Groundwater’s Place in the Earth Sciences - Hydrogeology: Objectives, Methods, Applications - page 10

aquifers), or even solutes plus suspended particles (clay, organic particles
such as parasites, bacteria, viruses) in the case of aquifers with distinct
drainages (gravel channels in alluvial aquifers, fractures in consolidated
rock, karst passages).
Within the water cycle, the division between runoff and infi ltration will
therefore have a high impact on the erosion of soil and rock in drainage
basins, on the steepness of fl ood peaks, and on the dry season discharge
of rivers. This division is a function of both the permeability of soils and
bedrock, and the type of precipitation. For example, a Mediterranean storm
in marly terrain will engender a violent, short-lived fl ood and high turbidity
in suface waters, while an oceanic precipitation event of the same magnitude
over a poorly-karstifi ed chalky plateau will provoke only a very dampened
increase in surface water levels, without increasing turbidity.
2 WATER AND OIL
In oil fi elds, hydrocarbon deposits (oil and gas), expulsed from the rock
in which they matured due to lithostatic pressure, migrate towards the
surface by replacing denser water in rock pores, until they encounter a trap:
a reservoir rock covered by a poorly permeable (clayey or compacted) unit,
in a favorable confi guration (pinchout, anticline, fault). Oil and gas follow
the lateral fl ow of groundwater; differentiated only by their density and
viscosity, they obey the same fl ow laws as water. Nevertheless, hydrocarbon
migration is slow, and grounwater circulation can modify the size of open
spaces within a rock (cementation or dissolution of grains—Machel, 1999).
In very poorly permeable environments, the chemical composition of
originally saline water can be modifi ed to become more concentrated (oil
fi eld brines).
3 WATER AND MINERAL DEPOSITS
As much in the domain of the great sedimentary basins as in that of
hydrothermalism (Garven et al., 1999), groundwater is a vector for
mineralisation. In the fi rst case, the slow fl ows in environments with
differing porosities, and in the second, the upwelling of deep water
through fractures, allows the crystallization of minerals. For example, brine
migration in the fractures of the Variscan (Hercynian) platform, thanks to
the difference in elevation, allowed the accumulation of giant lead-bearing
deposits in North American carbonates.