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

Oct 04, 2018

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  • 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: [email protected] 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,

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

  • 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)

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

  • 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 40C) 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 lC/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,

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