170
ENVIRONMENT, POLLUTION, DEVELOPMENT: THE CASE OF UZBEKISTAN
Ammonia (ammonium ions) is formed in water reservoirs as a
result of decomposition of organic nitrogenous substances. However,
more often its content levels are
determined by organic waste
(slurry, sewage, excrements), household and industrial waste inflow
into water bodies where nitrogen can be found in organic substances
(proteins, amino acids, amines, nucleic acids). Decomposition of
organic substances produces intermediates – relatively large quantity
and variety of nitrogen compounds, however, their accumulation in
water
does not occur, as their biological stability is low. Depending
on the pH of environ ment, the ammonia in water exists as NH
4
+
ion
(if the water pH <7) or non-dissociated NH
4
OH. If the pH reaction is
alkaline, then the share of water-dissolved ammonia increases in the
balance of this nitrogen compound form. Ammonium ions are toxic,
especially for fish. Binding with organic or suspended substances
reduces the bioavailability of ammonium ions. In
natural waters
ammonium ion content depends on the nature of biological processes
therein and thus the concentration of ammonium ions is affected by
the seasonal changes. Typically, in the summer season their intense
assimilation takes place, whereas in winter their concentration in
waters increases.
Nitrite ions (NO
2
–
) are mainly formed
as the intermediates
of nitrogen compound transformation – the oxidation of NH
4
+
or
reduction of NO
3
–
. In uncontaminated waters the nitrite ions can be
found in trace amounts (>0.001 mg/l NO
2
–
) and the increase in their
concentration is an important indicator of pollution.
Nitrate ions (NO
3
–
) can be found practically in any waters. The
clean surface water’s nitrate concentration is generally 0.4 to 8 mg/l,
but that of a contaminated water – up to 50 mg/l NO
3
–
. The nature of
seasonal variability of nitrate ions is similar to that of other biogenic
elements. The character of nitrate seasonal variability is affected
by the differences between their supply and consumption sources.
The main sources of pollution are chemical ferti lizers leaching
from the soil, organic and inorganic chemical trans formations and
reactions. Pollution intensity influences the differences in nitrate ion
concentration values in the river waters.
Nitrogen compound forms are
interconnected and these
compounds can change into one another. The nitrogen compound
cycle is determined by microorganism activity. The key process
in this cycle is the binding of nitrogen, which takes place in the
course of photosynthesis (assimilation). Assimilation leads to
forma tion of organic nitrogen compounds. As they decompose, the
ammonia – ammonium salts – are formed (ammonification), which
are subsequently converted into nitrates and nitrites (nitrification).
Nitrifi cation can also be defined as organic and inorganic nitrogen
compound oxidation.
8. WATER POLLUTION
171
Valency -3
-2
-1
0
+1
+2
+3
+4
+5
Biosynthesis
Ammonification
Nitrate
assimilation
Nitrification
Denitrification
N
2
fixation
N
2
N
2
O
NH
4
+
NH
2
OH
HNO
2
NO
2
–
NO
3
–
(NH
2
)
The primary source of nitrogen compounds is atmo spheric nitro-
gen fixation, which is performed by a few species of bacteria and
in the nitrogen fixation reactions in blue-green algae. It is be lie ved
that the internal waters, depending on the intensity of the biological
processes, the nitrogen fixation may form around 1 g N/m
2
/year.
Today, the biologically assimilable naturally bound nitro gen amount
is comparable to industrially implemented nitrogen fixation, whose
first stage is the synthesis of ammonia, which in turn is used as
a source of synthesis of nitrogen fertilizers and other nitrogen
compounds. Thus, the nitrogen compound source in waters is atmo-
spheric precipitation, direct nitrogen assimilation and sur face runoff.
The nitrogen fixation occurs as a result of NO
3
–
ions’ reduction under
the exposure to microorganisms (denitrification), under the formation
of N
2
, and the binding in the sediment. Nitrogen compound fall with
the atmo spheric precipitation determines their total balance.
The
role of Phosphorus compounds in the ongoing reactions in
the aquatic environ ment is determined by their function in hydro-
bi ont
metabolism and the fact that only relatively modest amounts
of phosphorus are available in hydrosphere. Compared with the
other elements required to be present for the development of
living organisms (C, H, O, N, Fe, S), phosphorus is considerably
less common and thus its limited availability
often restricts the
develop ment of living organisms. In water phosphorus can be found
in the form of many compounds. Increased amounts of
phosphate
ions (H
2
PO
4
–
, HPO
4
2–
, PO
4
3–
) enter the waters mostly due to human
economic activity. As a result of anthropogenic contamination
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