Geoscience Reference
In-Depth Information
studies provided some understanding of the evolution of hydraulic conductivity
caused by precipitation processes (Novak and Lake
1989
; Novak
1993
; Bolton
et al. 1996, 1997, Dijk and Berkowitz
1998
).
Loss or gain of dissolved chemical species from soil water by precipitation or
dissolution, respectively, usually is accounted for by adding a simple sink-source
term in the advection-dispersion equation:
X
n
ot
ð
hc
Þ
¼
o
o
ox
ð
vhc
Þþ
o
D
h
h
o
c
ox
Q
i
;
ð
11
:
7
Þ
ox
i¼1
where Q
i
denotes sink (or source) terms that account for, for example, contaminant
degradation, plant uptake, volatilization, or precipitation (dissolution). Alterna-
tively, focusing on precipitation-dissolution at the pore scale, the hydrodynamic
dispersion coefficient D
h
can be replaced by the coefficient of molecular diffusion,
so that Eq. (
11.7
) can be applied as an advection-diffusion equation.
Dissolution is a relatively simple mechanism, and so application of an advec-
tion-diffusion equation modified by a source term, or a similarly modified non-
Fickian (CTRW) transport equation (Hornung et al.
2005
), can effectively capture
chemical transport patterns. On the other hand, the interplay between the chemical
and physical aspects of mineral precipitation is more complex. In some instances,
during infilling of fissures and pore spaces by mineral precipitation, porosity is
reduced uniformly; in other cases, pockets of high porosity or unfilled fissures may
remain. In addition to the chemical kinetics of precipitation, the flow dynamics and
transport also are important factors in determining the resulting patterns of
porosity and mineral deposition (Dijk and Berkowitz
1998
). Note also that mod-
eling approaches similar to those for treating precipitating contaminants can be
employed to account for degradation and volatilization of contaminants in water.
For a given mineral to precipitate, a solution must be oversaturated with respect
to the mineral being deposited (i.e., the ion product must exceed the solubility
product), and the precipitation process must be kinetically favorable. The most
''conventional'' model for describing the deposition of minerals within rock for-
mations maintains that supersaturated fluids pass through a porous or fractured
rock, precipitating minerals along the way. However, recent experimental studies
demonstrate that a number of difficulties are inherent in such a simplified picture
of mineralization. Lee and Morse (
1999
) and Hilgers and Urai (
2002
) found that
when supersaturated fluids flowed through artificial fractures, most of the mineral
deposition occurred within several centimeters of the inlet.
Fluids become supersaturated, and thus, mineral deposition occurs, by three
main mechanisms: (1) changes in fluid pressures and temperatures during flow
through the porous medium, (2) dissolution of a particular mineral in the matrix
that results in the fluid being supersaturated with respect to another mineral (e.g.,
dissolution of calcite and precipitation of gypsum; Singurindy and Berkowitz
2003
), and (3) mixing-induced supersaturation. This third mechanism occurs when
two initially saturated or undersaturated fluids of different chemical compositions
