Simulation of Uranium Tailing Leaching using VS2DRT
Sosina S. Haile and Broder J. Merkel
Department of Geology, Technische Universität Bergakademie Freiberg, Gustav-Zeuner-Str. 12,09596 Freiberg/Sa., Germany, wedatnsos@yahoo.com and merkel@geo.tu-freiberg.deAbstract
Uranium contamination of surface and ground waters from uranium tailings is a serious
environmental problem in many countries that have uranium mining. Reactive transport models
can be used to predict the potential environmental impacts of new uranium tailing sites, as wellas to assess a level of contamination from existing uranium tailing sites. Hence, reactive
transport models are useful tools to visualize potential threats, as well as to emanate dangerous
situations, and to design preventive and remedial measures to protect and rehabilitate water
resources. In the present study, a newly developed, two-dimensional, multi-species coupled
reactive transport model, VS2DRT, is used to simulate uranium tailing leaching. The problem
was adapted from HYDROGEOCHEM 5.0 and HP2 manuals and deals with the release andsubsequent migration of uranium from a uranium tailing pile toward ground water and a nearby
river. The simulation does not involve redox reactions and shows the application of VS2DRT in
simulating uranium migration in the subsurface.

1. Introduction
Uranium is a natural, radioactive and hazardous element which exists in three isotope forms ofU238, U235, and U234 in trace amounts in the environment. The discovery of radium by Marie
Curie led to uranium mining in the first half of the 20th century for the production of radium(Carvalho, 2012). After World War II, uranium was mined widely for production of nuclear
weapons and later for nuclear power plants to produce energy. The radioactive and hazardous
nature of waste materials from uranium mining and milling was poorly recognized during thefirst uranium boom in the 1940s to 1960s (Robinson, 2004).
Uranium tailing refers to a uranium ore residue, which contains the radioactive decay products ofuranium and heavy metals that are deposited in ponds. There is a huge volume of tailing waste
around the world, with an estimated volume of 938 million cubic meters produced at more than
4000 mines (IAEA, 2004). Living in a uranium mining area may lead to exposure toradionuclides from mining, mill tailings, dust, and contaminated water and foodstuff (Matias,2008). Although the major environmental impacts of uranium tailings are the failure of tailingdams and exposure to radiation, contamination of groundwater and/or surface water by leakageof contaminated water from tailing ponds is a serious concern. For example, in Australia in 1994,
the uranium tailing Olympic Dam released up to 5 million m3 of contaminated water into the soil
(IAEA, 2004). More recently, in 2012 in Sotkamo, Kainuu province, Finland, leakage from agypsum pond resulted in higher uranium concentrations in the nearby Snow River, exceeding
more than tenfold the allowable threshold level (http://www.wise-uranium.org/mdaf.html).
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Reactive transport modeling of contaminants from uranium tailing dams or ponds could be used
as a predictive tool to estimate the extent of contamination and to take remedial measures torehabilitate the site. Hence, reactive transport models such as HYDROGEOCHEM 5.0 (Yeh etal., 2004), HP2 (Šimůnek et al., 2012), and VS2DRT (Haile, 2013) could be useful tools tosimulate possible leaching of uranium from passive as well as active tailing sites. In this paper,we introduce a new reactive transport model, VS2DRT, which could be used to simulateleaching of uranium and other heavy metals from uranium tailings. The HP2 test example
involving leaching of uranium tailing is adapted here to show the applicability of VS2DRT forsuch cases.

2. VS2DRT Simulation of Leaching of Uranium Tailings
2.1. Description of VS2DRT
VS2DRT is a new reactive transport model for variably-saturated porous media. VS2DRT
couples the single solute transport program, VS2DT (Healy R., 1990), and the heat transport
program, VS2DH (Healy and Ronan, 1996), with the hydro-geochemical program, PHREEQC(Parkhurst and Appelo, 1999). VS2DRT uses a non-iterative operator splitting approach tocouple the programs. This approach involves solving the transport and chemical reactions
sequentially in one time step. VS2DRT has capabilities to simulate the flow of water, heattransport, multi-solute transport, various equilibrium reactions such as cation exchange,adsorption, dissolution/precipitation, and various kinetic reactions in variably-saturated porous
media. However, VS2DRT does not consider gas phase flow, the effect of chemical reactions onhydraulic properties, and the density of water.
2.2. The Case of Leaching of Uranium Tailings
This problem was adapted from the HYDROGEOCHEM 5.0 and HP2 manuals. The problem
deals with a hypothetical case of a uranium mill-tailing disposal and a subsequent release andmigration of uranium to groundwater and a river. The schematic representation of the domain ofsimulation is presented on Figure 1. The tailings pile is located on a flat surface which slopes
down to a river. The hydraulic properties of soil and tailing are presented in Figure 2. The
surface of the tailing pile was defined as the flux boundary with an infiltration rate of 0.0139
m/day. The top boundary, except for the surface of the tailing pile, has a flux boundary condition
with an infiltration rate of 0.00139 m/day. The bottom boundary is considered to be impervious.
The left side of the simulation domain below the water table has a 5 m pressure head. The riverbottom and the vertical nodes on the right side of the river have a 4 m pressure head boundary
condition. The initial pressure head and uranium profiles are presented in Figures 3 and 4,respectively. The light green color on the top right corner in Figure 3 is out of the simulation
domain.

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Figure 1. Schematic representation of the uranium tailing problem.

Figure 2. van Genuchten hydraulic parameters used for the soil and tailing piles.

Figure 3. The pressure head profile at the beginning of the simulation (0 days).

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Figure 4. The uranium (mole/ kg of water) profile at the beginning of the simulation (0 days).

The list of chemical reactions and their thermodynamic equilibrium constants considered for thiscase are presented in Table 1. The simulation here does not consider redox reactions. The
aqueous solution inside the tailing piles, outside the tailing, and at the boundaries are presented
in Table 2. The simulation was conducted for 1000 days with a time step of 1 day.
Table 1. Reactions and log K used for a uranium tailing problem (Šimůnek et al., 2012).
Reaction
Aqueous Complexation Reactions considered
H2O ↔ H+ + OHCa2+ + CO32- ↔ CaCO3
Ca2+ + H+ + CO32- ↔ CaHCO3+Ca2+ + SO32- ↔ CaSO3
Ca2+ + 2H+ + PO43- ↔ CaH2PO4+Ca2+ + PO43- ↔ CaPO4Ca2+ + H+ + PO43- ↔ CaHPO4Ca2+ + H2O ↔ H+ + CaOHFe2+ + SO42- ↔ FeSO4Fe2+ + H2O ↔ H+ + FeOHFe2+ + 2H2O ↔ 2H+ + Fe(OH)2Fe2+ + 3H2O ↔ 3H+ + Fe(OH)3Fe2+ + 4H2O ↔ 4H+ + Fe(OH)42UO22+ + H2O ↔ H+ +(UO2)(OH)+
2UO22+ + 2H2O ↔ 2H+ +(UO2)2(OH)22+
3UO22+ + 4H2O ↔ 4H+ +(UO2)3(OH) 42+
3UO22+ + 5H2O ↔ 5H+ +(UO2)3(OH) 5+
4UO22+ + 7H2O ↔ 7H+ +(UO2)4(OH) 7+
3UO22+ + 7H2O ↔ 7H+ +(UO2)3(OH) 7UO22+ + CO32- ↔ (UO2)(CO3)
UO22+ + 2CO32- ↔ (UO2)(CO3) 22UO22+ + 3CO32- ↔ (UO2)(CO3) 342UO22+ + CO32- + 3H2O ↔ 3H+ + (UO2)2(CO3)(OH) 3UO22+ + SO42- ↔ (UO2)(SO4)
UO22+ + 2SO42- ↔ (UO2)(SO4) 222H+ + UO22+ + PO43- ↔ H2(UO2)(PO4)+
3H+ + UO22+ + PO43- ↔ H2(UO2)(PO4)2+Ca2+ + 4H+ + UO22+ + 2PO43- ↔ CaH4(UO2)(PO4) 22+4NoLog KR1R2R3R4R5R6R7R8R9R10R11R12R13R14R15R16R17R18R19R20R21R22R23R24R25R26R27R28-14
3.22
11.43
2.31
20.96
6.49
15.08
-12.582.2-9.5-20.57-31.00-43.00-5.3-5.68-11.88-15.82-21.9-28.34
9.65
17.08
21.7
-1.18
2.95
4.0
23.2
22.90
45.24

Ca2+ + 5H+ + UO22+ + 2PO43- ↔ CaH5(UO2)(PO4) 23+
H+ + CO32- ↔ HCO32H+ + CO32- ↔ H2CO3
H+ + SO42- ↔ HSO4H+ + PO43- ↔ HPO422H+ + PO43- ↔ H2PO43H+ + PO43- ↔ H3PO4
Precipitation-Dissolution Reactions
Ca2+ + SO42- ↔ CaSO4 (s)Ca2+ + CO32- ↔ CaCO3 (s)Ca2+ + 3PO43- + H2O ↔ H+ + Ca5(OH)(PO4)3(s)Fe2+ + CO32- ↔ FeCO3 (s)Ca2+ + 2UO22+ + 2PO43- ↔ Ca(UO2)2(PO4)2(s)4Ca2+ + H+ + 3PO43- ↔ CaH(PO4)3(s)Ca2+ + H+ + PO43- ↔ CaH(PO4) (s)Ca2+ + 2H2O ↔ 2H+ + Ca(OH)2 (s)3Fe2+ + 2PO43- ↔ Fe3(PO4)2(s)Fe2+ + 2H2O ↔ 2H+ + Fe(OH)2 (s)UO22+ + 2H2O ↔ 2H+ + (UO2) (OH)2 (s)UO22+ + CO32- ↔ (UO2)(CO3) (s)Fe2+ + 2UO22+ + 2PO43- ↔ Fe(UO2)2(PO4)2(s)H+ + UO22+ + PO43- ↔ H(UO2) (PO4)2(s)

R29
R30
R31
R32
R33
R34
R35

46.00
10.32
16.67
1.99
12.32
19.55
21.74

R36
R37
R38
R39
R40
R41
R42
R43
R44
R45
R46
R47
R48
R49

4.62
8.48
40.47
10.50
48.61
48.20
19.30
-21.9033.30-12.10
-5.40
14.11
46.00
25.00

Table 2. Initial and boundary solutions for a uranium tailing problem (Šimůnek et al., 2012).
Species
Ca2+CO32UO22+PO43SO42H+Fe2+Inside the Tailing(mole/kg of water)
1.0 x 10-2
1.0 x 10-2
5.0 x 10-4
1.0 x 10-6
2.0 x 10-1
3.5 x 10-2
2.0 x 10-1

Outside the Tailing(mole/kg of water)
1.0 x 10-2
1.5 x 10-3
1.0 x 10-7
1.0 x 10-6
2.0 x 10-2
1.0 x 10-7
1.0 x 10-3

Boundary solution(mole/kg of water)
1.0 x 10-3
1.5 x 10-3
1.0 x 10-8
1.0 x 10-6
1.0 x 10-4
1.0 x 10-7
1.0 x 10-3

3. Result and Discussion
The results of the VS2DRT simulation are presented in Figures 5, 6, and 7 for uranium at 250,500, and 1000 days, respectively. At 250 days, the uranium plume spreads toward the groundwater, which is located at a depth of about 12 m. At 500 days, the uranium plume continues tospread in ground water and moves toward the river, which is located about 100 m along the xaxis. At 1000 days, ground water is completely polluted with uranium with concentrations ashigh as 0.0004 mole/kg of water. Uranium continues spreading toward the river with aconcentration of 0.00005 mole/kg of water.

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Figure 5. The uranium (mole/ kg of water) profile at 250 days.

Figure 6. The uranium (mole/kg of water) profile at 500 days.

Figure 7. The uranium (mole/kg of water) profile at 1000 days.

Figure 8 shows the uranium profile at 250 and 500 days based on the HP2 simulation. Theuranium plumes show similar patterns for both HP2 and VS2DRT models. The difference
between HP2 and VS2DRT simulation results could be due to spatial and temporal
discretizations used. HP2 uses a finite element method, and for the current problem, it used atime step of 0.0001 day. However, VS2DRT uses a finite difference method, and for the current
problem, it used a time step of 1 day. The effect of the pumping well considered in the HP2
simulation is also not included in VS2DRT.
Generally, both HP2 and VS2DRT can be used for 2D reactive transport modeling in variablysaturated porous media. The advantage of VS2DRT is that it is a public domain program while
HP2 is a commercial one.
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Figure 8. Uranium (mole/kg of water) profiles at (a) 250 days and (b) 500 days based on the HP2
simulation (Šimůnek et al., 2012).

4. Conclusions
Reactive transport programs such as VS2DRT are useful tools to visualize the possible spatial
and temporal distributions of contaminants in the subsurface. Coupling of VS2DT and VS2DH
with PHREEQC has boosted the applicability of VS2DT to simulate multi-solute transport with awide variety of equilibrium and kinetic reactions. The hypothetical case presented here showsthat VS2DRT can be used to predict the uranium plume pattern at variable times. In real world
applications, the accuracy of the simulation depends on the hydraulic and chemical properties
and the initial and boundary conditions for flow and chemical solutions. Moreover, the grid size
and the time steps used for the numerical simulations play a huge role. It is good practice toverify the numerical simulations with measured or observed values whenever possible in order tohave more confidence in the obtained results and in decisions made based on numericalsimulations.

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http://www.wise-uranium.org/mdaf.html

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