Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 367

14 Chemical and Water Quality

Kinetic Characteristics
and Processes

14.1 DISSOLVED GASES
The solubility of gases in water is often an important consideration in the management of lakes
and reservoirs, both from a regulatory and an ecological perspective. Total dissolved gases (TDGs)
may be important, particularly when they are in excess, resulting in gas bubble disease. Other dis-
solved gases commonly of interest include nitrogen, ammonia, oxygen, carbon dioxide, methane,
and hydrogen sulide. Dissolved oxygen (DO) is always a critical water quality parameter. In many
lakes that have an anaerobic hypolimnion, methane, hydrogen sulide, ammonia, and other gases
may accumulate, creating problems associated with hypolimnetic releases.

14.1.1 a tMoSpHerIc c oMpoSItIon
A primary source of gases dissolved in water is the atmosphere and, under equilibrium conditions,
the amount of gas dissolved is proportional to the partial pressure of the individual gases. The atmo-
sphere is primarily composed of the gases listed, along with their properties, in Table 14.1 (AFS
1984). These gases comprise approximately 99.996% of the atmosphere and, with the exception of
carbon dioxide, are of relatively constant proportions. Other gases include neon (Ne, about 0.0018%)
and hydrogen (H 2 , about 0.00005%), with the remaining gases being made up of variable quantities
of water vapor, helium (He, about 0.0005%), and methane (CH 4 , about 0.00017%).

14.1.2 a tMoSpHerIc e xcHanGeS
At equilibrium with the atmosphere, the aqueous-phase concentration would be equal to the gas-
phase concentration. This equilibrium concentration is also called the saturation concentration ( c s ).
If the liquid concentration exceeds the saturation concentration ( c > c s ), the net movement will be
out of the liquid, and if the liquid concentration is less ( c < c s ), the net movement would be into the
liquid. That is, the rate of movement ( R ) is proportional to the gradient, or it is between the satura-
tion concentration ( c s ) and the liquid concentration c .

( )

RKcc
r

= − (14.1)

s

The rate constant ( K r ) is a transfer rate that is a function of luid (air and/or water) turbulence.
That is, the greater the turbulence is, the higher the rate will be.

14.1.3 o tHer S ourceS
While gas exchange with the atmosphere is a source of dissolved gases, other mechanisms impact
the gas concentration and the composition of lakes, including chemical and biological processes.