Page 79 - Soil and water contamination, 2nd edition

66 Soil and Water Contamination

the lower layer is denser and water currents in the upper layer usually generated by wind are
unable to cause eddies strong enough to penetrate the boundary between the two layers. In
regions with distinct cold and warm seasons, stratification is mostly the result of temperature
differences, as the upper layer of the lake warms up in summer due to solar radiation . Since
warm water is less dense, it tends to float, and the warm water concentrates at the surface. If
this happens, the lake is said to be thermally stratified. The upper, well-mixed, warm layer is
called the epilimnion and the lower, cold layer is called the hypolimnion (see Figure 3.6).
Between the epilimnion and the hypolimnion is a zone of rapid temperature change, the
thermocline or metalimnion, which may extend for several metres. The epilimnion is usually
well mixed due to wind-driven circulation, while the hypolimnion is isolated from wind
effects and is consequently rather quiescent. The temperature difference between epilimnion
and hypolimnion may be fairly large. During the summer in the temperate zone, it is not
exceptional for the epilimnion to warm up to between 15 °C and 25 °C, while the water
temperature in the hypolimnion remains considerably below 10 °C. During stratification,
little exchange of chemicals and heat occurs between the epilimnion and hypolimnion.
The only exchange occurs through settling of suspended particles, which causes a material
flux from the epilimnion to the hypolimnion. Thermal stratification is usually overturned
in autumn, which causes the lake to be well mixed over the entire depth (Figure 3.6).
During winter, lakes may stratify because ice covers the lake surface. In spring, as ice melts,
this winter stratification is removed. Shallow lakes are usually well mixed and may warm
throughout their depth and not exhibit a thermocline. In some shallow lakes, the bottom
layer may be fed by colder spring water; in such cases the thermocline may be quite abrupt.
In surface water, just as in soil and groundwater, oxygen is consumed by the
decomposition of organic materials, but because surface waters are in direct contact with the
atmosphere, they are usually well aerated. Moreover, photosynthesis by aquatic vegetation
and algae adds oxygen to the surface water. As a result, the redox potential in surface water is
usually high (Eh > 700 mV). The maximum total dissolved oxygen concentration in water is
determined by the atmospheric pressure and, as noted above, temperature . Cold water holds
more dissolved oxygen than warm water, and surface water bodies at sea level contain more
oxygen than water bodies at high altitudes. Water may become depleted in oxygen due to the
presence of abundant amounts of easily degradable organic matter originating from inputs of
domestic waste or extensive algal blooms (see Chapter 6). Because water in the hypolimnion

Figure 3.7 Discharge of the river Rhine near Lobith, the Netherlands between 1991 and 2000 (source: Waterbase,
2013).

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