Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 750

on earth, in some regions close to the surface such as in wetlands or marsh areas,
while in arid areas up to several hundreds of meters below ground. In many regions,
groundwater is directly linked to surface waters, forming complete water systems.
In highly permeable limestone catchments, rivers can be entirely supported by the
groundwater discharge. Fossil groundwater bodies are not, or are rarely connected
to surface water systems.
It is generally recognized by all segments of society that fresh water, although not
always associated with groundwater, is an immensely important resource. Although
70% of the earth's surface is covered by water, most of it is the salt water of seas
and oceans. Only 2.5% of all water on earth is fresh water, most of which exists as
glaciers and permanent snowfields, leaving only 0.26% of all fresh water available
to support mankind (Eurogeosurveys 2009 ). More than 90% of this small volume
is in the form of groundwater. The need of water for a rapidly growing human
population worldwide and for its activities such as non-controlled water use, pose
serious threats, along with industrial and agricultural contamination, to sustaining
this important volume of fresh water. At present, humans use some 10% of the
renewable fresh water resource, but it can be very variable depending on time of
year, and on geographical location (Optimum Population Trust 2009 ). Moreover,
humans must share it with all the other species on earth.
Water is part of a spectacular cycle known as the hydrological cycle (aka: water
cycle) that includes an exchange of water between the soil-groundwater system,
surface waters and the atmosphere (see Fig. 17.1 ).
Of all water volumes that play a role in the hydrological cycle, water in the soil
has the highest residence time. Water can reside in aquifers for hundreds of years;
see Fig 17.2 , which shows recharge and discharge areas, flow lines and residence
time of water in an aquifer (López-Geta et al. 2006 ).

precipitation

precipitation

infiltration

evaporation

percolation

(open water)
evaporation

surface runoff

leaching

•

transport

transport

surface
water

groundwater table

impermeable layer

Fig. 17.1 The hydrological cycle, including an exchange of water between the soil-groundwater
system, surface waters and the atmosphere