Environmental Engineering Reference
In-Depth Information
bodies, such as when untreated sewage or waste from pulp mills is discharged. This
organic matter settles to the sediments, stimulates decomposition, creates anaerobic condi-
tions, and cuts off coupled nitrification
denitrification.
Streams and Rivers
Streams and rivers are both the kidneys of the continents (e.g., they can remove signifi-
cant amounts of N from the water column) and the delivery system moving nutrients
from continents to the sea. Typically, the coupling between the water column and sedi-
ments is much stronger in small streams than in large water bodies because the water col-
umn is in close contact with the sediments. As streams get larger and depth increases, the
interactions between the water column and the sediment may diminish.
In rivers and streams, directional water flow creates additional complexity in N cycling.
The downstream movement of water and solutes results in N being pulled downstream as
it cycles and is referred to as nutrient spiraling (see Box 5.2 in Chapter 5). Streams and riv-
ers are both spatially and temporally heterogeneous with respect to N cycling and are
composed of patches of aerobic and anaerobic environments. For example, shallow areas
of rapid flow (riffles) are well oxygenated and support high rates of mineralization and
nitrification, while quiescent pool areas and accumulations of organic matter behind
obstructions in the stream channel can be anaerobic “hotspots” of denitrification. In addi-
tion, zones of upwelling, downwelling, and ground-water interactions in a stream can
strongly influence N transformations. Explorations of hotspots and hot moments (when N
transformations are maximized in time) are currently an area of fruitful research.
Small headwater streams have the capacity to remove N from the water column via assimi-
lation and denitrification, but this capacity is influenced by human activities such as land-use
change in the watershed and runoff of N. For example, streams draining agricultural water-
sheds can have high rates of uptake and denitrification, but can only remove a small fraction
of large amounts of N entering the systems from fertilizer. In contrast, streams draining for-
ested catchments do not have high rates of N removal, but can remove a larger fraction of the
total N load entering the stream. The extent to which large rivers process and retain N needs
further research, as these systems are less frequently studied.
Ground Water
There is great interest in nitrogen dynamics in ground water, because as noted earlier,
nitrate is readily transported through the soil profile into ground water. Nitrate is the
most commonly detected drinking water pollutant in ground water in the United States,
although it is usually below the regulatory threshold of 10 mg N/L. Ground-water con-
tamination with nitrate is widespread in agricultural landscapes and in rural/suburban
landscapes serviced by septic systems.
Much of the interest in ground-water nitrate dynamics focuses on the potential for deni-
trification to convert the nitrate into nitrogen gases, removing the water quality concerns.
Ground water can be anaerobic (necessary for denitrification) if there is sufficient microbial
respiration to consume the oxygen present. Relatively low rates of respiration (10 times less
than surface soil) can be sufficient to make ground water anaerobic because ground-water
often flows very slowly (months to years per km). It is also important to note that
