Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 100

Fig. 4.4 Porosity for a porous media is the volume of the void spaces
in a total volume of sediment. The effective porosity is the fraction of
porosity that is interconnected and can support groundwater flow.

Fig. 4.5 Generalized views of primary and secondary porosity in
porous media and fractured rock, respectively.

such as grains of uniform size distribution relative to grains
of non-uniform size, inversely proportional to its degree of
compaction, and not dependent on grain size. In other words,
a unit volume of ball bearings of the same diameter has the
same porosity as a unit volume of bowling balls.
This definition of porosity suggests that the volume of
voids accounts not only for water content but also for the
volume of flowing water. The latter does not hold, however,
because not all pores are interconnected to the extent that
water travels from one to the other through the material. For
example, an individual particle of water introduced at the
inlet, Q in , in Darcy's column filled with sand will travel a
longer path through the column length, l , filled with a porous
material relative to the flow path taken if no media were
present. This longer flow path, l t , relative to the straighter
path, l , is called the tortuosity, T , where T

media is called effective porosity, or n e . Higher values of
hydraulic conductivity usually mean higher values of n e .
Clays, however, that have higher porosity than sands actually
have much lower values of hydraulic conductivity and n e .
The porosity of a porous media may not be constant over
time. Porosity can be a function of the pore spaces that have
existed since the sediments were deposited or a function of
events that have happened since deposition. The first
instance, already discussed, is referred to as primary porosity
(Fig. 4.5 ). The second instance is called secondary porosity
and can occur to geologic materials such as limestone that
are weathered over time by the flow of groundwater that
contains carbonic acid from precipitation. Igneous and meta-
morphic rocks that essentially have no primary porosity can

( l t / l ) 2 . The
porosity that results in interconnected flow through porous

¼