Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 374

CO 2 = 5.6

SO 4 = 5.9

FeO (OH) = 10.1

O 2 = 29.9

MnO 2 = 23.3

NO 3 = 28.4

FIGURE 14.4 Relative BUE yield of various oxidants.

Typically in lakes and reservoirs, the ORP would be a high positive value in the oxic epilimnion
and would then decrease with depth. In an anoxic hypolimnion, and in the anoxic sediments, the
ORP would be relatively high and negative (reducing).
As oxygen (and organics) is consumed under oxic conditions (i.e., oxygen is the TEA), the ORP is
typically from about 400 to 300 mV. This oxidation also results in a decrease in pH, typically from
neutral to 6.5 (Gordon and Higgins 2007).
Once the oxygen is depleted, the chemical characteristics of the lakes or reservoirs are largely
impacted by the availability of TEAs. The end product of these processes may be problematic as
these materials are entrained into surface waters or they are released. For example, hypolimnetic
releases such as from hydropower facilities may not only be anoxic, but they may also contain high
concentrations of reduced materials, such as NH 4 , Mn 2+ , Fe 2+ , H 2 S, and CH 4 .
Somewhat surprisingly, the sequence in which the TEAs are utilized generally follows a thermo-
dynamically predicted sequence, illustrated by a BUE yield (Table 14.3). As indicated by Di Toro
(2001), it is not intuitively obvious why this should be the case, since all of these reactions are bac-
terially mediated. But, data, as reported by Di Toro, demonstrate that is the case, with the exception
of NO 3 , where the predicted route of denitriication is from NO 3 to NH + , where most if not all of the
reduced nitrogen appears as N 2 (Table 14.3).
Denitriication poises the ORP at about 200 mV and the pH may drop to 6 (Gordon and Higgins
2007). As will be discussed later, there are usually substantially elevated concentrations of ammonia
(NH + ) in the anoxic hypolimnion of lakes and reservoirs. However, this may occur due to ammoniica-
tion, which is the release of ammonia from decomposing organics. Bacteria irst strip off the ammonia
from nitrogenous organics before further oxidation; subsequently, the ammonia may build up under
anoxic conditions because the only sink is cell synthesis, which is minimal (Gordon and Higgins 2007).
Under anaerobic conditions and following the depletion of nitrate, manganese is the next most
thermodynamically preferable TEA. Manganese reduction is primarily associated with bottom
sediments, and, as a result, the ORP drops from 200 to 50 mV and the pH remains at about 6-6.5
(Gordon and Higgins 2007).
Next, solid-phase hydrous oxides of iron are used for TEAs with the soluble form released being
Fe 2+ , which accumulates under anaerobic conditions. The ORP must be at about 50 mV for 2 weeks
before reduction begins and the pH will be 6-6.5 (Gordon and Higgins 2007).