Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

Figure 8.5. 
Changes 
of chemical fertilizer 
consumption trends 
in industrialized and 
developing countries
Biogenic elements – 
nutrients whose 
presence is required 
for the development of 
living organisms.

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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.

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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