Evaluation of the radiological hazards from uranium mining and milling wastes (Urgeiriça – Central Portugal)

A.J.S.C. Pereira1, L.J.P.F. Neves1, J.M.M. Dias2, A.B.A. Campos3 & S.V.T. Barbosa4

1 Dpto. Ciências da Terra, Universidade de Coimbra, Largo Marquês de Pombal, 3000-272 Coimbra, Portugal.

E-mail: apereira@dct.uc.pt2 EXMIN consultant – Companhia de Indústria e Serviços Mineiros e Ambientais, S.A.

3 AMBICANAS, L.da, Urb. das Lameiras, Bloco 2 r/c, G, 3525-070 Canas de Senhorim, Portugal 4 EXMIN – Companhia de Indústria e Serviços Mineiros e Ambientais, S.A, Rua do Açucar, 86-C,

1950-010 Lisboa, Portugal.

Abstract. The Urgeiriça uranium mine was the most important exploitation in Portugal. It was an underground mine, mined by conventional techniques until 1973 and, after that, by in-situ leaching to recover low-grade ore; nearby, an uranium mill facility was also build for ore processing. As a consequence of the intense mining activities, that last for almost a century, large amounts of waste was produced, currently deposited in several tailings; the main one consists of the sludge produced by the mill facility. To evaluate the degree and extension of the contamination of the environment, the radionuclides of the U-chain as well as other chemical elements were measured in samples of water (114), stream sediments (9) and soils (22), collected in the mining area and its vicinity. The activity of the radionuclides in the different environmental compartments is highly variable, and can be related with mine workings as well as with natural processes. The modelling of the whole geochemical dataset, by multivariate techniques based on discriminant analysis, validate the empirical allocation of the samples in two groups (uncontaminated and contaminated by mine workings) previously carried out on the basis of geographical, geological, hydrogeological and geochemical criteria. Thus, the areas that show contamination induced by the old mine workings were clearly identified and mapped; moreover, the movement of the underground contaminant plume was also constrained.

1. Introduction The Urgeiriça mine is located in central Portugal and was the most important uranium exploitation of this country. The exploitation began in 1913 and until 1944 the ore was mined for radium extraction. After the II World War the goal of the exploitation changed and afterwards the uranium was the only recovered radioactive substance. Until 1973 the ore was mined by conventional underground mine techniques, through six shafts, to a maximum depth of almost 500m. After the end of the conventional exploitation, in-situ leaching techniques were used, with injection of sulphuric acid, to recover the low-grade ore still present in the abandoned mine galleries; this extraction continued until 1991. The ores of the Urgeiriça mine, as well as from all the other 62 mines exploited in Portugal, were processed in a uranium mill facility build in 1951 nearby Urgeiriça. Sludge from this facility includes the chemical elements and most of the radioisotopes contained in the ores, as well as several other components added by the technological procedures of selective uranium extraction, like sulphuric acid, manganese oxides, ammonia, sodium chloride, and others [1]. The described mine workings, active during almost a century in the Urgeiriça area, left a large amount of waste, deposited today in several places (figure 1). The most voluminous tailing (around 1 390 000 m3), consists of the sludge produced in the mill facility and occupies approximately 13.3 ha (see detailed description in [2]). Sludge was hydro-dynamically transported through conduits to the tailings, which grew by progressive accumulation of this waste. Waste rock mixed with portions of low-grade ore extracted inside Urgeiriça mine accumulates in a deposit located close to the main shaft (Stª Bárbara); this deposit has an estimated volume of 91000 m3 and occupies an area of 1.5 ha. A few tons of high-grade uranium ore, not milled in the plant, is still deposited nearby this facility. Ground water percolates the interior of the old mine and also the nearby surface tailings, where it can interact with the geological materials, promoting the transfer of the chemical and radioactive elements from the solid to the liquid phase. Previous data shows that these mine waters are of acidic nature and

FIG. 1. Geology of the Urgeiriça mine area and location of the tailings deposits as well as of the samples collected to evaluate the related environmental impacts. can transfer significant amounts of dissolved metals and radionuclides to the environment [3]. To prevent this possibility a drainage system collects the contaminated water from the old mine and also from the mill tailing and divert it to a wastewater treatment plant [1]. Here, pH is neutralized through the addiction of burning lime. Barium chloride is also used to induce the precipitation of radium compounds and also to remove other hazardous chemical elements, like heavy metals. Before the discharge in the natural environment, water is kept for some time in impervious settling basins, for depuration of the precipitated chemical compounds which gradually accumulate at the bottom. These settling basins occupy an area of around 19.6 ha. Therefore, the geological materials of the waste deposits and water that runs through them are the potential sources of contamination for the Urgeiriça environment. To evaluate the degree and extension of this contamination a set of samples of water (superficial and ground water), stream sediments and soils were collected, and their chemical composition analysed. In face of the nature of the solid and water phases, a complex chemical signature of the mine activities is expected, including radioactive and non-radioactive compounds. This study is focussed mainly on the dispersion of the radioactive components.

2. Geological setting The uranium-bearing ore of Urgeiriça is of vein-type, striking N60ºE with variable dip, comprised between 75º and 90º (figure 1); besides the major mineralogy, composed of siliceous minerals, it contains also pitchblende, uraninite, pyrite, fluorite, blende, native lead sulphide, chalcopyrite, calcite and several secondary uranium minerals [4]. It cuts hercynian porphyritic medium to coarse grained biotite granites which is the dominant rock type observed in the area. Porphyritic fine to medium grained two mica granite as well as tertiary sedimentary rocks are less abundant, and the last ones occur as a thin cover over the granitic bedrock. 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 “Ribeira da Pantanha”, and the corresponding watershed can be delimited as illustrated in figure 1.

3. Material and methods In face of the location of the different sources of pollution, as well as the geology and geomorphology of the region, the area delimited in figure 1 should have the highest potential of contamination. The sampling set is also located in the same figure and comprises stream sediments (9), from Pantanha and Mondego watercourses, soils (22), collected in the margins of the first watercourse, and water samples (108). The stream sediments refer to the dry season and different soil layers were sampled; the layer corresponding to the first 15 cm and the layer comprised between 15 and 30 cm. The water samples were collected in streams (16), wells (66) and holes (26), in two different campaigns (June and November of 2000). The sources of pollution were also sampled as described: geological materials from the mill tailing (32), wastewater of the mine and seepages of the mill tailing (4), and depurated water from the treatment plant discharge in the Ribeira da Pantanha (2). The uranium content in water samples was analysed by fluorometry and the 226Ra activity by scintillation techniques, after selective extraction by radiochemistry methods. The activity of the radionuclides in solids was measured by gamma spectrometry. The first technique was carried out on the Instituto Geológico e Mineiro (Portugal) and the others on CIEMAT (Spain). The analytical error in all cases is under 10%.

4. Results 4.1 Characterization of pollution sources In face of the nature, dimension and storage conditions of the tailings, the most important source of pollution is the mill tailing deposit, through solid dispersion and rainfall water percolation. Water percolation on the galleries of the old mine is also a likely source of contamination to the environment. The mill tailing deposit was studied in detail. Gamma radiation levels were determined at the surface and along drill holes in the deposit, and allowed to recognize a significant compositional heterogeneity. Three different units were recognized, two of detritic and one of chemical nature. Lowest radioactivity is shown by unit A, which is disposed in the margins of the deposit and is mainly composed of sand. This unit circumscribes other, more radioactive, that includes six cores composed of silt and clay materials (B). A third unit, middle radioactive (C), was only detected in two drill-holes, and has a chemical nature analogous to the mud materials currently deposited in the settling basins used for depuration of the wastewater. Radionuclide activity was measured in samples collected in all units; average values are shown in table I. A detailed analysis of the data available for each core of unit B shows that the gamma radiation levels increase towards the centre of the core and, thus, a subdivision in two subunits (B1 and B2) was considered appropriate [5].

Table I – Mean and standard deviation (in Bq.Kg-1) of the radionuclides from the U-chain measured in samples collected in drill-holes executed in the radiometric units mapped in the Urgeiriça mill tailing.

210Pb 226Ra 230Th 238UA 6880±2822 4898±2243 4733±905 1150±59

B1 26189±7953 22089±6767 12679±2781 2927±996

B2 61433±19868 51958±18948 3237±2838

C 5880±2844 3360±3293 65600±18559 12867±777

The radionuclide activity is, as expected, lowers in the unit A and shows a sharp increase in the silt-clay material of unit B, especially in the activity of 226Ra and its decay products. The isotopic disequilibria observed in these two units, more relevant for the relations 226Ra/230Th and 230Th/238U, is

not surprising and was induced by the technological procedures used in the milling facility for uranium extraction [3]. Other chemical elements show a similar variation pattern, as is the case of As, Pb, Cu and Zn, which are much more abundant towards the centre of unit B [5]. Unit C is interpreted has being the deposit of an old settling basin that was active in the past for treatment of the wastewater. Probably the treatment consisted only in pH control and not included the removal of radium, as the activity of this radioisotope is quite low in unit C. However, despite its simplicity, the wastewater treatment procedure was good enough to remove large amounts of 230Th and 238U, as well as other elements as Mn, Ca, P, S and heavy metals [5]. Waters that percolate the mine and the mill tailing deposit concentrate high U and 226Ra contents (table II), as well as other chemical hazards as SO4, Al, Mn, Ni, Zn, Be, Co, Cu and Cr [5]. In the case of the wastewater collected in the old mine, the 226Ra activity is much higher than the activity of the same radioisotope from a resurgence related to the mill tailing. The surprising low value observed in water from the tailing implies that radium should be concentrated in a mineralogical support that is not suitable to remobilization by ground water. On the contrary, the leach rate of U in the same geological materials is high and, if not properly controlled, the transfer for the environment can be very fast.

Table II – Analytical results obtained in wastewater collected in the old mine (WAM) and in a resurgence from the mill tailing (WAE), as well as treated water discharged to the Pantanha watercourse (WAL); U in ppb and 226Ra in Bq.l-1.

Campaign U 226Ra WAM June 140 4.45

November 117 5.58

WAE June 2340 0.16 November 1560 0.16

WAL June 0.8 0.96 November 17.0 0.26

On the basis of data obtained in treated water (table II), it can be concluded that the procedures used in the treatment plant remove U and 226Ra from the wastewater in an efficient way. At the same time, the concentration of other chemical elements also decreases, as is the case of Mn, Ca, P, S, Be, Co, Cu, Ni, Y, Zn and rare earth [5]. These elements are currently referred as being concentrated in samples of mud collected in settling basins of other mines [3] and, thus, it can be anticipated the same occurs in the solid phase deposited in the settling basins of Urgeiriça.

4.2 Evaluation of the contamination in the vicinity of the Urgeiriça mine 4.2.1 Water The data obtained in water samples (superficial and groundwater) relative to U and 226Ra is summarized in table III. As expected, the values are lower for superficial water compared to ground water; the highest values are found in the water holes. Seasonal variations are larger for superficial waters and more subtle in the other water points. In all cases it is clear that variance is very high for the same campaign, as can be observed in figure 2; values are dispersed in a large interval, with most of them concentrated in the classes comprising the lowest concentrations.

Table III – Mean and standard deviation for radioactive elements measured in waters collected around the Urgeiriça mine; U in ppb and 226Ra in Bq.l-1.

Campaign U 226Ra Streams June 8.5 ± 14.4 0.14 ± 0.18

November 19.9 ± 30.1 0.09 ± 0.12

Wells June 13.1 ± 40.7 0.14 ± 0.23 November 16.5 ± 48.8 0.13 ± 0.17

Water hole June 38.4 ± 71.3 0.20 ± 0.22 November 35.5 ± 82.6 0.20 ± 0.22

FIG. 2. Histogram relative to the water samples collected around the Urgeiriça mine; values in ordinate are absolute frequencies.

4.2.2 Stream sediments and soils The stream sediments from the main watercourse that drains the Urgeiriça mine show high average radionuclide activity as well as a notorious isotopic disequilibrium of the U-chain (table IV).

Table IV – Mean and standard deviation (in Bq.Kg-1) of radionuclides activity measured in stream sediments and soils collected in Urgeiriça are. (1) set of 11 samples and (2) set of 7 samples.

210Pb 226Ra 230Th 238UStream sediments 811 ± 904 282 ± 200 3465 ± 4912 1088 ± 1006

Soils (Layer A)1 303 ± 151 257 ± 133 978 ± 1182 1297 ± 1604 Soils (Layer B)2 342 ± 179 271 ± 143 1244 ± 1425 1860 ± 1620

These facts can be interpreted as an evidence of contamination from the mine workings; this can also be reinforced by the data of figure 3, which shows the relation between the activities of some radionuclides and distance to the old mine. The trend is analogous for all the radioisotopes, with a clear increase in the activities downstream, especially for the case of 230Th and 238U. The variability of the activity values observed explains the large standard deviation estimated from the sample set (table IV). The highest values in the sediments are observed not in the vicinity of the mine area but at a distance comprised between 1500 to 2000 m. Influence extended for more than 3000 m. The radionuclide signature in the soils is similar to that already described for the stream sediments. In the soil samples 230Th and 238U have average values higher than those observed in the sediments, but the 210Pb activity is lower. For both cases the average values are highest than expected for a soil evolved from a granite rock [6], which could be an evidence of contamination through the irrigation with polluted waters, probably collected in the Pantanha watercourse.

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FIG. 3. Relations between the activities of some radionuclides with distance to the old mine. The comparison between samples collected in soil layers A and B reveals that the differences are under the analytical error [5] which does not support the existence of a significant vertical gradient of concentration. Some chemical elements analyzed in the same sample set (sediments and soil) show also high variation, in particular Mn, Fe, P, S, As, Cu and Zn [5].

5. Discussion The occurrence in the dataset of samples with high concentrations in radionuclides of the U-chain, as well as other chemical elements, as is the case of chlorine, fluorine and sulphate compounds, Ca, Fe, Mn, P, Na, Al, As, Ni, Cu and Zn [5], all of them found also in high contents in the mine pollution sources, suggests that the area around the old mine of Urgeiriça could have been contaminated by the mine workings. However, a similar chemical signature can also result from the leaching of localized uranium-bearing rocks composed of an identical mineralogy to that contained in the Urgeiriça mineralized vein, which is a rather common situation in the area, since belongs to a metalogenetic province [7]; the mineralogy of those rocks also include sulphide minerals whose alteration can increase the acidity of the circulating waters, and thus enhance the leach rate of several chemical elements, as the heavy metals and radionuclides. Despite of the lack of economic significance of most of the occurrences, this type of rocks can however act as a source of contamination to the environment. Therefore, the identification of a contaminate sample by human activities should not be based only on the measured concentration of the chemical elements, but use instead an approach able to discriminate the chemical signatures of the mine workings from the natural variability imposed by geology; in the present study this goal was achieved with support on discriminant analysis, a multivariate technique already applied to the evaluation of environmental impacts issues [8]. To apply this technique it is necessary, on a first step, to allocate the analyzed samples in two different groups, one corresponding to the uncontaminated samples (I), and the other including those with chemical composition presumably changed by the mine workings (II). This grouping was carried out on the basis of several criteria, as follows: a) the correlation between the location of each sample to the pollution sources (it was assumed that samples collected upstream and to higher distances of those places have lower probability to be contaminated); b) the occurrence of faults that intersect the uranium ore and, thus, be able to carry ground water to the surrounding area; c) the shape of the water level surface; d) the occurrence of water samples enriched in sulphate anion and Ca cation, which can be used as tracers of the mine pollution; the last one is added to wastewater for pH control in the treatment plant, and the sulphate is in most part related with sulphuric acid used as a leaching agent to extract the uranium from ore. For the purpose of discriminant analysis modelling, some of the chemical elements of the dataset were excluded, as a rule when more than 80% of the samples showed concentrations under the detection limit. Moreover, a stepwise procedure was followed to ensure the exclusion from the model of variables without significance in the discrimination between the two

groups [9]. The soil radionuclide data was not used because this type of analysis is only available for part of the samples collected in layer B. The results obtained from the mathematical technique in water samples (including streams, wells and holes), stream sediments and soils are shown in table V; the calculated linear functions are all significant, as proved by the large gap between the averages of the discriminator values estimated for each group (Mahalanobis distance) and also by the percentage of samples correctly classified (always higher than 98.5%). Table V – Results of the application of discriminant techniques to samples of waters, stream sediments and soils collected in Urgeiriça area. RI and RII are the average of the discriminator values for each group (I – natural; II – contaminated) and R0 is the discriminator value between the two groups. The last column shown the percentage of samples correctly classified. CE is the electric conductance.

Linear discriminant function RI RII R0 %Streams R = -0.19U + 0.01CE – 3.33 -2.72 4.54 0.91 100

Wells R = 0.07U + 0.02Cl – 3.02F + 0.02SO4 – 0.03Ca – 1.12 -1.21 6.75 2.77 98.5

Water holes R = -0.04U + 14.02226Ra + 0.02CE + 0.03 Cl + + 1.72Fe – 11.23 -7.68 7.68 0.00 100

Stream sediments R = 0.10U – 2.32 -2.13 1.28 -0.83 100

Soils R = 57.88MnO - 0.88Be - 0.70As + 0.59Cs + 0.03Cu + + 0.28Zn – 104.6S – 22.87 5.64 -6.77 -0.56 100

Significant variables on the discriminant models include chemical elements (major and minor), 226Ra, and the electric conductance measured in water samples. All were detected in significant concentrations in the polluted waters from the mine and the mill tailing, as well as in the geological materials that compose the mill tailing. U is present in almost all equations showing, as expected, high mobility in the surface environment and also its importance as an indicator of the mining contamination. The only exception is soil samples, and a possible explanation for this will be discussed further ahead. The location of the samples according to its filiation (natural or contaminated) is shown on figure 4. For the case of water samples (figure 4a), it can be concluded that the major part of the sample set did not show evidence of contamination by the mine workings; the contaminated samples occur in the Pantanha watershed and downstream of the mill tailing deposit. The spatial distribution of these samples suggests that the plume of contamination runs from NW to SE. After the intersection with the mining area, water flowing into the Pantanha channel is contaminated along all of its extension until reaches the confluence with the Mondego river (figure 4a). Stream sediments and soils show a similar trend, with almost all the contaminated samples located downstream of the radioactive pile; the exception is one sediment sample collected in the Pantanha watercourse before it reaches the mill tailing (figure 4b). The chemical signature of this sample could have been changed by old mine workings related with a small exploitation located upstream, in a NE section of the Urgeiriça vein. As previously referred, the peak concentrations in the stream sediments are reached at a distance of around 1000-1500m from the mill tailing deposit (figure 3). In the Pantanha watercourse, downstream of this deposit, exists a few very small dams were built for agriculture purposes, and as a result they accumulate stream sediments. Moreover, the watercourse has, in general, a narrow channel and is of torrential type which means that the dam deposits are the only sampling points available to collect representative samples. This is a possible explanation for the peak concentrations observed at those places, and not closer to the pollution sources. The reduction in the water flow imposed by the dams increase the deposition of the carried sediments and, eventually, the physical and chemical environment of the settling basin can change; if so, new compounds can be formed from the chemical dissolved fraction and precipitate at the bottom of the basin. The removal of

FIG. 4. – Location of the samples (a – waters; b – stream sediments; c – soils) collected in Urgeiriça area considering the two groups validated by discriminant analysis. this contaminated fraction and the dilution effect promoted by mixing with fresh water induce, afterwards, a reduction in the contamination level of the flowing water, as shown in figure 3. The composition of the stream sediments of river Mondego is not affected by the sediments discharge from the Pantanha watercourse, as revealed by data obtained in samples collected before and after the confluence (figure 4b). The amount of sediments carried out by Pantanha is very small compared with those transported by the river Mondego and, thus, a quick dilution is expected. The spatial location of the soil samples also explains the reason why U was not considered as significant in the numerical discriminant model. Three of the soil samples were collected upstream the mill deposit, nearby a small stream that drains the mineralized uranium-bearing vein of Urgeiriça. As expected, the U content in these samples is higher than measured in the surrounding areas and is close to the average content of the contaminated group [5]. These data and the proximity of the tailing deposit located nearby the main shaft of the old mine (Stª Bárbara) suggests a possible human origin for the contamination. However, this was rejected by the multivariate model, which indicates, on the basis of the global geochemical signature, a provenance of U from natural sources. This is a good example that shows that the evaluation of the contamination from mine workings based only on the content of a particular chemical element can be misleading. The average results and also the standard deviation calculated for samples collected in each environmental compartment are shown in tables VI and VII.

a b

c

Table VI – Mean and standard deviation of the U and 226Ra results in water samples collected in the Urgeiriça area considering the two groups validated by discriminant analysis; I – uncontaminated samples, II – contaminated samples by mine workings.

U (ppb) 226Ra (Bq.l-1)Water points Campaign I II I II

Streams June 0.8 ± 1.1 21.3 ± 18.0 0.02 ± 0.05 0.28 ± 0.18 November 2.2 ± 3.8 49.3 ± 28.4 0.01 ± 0.02 0.23 ± 0.05

Wells June 0.8 ± 0.5 81.9 ± 79.3 0.07 ± 0.09 0.50 ± 0.39 November 1.1 ± 1.2 102.4 ± 90.4 0.08 ± 0.10 0.40 ± 0.24

Water holes June 9.3 ± 17.2 41.4 ± 77.3 0.08 ± 0.02 0.32 ± 0.29 November 2.6 ± 2.9 28.3 ± 57.7 0.09 ± 0.02 0.30 ± 0.30

Table VII - Mean and standard deviation of several radionuclides elements obtained in Urgeiriça area in stream sediments (SD) and soils (SL; layer A) considering the two groups validated by discriminant analysis; I – uncontaminated samples, II – contaminated samples by mine workings.

210Pb 226Ra 230Th 238UI II I II I II I II

SD 104±31 1164±926 79±16 384±165 -- 5198±5272 109±75 1578±869

SL 268±192 367±100 233±170 291±76 429±392 1723±1488 395±349 2485±1735

As expected, the average values are in all cases several times higher in the group of the contaminated samples comparatively to the uncontaminated group. The concentration factors are, however, highly variable, with the highest differences observed in the water samples, mainly in the surface waters and in wells; for example, the U content can be as high as 25 to 100 times of the average value observed in the uncontaminated group. In the same samples the variation of 226Ra is smaller, with concentration factors between 3 and 23; once again this suggests a significant difference on the behaviour of U and 226Ra in the surface environment of the Urgeiriça area. For samples of stream sediments and soils variations are less expressive, and only the 238U activity measured in the stream sediments approximates to the previously referred concentration factors for contaminated samples (14). 230Th is present in all the environmental compartments, sometimes in significant concentrations, which is not an expectable behaviour. When enclosed in the ore mineralogy, thorium is a relatively immobile chemical element [6]. However, during the milling procedures almost 50% of the 230Th can be remobilized and integrated later in the residual liquid fraction [1]. During the operational phase of the Urgeiriça mine, this residual fraction was added to the mill deposit (mixed with the solid fraction or decanted in the primitive settling basin) or included in the leaching solution used for extraction of the low-grade ore in the underground mine [1]. Thus, the results obtained for the spatial distribution of 230Th suggests that, through these pathways, the radionuclide can be remobilized and transferred between the different environmental compartments (water, stream sediments and soils). With exception of the soils located nearby the mill tailing deposit, subject to contamination by aerial deposition of dust particles carried out by the wind, the main pathway for the contamination of the soils located at some distance should be irrigation with contaminated water pumped from the Pantanha watercourse. Thus, it can be deduced that 230Th has been transported in dissolution or included in the suspended fraction of the flowing water. For a correct evaluation of the environmental impacts induced by the presence of high concentrations of toxic elements in the composition of the soils, it is necessary to obtain complementary data, as is the case of acidity [10]. Even so, a preliminary evaluation of the environmental impacts can be tentatively carried out. For the case of 226Ra, a maximum allowance activity of 185 Bq.Kg-1 above the background

was proposed by EPA for samples corresponding to the first 15 cm of soil (layer A). As shown in figure 5, only two samples, included in the contaminated group, don’t complain with the compulsory maximum value. One of the samples allocated to the uncontaminated group shows the same

behaviour, which, once again, strengthens the importance of the use of a multivariate approach for the purpose of delimitation of the area affected by mine workings.

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FIG. 5 – Comparison of the 226Ra activity measured in soil samples collected in the Urgeiriça area (layer A) with the maximum allowance limit proposed by EPA. The assumed background activity was of 115 Bq.kg-1 [5],

6. Conclusions The radiological and chemical data obtained in the Urgeiriça mining area in samples of water, stream sediments and soils, shows the occurrence of high contents of several elements, namely U, as well as high activities of the radionuclides included in its decay chain. The use of multivariate mathematical techniques, namely discriminant analysis, allowed identifying the areas contaminated by the mining activities, clearly separating anomalies due to natural and anthropogenic factors. The comparison of the concentrations or activities determined in the contaminated areas with the limits established in the legislation, allowed the identification of the appropriate sectors that require remediation measures.

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of a rehabilitation plan for a uranium mill tailing deposit (Urgeiriça – Central Portugal), this volume (2004)

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4. Neiva, J.M.C., Wall-rock metassomatism associated with the uranium ores in Portugal,International Geological Congress, Abstracts, 23, p.183, (1968)

5. Exmin, Estudo Director das áreas de minérios radioactivos, 2ª fase, (2002) 6. Neves, L.J.P.F., Godinho, M.M. & Pereira, A.J.S.C., Influência do processo de meteorização do

granito no equilíbrio secular das cadeias de decaimento de 238U e 232Th – estudo de um caso. II Congresso Ibérico de Geoquímica, Lisboa, 405-408, (1999)

7. Neves, L.J.P.F., Pereira, A.J.S.C., Godinho, M.M. & Dias, J.M., A radioactividade das rochas como um factor de risco ambiental no território continental português: uma síntese. Actas da V Conferência Nacional Sobre a Qualidade do Ambiente, edited by C. Borrego, C. Coelho, L. Arroja, C. Boia e E. Figueiredo, 1:641-649 (1996)

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resultante da exploração e abandono da mina de urânio da Cunha Baixa. II – Solos. Actas II Congresso Ibérico de Geoquímica, Lisboa, 189-192, (1999)