Page 26 - Hydrogeology Principles and Practice

HYDC01 12/5/05 5:44 PM Page 9

Introduction 9

Table 1.2 Agents of material transport to the oceans. After Garrels et al. (1975).

Agent % of total transport Remarks
Rivers 89 Dissolved load 17%, suspended load 72%
Glacier ice 7 Ground rock debris plus material up to boulder size. Mainly from Antarctica
and Greenland. Distributed in seas by icebergs
Groundwater 2 Dissolved materials similar to river composition. Estimate poorly constrained
Coastal erosion 1 Sediments eroded from cliffs, etc.
Volcanic 0.3(?) Dust from explosive eruptions. Estimate poorly constrained
Wind-blown dust 0.2 Related to desert source areas and wind patterns, e.g. Sahara, major source for tropical Atlantic

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3 −1
3 −1
(0.037 × 10 km a ) gives an average time that a discharge to the oceans of 2220 km a , the average
−1
water molecule spends in the ocean of about 37,000 dissolved solids concentration is about 585 mg L .
years. Lakes, rivers, glaciers and shallow ground- This calculation illustrates the long residence time of
water have residence times ranging between days groundwater in the Earth’s crust where its mineral
and thousands of years. Because of extreme variabil- content is concentrated by dissolution.
ity in volumes and precipitation and evaporation
rates, no simple average residence time can be given
for each of these reservoirs. As a rough calculation, 1.6 Groundwater as a natural resource
and with reference to Fig. 1.5 and Table 1.1, if about
3 −1
6% (2220 km a ) of runoff from land is taken as Groundwater is an important natural resource.
active groundwater circulation, then the time taken Worldwide, more than 2 billion people depend on
3
6
to replenish the volume (4.2 × 10 km ) of shallow groundwater for their daily supply (Kemper 2004).
groundwater stored below the Earth’s surface is of A large proportion of the world’s agriculture and irri-
the order of 2000 years. In reality, groundwater resid- gation is dependent on groundwater, as are a large
ence times vary from about 2 weeks to 10,000 years number of industries. Whether groundwater or sur-
(Nace 1971), and longer (Edmunds 2001). A similar face water is exploited for water supply is largely
estimation for rivers provides a value of about 20 dependent on the location of aquifers relative to the
days. These estimates, although a gross simplification point of demand. A large urban population with a
of the natural variability, do serve to emphasize the high demand for water would only be able to exploit
potential longevity of groundwater pollution com- groundwater if the aquifer, typically a sedimentary
pared to more rapid flushing of contaminants from rock, has favourable storage and transmission proper-
river systems. ties, whereas in a sparsely populated rural district
As an agent of material transport to the oceans more limited but essential water supplies might be
of products of weathering processes, groundwater found in poor aquifers, such as weathered basement
probably represents only a small fraction of the total rock.
transport (Table 1.2). Rivers (89% of total transport) The relationship between population and geology
represent an important pathway while groundwater can be inferred from Tables 1.3 and 1.4, which give a
accounts for a poorly constrained estimate of 2% breakdown of water use by purpose and type (surface
of total transport in the form of dissolved materials water and groundwater) for regions of England and
(Garrels et al. 1975). More recent estimates by Wales. Surface water abstraction for electricity gen-
Zektser and Loáiciga (1993) indicate that globally the eration is the largest category, but most of the fresh-
transport of salts via direct groundwater discharge is water abstracted for cooling purposes is returned to
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−1
approximately 1.3 × 10 ta , roughly equal to half rivers and can be used again downstream. In terms of
of the quantity contributed by rivers to the oceans. public water supply abstractions, groundwater is espe-
Given a volumetric rate of direct groundwater cially significant in the Southern (73% dependence