Environmental Engineering Reference
In-Depth Information
The significant contribution of nitrates from the farmland during all seasons and at
both points suggests a different pathway of transport, associated with a regular load from
seepage and ground water discharge. It is more pronounced at SW
2
, which could be
explained with the fact that the existing drainage structure has four points of discharge to
Marimba River and only one point of discharge contributing to SW
3
. The fact that this
impact was present during all seasons indicates that the soil is saturated with nitrates,
resulting in a constant leaching to both rivers through groundwater or tillage water.
However, a study performed later (Chapter 9), which investigated the ground water
quality at a level below the under drain system, shows a considerably lower nitrate
concentrations of the aquifer over the whole farm area. This suggests that the leaching of
nitrates from the topsoil layers is pronounced with respect to the drainage water, which
discharges into the surface water bodies, thus protecting the aquifer. The impact is more
pronounced during the wet season, due to increased tillage, caused by increased hydraulic
loads.
The degradable organic pollution, measured as BOD
5,
did not show a significant
increase in spatial variations, which indicates that biodegradable organic matter is
retained on the farmland and is not influencing the river water quality. Variations of TS
and fractions (Fig. 8.8) show a significant change in the fractional composition, where the
TS volatile fraction during November was drastically increased, compared to the dry and
wet season data. This suggests a contribution from the farmland, where the organic
materials are higher in percentage compared to the mineral fraction. However, this data
was not supported by a significant increase of BOD
5
during the same period. A
significant decrease in concentrations was detected in February from 7.7 mg/l to 3.1 mg/l
as the average of the mean values of all the points, which could be attributed to dilution.
4.4.2 Metals
The results obtained during this study point out to a significant seasonal variation of the
concentrations of heavy metals. Cd concentrations did not show a significant spatial
variation (Fig. 8.9a) at SW
2
and SW
3
indicating that despite the high loading rates the
pasture does not affect river water quality.
A well pronounced seasonal variation at all three points was measured, with a
significant difference from 0.03 mg/l in October to 0.07 mg/l and 0.02 mg/l in November
and February respectively, given as average values for the study period.
Ni spatial variation during October shows a significant increase at SW
2
and SW
3
compared to SW1 (Fig. 8.9b). No significant spatial difference was found during
November at the two examined points, and in February, a statistically significant but not
pronounced increase was found only at SW3.
The seasonal variation shows a significant difference during the three occasions. A
sharp increase in concentrations from 0.56 mg/l to 2.39 mg/l was measured in November,
with a significant decrease in February to 0.35 mg/l, given as the average values for the
three sampling points.
Significant increase in spatial variations of Zn, Cr
+6
and Pb was not observed during
the study period, indicating that the farmland is not affecting river water quality regarding
these metals. However, a significant seasonal variation of these parameters was detected
at all points. Variations between November and February, given as average of the means
