Radium isotopes in shallow aquifers (Comunicazione a convegno)

  • Radium isotopes in shallow aquifers (Comunicazione a convegno) (literal)
  • 2013-01-01T00:00:00+01:00 (literal)
Alternative label
  • Mario Voltaggio (2013)
    Radium isotopes in shallow aquifers
    in Indo-Italian Workshop"Tecnological solutions for environmental sustainability", Nagpur INDIA, 5-6 marzo 2103
  • Mario Voltaggio (literal)
  • http://www.indiaitaly.com/IndoItalianNew/Upload/EnvironmentBrochure_Indo_Italian_Workshop521312348882.pdf (literal)
  • Istituto di Geologia Ambientale e Geoingegneria (literal)
  • Radium isotopes in shallow aquifers (literal)
  • RADIUM ISOTOPES IN SHALLOW AQUIFERS Mario Voltaggio CNR- Istituto di Geologia Ambientale e Geoingegneria, Roma (Italy) E-mail: mario.voltaggio@igag.cnr.it Keywords: Radium isotopes, ground water residence time, retardation factor Radium isotopes are a quartet of radioactive isotopes with half lives which extend from some days up to thousands of years (224Ra t1/2 =3.66 days, 223Ra t1/2 =11.4 days, 228Ra t1/2 =5.75 years, and 226Ra t1/2 =1600 years). They are members of the natural radionuclide series of 238U(226Ra), 235U(223Ra) and 232Th(224Ra and 228Ra). The hydrogeochemistry of radium depends mainly on : ( 1) its low ionic potential (z/R) favouring strong adsorption of the element on the surface of the aquifer rocks, (2) on its coprecipitation with barium in sulphate and carbonatic solution, (3) on its mobility from the aquifer rocks through alpha recoil processes and (4) on salinity of the ground water determining the deadsorption of radium from the surface of aquifer rocks. Radium has a conservative behavior in carbonatic or karst aquifers and in highly fractured aquifers. In aquifers with porous rocks, radium is instead strong adsorbed displaying a very high retardation factor. The measurement of radium isotopes in ground water of unconfined aquifers gives indications about the residence time of the ground water in the aquifer if the dissolution rate of the radium isotopes from the aquifer rock, their recoil rate, and the content in uranium and thorium of the country rocks are known. There are different methods to get this information either in the field or in laboratory. In particular cases, dissolution or recoil rates can be also ignored because the ratio between radium isotopes eliminates these parameters in the equations used for determining the residence time. Retardation factor can be modeled through laboratory experiments by studying adsorption and deadsorption of radium isotopes in samples representative of the aquifer rocks . The measurement of the radium isotopes is efficiently performed using acrylic fibers coated with manganese dioxide which adsorbs strongly radium. The adsorption is performed passing a fixed volume of solution through filters of Mn fibers, calibrated in laboratory with solutions at known content of Ra and the radium isotopes can be measured individually through ?-spectrometry (van Beek et al., 2010) (Fig.1). Differently, the Mn fibers can be also kept for a fixed time in ground water, and, taking into account a correction due to the accumulation time, the ratio between the isotopes is measured by ?-spectrometry. In this last case the single values of the Ra isotopes are recalculated measuring 226Ra in a concentrated volume of groundwater, by measuring the 222Rn emanation by ?-spectrometry after 20 days. Fig. 1 - Gamma spectrum of radium isotopes adsorbed on MnO2 coated fibers Five applications of the radium quartet (Rama and Moore, 1996) will be discussed: 1,2) The method is very promising in carbonate or karst aquifers. In Italy, where this kind of aquifers is very spread, the method has been applied to the most important aquifer of the Latium (Peschiera aquifer) feeding the city of Rome (Civita and Fiorucci, 2010) and to one of the most famous mineral aquifers (San Gemini aquifer). The residence time of water, coupled to the known data on the annual ground water discharge of the aquifers, gives an estimate of the volume of the water of the aquifer and permits to monitor over time the exploitation of the reservoir (Barbieri and Voltaggio, 1998). 3) Another application regards the aquifers affected by a contribute of elements of geothermal origin (Voltaggio et al., 1987; Froehlich et al, 1995). The shallow volcanic aquifer of Mt. Amiata which supplies water to about 700,000 citizens in the provinces of Grosseto, Siena and Viterbo is connected, through faults and fractures, to a large geothermal field. The exploitation of the geothermal field causes the progressive lowering of the water reservoir and enrichment of heavy metals in drinking water. The measurement of the ground water residence time allows to estimate the volumetric reduction of the aquifer. 4) In the Lagoon of Pialassa Baiona (Ravenna, Italy) the San Vitale Pinewood is affected by major salinization processes induced by local subsidence and concerning underground water and soils. Ra isotopes give estimate of the rate of salinization in different points of the pinewood (Petrini et al., 2009). 5)The residence time of the water in the shallow unconfined aquifers of the fly ash deposits of the Koradi thermal power plant (Nagpur, India), is correlated with the TDS of the streams draining the fly ash deposits. In these shallow aquifers the retardation factor has been estimated by adsorption and deadsorption experiments in laboratory (Fig.2). The values of Ra isotope composition of draining streams, and of polluted and unpolluted wells of the zone ( Ramya et al., 2012 ) are part of a wider isotope-based model for the assessment of environmental hazard and ground water vulnerability from fly ash ponds ( Project: CNR /IGAG/- CSIR /NEERI/ Water Technology and Management Division/, Italy-India bilateral Programme). Fig. 2 - Adsorption-deadsorption experiments Barbieri M. & Voltaggio M. (1999) U-234/U-238 and Sr-87/Sr-86 applications to the hydrogeology of Sangemini Area (Terni), Central Italy. Min. Petr. Acta., 41, 119-126. Civita M, Fiorucci A. (2010) The recharge - discharge process of the Peschiera spring system (central Italy) AQUA mundi (2010) - Am02019: 161 - 178. Froehlich K, Battaglia A., Ceccarelli A., Ridolfi A and Panichi C. (1995) Radium Isotopes contribution to geothermal exploration in Italy. World geothermal Congress 1995. Section 4, Exploration and conceptual modeling. Petrini R., Pennisi M., Adorni-Braccesi A., Stenni B., Flora O., Voltaggio M.(2009)-Isotope-systematics to explore salinization and flushing in coastal aquifers: preliminary data from the Pialassa Baiona ecosystem (Adriatic Sea). In: EAQ III air, water and soil pollution, Imola 24-25 june,p.43-50 ISBN/ISSN 8890126175. Rama and W. S. Moore. (1996). Using the radium quartet for evaluating groundwater input and water exchange in salt marshes. Geochim. Cosmochim. Acta, 6023, 4645-4652. Ramya S.S., V. U. Deshmukh, Vijendra J. Khandekar , C. Padmakar , L. SuriNaidu , Piyush K. Mahore, Paras R. Pujari , D. Panaskar , P. K. Labhasetwar , V. V. S. G. Rao (2012) Assessment of impact of ash ponds on groundwater quality: a case study from Koradi in Central India. Environ Earth Sci.DOI 10.1007/s12665-012-2071-7 van Beek P, Souhaut M, Reyss JL.(2010) Measuring the radium quartet (228Ra, 226Ra, 224Ra, 223Ra) in seawater samples using gamma spectrometry. J Environ Radioact. 101(7), 521-9 Voltaggio M., Andretta D. and A, Taddeucci (1987) Dating of newly formed minerals in geothermal fields through 232Th series short lived isotopes: check on mineral of known age and implications to fluid-rock interaction .Geothermics,16,3,255-261. (literal)
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