Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 371

g is the gravitational acceleration (9.80655 m -2 s -1 )
ρ is the average water density (kilograms per cubic meter)
k is a conversion factor that converts kPa to mmHg (1 kPa = 7.50062 mmHg) (mmHg/kPa)
z is the depth (meters)

14.1.5.1 Bubble Formation
The relationships between depth and solubility discussed earlier were based on the assumption that
the water and an air bubble were in equilibrium. Often, equilibrium does not occur (such as due to a
pressure drop) and bubbles may form and rise up through water; if they reach the surface, the gases
are ventilated to the atmosphere. The rate of bubble rise is dependent on the bubble size, the proper-
ties of the luid (density and viscosity), the presence of surfactants, etc. In addition, mass exchange
occurs between the bubble and the water column. Therefore, the rate of bubble rise versus the rate
of dissolution impacts whether a bubble would reach the surface.
The characterization and prediction of bubble formation and transport are important in a number
of applications, such as in the design of oxygen injection systems. For those systems, if the bubbles
that are formed rise up and are ventilated, then their effectiveness is compromised. Examples of
studies of bubble formation and its impact on diffuser design include Little and McGinnis (2001),
Martin and Cole (2000), and McGinnis and Little (1998, 2002).

14.1.5.2 Limnetic CO 2 Eruptions
Carbon dioxide concentrations may increase at depth due to the dominance of degradation and
decomposition reactions in the hypolimnion, which consume electron acceptors (such as O 2 , NO − ,
and SO 2− ) and produce CO 2 and CH 4 . Other sources include the dissolution of calcium carbonate
and surface transfers. Carbon dioxide may also be introduced into the hypolimnion from ground-
waters and, in some lakes, from volcanic origin. The result can be supersaturated carbon dioxide
concentrations in the hypolimnion. The catastrophic release of this carbon dioxide, such as that
resulting from overturn or other factors bringing this highly supersaturated water to the surface,
may result in an explosive eruption, called a limnetic eruption.
In 1986, a tremendous explosion of CO 2 from Lake Nyos, a crater lake west of Cameroon, killed
more than 1700 people and livestock up to 25 km away. Lake Nyos has an area of 1.5 km 2 , its
depth exceeds 200 m, and it is strongly stratiied (Figure 14.2). Dissolved CO 2 seeping from springs
beneath the lake became trapped by the high hydrostatic pressure. While the exact mechanism is
not known, it is speculated that an overturn of the whole lake resulted in exposing the supersatu-
rated waters, with the consequent limnetic eruption of CO 2 on August 21, 1986 (Evans et al. 1993).
A CO 2 cloud formed, estimated to be 50 m thick, and since CO 2 is more dense than air, the cloud

FIGURE 14.2 Lake Nyos, Cameroon, following the gas eruption. (Courtesy of Wikimedia Commons.)