Our Environment - Mining and the Environment: From Ore to Metal - page 293

Evaporation and Evapotranspiration

Water is removed from oceans, lakes, and other surface bodies by evaporation. Evaporation
is the process where a liquid, in this case water, changes from a liquid to a gaseous state.
It is a misconception that at a pressure of 1 atm, water vapour only exists at 100ºC. Water
molecules are in a constant state of evaporation and condensation flux near the surface of
any water body. If a surface molecule receives enough energy, that is solar radiation, it will
leave the liquid and turn into vapour. At mine sites located in high temperature regimes
with low precipitation, most of the water used in mining will probably be lost to evapora-
tion, rather than returned to surface water or ground water systems.
Evapotranspiration is the loss of water to the atmosphere by evaporation from the sur-
face as well as by transpiration from vegetation growing on the soil. As for evaporation,
transpiration is measured in kg of H 2 O/m 2 /day, the loss of water to the atmosphere per
square metre of land surface per day. Evaporation and transpiration are critical hydrologi-
cal links between the Earth's surface and the atmosphere. Both are therefore important for
issues involving climate change and ecosystem response. Changes in vegetation may have
large impacts on transpiration and evaporation rates, with consequent effects on the water
balance. Conversely, if changes in the water balance are significant, major shifts in vegeta-
tion patterns and composition are a likely outcome. Equally, changes in transpiration are
likely to impact atmospheric composition of greenhouse gases, and climate, as the hydro-
logical cycle increases in intensity with warming.

At mine sites located in high
temperature regimes with low
precipitation, most of the water
used in mining will probably be
lost to evaporation.

The Atmosphere in Motion

Air moves because at one place less dense (warmer) air rises, while in another place denser
(cooler air) sinks towards the Earth. Between these two areas, the air flows horizontally
along the Earth's surface as wind. Simplified, air circulation is in the form of giant con-
vection cells. Less-dense, warmer air rises at the equator where temperatures are high,
and cooler, denser air sinks at both poles, where temperatures are very low. The Earth's
rotation, and the associated Coriolis effect , complicates this basic concept. The result is
three convection cells in each hemisphere wrapped around the rotating Earth ( Figure
7.2 ). Moving from this conceptual model to real air movement requires consideration of
three factors: (1) seasonal changes in temperature due to solar heating, (2) large continental
blocks with differences in elevations, and (3) the difference in heat capacity of land and
water, cause of the monsoon effect. While the general wind and pressure belts are still
identifiable over the Earth, actual air movement can fluctuate widely at any given time
and place, making forecasting wind and weather notoriously difficult. Local wind data
become especially important for mining projects with integrated power or pyrometallurgi-
cal processing plants. The surface footprints of gaseous emissions from these operations
will very much depend on the prevailing wind strength and direction.

Local wind data become
especially important for mining
projects with integrated power
or pyrometallurgical processing
plants.

Ambient Air Quality

The atmosphere has a finite ability to absorb anthropogenic emissions before those emis-
sions inflict harm on human health. In many parts of the world air pollution has already
become the environmental factor with the greatest impact on health and is responsible for
the largest burden of environment-related disease. As one example, recent estimates indi-
cate that 20 million Europeans suffer from respiratory problems every day (EEA 2005).