Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 404

The formation of carbonic acid can reduce the pH. As a result, during daylight hours when net
productivity by autotrophs is positive, the pH could increase and then decrease during periods
of nighttime respiration. Similarly, heterotrophic production and respiration impact carbon diox-
ide. Heterotrophic respiration under oxic conditions will produce CO 2 and decrease the pH, typi-
cally from neutral to 6.5 (Gordon and Higgins 2007). Under anaerobic conditions, the pH is also
impacted by denitriication, the formation of organic acids, and the reduction of nitrate, manganese,
iron, and sulfate.
External loads, such as from acidic streams or snowmelt, or from the atmosphere (e.g., nitrous
oxides and sulides), impact the pH of lakes. Other sources include the dissolution of calcium car-
bonate and the introduction of carbon dioxide from groundwaters and, in some lakes, from volcanic
origin. As discussed in Section 14.1.5.2, loads from volcanic sources can produce highly super-
saturated carbon dioxide concentrations in the hypolimnion, which have resulted in a catastrophic
release of carbon dioxide during overturn (a limnetic eruption).
One of the factors impacting the pH is the exchange of carbon dioxide between water and the
atmosphere. As atmospheric carbon dioxide concentrations increase, lake concentrations will
increase, with an impact a resulting impact (lowering) on the pH.
The susceptibility of a lake or a reservoir to pH changes is largely relected in its alkalinity,
where alkalinity is a measure of the buffering capacity or the ability to resist pH changes. For
example, lakes with low alkalinity and hence a low buffering capacity are much more susceptible to
acidiication. Alkalinity and pH relationships are discussed in Chapter 5.

14.8 SULFIDES AND SULFATES
Sulfur in freshwaters may exist in a number of forms, as listed in Table 14.7. Sulfur is common in
the earth's crust in the form of gypsum (CaSO 4 ) and pyrite (FeS 2 ). It is common in the atmosphere
in the form of sulfur oxides (SO 2 , SO 4 , such as forming sulfuric acid, H 2 SO 4 ), which can serve as a
source of sulfur to lakes and reservoirs in the form of dry or wet deposition, with the greatest rates
of deposition in the north-eastern United States (USEPA 1999). Mean annual atmospheric SO 4 con-
centrations for 2011 are illustrated by Figure 14.30. External loads of sulfur oxides may contribute
to lake acidiication, particularly in lakes with low alkalinity.
A generalized sulfur cycle is illustrated in Figure 14.31. Sulfur is taken up during the process of
building plant and animal cells and is also a component of waste. Therefore, organic forms in the water
column and sediment impact sulfur cycling, via production, degradation, and excretion (e.g., of SO 4 ).
Under aerobic conditions, sulides may be microbiologically oxidized to form sulites and sul-
fates. Under anaerobic conditions, sulfates are utilized as an oxidant (TEA; Table 14.3), ultimately
resulting in a reduction to sulides. Sulfates follow nitrates, iron, and manganese oxides in terms of
biologically usable energy yield for oxidation.

TABLE 14.7
Common Oxidation States for Sulfur
Oxidation
State of S

Increasing Levels of Sulfur Oxidation

Form −2 −1 0 +4 +8

Aqueous
solution and
salts

H 2 S H 2 SO 3 H 2 SO 4

HS − HSO 3− HSO 4−

S 2− S 2− SO 2− SO 2−

Gas phase H 2 S SO 2 SO 3

Molecular solid S 8