Page 25 - Hydrogeology Principles and Practice

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8 Chapter One

Table 1.1 Inventory of water at or near the Earth’s surface. After is locked in glacial ice or is saline (Fig. 1.6). The relat-
Berner and Berner (1987). ive importance of groundwater can be realized when
it is considered that, of the remaining quarter of water
Reservoir Volume % of total
×
6
3
(× 10 km ) in land areas, around 98% is stored underground. In
addition to the more accessible groundwater involved
Oceans 1370 97.25 in the water cycle above a depth of 4 km, estimates of
Ice caps and glaciers 29 2.05 the volume of interstitial water in rock pores at even
Deep groundwater (750–4000 m) 5.3 0.38 greater depths range from 53 × 10 km (Ambroggi
6
3
Shallow groundwater (<750 m) 4.2 0.30 6 3
Lakes 0.125 0.01 1977) to 320 × 10 km (Garrels et al. 1975).
Soil moisture 0.065 0.005 Within the water cycle, and in order to conserve
Atmosphere* 0.013 0.001 total water, evaporation must balance precipitation
Rivers 0.0017 0.0001 for the Earth as a whole. The average global precip-
Biosphere 0.0006 0.00004
itation rate, which is equal to the evaporation rate, is
3 −1
Total 1408.7 100 496,000 km a . However, as Fig. 1.5 shows, for any
one portion of the Earth, evaporation and precip-
* As liquid equivalent of water vapour.
itation generally do not balance. The differences
comprise water transported from the oceans to the
continents as atmospheric water vapour and water
returned to the oceans as river runoff and a small
amount (~6%) of direct groundwater discharge to
the oceans (Zektser & Loáiciga 1993).
The approximate breakdown of direct groundwa-
ter discharge from continents to adjacent oceans and
3 −1
seas is estimated as follows: Australia 24 km a ;
3 −1
3 −1
Europe 153 km a ; Africa 236 km a ; Asia 328 km 3
3 −1
−1
a ; the Americas 729 km a ; and major islands
3 −1
914 km a (Zektser & Loáiciga 1993). The low con-
tribution from the Australian continent of direct
groundwater discharge, despite its relatively large
territory, is attributed to the widespread occurrence
of low-permeability surface rocks that cover the con-
tinent. At the other extreme, the overall proximity of
recharge areas to discharge areas is the reason why
major islands of the world contribute over one-third
of the world’s direct groundwater discharge to the
oceans. The largest direct groundwater flows to oceans
Fig. 1.6 The distribution of water at or near the Earth’s surface.
Only a very small amount of freshwater (<0.3% of total water) are found in mountainous areas of tropical and
3 −1
−3
−2
is readily available to humans and other biota. After Maurits la humid zones and can reach 10–15 × 10 m s km .
Riviére (1987). The smallest direct groundwater discharge values
3 −1
−2
−3
of 0.2–0.5 × 10 m s km occur in arid and arctic
regions that have unfavourable recharge and perme-
Of the total water in the global cycle, Table 1.1 ability conditions (Zektser & Loáiciga 1993).
shows that saline water in the oceans accounts for Taking the constant volume of water in a given
97.25%. Land masses and the atmosphere therefore reservoir and dividing by the rate of addition (or loss)
contain 2.75%. Ice caps and glaciers hold 2.05%, of water to (from) it enables the calculation of a resid-
groundwater to a depth of 4 km accounts for 0.68%, ence time for that reservoir. For the oceans, the vol-
3
6
freshwater lakes 0.01%, soil moisture 0.005% and ume of water present (1370 × 10 km ; see Fig. 1.5)
rivers 0.0001%. About 75% of the water in land areas divided by the rate of river runoff to the oceans