POLYMIN - University of Waterloo

POLYMIN 2005
where S k is the solid phase concentration of the k th solid component, [ML -3 ], and R k S represents
the change in the k th solid component concentration [ML -3 T -1 ]. Further details are provided by
Walter et al. (1994a).
Oxygen Diffusion
Assuming an immobile air phase (convection is neglected), oxygen transport through the
unsaturated porous medium is governed by the 2D equation for oxygen diffusion, expressed as:
θ
eq
2 2
∂ [ O2 ]
a
⎛ ∂ [ O2 ]
a ∂ [ O2
]
a
⎞
= D e ⎜ + - Q
2 2 ⎟
∂t ⎝ ∂ x ∂ z ⎠
O 2
(4)
where θ eq (x,z,t) is the spatially and temporally-variable water phase corrected volumetric air
content [-], D e (x,z,t) is the effective oxygen diffusion coefficient [L 2 T -1 ], and Q O2 (x,z,t) is the
sink term for oxygen consumption due to sulphide mineral oxidation (M O2 /L 3 /T) (see below).
The equivalent air content θ eq in Equation (4) is defined here as θ eq = θ a + Hθ w where θ a is the
air-filled porosity and H is Henry’s Law coefficient for equilibrium oxygen partitioning between
air and water (-). The oxygen diffusion coefficient D e is represented in POLYMIN by the Aachib
et al. (2002) model:
D
e
1 ⎡ p D
a w pw
= ⎢Daθ
a
+ θ
w
θ 2
⎣ H
s
⎤
⎥
⎦
(5)
where D a and D w are the diffusion coefficients in air and water, respectively, and p a and p w are
fitting coefficients; here p a = p w = 3.3 is assumed, as suggested by Aachib et al. (2002).
The coefficient D e , representing diffusion within the air-filled porosity of the unsaturated spoil,
can be represented using models presented by Elberling & Nicholson (1993), Millington &
Quirk (1961), or Aachib et al. (2002).
Sulphide Oxidation
Acid mine drainage is controlled by the oxidation of sulphide minerals, mostly pyrite and
pyrrhotite, within the waste rock. The oxidation of pyrite, for example, can be described in
simplified form by the following stoichiometric equations (Wunderly et al, 1996):
2+
2- +
Fe S 2 + H 2 O +7/2 O2
⇒ Fe + 2 SO4
+ 2 H
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