Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 95

4

Fundamentals of Groundwater Hydrogeology

some channel or other opening, they flow out as fountains or
brooks or rivers according to the size of the opening and
receptacles

Groundwater composes more than 98% by volume of the
available freshwater on earth. Available does not mean
readily accessible, however, because more than half of
the available freshwater is held too tightly onto subsurface
sediments by molecular attractiontoflowtowells.In
general, groundwater is the water that completely fills
the void spaces in rocks, sediments, and soils and can
move under the influence of gravity. The ultimate
source of groundwater is precipitation as was described
in Chap. 2.
This connection between precipitation and ground-
water was not always clear. For example, Seneca
(4 BC -65 AD ) wrote that precipitation didn't soak deeply
into the ground but only penetrated the upper layers of soil
because
First of all, being a diligent digger among my vines, I can affirm
from observation that no rain is ever so heavy as to wet the
ground to a depth of more than 10 feet. All the moisture is
absorbed in the upper layer of earth without getting down to
the lower ones.

...

(Palissy 1580)

Water moves through porous sediments because water
molecules in contact with soil particles lose their cohesive-
ness and, therefore, can flow around the surface of soil
particles. This relation between water and geology forms
one of the foundations of the discipline of hydrogeology.
Although a few scientists and engineers in the 1800s had
begun to expand on the relation between precipitation, geol-
ogy, and groundwater, such as Joseph Elkington and
William Smith (Stephens and Ankeny 2004 and references
therein), the first to explain the movement of groundwater in
aquifers in a useful manner was an engineer named Henri
Darcy. His work is widely used in the hydrogeological and
hydrological sciences, and can be applied to address hydro-
logic issues regarding the phytoremediation of contaminated
groundwater.

(Kramer and Boyer 1995)

Ironically, what Seneca perhaps was observing in his
vineyard was the rapid uptake of precipitation by his vines
before it could become recharge, an observation he may
have made had he dug where there were no plants.
How, then, could the water collected from much deeper
wells be explained if water did not penetrate beyond the
upper soil? Seneca had suggested that the element earth
was being converted to the element water or that air in the
earth was condensed into water.
Bernard Palissy, introduced in Chap. 2, was one of the
first to deduce the correct flow of water in the hydrologic
cycle and provided perhaps the first definition of the relation
between groundwater and springs.

4.1 Henri Darcy and Darcy's Law

The concept that has become the fundamental basis of
groundwater flow and quantity investigations actually
started as part of an investigation into surface-water quality.
In 1856, Henri (Henry) Darcy (1803-1858), a hydraulic
engineer, was tasked by his native city of Dijon, France, to
investigate the city's drinking-water supply system. The city
had been diverting surface water and having it flow through
large beds of sand. The sand filtered out the larger particles
and thereby greatly improved the quality of the surface water
used for drinking water; this process is still being used by
many municipalities around the world for water treatment.
Being an engineer Darcy set up a laboratory-scale experi-
ment of the sand beds in the basement of a local hospital to
examine how different sizes of sand affected the flow of

... these waters (rain), falling on these mountains through the
ground and cracks, always descend and do not stop until they
find some region blocked by stones or rock very close set and
condensed. And they rest on such a bottom and having found