Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 372

FIGURE 14.3 Degassing project for Lake Nyos. (Courtesy of Wikimedia Commons.)

sank downslope, killing people and animals in its path. A similar but smaller event occurred at
Lake Monoun, Cameroon, on August 15, 1984 (Sigurdsson et al. 1987). The recharge of Lake Nyos
is of concern and was measured by Evans et al. (1993). Degassing methods have been used to aid
in preventing a recurrence of the limnetic eruption, and include a pipe from the surface to the lake
bottom allowing excess gas to be released to the atmosphere (Figure 14.3).
Conditions similar to those in Lake Nyos, which may potentially result in limnetic eruptions,
occur in other lakes, including Lake Kivu. Lake Kivu is located in a heavily populated area situ-
ated on the border of Rwanda and the Democratic Republic of the Congo. Both carbon dioxide and
methane are problems in Lake Kivu.

14.2 TOTAL DISSOLVED GAS
TDG refers to the total pressure of all gases and is a concern particularly below reservoir spillways
where increases in pressure, due to the spillage of water, cause increases in gas concentrations. This
is typically a transient condition, therefore as the pressure decreases, the water will degas. Similarly,
ish exposed to excessive dissolved gas pressure or tension also have excess gas dissolved in their
circulatory system. When the gas comes out of a solution it may form bubbles (emboli), which
block the low of blood through the capillary vessels, causing distress or death; this is called “gas
bubble disease” (discussed in more detail in Chapter 18). Water quality criteria to protect aquatic
life generally limit the TDG concentration to 110% of the saturation value for gases at the existing
atmospheric and hydrostatic pressures.

14.3 OXIC VERSUS ANOXIC RESERVOIR PROCESSES
The water quality of lakes and reservoirs is controlled to a large degree by the transport and distribu-
tion of light and heat, in addition to chemical and biological processes (e.g., productivity, respiration,
and decomposition). The classical characterization of the seasonal cycle in deep dimictic lakes is that
following spring overturn a seasonal stratiication pattern develops where a warm, well-oxygenated
epilimnion lies over a cold, less oxygenated (and often hypoxic) hypolimnion. For dimictic lakes,
there is then a fall overturn followed by an inverse stratiication period in the winter months, where
due to reduced exchange through ice and snow, deoxygenation can again occur in deeper waters.
The water quality in a stratiied reservoir depends on time, temperature, environmental condi-
tions, and terminal electron acceptors (TEAs). TEAs are necessary in the metabolic process of oxi-
dizing organic matter to provide food for aquatic life. When DO is present (oxic conditions), oxygen
serves as the TEA, while under anoxic conditions a different set of microorganisms emerge that can
use other forms of oxygen (e.g., NO 3 , MnO 2 , SO 4 , and Fe 2+ ).