Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 65

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Water and theHydrogeologyofWatersheds
3.1 Introduction
The importance of water on Earth cannot be overstated. Water, along with energy and
organic molecules, was the prerequisite for the origin of life here billions of years ago.
Today, water plays a central role in maintaining our survival as a species, preservation of
the natural environment, and therefore achieving a sustainable planet (United Nations
2003). Yet, there are immense challenges facing humans with respect to securing water for
their basic needs and long-term quality of life. Although almost three-fourths of Earth's
surface is covered by water, most of this water is not potable, and a high percentage of the
fresh stuff is either frozen, underground, or in a gaseous phase.
Wait—there are other complications. Because of the spherical nature of the Earth and
its axial tilt, the arrangement of land and water, and differences in surface elevation, it is
not possible to achieve a uniform distribution of the incoming solar energy we receive.
Surpluses and deficits of energy arise at different locations and create uneven atmospheric
pressures and densities. In a thermodynamic system, these inequalities try to even them-
selves out, so fluids move in a quest to achieve overall energy equilibrium. Air (via wind)
transports part of the solar energy it has absorbed and moves it from zones of higher pres-
sure to zones of lower pressure. Water moves the excess energy it has absorbed from the
equatorial region in a general poleward direction via ocean currents. All of this movement
results in Earth's topside having a peculiar and unpredictable precipitation pattern: some
areas receive precipitation almost daily; others may not get any for years; and many places
lie between these extremes.
Precipitation from the atmosphere recharges the underground water reservoirs and
plays a central role in shaping the geology of urban areas through weathering, sediment
transport and deposition, chemical precipitation and dissolution, and erosion. Given the
haphazard nature of precipitation, it is a good thing that nature is so organized. Surface
flows of water are controlled hierarchically by a system of topographically bounded spa-
tial units called watersheds, which are really just water collection bowls. Small watersheds
consisting of small streams feed larger watersheds with larger streams; for example, the
Scioto River watershed in Ohio delivers its flows into the Ohio River, which then flows
into the larger Mississippi River. Some of the precipitation is intercepted by vegetation,
some pools on the surface, and a good part of it moves downward through the soil where
it recharges the groundwater reservoir.
This chapter opens with a brief discussion of basic water chemistry and the geog-
raphy of Earth's water, and how it moves cyclically between the atmosphere and the
oceans. Understanding basic water chemistry and refreshing ourselves about the global
water cycle provide the knowledge base for the more detailed material later in the chap-
ter and later sections of this topic. Key objectives of this chapter include an investiga-
tion of the multifaceted nature of surface water and groundwater as discrete entities,