Geoscience Reference
In-Depth Information
(Fig.
10.1
a). Here, the average contaminant advance, given by the point at which
the relative concentration c/c
o
= 0.5, corresponds to the average water velocity, v.
Figure
10.1
b shows spatial concentration profiles within the column, at dif-
ferent ''snapshots'' in time. Note that the profile spreads with increasing travel
distance (and thus with increasing time). The positions (distances) noted by the
points t
1
and t
2
correspond to the times t
1
and t
2
shown in Fig.
10.1
a. The effect of
retardation, caused by the additional mechanism of chemical adsorption, is shown
in Fig.
10.1
c; both the average velocity of the contaminant (corresponding to the
point c/c
o
= 0.5) and the degree of spreading around this value are reduced. This
behavior is discussed further in
Sect. 11.1
.
While the advection-dispersion equation has been used widely over the last half
century, there is now widespread recognition that this equation has serious limi-
tations. As noted previously, laboratory and field-scale application of the advec-
tion-dispersion equation is based on the assumption that dispersion behaves
macroscopically as a Fickian diffusive process, with the dispersivity being
assumed constant in space and time. However, it has been observed consistently
through field, laboratory, and Monte Carlo analyses that the dispersivity is not
constant but, rather, dependent on the time or length scale of measurement (Gelhar
et al.
1992
), leading to what has been referred to as the scale effect. The spatial
dependence of transport coefficients is usually attributed to the existence of
hydraulic conductivity fields with coherence (correlation) lengths varying over
many scales. We observe that chemical transport at the field scale now is often
recognized to be non-Fickian, so that one can argue that the ''anomalous,'' non-
Fickian transport actually is the norm rather than the exception.
Therefore, Eq.
10.5
is limited in its applicability, as are variations of this
equation such as the mobile-immobile one (see
Sect. 10.2
). We discuss non-
Fickian transport in detail in
Sect. 10.3
.
10.2 Preferential Transport
As noted in
Sect. 10.1
, heterogeneities play a dominant role in the migration of
contaminants in the subsurface. Nonuniform, preferential patterns of flow and
transport are ubiquitous. It is important to recognize that, at the field scale, con-
taminant movement generally is very difficult to anticipate. In natural soils and
aquifer materials, macropores, soil cracks and aggregates, fissures, solution
channels, root paths, and wormholes, as well as variable mineral composition (e.g.,
clay aggregates and lenses) all serve to make nonuniform patterns of flow highly
contents are subject to shrinkage and cracking under repeated cycles of wetting
and drying. During contaminant infiltration, cracks and larger pores control the
water distribution and limit reaction surfaces to smaller areas. As a consequence,
downward migration of contaminants from the land surface usually is highly
nonuniform, influenced strongly by movement through preferential pathways.
