Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 99

4.1.3 Anisotropy

property of absorption occurs at the ends of the roots. When
the water present in the soil is full of soluble materials and
enters into the rootlets, the water becomes part of the plant
juice and this substance is called sap, properly speaking.
The rising sap reaches the leaves and undergoes several
modifications with which we shall not concern ourselves
here. We will say only that it gives up a large part of its
humidity there, which is released into the atmosphere as aque-
ous vapor through all the green parts, and especially through
the pores that cover the lower side of the leaves. Sometimes
this transpiration is so abundant that it becomes noticeable as
sweat in the form of droplets. The measurement of the product
of this transpiration, or of the excess of total aqueous volume
absorbed over the amount that the plant assimilates, gives us
an idea of the importance of the first volume. The famous
physiologist Hales found that the average transpiration of a
sunflower was 20 ounces (1.25 pounds) during the 12 hours of
a dry and hot day, and up to 3 ounces during a dry, hot night
without dew. He also found that a dwarf apple tree can exhale
15 pounds of water during 10 hours of the day. In Sologne,
I have seen very wet and, as a result, very unhealthy land
completely desiccated and drained by planting green trees.
Darcy (1856), translated by Bobeck (2004)

The distribution of hydraulic conductivity is never uniform
as required by Darcy's Law. Even in Darcy's column
experiments, the sand grains are not all uniform in size and
water flow that occurs through the middle of the column will
be different from water flow that occurs through sand in
contact with the sides of the container. As can be imagined,
this difference in hydraulic conductivity and its effect on
flow in the space of a sand-filled column can be much larger
when expanded to the field scale.
Under ideal, homogeneous conditions hydraulic conduc-
tivity would remain uniform throughout the aquifer. Because
of the orders-of-magnitude variation inherent in the grain
sizes of complex geologic settings, it follows that the
hydraulic conductivity of a particular area varies with
respect to space. This variation is called aquifer heterogene-
ity, from the Greek hetero meaning different and gene
meaning birth. If the variation in hydraulic conductivity
does not impart a preferential groundwater-flow direction,
conditions are called isotropic. If hydraulic conductivity
varies along a direction, this variation is called anisotropy,
from the Greek an meaning without, and iso meaning equal.
Because most unconsolidated aquifers are geologic units
created by sedimentary processes that result in layered depo-
sitional sequences, most aquifers have some degree of
anisotropy as a result of the orientation of sediments during
original deposition. The vertical hydraulic conductivity, K 0 ,
is often much lower than the horizontal hydraulic conduc-
tivity, both of which may vary directionally. Aquifer
systems that have undergone some degree of weathering
since deposition or formation, such as karst in limestone
aquifers, tend to have anisotropic conditions. Anisotropic
flow can be enhanced by continued groundwater flow to
wells during pumping, especially if the influence of the
well causes surface water of a different geochemistry to
enter groundwater and cause dissolution or precipitation
reactions to occur along flow paths.

Darcy also recognized that transpiration of groundwater
by certain phreatophytes was the cause not only of making
wet ground dry but behind the rapid tree growth he had
observed near springs that subsequently went dry. In his
own words, Darcy stated that
A spring called the Fountaine des Suisses (Swiss Spring) in
Dijon had almost entirely disappeared; it produced only 1/5
liter per minute. Two lines of poplars planted along the small
valley where the spring occurred showed the following phenom-
enon. The poplars had been planted at the same time; however,
the first of each line showed more than double the growth of the
following ones. I had a trench dug in the place where the spring
was thought to emerge, and I noticed the roots of the two first
trees had already advanced 8 to 10 meters toward this spring in
the middle of the natural basin where they had grown, and were
in the process of taking it over entirely. After some work to
modify the course of the spring, its volume again equaled 12 to
13 liters per minute.

Darcy (1856), translated
by Bobeck (2004)

Darcy stated that this relation between plants and ground-
water could be used to locate springs (Sharp and Simmons
2005). Interestingly, Darcy foretold of future investigations
into the consumptive use of water by plants in arid areas of
the United States, which were examined more than 50 year
later by the USGS. Darcy's interest lay in using plants to find
springs that could then be diverted to supply man's needs.
Darcy stated

4.1.4 Porosity

In Darcy's experiments, the volume of water added to the
sand-packed column represented the gross porosity of the
saturated material. In other words, the total porosity, n ,ofa
particular volume of the column media can be described as
the ratio of the volume of voids, V v , to the total volume, V t ,
where V t ¼

when brought to the surface this water has real usefulness
rather than contributing to making naturally rich terrain even
richer in vegetation as a result of its mineral composition and its
location in a swamp.

...

V v / V t (Fig. 4.4 ). This also holds
true for most geologic media. The porosity of a porous
medium is directly proportional to its degree of sorting,

V v + V s ,or n ¼

Darcy (1856), translated
by Bobeck (2004)