Environmental Engineering Reference
In-Depth Information
Count
9
8
9
8
9
8
9
8
1997
Standard Error
0.118
0.097
7.729
0.073
0.019
0.013
3.254 0.472
Count
12
13
12
13
12
13
12
13
1998
Standard Error
0.153
0.271
1.955
0.814
0.022
0.145
0.242 0.230
Count
10
11
10
11
10
11
10
11
1999-2000
Standard Error
0.271
0.130
0.284
0.931
0.521
0.448
0.574 0.369
Count
5
7
5
7
5
7
5
7
the prescribed limit. The median phosphate values are below or equal to the background
pollution and are much lower than the reported river water quality upstream the discharge
point as given in Table 3.4, Point 3. Mean values exceed up to two times the prescribed
values throughout the study period with peak mean and median values in 1996 and 1997.
Nitrate variations show a similar pattern in both channels and are relatively low, with
occasional high values measured during the period 1998-2000.
Spatial variations were analyzed by comparison of the means of the two data sets for
each specific year, regarding pH and phosphates only. Nitrogen compounds were not
tested because of the high irregularity of the data sets. The test results show no significant
difference for the annual mean pH values at DR1 and DR2. The test results regarding
phosphates show a significant difference between DR1 and DR2 annual mean values only
for the years 1996, 1998 and 1999-2000. DR1 shows a much higher annual mean and
median values of phosphates and there is a well-defined trend of increase of these
concentrations after 1995. The lack of a significant difference between the annual values
in 1997 could be attributed to the irregularity of the data set (SE = 3.24) but not to
reduced pollution at DR1.
During 1998-2000 the median values at DR1 are closer to the mean values, suggesting
that the process of pollution has a constant nature. However, the reduced frequency of
sampling during 1999-2000 might have resulted in data, which does not reflect the real
status of the water quality. The significant difference in the level of phosphate
concentrations between DR1 and DR2 could be associated with the difference in the land
use pattern of the drained area. DR1 drains three different types of areas - residential,
industrial and open undeveloped grounds. The residential part consists of medium density
one story housing units, where the roof runoff is discharged and infiltrated to gardens.
Therefore, its contribution in terms of the quantity and quality of the total runoff could
not be significant. The industrial area has a much higher level of impermeable surfaces,
thus in terms of runoff quantity its contribution is expected to be much higher than the
contribution from the rest of the area. A considerable portion of the open undeveloped
areas is used for informal agricultural practice, most often for maize production. Possible
sources of phosphate pollution to DR1 could be associated with:
