Environmental Engineering Reference
In-Depth Information
• Liquid water is denser than ice.
• It has a high specific heat capacity .
• It has a very high surface tension.
Water is unique, because it is the only substance on Earth found in all three states (liquid,
solid, and gas) within our planet's temperature ranges. Liquid water is essential to life, as it
comprises approximately 60% of the human body by weight and 70% of the human brain.
Some organisms are 90% liquid water by weight.
Without water, humans simply would not exist. The ability of water to dissolve so many
different substances allows cells to use the nutrients, minerals, and chemicals in biologi-
cal processes. In natural systems, wherever water goes—the air, the ground, streams and
lakes, biota, or through our bodies—it takes valuable chemicals, minerals, and nutrients
picked up along the way. This transport ability also means that water can carry substances
harmful to humans and the environment. If these contaminants are present at a suffi-
cient concentration and the exposure is long enough, potentially harmful effects can occur.
The damage can be immediate and obvious, as when oil is washed up on a beach and
kills waterfowl—or slow developing and silent—a scenario represented all to often by the
benzene that originated from a leaking gasoline tank and carried by groundwater into a
drinking water well.
This is why we spend so much time and effort studying water.
3.2.2 Water Cycle
Water is constantly on the move through the four spheres of the geosphere—the atmo-
sphere, biosphere, hydrosphere, and lithosphere. This movement of water is cyclical and is
called the hydrologic cycle or water cycle. In this cycle, water changes phase many times
and exists as a liquid (surface waters and groundwater), solid (snow and ice), or gas (water
vapor). The global movement of water is initiated by solar energy, which evaporates surface
water into the atmosphere. Much of this water vapor condenses and falls as some form of
precipitation on a distant land surface where it either evaporates, flows back into the oceans
through rivers and streams, is taken up by vegetation and slowly released into the atmo-
sphere as evapotranspiration, or infiltrates into the ground. Groundwater also migrates
back to the oceans (Jones 1997; Alley et al. 1999). Figure 3.5 depicts the water cycle on Earth.
As with all physical systems, the water cycle performs some work. These three main
chores are accomplished: (1) evaporation desalinates large quantities of ocean water and
creates freshwater that is potentially potable; (2) nutrients are transported—a process
called the biogeochemical cycles—which moves carbon, phosphorus, and other nutrients
between their reservoirs. For example, the carbon cycle transports enormous quantities
of carbon between the atmosphere and the oceans by precipitation, with much of this
carbon held within the carbonic acid (H 2 CO 3 ) formed in the atmosphere by the combina-
tion of CO 2 and water vapor; and (3) landscapes are altered by flowing water (erosion and
sedimentation) and water's phase changes between ice and liquid (ice wedging and frost
heaving). Water, with its higher viscosity and weight per unit volume than air, is the key
agent of landscape change on Earth.
3.2.3 Distribution of Water on Earth
The vast majority of water on the Earth is saline and resides in the oceans. Although the
water in the oceans is important for life on Earth, it is the freshwater contained in ice and
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