Radionuclide groundwater

Figure 2. (A.) The radionuclides in an aquifer are divided into three reservoirs groundwater, the host aquifer minerals, and adsorbed onto active surfaces. Also shown are the processes adding to a daughter nuclide (closed circles) in the groundwater of weathering, advection, recoil from decay of parent atoms ( P ) in the aquifer minerals, and production by parent decay, the processes of losses of a radionuclide of advection and decay, and exchange between dissolved and adsorbed atoms.

Th do not reach constant concentrations over any reasonable distances, Ra does over several hundred meters, and all the other radionuclides require less than a few meters. Constant concentrations are achieved when the inventories of a radionuclide in both groundwater and adsorbed on surfaces decay at the same rate at which this radionuclide is... 

Figure 3. Systematics of radionuclides along the series. The major and minor fluxes to each nuclide can be readily seen from the arrows showm The behavior of each nuclide can be evaluated by considering the surface and groundwater populations individually, or together as the mobile pool. Nuclides in the decay series within the host rock minerals supply atoms at the surface and in the groundwater by recoil during a decay, so that there are greater abundances in the mobile pool of nuchdes progressively along the series, a decay of nuclides at the surface injects atoms back into the minerals as well as into groundwater.

As a noble gas, Rn in groundwater does not react with host aquifer surfaces and is present as uncharged single atoms. The radionuclide Rn typically has the highest activities in groundwater (Fig. 1). Krishnaswami et al. (1982) argued that Rn and all of the other isotopes produced by a decay are supplied at similar rates by recoil, so that the differences in concentrations are related to the more reactive nature of the other nuclides. Therefore, the concentration of Rn could be used to calculate the recoil rate for all U-series nuclides produced by a recoil. The only output of Rn is by decay, and with a 3.8 day half-life it is expected to readily reach steady state concentrations at each location. Each measured activity (i.e., the decay or removal rate) can therefore be equated with the input rate. In this case, the fraction released, or emanation efficiency, can be calculated from the bulk rock Ra activity per unit mass ... 

Since Ra and " Ra are both produced by recoil from the host mineral, it might be assumed that the production rates are equal. However, the relative recoil rates can be adjusted by considering that the parent nuclides near the mineral surface may not be in secular equilibrium due to ejection losses i.e., the activity of Th may be lower than that of Th due to recoil into groundwater of the intermediate nuclide Ra. Krisnaswami et al. (1982) calculated that the recoil rate of " Ra is 70% that of Ra if radionuclides are depleted along the decay chain in this way. 

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

Cherdyntsev W (1971) Uranium-234. Israel Program for Scientific Translations, Jerusalem Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Copenhaver SA, Krishnaswami S, Turekian KK, Shaw H (1992) and Th series nuclides in groundwater from the J-13 well at the Nevada test site implications for ion retardation. Geophys Res Lett 19 1383-1386... 

Dearlove JLP, Longworth G, Ivanovich M, Kim Jl, Delakowitz B, Zeh P (1991) A study of groundwater colloids and their geochemical interactions with natural radionuclides in Gorleben aquifer systems. Radiochim Acta 52/53 83-89... 

Hussain N (1995) Supply rates of natural U-Th series radionuclides from aquifer solids into groundwater. Geophys Res Lett 22 1521-1524... 

Luo SD, Ku XL, Roback R, Murrell M, McLing XL (2000) In-situ radionuclide transport and preferential groundwater flows at INELL (Idaho) decay-series disequilibrium studies. Geochim Cosmochim Acta 64 867-881... 

Bischoff JL, Fitzpatrick JA (1991) U-series dating of impure carbonates An isochron technique using total-sample dissolution. Geochim Cosmochim Acta 55 543-554 Bonotto DM (1998) Implication of groundwater weathered profile interactions to the mobilization of radionuclides. J South Am Earth Sci 11 389-405... 

Understanding the behavior of radionuclides in estuaries, as the dynamic interface between the continental hydrochemical systems and the ocean basins, requires consideration of broader chemical cycling in the hydrosphere. In this volume, the behavior of U- and Th-series isotopes in rivers is discussed by Chabaux et al. (2003), that in groundwaters by Porcelli and Swarzenski (2003), and that in oceans by Cochran and Masque (2003). General background information is provided by Bourdon et al. (2003). 

Cochran JK (1984) The fates of U and Th decay series nuclides in the estuarine environment. In The Estuary as a Filter. Kennedy VS (ed) Academic Press, London, p 179-220 Cochran JK (1992) The oceanic chemistry of the uranium - and thorium - series nuclides. In Uranium-series Disequilibrium Applications to Earth, Marine and Environmental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 334-395 Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cochran JK, Carey AE, Sholkovitz ER, Surprenant LD (1986) The geochemistry of uranium and thorium in coastal marine-sediments and sediment pore waters. Geochim Cosmochim Acta 50 663-680 Corbett DR, Chanton J, Burnett W, Dillon K, Rutkowski C. (1999) Patterns of groundwater discharge into Florida Bay. Linrnol Oceanogr 44 1045-1055... 

Moore WS (1992) Radionuclides of the uranium and thorium decay series in the estuarine enviromnent. In Uranium-series Disequilibrium Applications to Earth, Marine and Enviromnental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 396-422 Moore WS (1996) Large groundwater inputs to coastal waters revealed by Tla emichments. Nature 380 612-614... 

Reay WG, Gallagher DL, Simmons GM (1992) Groundwater discharge and its impact on surface water quality in a Chesapeake Bay inlet. Water Res Bull 28 1121-1134 Santschi PH, Li YH, Bell J (1979) Natural radionuclides in the water of Narragansett Bay. Earth Planet Sci Lett 45 201-213... 

ToranL. 1994. Radionuclide contamination in groundwater Is there a problem In Environmental science pollution control series. New York, NY M. Dekker, 437-455. 

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