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A useful start in promoting a holistic approach to linking ground and surface waters is to adopt the hydrological cycle as a basic framework. The hydrological cycle, as depicted in Fig. 1.8, can be thought of as the continuous circulation of water near the surface of the Earth from the ocean to the atmosphere and then via precipitation, surface runoff and groundwater flow back to the ocean. Warming of the ocean by solar radiation causes water to be evaporated into the atmosphere and transported by winds to the land masses where the vapour condenses and falls as precipitation. The precipitation is either returned directly to the ocean, intercepted by vegetated surfaces and returned to the atmosphere by evapotranspiration, collected to form surface runoff, or infiltrated into the soil and underlying rocks to form groundwater. The surface runoff and groundwater flow contribute to surface streams and rivers that flow to the ocean, with pools and lakes providing temporary surface storage.

Of the total water in the global cycle, Table 1.1 shows that saline water in the oceans accounts for 97.25%. Land masses and the atmosphere therefore contain 2.75%. Ice caps and glaciers hold 2.05%, groundwater to a depth of 4 km accounts for 0.68%, freshwater lakes 0.01%, soil moisture 0.005% and rivers 0.0001%. About 75% of the water in land areas is locked in glacial ice or is saline (Fig. 1.9). The relative importance of groundwater can be realized when it is considered that, of the remaining quarter of water in land areas, around 98% is stored underground, and so making groundwater the second largest store of freshwater in the global cycle. In addition to the more accessible groundwater involved in the water cycle above a depth of 4 km, estimates of the volume of interstitial water in rock pores at even greater depths range from 53 × 10⁶ km³ (Ambroggi 1977) to 320 × 10⁶ km³ (Garrels et al. 1975).

Fig. 1.8 The hydrological cycle. The global water cycle has three major pathways: precipitation, evaporation and water vapour transport. Vapour transport from sea to land is returned as runoff (surface water and groundwater flow). Numbers in ( ) represent inventories (in 10⁶ km³) for each reservoir. Fluxes in [ ] are in 10⁶ km³ a⁻¹ (Berner and Berner 1987).

(Source: Berner, E.K. and Berner, R.A. (1987) The Global Water Cycle: Geochemistry and Environment. Prentice‐Hall, Inc., Englewood Cliffs, New Jersey. © 1987, Pearson Education.)

Within the water cycle, and in order to conserve total water, evaporation must balance precipitation for the Earth as a whole. The average global precipitation rate, which is equal to the evaporation rate, is 496 000 km³ a⁻¹. However, as Fig. 1.8 shows, for any one portion of the Earth, evaporation and precipitation 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 and Loaiciga 1993).

Table 1.1 Inventory of water at or near the Earth's surface (Berner and Berner 1987).

(Source: Berner, E.K. and Berner, R.A. (1987) The Global Water Cycle: Geochemistry and Environment. Prentice‐Hall, Inc., Englewood Cliffs, New Jersey. © 1987, Pearson Education.)

Reservoir	Volume (×106 km3)	Percentage of total
Oceans	1370	97.25
Ice caps and glaciers	29	2.05
Deep groundwater (750–4000 m)	5.3	0.38
Shallow groundwater (<750 m)	4.2	0.30
Lakes	0.125	0.01
Soil moisture	0.065	0.005
Atmospherea	0.013	0.001
Rivers	0.0017	0.0001
Biosphere	0.0006	0.00004
Total	1408.7	100

Note:

^a As liquid equivalent of water vapour.

Fig. 1.9 The distribution of water at or near the Earth's surface. Only a very small amount of freshwater (<0.3% of total water) is readily available to humans and other biota (Maurits la Rivière 1989).

(Source: Maurits la Rivière, J.W. (1989) Threats to the world’s water. Scientific American 261, 48–55.)

The interfaces between hydrological compartments in the water cycle have important implications for water quantity and quality. Processes at the interfaces between the hydrological compartments (for example, soil‐atmosphere or soil‐groundwater) determine the age distribution of the water fluxes between these compartments and can, thus, greatly influence water travel and residence times (Sprenger et al. 2019). The age distribution of water spans over a wide range of temporal scales. In the ‘critical zone’, the Earth's boundary layer ranging from the top of the vegetation layer to the bottom of the groundwater storage, water ages range from hours to millennia (Fig. 1.10).

By taking the constant volume of water in a given reservoir and dividing by the rate of addition (or loss) of water to (from) it enables the calculation of a residence time for that reservoir. For the oceans, the volume of water present (1370 × 10⁶ km³; see Fig. 1.8) divided by the rate of river runoff to the oceans (0.037 × 10⁶ km³ a⁻¹) gives an average time that a water molecule spends in the ocean of about 37 000 years. Lakes, rivers, glaciers and shallow groundwater have residence times ranging between days and thousands of years. Because of extreme variability in volumes and precipitation and evaporation rates, no simple average residence time can be given for each of these reservoirs. As a rough calculation, and with reference to Fig. 1.8 and Table 1.1, if about 6% (2220 km³ a⁻¹) of runoff from land is taken as active groundwater circulation, then the time taken to replenish the volume (4.2 × 10⁶ km³) of shallow groundwater stored below the Earth's surface is of the order of 2000 years. In reality, groundwater residence times vary from about 2 weeks to 10 000 years (Nace 1971), and longer (Edmunds 2001). A similar estimation for rivers provides a value of about 20 days. These estimates, although a gross simplification of the natural variability, do serve to emphasize the potential longevity of groundwater pollution compared to more rapid flushing of contaminants from river systems.