Environmental Engineering Reference
In-Depth Information
groundwater, lakes, reservoirs, rivers, and oceans. Differences in density
are important because lower density water floats on top of higher density
water. Such density differences can maintain stable layers. Formation of
distinct stable layers is called
stratification
. Stratification is discussed in de-
tail in Chapter 6 because it can control water movement and distribution
of chemicals and organisms in lakes.
Maximum density
of water occurs at
3.98°C (Fig. 2.3A). Water has a continuously greater decrease in density
per degree temperature increase above 3.98°C (Fig. 2.3B). Dissolved ions
also increase water density. This density increase can easily overcome or
enhance temperature effects on stratification at ionic concentrations that
can occur in some natural lakes (Fig. 2.4).
Water is also one of the best
solvents
known and can dissolve both
gasses and ions. The solvent properties of water have greatly influenced ge-
ologic
weathering
of the earth's surface by dissolving ions from rocks.
Weathering is responsible for most nonhuman-caused nutrients that enter
the biosphere. Weathering also alters geomorphology. For example, about
20% of the continental land is karstic terrain (White
et al.,
1995), a geo-
logical formation caused by rainwater dissolving limestone and leaving
very rough land topography.
Most solids dissolve in water more readily as temperature increases.
For example, this temperature effect on dissolved ions causes sugar to dis-
solve more readily in hot than in iced tea. Conversely,
solubility
of gasses
1.000
A
0.999
0.998
0.997
0.996
0.995
0.925
0.900
0.03
B
0.02
0.01
0.00
-0.01
0
10
20
30
Temperature (
0
C)
FIGURE 2.3
The density of water as a function of temperature (A) and the % decrease in
density with each 1°C warming (B). The rate of change in density per degree warming in-
creases with increasing temperature. At 0°C, ice forms with a density of 0.917 g milliliter
1
(data from Cole, 1994).
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