Geoscience Reference
In-Depth Information
Another source of nitrates in river systems is from
animal wastes, particularly in dairy farming where
slurry is applied to fields. This is organic nitrogen
(frequently with high urea content from urine)
which will break down to form nitrates. This is part
of the nitrification process described earlier.
A fourth source of nitrates in river systems is from
plants that capture nitrogen gas from the air. This
is not strictly true, as it is actually bacteria such as
Rhizobium, attached to a plant's root, that capture
the gaseous nitrogen and turn it into water-soluble
forms for the plants to use. Not all plants have this
ability; in agriculture it is the legumes, such as
clovers, lucerne (or alfalfa), peas and soy beans, that
can gain nitrogen in this way. Once the nitrogen
is in a soluble form it can leach through to a river
system in the same way that fertilisers do. Over a
summer period the nitrogen levels in a soil build up
and then are washed out when autumn and winter
rains arrive. This effect is exacerbated by ploughing
in the autumn, which releases large amounts of soil-
bound nitrogen.
There is one other source of nitrates in rivers:
atmospheric pollution. Nitrogen gas (the largest
constituent of the atmosphere) will combine with
oxygen whenever there is enough energy for it to
do so. This energy is readily supplied by combus-
tion engines (cars, trucks, industry, etc.) producing
various forms of nitrogen oxide gases (often referred
to as NO
x
gases). These gases are soluble to water
in the atmosphere and form nitrites and nitrates in
rainwater. This is not a well-studied area and it is
difficult to quantify how much nitrogen reaches
rivers from this source (see p. 35).
The different sources of nitrate in a river add
together to give a cycle of levels to be expected in a
year. Figure 7.5 shows this cycle over a three-year
period on the river Lea, south-east England. The low
points of nitrate levels correspond to the end of a
summer period, with distinct peaks being visible
over the autumn to spring period, particularly in the
spring. The Lea is a river that has intensive arable
agriculture in its upper reaches, but also a signifi-
cant input from sewage effluent. At times during
the summer months the Lea can consist of com-
pletely recycled water, and the water may have been
through more than one sewage works. This gives a
background nitrate level, but it is perhaps surpris-
ing that the summer levels of nitrate are not higher,
compared to the winter period. Partly this can be
attributed to the growth of aquatic plants in the
summer, which remove nitrate from the water. The
peaks over the autumn-spring period are as a result
of agricultural practices discussed above. The
example given here is specific to the south-east of
England; in different parts of the world the cycles
will differ in timing and extent.
Nitrates are relatively inert and do not create a
major health concern. An exception to this is
methaemoglobinaemina ('blue baby syndrome').
Newborn babies do not have the bacteria in their
stomach to deal with nitrates in the same manner
as older children and adults. In the reducing sur-
roundings of the stomach the nitrate is transformed
into nitrite that then attaches itself to the haemo-
globin molecule in red blood cells, preferentially
replacing oxygen. This leads to a reduction in
oxygen supply around the body, hence the name
'blue baby syndrome'. In reality methaemoglobin-
aemina is extremely rare, possibly coming from
nitrate-polluted well supplies but not mains-
supplied drinking water. The drinking water limit
for the European Union is 50 mg/l of nitrate (44
mg/l in the USA). In rivers it is rare to have nitrate
values as high as this. In a study of streams draining
intensively dairy-farmed land in the North Island
of New Zealand, Rodda
et al
. (1999) report maxi-
mum nitrate levels of 26.4 mg/l. These are reported
as being 'very high by New Zealand standards'
(Rodda
et al
., 1999: 77). In Figure 7.5 the peak
nitrate level for the river Lea in England is 21 mg/l,
with the norm being somewhere between 5 and
10 mg/l.
The biggest concern with nitrates in a river
system is
eutrophication
. In exactly the same
way that the nitrogen enhances the growth of land-
based plants, it will also boost the growth of aquatic
plants, including algae. This creates a problem of
over-production of plant matter in river systems.
This is discussed in more detail on pp. 142-143.
