Characteristics of the Environment - Field Sampling: Principles and Practices in Environmental Analysis - page 28

freeze in the winter. During the freezing process the water and the subsequent ice is less
dense than the underlying warmer water. During the winter the lower layers gradually
cool and become more dense. In the spring the surface melts and the water warms,
becoming more dense than the underlying water. At this point the lake turns over, with
the upper layer sinking to the bottom and the bottom layer coming to the top.
Air is more soluble in cold water than in warm water. However, water that has been
trapped at the bottom of a lake under ice for an extended period of time becomes anoxic
(lacks oxygen). When a lake turns over in the spring, oxygen-rich surface water moves to
the bottom of the lake and there is a flurry of oxidative reactions. The composition of the
water and sediments in terms of oxidized and reduced species changes dramatically
during this time [12].
In temperate and tropical lakes layers with varying amounts of oxygen are also
possible, along with areas of differing temperature. These lakes can also turn over,
sometimes with catastrophic effects. Turnover can be due to
FIGURE 2.3 Water molecules hydrogen bonded to each other and attracted to
the oxygens in a soil particle.
incoming streams and rivers, springs, uneven heating caused by currents, and underlying
geothermal activity. For these reasons changes in the oxidative state can occur. For areas
in which there is a pronounced dry period during the year the bottom of a lake may
become anoxic. When the rains come oxygen-rich, water flows into the lake, changing its
oxidative status [13].
Even in situations in which turnover or incoming sources of oxygenated water are not
of concern, the solubility—even though it is low—of air in water is important. As noted
above, air is less soluble in warm water than in cold water, and thus its concentration is
dependent on the temperature of the water being sampled. No matter what the
temperature is, however, the deeper the water the lower the oxygen content. Thus, at the
bottom of bodies of water there is very little (or no) oxygen, producing anaerobic or
reducing conditions. Not only are sediments anaerobic and reducing, but so is any portion
of the lithosphere saturated with water for a period of time. This condition routinely
occurs in soil and in any material covered by floodwaters. Sampling of any water-
saturated soils and sediments must thus be done carefully to maintain their physical and