2,001(8)P9 7.9 3.9 18,54(7) 15,69(6) 38,23(15) 1,181(3) 2,062(7)

wells > 15 metersF2 16.8 358 18,55(7) 15,65(5) 37,79(22) 1,186(4) 2,042(9)F3 11.5 54.5 19,13(7) 15,76(8) 38,21(10) 1,214(4) 1,997(6)F10 245 65.9 17,53(5) 15,56(3) 37,26(9) 1,127(4) 2,125(6)F21 3.7 2.3 19,33(17) 15,82(10) 38,22(33) 1,223(6) 1,982(9)

Cores deposit (Suspension Material)PZ 1 SM 338 354 22,75(6) 15,88(5) 38,67(22) 1,433(5) 1,698(6)PZ 3 SM 1.3E+03 450 23,21(7) 15,89(7) 38,56(15) 1,460(7) 1,662(4)PZ 4 SM 199 344 37,55(7) 16,64(6) 39,14(8) 2,256(5) 1,042(2)PZ 5 SM 385 465 48,44(9) 17,30(5) 38,91(6) 2,799(6) 0,803(2)PZ 7 SM 87 277 22,12(8) 15,85(4) 38,65(16) 1,395(2) 1,749(6)PZ 10 SM 155 552 23,34(11) 15,85(7) 38,89(26) 1,473(3) 1,665(7)

source was rejected too, because these samples do not have a higher radiogenic behaviour than that of Fonte 5.

Close examination of the isotope diagram (Fig. 6) reveals that all the tailings deposit samples resulted from a mixture between two end-members of the three selected Pb sources: background – pitchblende; background – Fonte 5 or pitchblende – Fonte 5.

According to the position in the isotope diagram, some profiles 4/3, 4/4, 4/5 of Pz4 core and 5/3, 5/4 from Pz5 core, could result from a probable mixture between pitchblende and Fonte 5, or alternatively, between background and Fonte 5. For the rest of the profiles it seems that a mixture between pitchblende and background could explain the obtained isotopic ratios.




binary mixing modelAfter the identification of the probable Pb sources, the data can be basically treated as a two-component mixture following the conventional law (Faure 1986) to calculate the fractional contribution of each source to the Pb in the various profiles.

Consider a profile sample having Pb of two isotopically dif-ferent signatures with measured Pb isotopic ratios of 206Pb/207Pb and 206Pb/204Pb, as an example. Let F1 and F2 be the fractions of the respective Pb Type 1 and 2 that have pro-duced the measured Pb isotopic composition. The following algebraic mass balance constraints apply:

The isotopic ratios chosen for Pb Type 1 and 2 are groups of two of the three Pb sources selected. According to the

isotopic data and the Pb concentration found, we calculate the corresponding Pb source percentage in each profile.

Some curious considerations were found in the different layers of the tailings deposit (Fig. 7), as follows: (1) the major-ity of the profiles were composed of natural Pb (background); (2) in almost all profiles pitchblende was present, since it was the main uranium mineral exploited; (3) Pz4 and Pz5 revealed the presence of another, and more radiogenic, Pb source (Fonte 5), to be more precise, between 2.1–15.7 and 9.1–16.9 m, respectively; and (4) although the former industrial processing plant was optimized for uranium recov-ery, some layers were highly enriched in pitchblende (e.g. lay-ers 4.3–9.1 m on Pz5).

Fig. 6. 206Pb/204Pb versus 206Pb/207Pb for all the tailings samples including the Pb sources: background; pitchblende and Fonte 5 (Pz5 water).

Fig. 7. Fraction of Pb (%) attributed to different sources: background; pitchblende and Fonte 5 in all the cores.




The provenience of Fonte 5 could be related to the ores’ residues treatment that came from other uranium mines, or even from different mixtures of ores sent at that time to the industrial Urgeiriça processing plant.

From a global point of view, Figure 8 shows that 69 % of the 5 Mton in the tailings deposit were constituted by granites (background); 25 % of the rejects derived from the main ura-nium mineral (pitchblende) and 6 % came from from the other radiogenic Pb source (Fonte 5).

impact on the water environmentIn order to assess the impact on the water environment by the tailings deposit, different kinds of water samples were col-lected: (1) 4 wells up to 60 m in depth from the underground water system; (2) 8 small wells, up to 15 m in depth for charac-terization of sub-superficial waters; (3) 4 collected water points in the full riverbed (Rib.ª da Pantanha) for the evaluation of the quality of surface waters; and (4) four regional waters from the hydrographic basin where the polluting focus was located. In a

similar way to the Pb source identification in the tailings deposit, the isotopic data (206Pb/204Pb and 206Pb/207Pb) of col-loidal and dissolved Pb (Table 2 and 3) were compared to lithological background and pitchblende.

Figure 9 suggests that most Pb isotopic ratios associated with the water samples (dissolved and colloidal phases) resulted from a mixture of the selected two sources (background and pitchblende). However, it should be noted that various Pb iso-topic ratios show values below the defined region background (mean of the seven granites). This fact could suggest poor rep-resentativeness of the region background definition or, on the contrary, could indicate the presence of another anthropogenic Pb source, possibly related to organic matter decomposition, fertilizers, motor vehicles gaseous emissions, etc.

At the time of this study it was impossible to collect other kinds of samples that could represent other Pb sources. However, we consider that organic matter decomposition or fertilizers could be the most probable anthropogenic Pb sources because sampling sites are very close to the agricultural fields. The impossibility of collecting other possible Pb sources samples were not considered prejudicial to the main purpose of this study since it would only lead to the reformulation of the region background value. So, we decided to select the less radiogenic granite (GR3) as a background value, instead of the seven regional granites average value. We achieved a better contrast between samples by lowering the background limit. But even in this way, some samples (noted with an asterisk) could carry a less radiogenic anthropogenic Pb than GR3.

Despite the serious difficulties experienced during the ana-lytical phase of Pb isotopic ratio determination caused by low Pb concentration found in most water samples, it was still pos-sible to collect a significant isotopic data-set for the water envi-ronment around Urgeiriça’s mine. Table 4 shows the contribution of the two selected Pb sources according to the

Fig. 8. Contribution (%) of different Pb sources in the tailings deposit.

Fig. 9. Isotopic diagram: 206Pb/207Pb vs 206Pb/204Pb of the water samples (all the wells) collected on the tailings surroundings compared to the background and pitchblende Pb sources.

Table 4. Contributions of Pb sources in water samples

WATER SAMPLES

Dissolved Pb Colloidal Pb (SM)

% Background % Pitchblende % Background % Pitchblende

Regional watersB1 100* 0 100 0B2 100* 0 95 5B3 100* 0 100* 0BP24 b b 100 0

wells < 15 metresM1 a a 100 0R1 a a 98 2R2 a a 90 10R3 a a 81 19P2 96 4 85 15P3 b b 100 0P4 a a 100 0P5 100* 0 100* 0P6 a a 100* 0P7 100* 0 100* 0P8 a a 100 0P9 b b 100* 0

wells > 15 metresF2 93 7 100* 0F3 a a 100 0F10 100* 0 100* 0F21 100* 0 100 0

*Presence of another lead anthropogenic sourceaLead below the quantification limitbLead concentration too low for accurate isotopes ratio determination




previous mixing model used previously. Detailed analysis of these data provide the following information: (1) about half the samples analysed (48%) display isotopic signatures reflect-ing the existence of an anthropogenic Pb source which could not be identified (only 7% of the Pb present in samples came uniquely from the region’s background); and (2) the same per-centage of samples carrying Pb from the tailings deposit (mix-ture of background and pitchblende), reaching proportions between 2 and 19% of the main uranium mineral.

Samples P2 and F2 reveal a direct influence of pitchblende estimated at 4 and 7%, respectively, on the dissolved Pb. In all the other samples, dissolved Pb results from the unknown anthropogenic Pb source.

Colloidal Pb isotope analyses reveal contributions of pitch-blende signatures found in samples B2, R1, R2, R3 and P2, reaching 19% in R3. By the fact of being present in a colloidal phase, we assume that this Pb provenance is a result of remo-bilization phenomena that occurred in water/sediment inter-face where the Pb is preferably retained since it forms part of the iron-manganese oxide/hydroxide coprecipitate. Sample P2 reveals the presence of a pitchblende signature in both dis-solved and colloidal Pb, with contributions of 4 and 15%, respectively. Such facts support the theory that an under-ground water system coming from the tailings environs occurs in the surrounding area.

The samples collected along the main watercourse (Rib.ª da Pantanha) were too low in the Pb concentration required for accurate isotopic analyses. In such samples, it was only possi-ble to determine Pb isotopic ratios in the colloidal phase, which clearly marked the influence of pitchblende and revealed possible sediment remobilization phenomena along the bed-side and margins of the watercourse.

ConClUSionSThe present study demonstrates the existence of several Pb sources contributed to the tailings deposit. The granite rocks of the region represent 69% of the Pb in the 5 Mton deposit and correspond to the main geogenic Pb source for the tailings deposit. About 25% of the Pb in the tailings is sourced from the extraction of the main uranium mineral (pitchblende) exploited. Another higher radiogenic Pb source was found in c. 6% of the water cores, which could be related to other ura-nium mines in the region. The different Pb isotope composi-tion of the tailings deposit facilitates the investigation of its environmental impact on the surroundings.

The environmental impact study revealed that 48% of the water samples analysed have different isotopic signatures from the Pb sources identified. This fact can be better explained by the presence of other Pb anthropogenic source(s), which could not be identified at the time of the study.

The dissolved and colloidal Pb with a pitchblende isotopic signature revealed not only the remobilization phenomena along the main watercourse that occurred in the water/sedi-ment interface, but also that the pollution from the tailings deposit was limited to the beginning of the Rib.ª da Pantanha.

The authors want to thank Empresa Nacional de Urânio (ENU) for the permission to access the mining installations and the tailing deposit, to Dr. Guimas (ENU-Canas de Senhorim, Portugal) for his help during sample collection. Special thanks are due to Professors Izabel Ruiz and Kei Sato (Universidade de São Paulo, Brasil) for the support and for making possible the analyses in CPGeo Laboratory. To Prof. Macha-do Leite (LNEG - Lab. de S. Mamede de Infesta, Portugal) our gratitude for his careful review that helped to improve the first manuscript

and to Dr. Maria José do Canto for all her devotion in the scientific discussions during nine years of cooperation. To the Fundação para a Ciência e Tecnologia de Portugal (FCT), for the Ph.D. scholarship of Rui Santos.

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Received 19 December 2011; revised typescript accepted 3 November 2011.



different lead sources in an abandoned uranium mine ......rui m.p. santos1,* & colombo c. g....

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