Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 82

• Each geologic unit has a unique arrangement and network of interconnecting fea-
tures created by the depositional method. Karst aquifers are demonstrably differ-
ent in this respect than sand and gravel aquifers.
• Comparison of heterogeneity and homogeneity is possible at several different geo-
graphical scales. The mega scale is a geologically dependent formational scale and
is regional in extent. An example here is the heterogeneity of large river deposits
compared to a glacial moraine deposit. The macroscale refers to the variability
within the formation itself. In this case, you could compare the variability of two
sand bars deposited by the same river. The microscale refers to the variability and
arrangement of depositional layers and individual grains within a specific deposit.

When an aquifer has high variability, it is called heterogeneous. An aquifer of low vari-
ability may be labeled as a homogeneous or relatively homogeneous aquifer. The prob-
ability of encountering a truly homogeneous aquifer is effectively zero, since all aquifers
demonstrate some degree of variability, with most displaying a high degree of variability.
The terms isotropic and anisotropic are used in describing the degree of variability
related to a given direction for a specified geologic feature. For instance, a geologic forma-
tion may exhibit a more homogeneous texture within a specific bedding layer trending in
a certain direction. This phenomenon then qualifies as a layer displaying either more of
an isotropic tendency or anisotropic tendency in a given direction from a defined location.
Figure 3.17 has examples showing the different combinations of homogeneous, heteroge-
neous, isotropic, and anisotropic concepts.
The concept of heterogeneity describing the variability within an aquifer is significant
and requires a sound scientific understanding. Heterogeneity has a profound effect on
the migration and fate of contaminants present in groundwater and influences how those
contaminants are removed, destroyed, or chemically altered through specially designed
remedial technologies (Payne et al. 2008).

Isotropic
homogeneous

Isotropic
heterogeneous

Anisotropic
homogeneous

Anisotropic
heterogeneous

FIGURE 3.17
Visualization of variability concepts.