Porous Aquifers and Groundwater Circulation - Hydrogeology: Objectives, Methods, Applications - page 41

fl uvial or deltaic alluvial aquifers (lower Rhône or Var valleys), and the
qualitative consequence is the isolation of the aquifer from atmospheric
oxygen. Lacking exchange with the atmosphere, such an aquifer fi nds itself
placed under reducing conditions favorable to the dissolution of undesirable
metals, such as iron and manganese, as well as toxic heavy metals.
1 HYDRODYNAMIC CHARACTERISTICS
The major hydrodynamic characteristic of porous environments is that,
their interconnected porosity being a sedimentologic property, drainage
occurs over the entire cross-section of the aquifer, with relatively low,
homogeneous velocities due to the large fl ow cross-section. The consequence
for exploitation is the high probability of fi nding the same hydrodynamic
characteristics within a certain radius around a catchment project. However,
this observation cannot be applied to a deltaic aquifer, within which
drainage is highly channelized, and therefore where the distribution of
fl ow velocities will be be very heterogeneous: very high in gravel channels,
quasi-null in silty, clayey banks.
2 DARCY'S LAW AND ITS APPLICATIONS
The basic experiment describing hydrogeology in a continuous medium
was performed by Darcy in 1856.
In a vertical cylinder with a cross-sectional area A, a height Δh of water
percolates through a thickness L of sand. Darcy observed that the discharge
Q of water passing through the sand section is proportional to the value of
Δh/L. This relationship is called the head loss, or hydraulic gradient i, and
the proportional factor between the hydraulic gradient and the discharge
is termed the coeffi cient of fi ltration K.
Figure 17 Darcy's experiment applied to horizontal fl ow in an aquifer.