Introducing of Ce-Qual-W2 Model and Its Application - Water Quality Management: Assessment and Interpretation - page 110

Fig. 6.14 Dissolved oxygen (DO) in the study area during normal conditions (mg/l)

0.3 mg/l, respectively, for
CBOD, and DO. The negative values indicate that the measured data are higher
than the model results.
Analyzing the values of Dissolved Oxygen in normal conditions in the river
(Fig. 6.13 ) indicated a general decrease, as a result of agricultural, industrial and
municipal polluting sources, and then a sudden peak resulting from the Gotvand
dam. A decrease was again witnessed after the dam from agricultural sources and
two municipal sources. The lowest the DO reaches was 6 mg/l which is acceptable,
but could endanger aquatic life in the early stages of life based the Dissolved
Oxygen Levels for Aquatic Life in Table 6.1 . Fish deaths have occurred in the
K

The errors averaged over all stations were 0.5 and

−

in River in recent years, causing a stench and raising concerns. The main
streams that

ā r ū

fish kills happen are in cities that high volumes of wastewater enter the
river, especially in the region of Ahvaz City. The fish in these regions if not dead

are is not edible due to the pollution in the river.
Investigation of fish deaths by researchers has shown that low Dissolved Oxygen

(DO) was the main reason for this problem to occur (Meyer and Barclay 1990 ).
Oxygen is essential for

fish to survive. Fish deaths will occur if oxygen levels
drop below a certain critical concentration that depends on the fish species. For

example Gheze Ala fish can live when the DO concentration in the river is

maintained above 5 mg/L (Fig. 6.14 ).
Figure 6.15 presents the changes in BOD during normal conditions in the K

ā ū

r n

River. The main locations affecting the quality are labeled in the gure. The BOD

changes in the stream are affected as a result of the receiving pollutants; however,
due to the relatively high water flow the changes are not considerable and the rivers
self-restoration is affecting the quality more and is creating an acceptable water
quality with a BOD of less than 3 mg/l, which is acceptable by Iran

' s standards

based on Table 6.2 and most International Water Quality standards.