Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 396

14.5.1.6 Denitriication
Denitriication (Figure 14.22) is the process by which nitrate nitrogen is converted to molecular
nitrogen (N 2 ). As discussed in Section 14.3, once oxygen is depleted, nitrate is the next most ther-
modynamically favorable oxidant for heterotrophic organisms (Table 14.3). That is, in addition to
oxygen, it produces the most energy in the processing of food (organic matter). Since it occurs as
oxygen is depleted, it is generally assumed that denitriication only occurs under hypoxic (reduced
oxygen) conditions.
Denitriication was long thought to be the only pathway by which the oxidized forms of DIN
were converted to N 2 . However, a relatively recent process has been discovered, known as anaerobic
ammonium oxidation (anammox; Figure 14.22), whereby the anaerobic oxidation of ammonium
(NH 4 ) with nitrite (NO 2 ) generates N 2 (Thamdrup and Dalsgaard 2002). This process has been
found to be very important in marine sediments, but its importance in anaerobic waters or sedi-
ments of freshwater systems is not well understood (Hamersley et al. 2009).

14.5.1.7 Sediment Release
Particulate forms of organic nitrogen may settle from the water column to the sediment bed, where
they undergo decomposition. This process is known as sediment diagenesis, as described by Di Toro
(2001). As a result of the ammoniication (diagenesis) of particulate organic carbon, ammonia can
be produced and the lux of sediments serves as a source to the water column. This process is of
particular importance because of the relatively long timescales associated with sediment diagenesis.
For example, the sediments may serve as a source of nutrients to the water column long after the
external loads to a lake or reservoir have been reduced.

14.5.2 n ItroGen : r eGuLatory e nVIronMent
Nitrogen has been the subject of regulation for a number of reasons, including:

• Impact on acidiication

• Impact on oxygen

• Tox ic it y

• Impact on plant growth (as a nutrient)

Lake acidiication has been a major problem, resulting primarily from emissions of sulfur and
nitrogen oxides into the atmosphere and their subsequent introduction into lakes and reservoirs as
mist, fog, rain, or snow. Their impact has been greatest in lakes with low alkalinity or buffering
capacity. The regulatory focus has been on limiting these emissions.
Nitriication impacts oxygen concentrations, so nitrogen has been a common component of waste
load allocations and TMDLs where low DO concentrations are typically the problem. This may be
a problem in some lakes and reservoirs, but it is more commonly a regulatory target for streams and
rivers.
Similarly, ammonia is regulated as a result of the toxicity of unionized ammonia, as described in
the U.S. EPA's ambient water quality criteria for ammonia (USEPA 1999b) (Figure 14.25). Ammonia
toxicity may be a problem, particularly in highly eutrophic alkaline lakes.
One of the major regulatory focuses on nutrients (including nitrogen) is on developing nutrient
criteria, with the goal of reducing eutrophication at the national scale. The U.S. EPA developed its
irst guidance document on developing nutrient criteria for lakes and reservoirs in 2000 (USEPA
2000a). The general approach is to develop speciic guidance documents for speciic ecoregions
(14 in total) as illustrated by Figure 14.26 for the contiguous states. The U.S. EPA has provided
this guidance as a starting point to states, and all states are presently in the process of developing
or inalizing nutrient criteria. For nitrogen, the criteria have typically been developed in terms of
total nitrogen concentrations, such as 0.36 mg L -1 for ecoregion 9 (USEPA 2000b) (Table 14.6).