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					                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Radon requires the presence of a Th-rich surface layer

                    U and Th radionuclides in a sandy aquifer                                                                                                                                                                                                                                                              Results       Uranium in the vadose zone                                                                                                                                                             222
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Rn activities are 103 times higher than those of parent 226Ra in
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               the water.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     mineral without surface coating
                                                       A. Tricca, G.J. Wasserburg                                                                                                                                                                                                                                                        The U activity is variable in the vadose zone and indicates high weathering rates.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Rn activities in the water (100-300 dpm/kg) represent ~5% of
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               the rock production by α−recoil.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     Rn activity 100 times too low
                                           California Institute of Technology, Pasadena, CA                                                                                                                                                                                                                                              The 234U/238U activity ratios are also variable and proportional to ε234Th/w238U                                                                                                      Such high 222Rn activities cannot be produced by a homogenously
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               distributed aquifer solid of grain size >10 µm
                                                                                                                                                                                                                                                                                                                                         U in the vadose zone is the dominant input for the U in the aquifer.

                                                                                                                                                                                                                                                                                                                                       Uranium in the aquifer                                                                                                                                                                  The 226Ra necessary to produce the 222Rn measured in the water                        steady-state surface coating
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               must be located on the surface coating or at grain boundaries
                                                                                                                                                                                                                                                                                                                                       U activity                                                                                                                                                                              This cannot be attained by reversible precipitation of Ra; the
  Objectives:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  alternative, irreversible precipitation of Ra, is not compatible with                                                 α−recoil
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 Rn activity 10 times too low
                                                                                                                                                                                                                                                                                                                                         - 238U activities indicate a small added contribution from the aquifer (low weathering rates)
  To understand the transport of natural radionuclides in ground water                                                                                                                                                                                                                                                                                                                                                                                                                                                         other Ra data
                                                                                                                                                                                                                                                                                                                                         - 238U and 234U activities are variable and reflect input from the vadose zone
  Measurements of full decay series provide information on water-rock                                                                                                                                         In a closed system, all                                                                                                    - the wide variations of 238U activities accross the site indicate that dispersive mixing is not important                                                                                                                                                                  saturation of Th
  interactions                                                                                                                                                                                                nuclides of the same
                                                                                                                                                                                                                                                                                                                                         - there is no evidence of U adsorption (Th/U ratio in the water ~10-3crustal ratio)                                                                                                   As Th is at saturation, we show that th is precipitated irreversibly.
  We used a two-sided approach to understand the mechanisms:                                                                                                                                                  series have the same
                                                                                                                                                                                                              activities                                                                                                                                                                                                                                                                                                       We infer that the surface coating in the aquifer contains ThO2                         10%of Th, Ra                               Rn activity supplied
     - experimental: data collected from the Long Island Aquifer                                                                                                                                                                                                                                                                                                                                                                                                                                                               (~10% of the Th rock inventory) deposited in earlier weathering
                                                                                                                                                                                                                                                                                                                                       U isotopic composition
     - theoretical approach: theoretical model of activities along a flow line
                                                                                                                                                                                                                                                                                                                                           - the U isotopic composition reflects the ratio recoil/weathering input rates (ε234Th/w238U)                                                                                        cycles. This forms the major source for 222Rn.
                                                                                                                                                                                                                                                                                                                                          - the increase in 234U/238U shows evidence of recoil contribution within the aquifer for waters with
                                                                                                                                                                                                                                                                                                                                          low U activity input from the vadose zone

                                                                                                                                 The decay series include isotopes with a wide range of
                                                                                                                                 half-lives that reflect different time-scales                                                                                                                                                         Only high δ234U and low 238U activities or low δ234U and high 238U activities can be generated in the
The sampling field                                                                                                                                                                                                                                                                                                                     aquifer at flow distances < 100km. δ234U and 238U activities monitor the aquifer input over long                                                                                         Radium
                                                                                                                                 different elements with different chemical properties                                                                                                                                                 distances
                                                                                                                                 nuclides of different elements that are connected by                                                                                                                                                                                                                                                                                                                                           Ra sources
The aquifer at Brookhaven National Laboratory
                                                                                                                                 decay reactions                                                                                                                                                                                                                                                                                                                                                                                 - Ra in the vadose zone is provided by weathering and                      Vadose zone                                              Aquifer
The unconfined Upper Glacial aquifer at Brookhaven                                                 Long Island Sound                                                                                                                                                                                                                                                                                                                                                                                                             α−recoil
                                                                                                                                                                                                                                                                                                                                                                                                                                         1400                         W4(D)
                                                                                                                                                                                                                                                                                                                                             N                                                                              S
National Laboratory was chosen for study:                                                                                                                                                                                                                                                                                                                                                                                                                                           deeper wells                                 - Ra in the aquifer is provided by α−recoil                     Sorption   k1     k-1 Desorption             Sorption          k1      k-1 Desorption
                                                                                                                                                                                                                                                                                                                                                                                                                                                                  10 km             water table wells
     it is an unconsolidated sandy aquifer;                                                                                                                                                                                                                                                                                                                                                                                              1200                                           -3 234
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  ε=5x10 , δ Uw0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               =0                                                                                                                       α−recoil
     it has a well-characterized hydrology;                                            New York                                                                                                                                                                                                                                                                                                         Vadose zone                                         (b)                                           (c)                   The Ra nuclides are associated with the surface coating
                                                                                                                                                                                                                                                                                                                                        0’                                                                                      0 m      1000
     it has an extensive groundwater monitoring network                                                        Brookhaven                                                                                                                                                                                                                                           238
                                                                                                                                                                                                                                                                                                                                                                          U                                                                                 5km                                                                 that is in local equilibrium with the water                                                                                               α−recoil
                                                                                                                                                                                                                                                                                                                                                                                                                                          800 W2(I)                                                                             The atom ratio of the surface coating to the water is ~104
                                                                                                     Atlantic Ocean
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               20 km
The cross section beneath shows                                                                                                                                                                                                                                                                                                                   produced by α−recoil
                                                                                                                                                                                                                                                                                                                                                                                    U produced by weathering
    -The vertical sequence including the vadose zone,                                                                                                                                                                                                                                                                                                                                                                                               W5(D)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       10 km
     Upper Glacial and Magothy Aquifers. Note that both                                                                                                                                                                                                                                                                                                                                                     Upper Glacial                 400 E3(D)
     aquifers are separated by a layer of clay minerals, the                                                                  The Model
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   5 km             M1
     Gardiner's Clay                                                                                                                                                                                                                                                                                                                                                                    Activity                                          200                             W3(D)               (a)
    - The water table and a flow line                                                                                         Assumptions of the model                                                                                                                                                                                                                                                                                                                E2(D)

                                                                                                                              The theoretical model used to interpret the data considers one dimensional flow along a flow line without dispersion.                                                                                                                                                                                         0
    - An example of well lines designed to collect water                                                                                                                                                                                                                                                                                                                                                                                        0   2     4       6   8   10 12 14 16 18 20 22
                                                                                                                              The initial activity at the water table is inherited from interaction of the water with the vadose zone
     along a flow line                                                                                                                                                                                                                                                                                                                                                                                                                                  238U              -3
                                                                                                                              The reactions and the resulting equations for the model aquifer are shown below.                                                                                                                                                                                                                                                 Aw (10 dpm/kg)                                                The Magothy aquifer
                                                                                                                                          3 Phases                              Groundwater                                                                        Transport Equations
                                               Vadose zone                                                                                                                                                                                                                                                                                                                                                                                                                                                                   The Magothy aquifer solid is similar to that of the Upper Glacial.
         N                                                         S                                                                                                         Outputs   Inputs                                                                      diAw =
                           BNL                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               The water of the Magothy shows different characteristics:
  100 ’                                                                30 m                                                                                                                               L
                                                                                                                                                                                                              α−recoil                                                                                                                                                                                                                                                                                                            1) the pH value is higher (6.77 against 4-5.5 for the Upper Glacial)
                                                                                                                                                                                                                                                                     diAw                                                              Thorium                                                                                                                                                                                    2) the water is less oxidizing
    0’                                                                 0 m                                                                                                                                     ε= 3L                                                                                                                                                                                                                                                                                                              3) the solution contains very high amount of Fe-rich colloids
                                                                                                                                                 Active Surface                              D                     D
                                                                                                                                                  Layer                                                                                                                                                                                232
  100 ’ Upper Glacial                                                  30 m                                                                                                                  k1      k-1 Desorption                                                                                                                       Th activity is at the saturation limit of ThO2, the 232Th activities higher than the
                                                                                                                                                                              Sorption                                                                                                                                                                                                                                                                                                                                       The chemical behaviour of the other radionuclides in the Magothy are the same as for the Upper Glacial
                                   Flow line (60-200m/yr)                                                                                                                                                                                        ww
                                                                                                                                                                                                                                                 iriir-1sciiscw1w + kA
                                                                                                                                                                                                                                                                                                                                       saturation limit are associated with colloids.
                                                                                                                                                Grains                                                                                                                                                                                                                                                                                                                                                                       U activity is similar to that of the vadose zone, there is no evidence of important aquifer contribution
                                                                                                                                                                                         x                                                                                                                                             the Th/U ratio in the water ~10-3crustal ratio, indicates that Th adsorbs strongly
  200 ’                                          Gardiner’s Clay 60 m
                                                                                                                                          (U/Th-Bearing                                                                                                                                                                                                                                                                                                                                                                      The low δ234U value shows a recoil comparable to that in the Upper Glacial
             Magothy                                                                                                                      Mineral)                                                              α−recoil
                                                                                                                                                   Ground                                                                                                                                                                                                                                                                                                                                                                    The very high 232Th activity (103 above the solubility limit) is associated with colloids (the Th/U ratio equals the ratio in the crustal rock)
  300 ’                                                                90 m                                                                                                                                                                                                                                                            The Th isotopic ratios reflect the ratios of the input rates: weathering for 232Th
             0 km      1 km        2 km         3 km        4 km                                                                                                                                              Weathering                                                                                                               and 228Th, recoil for 230Th and 234Th
                                                                                                                                                                                                                                                                                                                                       The 230Th/232Th atomic ratio is ~5x10-5 (10 times the crustal ratio) due to recoil

                                                                                                                                                                               Decay                          Parent Decay                                                                                                             Th adsorbs quasi irreversibly onto the surface coating. The ratio of number of
The samples                                                                                                                                                                                      P    i
                                                                                                                                                                                                                                                                                                                                       atoms in the surface coating to those in the water is ~107
Samples from shallow and deeper wells have been collected along two well lines
                                                                                                                                                                                                                                                                                                                                       The 234Th activities indicate a residence time in the water is ~1h                                                                                           Conclusions:
(Western and Eastern)
                                                                                                                                                                                                                                                                                                                                       The 228Th/234Th activity ratio is lower than unity and indicates slow desorption
A sample from the confined Magothy aquifer has also been collected
                                                                                                                                                                                                                                                                                                                                       kinetics.                                                                                                                                                    - Uranium
                                                                                                                                                                                                                                                 0.08                                                                                  The residence time of Th in the surface layer ~6000 year                                                                                                         the data provide a monitor for water-rock interaction in the aquifer over long time-periods (>2000years)
                                                                                                                            Solutions to the model equations                                                                                                                                                                                                                                                                                                                                            the U activity reflects dominantly the vadose zone input
                                                                                                                                                                                                                                                                                                                                                                                       Sorption                  k-1        Desorption                                                                  234 238
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           U/ U isotopic composition depends on the vadose zone input and on aquifer mineral grains input and travel time.
                               BNL Upper Glacial Contour Map                                                                3 patterns may occur depending on the half-life and the sorption                                                                                                226          222
                                                                                                                                                                                                                         Activity in water iAw
                                                                                                                                                                                                                                                                                           ( Ra, Rn)                                                                                                   k1
                                    Unconfined Aquifer                                                                      properties of the nuclide                                                                                            0.06                                                                      (3)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        - Radon
                                                                                                                              -linear evolution with distance (238U, 234U, 232Th) if the                                                                                                                                                                                                                                                                                                                   222
                                                                                                                              removal time-scaleof the nuclide or its parent (half-life and                                                                                                                                                                                                                                                                                                                    Rn is a monitor for local processes on very short time-scale
                                 Brookhaven Lab.                             N                                                                                                                                                                   0.04                                                                                                                                                                                                                                                      the high activity levels reflect the importance of the surface coating
                                                                                                                              adsorption time-scale) is greater than ground water age (age                                                                                    234    230          228          224
                                                                                                                                                                                                                                                                                                                                                                               Weathering                                 α−recoil
                                                                                 Additional                                                                                                                                                                                  ( Th,         Th,          Th,          Ra)
                                                                                                                              = distance/ velocity)                                                                                                                                                                        (2)                                                                                          230                                                                                to provide the measured 222Rn, a 232Th and 230Th-rich surface coating (containing ~10% of the rock inventory) must
                                                                                 water table                                                                                                                                                                                                                                                                                          232
                                                                                                                                                                                                                                                                                                                                                                                            Th,228Th                        Th,234Th
     Western Line of                                                                                                          -constant activity (234Th, 230Th, 228Th, 224Ra), if the removal                                                                                                                                                                                                                                                                                                               have been deposited during a previous weathering cycle
                                                                                 samples                                                                                                                                                         0.02
      Monitor Wells              40                                                                                           time-scale is very short compared to ground water age (age =                                                                                          238     234         232
                                                                                                                                                                                                                                                                                    ( U,          U           Th)              (1)                                                                                                                                                                  - Thorium
                                                                       Peconic                                                distance/ velocity)
                                                                        river                                                                                                                                                                                                                                                                                                                                                                                                                           Th is at ThO2 solubility limit in absence of colloids
                                                                                                                              -exponential evolution towards iAwinf (226Ra, 222Rn) if the                                                        0.00                                                                                References for Brookhaven National Laboratory:
                                                                                 Eastern Line of
                                                                                                                              removal time-scale of the nuclide or its parent is comparable                                                                0             1     2           3                  4            5         CopenhaverS.A., Krishnaswami S., Turekian K.K., Epler N., Cochran J.K. (1993)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Th and 228Th activities provide a residence time in the water ~1h and in the surface coating ~ 6000 years
          Magothy sample                                    30                   Monitor Wells

                                                                                                                              to ground water age (age =distance/ velocity)                                                                                    x226Ra        Distance(km)                                            de LagunaW. (1963)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    - Radium
                                                                                                                                                                                                                                                                                                                                     Geraghty and Miller (1996)
                              Carman's                      20                                                                                                                                                                                                                                                                       Tricca A., Wasserburg G.J., Porcelli D., Baskaran M. (2000)                                                                                                        Ra is strongly adsorbed on the mineral surface and is in exchange equilibrium with the water
                                river                                                                                                                                                                                                                                                                                                Tricca A., Porcelli D., Wasserburg G.J., (2000)                                                                                                                    224
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Ra, 226Ra, 228Ra activities input occurs by α-recoil from the rock in the aquifer, by weathering in the vadose zone

              Piezometric contours                                                    3 km
          in feet above mean sea level

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