Applied Radiation and Isotopes 97 (2015) 24–33

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Applied Radiation and Isotopes

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Uranium isotopes in groundwater occurring at Amazonas State, Brazil

Márcio Luiz da Silva a, Daniel Marcos Bonotto b,n

a Coordenação de Pesquisas em Clima e Recursos Hídricos, CDAM-INPA, Av. André Araújo, 2936, Petrópolis, CEP 69080971 Manaus, Amazonas, Brazilb Departamento de Petrologia e Metalogenia, IGCE-UNESP, Av. 24-A, 1515, CP 178, CEP 13506-900 Rio Claro, São Paulo, Brazil

H I G H L I G H T S

� U-isotopes data in important aquifer systems in Amazon area.

� Application of the U-isotopes data to investigate the groundwater flow direction.� Evaluation of the drinking-water quality in terms of dissolved uranium.

a r t i c l e i n f o

Article history:Received 6 November 2013Received in revised form18 November 2014Accepted 9 December 2014Available online 11 December 2014

Keywords:GroundwaterTube wellsDissolved uranium234U/238U activity ratioAmazon area

x.doi.org/10.1016/j.apradiso.2014.12.01243/& 2014 Elsevier Ltd. All rights reserved.

esponding author.ail addresses: marciols@inpa.gov.br (M.L. da Sonotto@yahoo.com.br (D.M. Bonotto).

a b s t r a c t

This paper reports the behavior of the dissolved U-isotopes 238U and 234U in groundwater providing from15 cities in Amazonas State, Brazil. The isotope dilution technique accompanied by alpha spectrometrywere utilized for acquiring the U content and 234U/238U activity ratio (AR) data, 0.01–1.4 mg L�1 and 1.0–3.5, respectively. These results suggest that the water is circulating in a reducing environment andleaching strata containing minerals with low uranium concentration. A tendency to increasing ARs valuesfollowing the groundwater flow direction is identified in Manaus city. The AR also increases according tothe SW–NE directions: Uarini-Tefé; Manacapuru-Manaus; Presidente Figueiredo-São Sebastião doUatumã; and Boa Vista do Ramos-Parintins. Such trends are possibly related to several factors, amongthem the increasing acid character of the waters. The waters analyzed are used for human consumptionand the highest dissolved U content is much lower than the maximum established by the World HealthOrganization. Therefore, in view of this radiological parameter they can be used for drinking purposes.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Naturally-occurring uranium comprises the isotopes 238U, 235Uand 234U, whose relative abundances are 99.28%, 0.72% and0.0054%, respectively, under radioactive equilibrium conditions(Ivanovich and Harmon, 1992). 238U and 235U originate the (4nþ2)and (4nþ3) radioactive decay series, respectively.

In sandy sediments, uranium levels are often o1 mg g�1 due totheir easy leaching capability (Ivanovich and Harmon, 1992).However, in sediments derived from igneous rocks, enriched in Uand Th, enhanced concentrations of these elements may be found(Ivanovich and Harmon, 1992). Uranium exhibits a great tendencyto oxidize to U6þ that is very soluble, favoring its mobility in su-perficial soil layers or in oxidizing groundwater until reaching thereducing environments in which it precipitates as pitchblende or

ilva),

coffinite (USiO4 �nH2O) (Osmond and Cowart, 1976; Krauskopf andBird, 1995).

The main factors affecting the uranium distribution in naturalwaters are: the uranium content in the rock matrix, sediments orsoils and its leaching capacity; the grade of the water hydraulicconfinement in relation to dilution effects by meteoric waters;climatic effects; seasonal variability and evapotranspiration influ-ence; the pH and water oxidation state; the concentrations ofcarbonate, phosphate, vanadate, fluoride, sulfate, silicate, calcium,potassium and others constituents able to form uraniferous com-plexes or uranium insoluble minerals; the presence of materialswith capacity of adsorbing uranium like organic matter, clays, ironoxi-hydroxides, manganese and titanium (Langmuir, 1978). Theuranium content in groundwater is greatly variable, depending ofthe lithology and/or proximity of the deposits of this element,where, in many geological contexts values 44 mg L�1 have beenconsidered anomalous (Osmond and Cowart, 1976; Ivanovich andHarmon, 1992).

The 238U decay series finishes at 206Pb after 14 disintegrations-eightalpha-type (238U, 234U, 230Th, 226Ra, 222Rn, 218Po, 214Po, 210Po)

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–33 25

and six beta-type (234Th, 234mPa, 214Pb, 214Bi, 210Pb, 210Bi). Thus,234U is radiogenic in the 238U-series and both 238U and 234U are insecular equilibrium in all minerals and rocks older than 1 Ma if thesystems are closed for U. Therefore, the 234U/238U activity ratio(AR) is unity in them. However, rock/soil–water interaction fre-quently results in AR for dissolved uranium that is 41 (Osmondand Cowart, 1976; Ivanovich and Harmon, 1992). In general, thesize, climate, lithology, stratigraphy, hydrogeology, geochemicalconditions, and extent of rock-/soil water interactions are amongthe factors responsible for AR41 values in the aquifer systems.Such enhanced ratios may be caused by preferential chemicaldissolution of 234U (Rosholt et al., 1963) and alpha-recoil release of234Th at the rock/soil–water interface (Kigoshi, 1971), among otherfactors. The dissolved U content and AR data have been extensivelyutilized for different applications in environmental studies (Iva-novich and Harmon, 1992).

Alongside its economic importance, uranium in the hydro-logical compartment also should be carefully investigated due toits radiological implication as a consequence of its great mobilityin the waters and accentuated potential to cause biological da-mage. The human organism contains an average of 90 mg of U thathas been incorporated through the normal water and food inges-tion and breathed air, approximately 66% in the skeleton, 16% inthe liver, 8% in the kidneys and 10% in other tissues. Most of the Ureaching the organism (circa 95%) is absorbed. The provisionalguideline value for uranium in drinking water is 30 mg L�1 basedon its chemical toxicity for the kidney (WHO, 2011).

Rule no. 2914 of the Health Ministry in Brazil published in 12December 2011 established guideline reference values of the ac-tivity concentration in drinking water corresponding to 0.5 Bq/Lfor gross alpha and 1 Bq/L for gross beta (MS, 2011) as proposed byWHO (2011). However, no reference specifically for uranium wasmade by MS (2011); limits of 5�105 Bq and 4�105 Bq for theannual intake of 238U and 234U, respectively, were proposed byCNEN (1988).

Few studies have focused on investigating the presence of U ingroundwater in the Amazon area because of its large surface areaand abundant surface waters resources. The current paper reportsa novel database of the dissolved uranium content and 234U/238Uactivity ratio in groundwater from different localities in AmazonasState. Water quality has been evaluated from the dissolved Ucontent data as well the ARs variability according to the ground-water flow direction.

2. Study area

A vast plain straddling the Equator and almost at the sea levelcharacterizes the Amazon region in north Brazil. The climate(temperature426 °C) is hot over practically the whole area, andaverage precipitation is 2.3 myr�1 (Nimer, 1989). Amazonas Statein Brazil is underlain by Phanerozoic sedimentary cover depositedover pre-Cambrian substrates. The study area comprises strati-graphic units occurring in the Amazonas and Solimões basins e.g.the Nhamundá Formation as reported by Brito (1979), Petri andFúlfaro (1988) and Cunha et al. (1994). The basins have beenfractured, uplifted and eroded by N–S strains accompanied byfurther E–W distension followed by basic magmatism in the N–Sdirection (Cunha et al., 1994). The relaxation of the ENE–WSWcompressional strains created depositional sites resulting in sedi-ments of the Alter do Chão Fm. (Cretaceous) and Solimões Fm.(Tertiary), which borders the western portion of the Amazonassedimentary basin. The basin silted in the Pleistocene with abun-dant sediment inputs of the Içá Formation from Andean areas(Lourenço et al., 1978; Petri and Fúlfaro, 1988; Eiras et al., 1994;Fernandes Filho et al., 1997).

The Nhamundá Formation is predominantly composed of fineto coarse rounded and disseminated white-greyish colored quartzsandstone grains (Caputo, 1984). The Alter do Chão Formationspreads over the western Amazon area and comprises inter-digitated red-colored clay, silt and sand containing poorly selectedfeldspatic grains with quite variable mica content (Lourenço et al.,1978; Petri and Fúlfaro, 1988; Eiras et al., 1994; Fernandes Filhoet al., 1997).

Rivers in the Amazon area frequently exhibit linear, parallel,and angular flow patterns due to diagonal intercrossing, with thelineaments often linked to buried structures, mainly faults (An-drade and Cunha, 1971). The mixture of waters from Solimões andNegro rivers close to Manaus city occurs at a restraint zone formedin the N40E–N65W preferential directions crossing. It is an arealocated at a transcurrent neotectonic belt extending and control-ling the western central portion of the Amazon Plain (Igreja, 2012).Neotectonics here has caused folds, faults, fractures, and joints thataffected the bedrock, laterites, and soil beds, also affecting theunderground flow and also the direction of the Solimões channel(Igreja, 2012).

The Alter do Chão, Içá and Solimões formations form the largestgroundwater reservoirs in Amazon and Solimões basins. The Alterdo Chão Aquifer System (ACAS) is inserted in the Amazon Domi-nant Hydrographic Region (Imbiriba and Melo, 2012). It is char-acterized by unconfined and confined aquifers, whose transmis-sivity varies between 1.5 and 9.1�10�3 m2/s (Souza et al., 2013).Rebouças (2014) estimated for ACAS an outcrop area of438,000 km2, an average thickness of 545 m and a water storagecapacity of 86,000 km3. It is confined by Solimões Aquiclude inSolimões basin and covered by the Içá-Solimões Aquifer System(ISAS), whose main characteristics are (Souza et al., 2013): outcroparea¼948,600 km2, transmissivity¼3�10�3 m2/s, water storagecapacity¼7200 km3.

Groundwater sampling in this study was performed in tubewells situated at 15 localities in Amazonas State (Fig. 1). Thesehave been drilled in Icá, Alter do Chão and Nhamundá formationsaccording to the lithological profiles provided by the BrazilianGeological Survey (CPRM). All wells are utilized by the cities inwater-supply systems, with most of them exploiting the ACAS.Well selection was made on the basis of ease of access and op-erational facilities. Water samples (20–25 kg) were stored inpolyethylene bottles and transported up to the laboratory forevaluating the U-isotopes.

3. Analytical methods

The samples for U analysis were acidified to pH less than 2 onusing HCl, about 500 mg of FeCl3 plus 3.39 dpm of 232U were ad-ded, and U was co-precipitated on Fe(OH)3 by increasing the pH to7–8 through addition of concentrated NH4OH solution; the pre-cipitated was recovered, dissolved in 8 M HCl and Fe3þ was ex-tracted into an equal volume of isopropyl ether. The acid U-bearingsolution was purified by anion exchange, first on a Cl� and then ona NO3

� column of 50–100 mesh Rexyn 201 resin. U was finallyeluted from the NO3

� column with 0.1 M HCl and after evapora-tion to dryness was dissolved in 10 mL of 2 M (NH4)2SO4 electro-lyte and transferred to an electrodeposition cell. The pH was ad-justed to 2.4 and electrodeposition of U on a stainless steel plan-chet was complete after 3 h at a current density of 1 A cm�2.

The U content was measured by alpha spectrometry. The α-activities were determined with four 0.1 mm depletion depth,200–450 mm2 area silicon surface barrier detectors. The spectrafor natural U and 232U tracer extracted were recorded on an EG&GORTEC 919 Spectrum Master Multichannel Analyser. The con-centration data were calculated by isotope dilution from the

Fig. 1. Geological map of the study area and location of the cities in Amazonas State, Brazil, where groundwater sampling was performed for U-isotopes analysis. 1-Uarini;2-Alvarães; 3-Tefé; 4-Manacapuru; 5-Iranduba; 6-Manaus; 7-Presidente Figueiredo; 8-Rio Preto da Eva; 9-Itacoatiara; 10-São Sebastião do Uatumã; 11-Boa Vista do Ramos;12-Urucará; 13-Maués; 14-Barreirinha; 15-Parintins.

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–3326

counting rates of 238U and 232U peaks, whereas the 234U/238U ac-tivity ratio (AR) was found from the counting rates of 234U and 238Upeaks. The AR uncertainty (sAR) was obtained by the equation(sAR/AR)2¼(sA4/A4)2þ(sA8/A8)2, where s2A4¼A4þ2B4 ands2A8¼A8þ2B8 (A4 and A8 are the net counts in the 234U and 238Upeaks, respectively, and B4 and B8 their corresponding back-grounds). Thus, the recorded errors were 1 sigma based on Poissoncounting statistics. The Decision Level Lc (Currie, 1968) for accep-tance of a positive measurement in the 238U and 232U energy re-gions was 0.00082 and 0.00225 cpm, respectively.

4. Results and discussion

4.1. Dissolved U content data and redox conditions

The results of the U-isotopes analysis are given in Table 1. Thedissolved U content ranged from 0.01 mg L�1 at Urucará city to1.36 mg L�1 at Itacoatiara city similar to the range of 0.003–1.1mg L�1 reported by Silva (1999) in a study of the U isotopes oc-currence in Manaus city groundwater. Silva (1999) related thisrange of dissolved U contents to the groundwater flow dynamicsin the heterogeneous sedimentary strata of the geological forma-tions at Manaus city area. In the current paper, the highest averageU content was 0.39 mg L�1 at Parintins and Barreirinha cities. Ac-cording to the geology, the highest mean U content was 0.19 mgL�1

in Alter do Chão Formation, whereas the lowest was 0.04 mg L�1 inNhamundá Formation (Table 1). Sampling points No. 36 (Itacoa-tiara city), No. 16 (Iranduba city), and Nos. 59, 61 and 63 (Parintinscity) exhibited enhanced U levels compared to other samples in

the same cities. This could be related possibly to a heterogeneoussediments composition in Alter do Chão Formation at the studyarea.

All waters in Table 1 are utilized for human consumption. Theirpotability can be assessed against the established guideline re-ference values for U as its ingestion in waters is a health risk. Thehighest dissolved U content corresponding to 1.36 mg L�1 is wellbelow the maximum allowed in drinking water of 30 mg L�1(WHO,2011).

Oxidizing conditions generally prevail in the recharge zone ofan aquifer and the most active U leaching occurs in this zone. Agroundwater dissolving U close to recharge may undergo furtherchange in its U chemistry due to additional leaching or, undermore reducing conditions, due to U precipitation (Ivanovich andHarmon, 1992). Silva (1999) reported a pH range of 4.1–5.4 and Ehredox potentials from þ93 to þ256 mV in groundwaters ofManaus city, yielding a “reducing acid” classification according tothe Eh–pH diagram (Krauskopf and Bird, 1995). The available pHand Eh data in groundwater samples from Manaus and other citiesin Amazonas State is shown in Fig. 2, indicating this same classi-fication (reducing acid), except for those occurring at Içá Forma-tion (Uarini, Alvarães and Tefé cities) that could be considered“transitional-reducing” rather than “reducing” in character. Thishappened despite Imbiriba and Melo (2012) measured dissolvedoxygen content of 2.9–5.5 mg/L in groundwater occurring in theAlter do Chão Formation, which could suggest oxidizing waters.These findings suggest that Eh is determined by the overall com-position of a solution and not just by oxygen content as alreadypointed out by Brownlow (1979) when discussing (for pH¼7 andEh¼þ0.36 V) how the presence or absence of oxygen in water of

Table 1U-isotopes analysis of groundwater occurring at Amazonas State, Brazil.

Sample Geological Formation Citya 234U/238U AR U(lg L�1)

Sample GeologicalFormation

City 234U/238U AR U(lg L�1)

01 Içá Uarini 1.4070.08 0.18 34 Alter do Chão Itacoatiara 1.1370.12 0.1202 Içá Alvarães 1.1270.10 0.16 35 Alter do Chão Itacoatiara 1.4270.11 0.0603 Içá Alvarães 1.2570.12 0.05 36 Alter do Chão Itacoatiara 2.1470.05 1.36Mean (N¼2) 1.1970.11 0.11 37 Alter do Chão Itacoatiara 1.5570.11 0.0404 Içá Tefé 2.8570.11 0.21 38 Alter do Chão Itacoatiara 1.9170.06 0.0405 Içá Tefé 3.4070.12 0.45 Mean (N¼5) 1.6470.09 0.3206 Içá Tefé 2.1070.11 0.14 39 Alter do Chão São Sebastião do Uatumã 1.5770.10 0.0207 Içá Tefé 1.1170.13 0.19 40 Alter do Chão São Sebastião do Uatumã 1.1470.09 0.3708 Içá Tefé 1.3370.16 0.11 41 Alter do Chão São Sebastião do Uatumã 2.6670.09 0.11Mean (N¼5) 2.1670.13 0.22 Mean (N¼3) 1.7970.11 0.1709 Alter do Chão Manacapuru 1.0070.07 0.16 42 Alter do Chão Boa Vista do Ramos 1.2270.11 0.0110 Alter do Chão Manacapuru 1.3070.11 0.4311 Alter do Chão Manacapuru 1.0570.10 0.50 43 Alter do Chão Urucará 2.3070.11 0.0712 Alter do Chão Manacapuru 1.4470.10 0.08 44 Alter do Chão Urucará 1.0170.11 0.2213 Alter do Chão Manacapuru 1.2170.08 0.07 45 Alter do Chão Urucará 1.0170.10 0.25Mean (N¼5) 1.2170.09 0.25 46 Alter do Chão Urucará 1.2070.05 0.1714 Alter do Chão Iranduba 3.5370.12 0.04 47 Alter do Chão Urucará 1.1170.05 0.2115 Alter do Chão Iranduba 1.0070.07 0.01 48 Alter do Chão Urucará 1.3870.05 0.1916 Alter do Chão Iranduba 1.2670.05 0.92 49 Alter do Chão Urucará 1.0070.16 0.07Mean (N¼3) 1.9370.08 0.32 50 Alter do Chão Urucará 1.3370.11 0.0117 Alter do Chão Manaus 2.6270.09 0.04 Mean (N¼8) 1.2970.11 0.1518 Alter do Chão Manaus 2.4670.05 0.10 51 Alter do Chão Maués 1.0170.10 0.2219 Alter do Chão Manaus 1.4670.07 0.19 52 Alter do Chão Maués 1.0070.11 0.2520 Alter do Chão Manaus 2.5270.08 0.08 53 Alter do Chão Maués 1.3870.08 0.0521 Alter do Chão Manaus 3.0670.08 0.01 54 Alter do Chão Maués 2.0070.11 0.0322 Alter do Chão Manaus 1.5170.07 0.08 55 Alter do Chão Maués 1.6070.13 0.0423 Alter do Chão Manaus 1.3670.07 0.17 56 Alter do Chão Maués 1.6170.13 0.2924 Alter do Chão Manaus 2.7870.08 0.05 Mean (N¼6) 1.4370.11 0.1525 Alter do Chão Manaus 1.2770.06 0.03 57 Alter do Chão Barreirinha 1.7970.13 0.0426 Alter do Chão Manaus 2.6270.08 0.06 58 Alter do Chão Barreirinha 2.9870.14 0.4527 Alter do Chão Manaus 1.0070.07 0.20 Mean (N¼2) 2.3870.10 0.3928 Alter do Chão Manaus 1.9570.05 0.03 59 Alter do Chão Parintins 1.3270.06 0.8029 Alter do Chão Manaus 1.7270.07 0.09 60 Alter do Chão Parintins 1.9170.16 0.0730 Alter do Chão Manaus 1.6670.09 0.04 61 Alter do Chão Parintins 2.5070.06 0.84Mean (N¼14) 2.2470.10 0.08 62 Alter do Chão Parintins 2.5770.08 0.1931 Nhamundá Presidente Figueiredo 1.0470.07 0.05 63 Alter do Chão Parintins 3.2770.12 0.4032 Nhamundá Presidente Figueiredo 1.2070.08 0.04 64 Alter do Chão Parintins 1.2770.10 0.04Mean (N¼2) 1.1270.08 0.04 Mean (N¼6) 2.2470.10 0.3933 Alter do Chão Rio Preto da Eva 1.4570.09 0.05

Geological FormationAlter do Chão (N¼54) Içá (N¼8) Nhamundá (N¼2)

234U/238U ARU (lg L�1)234U/238U ARU (lg L�1)234U/238U ARU (lg L�1)

Highest 3.5370.12 1.36 3.4070.12 0.45 1.2070.08 0.05Lowest 1.0070.07 0.01 1.1170.13 0.00 1.0470.07 0.04Mean 1.7170.09 0.19 1.8270.12 0.18 1.1270.08 0.04

a Names and numbers, according to Fig. 1: 1-Uarini; 2-Alvarães; 3-Tefé; 4-Manacapuru; 5-Iranduba; 6-Manaus; 7-Presidente Figueiredo; 8-Rio Preto da Eva;9-Itacoatiara; 10-São Sebastião do Uatumã; 11-Boa Vista do Ramos; 12-Urucará; 13-Maués; 14-Barreirinha; 15-Parintins.

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–33 27

sediments affects the Eh. Brownlow (1979) used the concepts ofelectrochemical cells, Gibbs free-energy changes and Henry's lawto calculate an amount of 10�17 mol L�1 of dissolved oxygen in thewater of the sediments under such conditions. Such value in-dicated that a relatively high oxidation potential of þ0.36 V (at apH of 7) did not imply necessarily a significant amount of oxygenpresent in the environment. Brownlow (1979) also considered thatif we take water that is saturated with oxygen (Eh�0.45 V for pHof 7) and reduce its oxygen drastically, there would be a very smalllowering of the water Eh value.

Groundwater at Manaus city area is weakly mineralized (meanconductivity¼32 mS/cm; Silva, 1999) as are surface waters of theSolimões and Amazonas rivers (Furch, 1984). This and the heavyrainfall in the Amazon region suggest potentially a strong influ-ence of the rainwater and surface waters in the ground-water composition at the study area, with small amounts of silicaadded when the water interacts with the aquifer rock matrices, aswell as surface infiltration of organic compounds from the abun-dant vegetation debris. However, the pattern of the available

hydrochemical data is not much amenable to plot on a standardPiper (1944) diagram, because there is a lack of the preponderanceof typical anions and cations, and a mixed character is identified inseveral situations. They are also not favorable to model geo-chemically the equilibrium U-speciation utilizing traditional codeslike WATEQ4F (USGS) and to identify potential mineral saturationcontrols alongside complex formation. In addition, the large ex-tension (�950 km in straight line) of the area studied here makesdifficult the proper evaluation of the groundwater evolution. Thus,unidentified processes may also affect the physical and chemicalparameters, causing differential behavior of the dissolved ions andimpacting the changes in hydrochemical facies.

4.2. Acidity and spatial distribution of the U content AR data

Water–soil/rock interactions during infiltration of meteoricwaters may cause ARs41 for dissolved U due to the preferential234U leaching in relation to that of 238U (Osmond and Cowart,1976). The ARs of the waters analysed in this paper ranged from

Fig. 2. Plotting of the pH and Eh data in the study area as reported by Silva (1999,2005) in an Eh–pH diagram (Krauskopf and Bird, 1995). 1-Uarini; 2-Alvarães;3-Tefé; 4-Manacapuru; 5-Iranduba; 6-Manaus; 7-Presidente Figueiredo; 8-RioPreto da Eva; 9-Itacoatiara.

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1.0070.11 to 3.5370.12 (Table 1). The lowest average AR valuewas 1.1270.08 in Presidente Figueiredo city, whereas the highestwas 2.3870.14 in Barreirinha city. According to the geology, thehighest average AR value was 1.8270.12 in Içá Formation,whereas the lowest was 1.1270.08 in Nhamundá Formation (Ta-ble 1). The ARs measured here agree with the great majority of thevalues reported in the literature (Osmond and Cowart, 1976; Iva-novich and Harmon, 1992, etc.).

Moore (1967) reported a mean dissolved U content and AR inwaters of Solimões and Amazonas rivers corresponding to 0.043mg L�1 and 1.10, respectively, to the mean values measured inwaters providing from wells exploiting the Nhamundá Formationat Presidente Figueiredo city (Table 1). However, Moore (1967) didnot find any accentuated difference in the U content and AR valuesin waters of the Solimões and Amazonas rivers as identified herein the aquifer systems studied in the current paper.

Most of the dissolved U content and AR data given in Table 1are in the urban area of Manaus city. Gomes (1978) showed atendency of decreasing AR according to the groundwater flowdirection in a hydrochemical study of groundwaters in the BambuíGroup, Bahia State, Brazil. Such behavior, however, is not con-firmed at Manaus city, as instead there is a trend of increasing ARaccompanying the groundwater flow direction (Fig. 3).

The average dissolved U content in the whole study area ten-ded to increase following the SW–NE direction along three majorsections: Uarini-Iranduba; Manaus-São Sebastião do Uatumã;and Boa Vista do Ramos-Parintins (Fig. 4). The mean AR in thearea also tended to increase according to this SW–NE direction,including for the section Boa Vista do Ramos-Parintins (Fig. 4).However, three different sections are identified when looking atthe spatial distribution of the increasing mean AR data: Uarini-Tefé; Manacapuru-Manaus; and Presidente Figueiredo-São Se-bastião do Uatumã (Fig. 4).

The detailed evaluation of processes affecting the AR and dis-solved U content along the W–E direction (Uarini to Parintins) is not

feasible due to the size area, lack of constraints on reactions takingplace there, ground slope difference in the basin or buried structures(structural highs and lows) that could impact the water movement.Vertical lines highlighting the Purus, inferred and structural arches(Fig. 1) could be plotted in the diagram of the mean dissolved Ucontent and AR of groundwater samples shown along the W–E di-rection in Fig. 4. This would indicate no clear relation of the archesposition to the U-ARs increase/decrease, denotating that the struc-tural controls (Fig. 1) are not exerting much influence on the behaviorof the U-tracers. Nevertheless, the trends in Fig. 4 imply a directrelationship between the mean AR and dissolved U content (Fig. 5) asalso found by Bonotto (1993) in groundwaters of Águas da Prata city,Poços de Caldas plateau, Brazil, who associated it to different hy-drochemistries and leaching rates affecting the rock surfaces.

The Sr and U isotopes have been used to investigate the mixingof materials in common geological processes and in groundwaterstudies, considering mixtures of two components A and B and thereciprocal of the Sr or U concentration (Osmond et al., 1974; Os-mond and Cowart, 1976; Faure, 1977; Faure and Mensing, 2004).Whilst evolutionary sequences might follow straight line trends ofspecific sign, mixing components (assuming conservative mixing)could not be necessarily constrained by the sign of the linear slopebut the relative end-members of mixing (Osmond and Cowart,2000). The database plotted in Fig. 5 indicates that the slope isnegative for the reciprocal of the U content, contrarily to all majorcases reported in the literature (e.g., Osmond et al., 1974; Osmondand Cowart, 1976; Faure, 1977; Faure and Mensing, 2004). How-ever, the standard plot of AR vs. 1/U for a two-component mixingequation will not be used in the current paper.

It is possible speculate that the increasing acid character of thewaters can enhance the dissolution of uranium and more labile234U from the sediments, justifying the linear relationship shownin Fig. 5. The average pH equal 4.1 in groundwaters along thesection Boa Vista do Ramos-Parintins is more acid than the meanvalue of 5.4 along the section Uarini-Tefé, thus, confirming thetrends verified for the AR and dissolved U content data in them.

Bonotto and Andrews (1993, 2000) reported results of labora-tory time-scale experiments conducted on limestone and dolomitegravels from the Mendip Hills area, England, with the purpose ofevaluating the release of 238U and 234U to different aqueous so-lutions. Etching/leaching was conducted with distilled waterequilibrated with the atmosphere (pH¼4.9; Eh¼þ0.15 V), anddistilled water saturated by CO2 at 1 atm (pH¼3.5; Eh¼þ0.14 V).According to the Eh–pH diagram, these solutions are “reducingacid” like the groundwater analyzed in this paper. The results ofthe analyses of etch/leach solutions demonstrated that significantreaction took place in each set of experiments, where the U dis-solution was more accentuated for the more aggressive (acid)solution, CO2-equilibrated water. The 234U/238U activity ratio (AR)of dissolved U in leach/etch solutions generally increased as leach/etch proceeded. The obtained values did not indicate a reductionin the amount of dissolved U and an increase in the AR of theremaining dissolved U as commonly observed for groundwatersystems close to redox boundaries, also supporting the direct re-lationship between these parameters given in Fig. 5.

4.3. The role of humic substances (HS)

Humification corresponds to the transformation of macro-morphologically identifiable matter into amorphous compounds,as a rule involving the changes that occur in vegetal residues orsoil organic matter (Oliveira et al., 2007). It has been related to thepreferential oxidation of plant polysaccharides, the selective pre-servation of more recalcitrant organic compounds such as ligninand phenolic structures, and to the incorporation of organiccompounds of microbial origin (Oliveira et al., 2007).

Fig. 3. The contour map representing the groundwater flow direction and 234U/238U activity ratio (AR) at Manaus city, Amazonas State, Brazil.

Fig. 4. The mean dissolved U content and 234U/238U activity ratio (AR) of groundwater samples analyzed in this study plotted along the W–E direction.

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–33 29

Fig. 5. The mean 234U/238U activity ratio (AR) of groundwater occurring in each cityplotted against the respective average dissolved U content.

Fig. 6. Block diagrams representing, along the W–E direction, the distribution of a)the mean dissolved uranium content, b) the mean 234U/238U activity ratio (AR) andc) the altitude of the cities.

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–3330

Humic substances (HS) are amorphic, dark and resistant tobiological and chemical degradation, possessing a large content ofoxygenated organic groups, such as carboxylic, phenolic, enolic,alcoholic and quinone groups (Oliveira et al., 2007). The presenceof humic substances in surface Amazon waters has been re-cognized, as well the acid character of forest streams (pH¼4.5;Furch and Junk, 1997) and the Negro River (pH¼4.5–5.0; Oliveiraet al., 2007). The Negro River is a tributary of the Amazon Riverwhich contains a high concentration of Dissolved Organic Material(DOM) (600–900 μmol L�1) compared to that of other tributaries(300–500 mmol L�1) in the locale (McClain et al., 1997). Accordingto Ravichandran (2004), about 80% of the DOM is in the form ofHS. The movement of OM derived mainly from extensive areas ofhydromorphic podzols from the upper basin can cause acidity inthe Negro River waters (Oliveira et al., 2007).

HS have been extracted by Oliveira et al. (2007) from watersamples collected monthly from the Negro River basin in Ama-zonas State. Statistical analyses showed that when the pluvio-metric index was greater and the fluviometric index was smaller,the degree of humification of aquatic substances was greater.Leenheer (1980) pointed out that groundwater drainage frompodzol soils is the primary source of black waters in the Amazonarea and that organic materials responsible by coloration of thesewaters have chemical and physical properties similar to humicsubstances in soil.

These black waters generally are characterized by high per-centages of organic acids, where the highest Dissolved OrganicCarbon (DOC) (52%) is found in the hydrophobic-acid fractioncontaining humic and fulvic acids (Leenheer, 1980). Ratios of hu-mic acids to fulvic acids in soil and water extracts have been de-termined by Leenheer (1980) to investigate the relative mobility ofthese natural organic acids in Amazon soils and water. In thepodzol soil where black water originates, about 75% of the ex-tractable humic solutes in the surface litter layer were humic acids.The chemical data in black water from a spring draining podzolindicated that preferential leaching losses of fulvic acid resulted ina correspondingly high concentration of fulvic acid (65% of allhumic-type solutes) (Leenheer, 1980).

The HS characterization of the waters studied was beyond thescope of this paper, however, such findings indicate how relevantdissolved HS contents are to cause acidity in the groundwater.Their occurrence is likely due to the abundant vegetation debris,whose infiltration into the aquifer systems would be facilitated bythe accentuated rainfall and abundant surface water resources.

The hydrogeology of Amazon area has not been carefully in-vestigated yet due to the little interest historically in groundwaterresources in the face of the abundance of surface waters availablein the region. Thus, there is not enough information on the sub-surface flow directions in the area spreading along the W–E di-rection (Uarini to Parintins). In order to spatially represent the

acquired data in this sector, it is assumed that the regionalgroundwater flow follows the topography, despite a series ofstructural arches trending SE–NW which may inhibit flow (Fig. 1).This approach allows the construction of block diagrams to displaythe average AR and dissolved U content data, as well the localtopographic altitude (Fig. 6). It is seen that the variations in the ARand dissolved U content values do not exactly coincide perhapssuggesting that more hydrogeological investigation then is neededto explain the physical location of these trends. However, thehighest dissolved U content and AR values do occur at Barreirinhaand Parintins cities where the terrain slope is lower (Fig. 6) andwhere the Amazonas River makes 81% of its total discharge

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–33 31

(Goulding et al., 2003). Such conditions are favorable then for theinfiltration of HS, potentially contributing to the enhanced U-iso-topes release into groundwater occurring there. This is supportedby a wide range of studies reported in the literature focusing therole of colloids for the transport of U-isotopes (see, for instance,Porcelli et al. (1997), Zänker et al. (2007) and Crançon et al.(2010)).

4.4. Uranium isotopes as prospecting tool

238U and 234U have been considered useful isotopes for thehydrogeochemical prospection for concealed U deposits. The datafor dissolved U content and AR in groundwaters are plotted on atwo-dimensional diagram containing several areas of associativesignificance (Cowart and Osmond, 1980; Osmond and Cowart,1981; Chatam et al., 1981). In terms of dissolved U content data,the main categories defined are: oxidized aquifer containing“normal” U content (values of 1–10 μg L�1); oxidized aquifercontaining “enhanced” U content (values 410 μg L�1); reducedaquifer or strata with low U content (values o1 μg L�1). In termsof AR data, values between 1 and 2 define “normal” worldwidesituation, values 42 suggest the formation of a U-accumulation,and values o1 indicate the remobilization of a U-accumulation.This modeling is potentially useful in the study area because aU-deposit economically feasible as by-product of Sn, Zr and Ta-Nbalready has been identified in Pitinga mine (a fluvial placer de-posit) situated just 300 km from Manaus city (MBendi, 2014).Thus, the data reported in this paper could be used for identifyingother hidden U-deposits.

The U content and AR values reported in this paper are plottedin Fig. 7 together with the data interval obtained by Gomes (1978),Bonotto (1982, 1986) and Silva (1999) for groundwaters analyzed

Fig. 7. Plotting of the U-isotopes. data found in this study in the Cowart and Osmond ((1982, 1986) (sandstoneþvolcanic rocks, alkaline rocks) and Silva (1999) (Alter do Chão

in previous studies. The values given in Table 1 are inserted in thedata interval reported by Silva (1999) for groundwater providingfrom Alter do Chão Formation in Manaus city. However, they differsubstantially from the data set reported by Gomes (1978) andBonotto (1982), as the associated aquifers lithologies and climaticconditions are quite variable.

Most of the U content and AR data plotted in Fig. 7 fit the field“normal reduced”, conforming with the “reducing acid” classifi-cation based on the Eh–pH diagram (Krauskopf and Bird, 1995).Thus, reducing conditions are also suggested by the dissolved Ucontent vs. AR diagram in Fig. 7, which should dominate thegroundwater circulation environment, where lithologies contain-ing low uranium concentration would be leached during the wa-ter–soil/rock interactions.

An aspect that could be argued is the usefulness of the U-iso-topes diagram as a prospecting tool in systems where U may beassociated particularly with HS. Bonotto (1989, 2010) has eval-uated such a possibility at the Th-REEs ore body zone in Morro doFerro area, Poços de Caldas plateau, Brazil. Lei (1984) had sug-gested that natural fulvic and humic acids occurring there wereimportant agents responsible for the Th presence in groundwater.Considering the total DOC and U content data in groundwater,Bonotto (1989, 2010) proposed that organic soluble compoundscould be important for the U-transport in solution, because: a)soluble organic complexes are commonly formed with uranium(Pauli, 1975); b) no significant relationships were found in thiscase among the dissolved uranium and typical inorganic com-pounds that form soluble complexes with the uranyl ion, which isgenerally responsible by the U-migration in most groundwatersystems (Langmuir, 1978). The U-isotopes model for hydro-geochemical prospection of U-deposits is appropriate to this par-ticular site if the dissolved U content range in the “normal

1980) diagram. The data intervals obtained by Gomes (1978) (limestone), BonottoFm., Manaus city, Amazonas State) are also shown for comparison.

M.L. da Silva, D.M. Bonotto / Applied Radiation and Isotopes 97 (2015) 24–3332

oxidized” field is 0.01–1 μg L�1 rather than 1–10 μg L�1.Finally, in the current paper, a small number of points ex-

hibiting ARs values 42 in a reducing environment which couldsuggest the formation of a U-accumulation, but further studieswould be necessary to confirm this possibility as the increasedwater acidity could cause this AR enhancement into solution.Nevertheless and generally, in principle, the database reportedhere does not indicate the presence of any hidden U-accumulationin the study area.

5. Conclusion

This study has shown that the waters analyzed in 15 cities lo-cated at Amazonas State, Brazil, are appropriate for drinking pur-poses in terms of the presence of dissolved uranium. The use ofthe U content vs.234 U/238U activity ratio (AR) diagram indicatedthat the waters studied are leaching lithologies but exhibiting lowU concentration. These parameters yielded different values ingroundwater of Alter do Chão, Içá and Nhamundá formations,suggesting they are a good tool for discriminating the variouslithologies and confirming their applicability in hydrological stu-dies. The AR of dissolved uranium in Manaus city increased ac-cording to the groundwater flow direction, whilst four regionshave been also identified in which the AR tended to increase in theSW–NE direction: Uani to Tefé; Manacapuru to Manaus; Pre-sidente Figueiredo to São Sebastião do Uatumã; and Boa Vista doRamos to Parintins. The average dissolved U content in the wholestudy area also tends to increase following the SW–NE direction,implying a direct relationship between the mean AR and dissolvedU content. A possible explanation for such finding would be thefact that the increasing acid character of the waters related to thepresence of humic substances could enhance the dissolution ofuranium and more labile 234U from the sediments. Overall, a usefulU-isotopes database has been generated in a wide area occurringin the Amazon region, Brazil, contributing for the development offurther detailed surveys there.

Acknowledgments

FAPESP (Foundation Supporting Research in São Paulo State)and CNPq (National Council for Scientific and Technologic Devel-opment) in Brazil are greatly thanked for financial support of thisinvestigation. Prof. Dr. Trevor Elliot is thanked very much forhelpful comments that improved the readability of themanuscript.

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