basin, South America, and comprising southern Brazil ... · Uranium in groundwaters from Botucatu-Piramboia aquifer, Brazil D.M. Bonotto Departamento de Petrologia e Metalogenia,

Uranium in groundwaters from Botucatu-

Piramboia aquifer, Brazil

D.M. Bonotto

Departamento de Petrologia e Metalogenia, Institute de Geociencias eCiencias Exatas-UNESP, C.P. 178, 13506-900 Rio Claro, Sao Paulo,BrasilEmail: dbonotto@dpm. igce. unesp. br

Abstract

Groundwater from 60 pumped tubular wells of the Botucatu-Piramboia aquifersystem located at the Parana sedimentary basin in Brazil were chemicallyanalysed with the aim of evaluating if the mechanisms related to the migration ofuranium can generate concentrations greater than the maximum permissible limitin drinking water, as defined by the Brazilian national standards.

1 Introduction

The natural radioelement uranium is widely distributed in crustal rocks, andunder conditions present at the earth's surface, it tends to be a mobile element.^*U is the principal isotope of natural U (99.3% abundance) and is the parentnuclide in the mass number (4n+2) decay series, the longest known series.Uranium in the hydrologic environment is of special interest because of itseconomic importance and its chemical and radiotoxicity and that of some of itsdaugther nuclides. Worldwide soluble concentrations generally range from 0.1 to10 |uigL"* in rivers, lakes and groundwaters [1,2]. The purpose of thisinvestigation is to identify the mechanisms of mobilization of *U in the hugeaquifer Botucatu-Piramboia located in the South American continent, whosewaters are extensively used for drinking purposes, as well to evaluate if themeasured concentrations in groundwaters are greater than the maximumpermissible concentration limit defined by the national standards.

The Botucatu-Piramboia aquifer of Triassic-Jurassic age has continentaldimensions, extending over some 950,000 km^ within the Parana sedimentary

Transactions on Ecology and the Environment vol 26, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

282 Water Pollution

basin, South America, and comprising southern Brazil (states of Mato Grosso,Mato Grosso do Sul, Goias, Minas Gerais, Sao Paulo, Parana, Santa Catarina andRio Grande do Sul), eastern Paraguay, NW Uruguay and the northeasternextreme corner of Argentina. The aquifer has an average thickness of 300-400 m,being composed of silty and shaly sandstones of fluvial-lacustrine origin (thePiramboia formation), and variegated quartzitic sandstones accumulated byeolian processes under desertic conditions (the Botucatu formation) [3]. A thickbasaltic package of the Serra Geral formation (up to 1,500 m) overlies thisaquifer, reducing its exposed areas in non-continuous elongated strips, 10-100km wide, along the edges of the proper Parana basin, where the longest stripstretches between the states of Sao Paulo and Santa Catarina, a portion of whichis shown in Fig. 1. The aquifer overlies previous formations ranging from theigneous basement to the Paleozoic sediments of the Passa-Dois and TubaraoGroups, being covered by Cretaceous sediments of the Bauru-Caiua formations.

Situated within an intercratonic basin, the aquifer is almost undisturbed,data on water potentials were obtained from exploration boreholes drilled forpetroleum research and from some water wells that tapped the aquifer. Thepotentiometric surface of the water shows that about 70% of its total area hasartesian conditions [4], and recharge occurs by direct infiltration of rainwater inthe outcrop area, which is about 98,000 knf [5]. The percolating water movesfrom the phreatic exposed areas that surround the entire basin (Fig. 1) towards itscentral part, and, in spite of the great distances separating the existingexploitation centres, data obtained sporadically indicate hydraulic conductivityvalues of 10"MO~* m/s, effective porosity values of 10-20%, storage coefficientvalues of 10~MO~* and average transmissivity of 10~* nf/s [5]. The yields of thewells vary from 10-150 nf/h in the phreatic exposed parts of the aquifer to morethan 300 mVh in the confined artesian wells [3].

2 Sampling and analytical methods

The sampling of the Botucatu-Piramboia aquifer was performed at 51 localitiesin Sao Paulo, Mato Grosso do Sul and Parana States, where 60 groundwatersamples (Fig. 1 and Table 1) were collected from pumped tubular wells fortemperature, bicarbonate, carbonate, and uranium analyses. The available datadescribing the wells allowed to estimate the geostatic pressure, P, from theequation [7]: P = Pa + pgh, where Pa is the atmospheric pressure, p is theaverage density of the terrain, g is the gravity acceleration, and h is the depth ofthe top of the aquifer. Table 1 reports the results of these calculations.

The groundwater samples (19-20 kg) were stored in polyethylenebottles, one unfiltered and unpreserved aliquot was used for temperature,


GROUP/FORMATIONBauru/Serra GeralBotucatu/PiramboiaPassa-Dois/TubaraoCryst. basement

z Drainagey< Groundwater^ flowline

c,0 Equipotential line

40 Sampling point0 60km

Brasilia

) HSacPauloState

Figure 1: The groundwater flow in the Botucatu/Piramboia aquifer [6] and location of the sampling points for U analysis.

to00U)


284 Water Pollution

bicarbonate, and carbonate determinations, whereas other aliquot was filteredthrough 0.45 |im membrane and preserved with HC1 for the evaluation ofuranium. The methyl orange end-point titration standard analytical technique wasutilized for characterizing the bicarbonates and carbonates, data that allowed toestimate the CO] partial pressure by using the WATEQ 4F geochemical software[8].

The aliquots for U analysis were acidified to pH less than 2 on usingHC1, about 500 mg of FeClg plus 3.39 dpm of U were added, and U was co-precipitated on Fe(OH)3 by increasing the pH to 7-8 through addition ofconcentrated NI OH solution; the precipitated was recovered, dissolved in 8MHC1 and Fe^ was extracted into an equal volume of isopropyl ether. The acidsolution U-bearing was purified by anion exchange, first on a Cl" and then on aNOg" column of Rexyn 201 50-100 mesh resin. U was finally eluted from theNO]" column with O.I M HC1 and after evaporation to dryness was dissolved in10 ml of 2M (NH4)2SO4 electrolyte and transferred to an electrodeposition cell.The pH was adjusted to 2.4 and electrodeposition of U on a stainless steelplanchet was complete after 3 hours at a current density of 1 Acm" .

The U content was measured by alpha spectrometry. The a-activitieswere determined with two 0.1 mm depletion depth, 200-450 mnf area siliconsurface barrier detectors. The spectra for natural U and U tracer extracted wererecorded on a EG&G ORTEC 919 Spectrum Master Multichannel Buffer. TheDecision Level L^ [9] for acceptance of a positive measurement in the *U andU energy regions was 0.00082 and 0.00225 cpm, respectively. Theconcentration data were calculated by isotope dilution from the counting rates of^*U and U peaks, where the analytical details for these measurements werereported elsewhere [2]. The results obtained in this investigation are reported inTable 1.

3 Migration of uranium in the aquifer

The occurrence of water at high temperatures (above 40°C) was verified due tothe great depths the aquifer reaches (almost 2 km) and the thick confiningbasaltic cover. A linear relationship (r = 0.94) was observed between thetemperature and geostatic pressure (Fig. 2), confirming the results obtained byprevious investigators [6, 10] and relating the groundwater flow from the borderof the basin towards its central part, in the direction of the dip of the geologicalunits, according to the natural geothermal gradient of about l°C/35 m. Asaconsequence of such relationship, it was also observed an inverse linearrelationship between the COz partial pressure and geostatic pressure (r = -0.69)(Fig. 3), suggesting that the increase in temperature causes a decrease in thepartial pressure of CO2 in the analysed thermal and non-thermal groundwaters.

Aqueous-speciation modeling using WATEQ 4F geochemical computercode [8] allowed to evaluate the mineral saturation index (SI) for carbonates,


ie tubular wells drilled at the Botucatu/Piramboia aquifer and chemical analyses of groundwaters.

Li

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4679

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Longitude

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51°09'49"W50°3338"W

52°0577"W

51°22'43"W

50°26'21"W50°26'21"W47°41'00"W47°49'05"W47°49'05"W50°1432"W50°1432"W47°5338"W47°5338"W51°41'49"W50°3338"W48°5433"W49°22'44"W

Altitude(m)

650

586

450474

258

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355410835775880440480835805315433499490

Depth(m)

1082

979

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3953

1800

969120086130129146016831401304582132325671136

G. Pres.(bar)

228.5

214.2

224.8258.7

430.4

382.0

250.8257.614.10.90.9

341.0354.721724.7218.7266.9228.5221.6

Temp.

45

47

3843

70

63

42422526255956222346524545

HCCV(mg/L)

83

5897

189

200

11912210052928111211329895109

CO]'(mg/L)

101

114144

33

16

574840

3038

44366865

COz p.pn(x!0~* atr

0.04

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0.84

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the tubular wells drilled at the Botucatu/Piramboia aquifer and chemical analyses of groundwaters.

Latitudi

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Altitude(m)

496451460460417510480450495630590613600645700510507501560540

Depth(m)

8606002774343459295140298176134150145196120266222225208353266

G. Pres.(bar)

121.9112.174.077.7158.415.213.613.434.32.50.90.915.54.927.911.515.29.436.967.1

Temp.(°C)

3736293541302730302732272828313131313329

HC(V(mg/L)

104102861288562331045956122123331413181195

CCY(mg/L)

6869

62145617

CO% p.pn(xlO'3 atr

0350.1634.300.540.020200.1216.20

23.709.05159.00145.0073.606.2364.2036.7055.600.680.29


Water Pollution 287

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288 Water Pollution

O70

CK

Z 45LLJQ_2 20

100 200 300

G.PRESSURE (bar)

400

Figure 2: The temperature of groundwater from Botucatu/Piramboia aquiferplotted against the geostatic pressure in the aquifer.

10-'

O/

£2

oO

10 =

10"

10'=1 10 100 G.PRESSURE (bar)

Figure 3: The CO? partial pressure of groundwater from Botucatu/Piramboiaaquifer plotted against the geostatic pressure in the aquifer.


Water Pollution 289

where negative values obtained for most of the samples indicated the capabilityof the water for dissolving (hydro)magnesite, dolomite, calcite, aragonite,nahcolite, trona, natron, thermonatrite, huntite, nesquehonite, and artinite,whereas positive values obtained for certain samples suggested that water issupersaturated with respect to some of these mineral phases. These resultsshowed that, depending of the temperature of the water, kinetics and propertiesof the carbonate phase considered, then, dissolution or precipitation of themineral may be occurring. For instance, when calcite is taken into account, asaturated solution of this mineral in equilibrium with CO] at 3x10"^ atm containsabout 75 mg/L of Ca^ at 5°C, but only 40 mg/L at 30°C [11], showing, as aresult, that dissolution of this mineral is favored at lower temperatures, whereasprecipitation is expected at higher temperatures. In spite the solubility of calcitedecreases with increasing temperature, this, in fact, is not the rule for allcarbonates, since some of them exhibit an opposite behavior.

Uranium that goes into solution can migrate over long distances, mainlydue to its ability to form complexes with bicarbonate and carbonate ions, whichare most stable in solutions whose pH is greater than 7.5 [12]. Such typicalbehaviour of uranium was evidenced in the studied aquifer by two linearrelationships: bicarbonate + carbonate vs. geostatic pressure (r = 0.71) (Fig. 4)and uranium vs. geostatic pressure (r = 0.46) (Fig. 5). Obviously, as aconsequence of them, it was also found a linear relationship between thedissolved bicarbonate/carbonate ions and dissolved uranium (r = 0.47) (Fig. 6).Therefore, complexation of the uranyl ion (UO] ) with bicarbonate/carbonateions must be largely occurring at the studied aquifer, being again recognized theimportance of this mechanism for the transport of U in solution.

U

oU

0.1

0.02 -L-,

1 10 100 G. PRESSURE (bar)Fig. 4: The dissolved bicarbonate + carbonate in groundwater fromBotucatu/Piramboia aquifer plotted against the geostatic pressure in the aquifer.


290 Water Pollution

O)A

1

0.01

1 10 100 G. PRESSURE (bar)Figure 5: The dissolved uranium in groundwater from Botucatu/Piramboiaaquifer plotted against the geostatic pressure in the aquifer.

(No"U

' <r>O

o.,

0.01 —

0.01~r~0.1 1

URANIUM (n g/L)Figure 6: The relationship between dissolved bicarbonate+carbonate anduranium in groundwater from Botucatu/Piramboia aquifer.


Water Pollution 291

4 Radioactivity due to uranium and water quality

Some national standards for limiting radiation exposure establish maximumpermissible U concentration in drinking water, since anomalously high U levelsmay occur. However, gross-alpha and gross-beta radioactivity measurementshave been more commonly used in setting permissible standards for members ofthe public. The criterion used in Brazil as defined by the Rule No. 36 (19January 1990) of the Health Ministry establishes 0.1 BqL"' for the total alpharadioactivity and 1 BqL"* for the total beta activity, where the identification ofthe dissolved radionuclides and the measurement of their concentrations in thesamples must be performed only when the values found in them are greater thanthe gross-alpha and gross-beta activity contaminant limits. Under suchcircumstances, the maximum annual limit of intake of ^U as dissolvedinorganic uranium compounds corresponds to 5x10^ Bq [13].

The dissolved uranium content in groundwaters of the Botucatu-Piramboia aquifer varied between 0.01 and 4.82 jiglA If it is assumed that 2liters of water are ingested daily by every individual, and since 0.7336 is the ^Udisintegration rate for natural U (min~* fig"'), then, it is possible to estimate that44 Bq corresponds to the maximum annual intake of *U. Such value isconsiderably lower than the maximum annual limit established by the Brazilianstandard for the intake of *U, and, therefore, it is possible to conclude that theactive dissolution and migration of U which are taking place in the studiedaquifer don't affect the quality of the underground hydrological resources. Thus,the results obtained in this investigation allow to assert that the analysed waters,in terms of the presence of soluble uranium, may be used for drinking purposeswithout health risks.

Acknowledgments

The author thanks the International Atomic Energy Agency (IAEA ResearchContract No. 9723/Regular Budget Fund) and CNPq (Conselho Nacional deDesenvolvimento Cientifico e Tecnologico)-Brasil for financial support of thisinvestigation.

References

1. Fritz, P. & Fontes, 1C. (eds.), Handbook of Environmental IsotopeGeochemistry, Elsevier, Amsterdam, pp. 259-282, 1980.

2. Ivanovich, M. & Harmon, R.S. (eds.) Uranium series disequilibrium:applications to environmental problems, Clarendon Press, Oxford, 1982.

3. Gilboa, Y., Mero, F. & Mariano, I.B. The Botucatu aquifer of South America,model of an untapped continental aquifer, J. Hydrol, 29, pp. 165-179, 1976.


292 Water Pollution

4. Rebougas, A.C. Groundwater in Brazil, Episodes, 11, pp. 209-214, 1988.

5. Kimmelmann e Silva, A.A., Rebougas, A. C. & Santiago, M.M.F. '*Canalyses of groundwater from the Botucatu aquifer system in Brazil,Radiocarbon, 31, pp. 926-933, 1989.

6. Silva, R.B.G. Estudo hidroquimico e isotopico das dguas subterrdneas doaquifero Botucatu no Estado de Sao Paulo, USP, Sao Paulo, 1983.

7. Castany, G. Principes et methodes de I 'hydrogeologie, Dunod, Paris, 1982.

8. Truesdell, A.H. & Jones, B.F. WATEQ, a computer program for calculatingchemical equilibria of natural waters, Jour. Research U.S. Geol. Survey, 2, pp.233-248, 1974.

9. Currie, L.A. Limits for qualitative and quantitative determination, Anal.Chem.,40, pp. 586-593, 1968.

10. Teissedre, J.M. & Barner, U. Comportamento geotermico e geoquimicodas aguas do aqiiifero Botucatu na bacia do Parana, Revista AguasSubterrdneas,4, pp. 85-95, 1981.

11. Faure, G. Principles and applications of inorganic geochemistry, MacMillanPublishing Company, New York, 1991.

12. Langmuir, D.Uranium solution-mineral equilibria at low temperatures withapplications to sedimentary ore deposits, Geochim. Cosmochim. Ada, 42, pp.547-569, 1978.

13. CNEN (Comissao Nacional de Energia Nuclear) Diretrizes bdsicas deradioprotecao, Rio de Janeiro, 1988.


basin, South America, and comprising southern Brazil ... · Uranium in groundwaters from Botucatu-Piramboia aquifer, Brazil D.M. Bonotto Departamento de Petrologia e Metalogenia,

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basin, South America, and comprising southern Brazil ... · Uranium in groundwaters from Botucatu-Piramboia aquifer, Brazil D.M. Bonotto Departamento de Petrologia e Metalogenia,