Environmental Engineering Reference
In-Depth Information
The possible pathways of metals from the soil to surface water are through surface
runoff, containing particulate matter with adsorbed metals, or through seepage and
ground water, contaminated with leached metals. Impact from the farmland was found at
both SW
2
and SW
3
, regarding Ni only and it was pronounced during October. It could be
associated with the acidic conditions described above and suggests that the most likely
pathway is through a direct leak from the wetland. The high application rates, shown in
Table 8.1, support this finding, indicating the high pollution loading rates with respect to
this metal. It could be expected that the retention capacity of the soil, with respect to Ni,
should have been exhausted. In November and February, the impact of the farmland is
reduced and this could be associated with the increased concentrations at the control
point, indicating to a higher pollution load, which has been contributed upstream of the
control point. The farmland contribution is relatively low compared to the upstream load.
Heavy metals concentrations show distinct seasonal variations at all points, with peak
values for Cd, Ni and Pb during November and a decrease of concentrations in February,
which could be associated with a first flush effect over the drainage area. The river is
relatively small and shallow, thus turbulent flow conditions in November allow for the
lifting and transport of sediments and mixing them with the water flow, which could
contribute to increased metal concentrations as well. Similar seasonal patterns of diffuse
pollution, associated with heavy metals, have been reported earlier (Baily et al. 1999,
Gray 1998) and specific attention is given to the fact that seasonal variations in
concentrations, together with other factors as hardness and the ratio of different metals
concentrations, are leading to an increase of the toxic effect of these cations. Therefore, it
is advisable that regulations for heavy metals concentrations in effluents or in natural
water bodies should consider event-focused toxicity, which more accurately represents
the acute toxicity of wet weather events (Brent & Herricks 1999).
Table 8.4. Summary of water quality characteristics
of Marimba River during the period of study.
Parameter in mg/l
SW1
SW2
SW3
Safe regulated limit
1
2
3
4
5
Cadmium
0.036
(0.065)
0.042
(0.072)
0.039
(0.065)
0.01[0.3]
Chromium (VI)
0.07
(0.1)
0.07
(0.11)
0.072
(0.1)
0.05[0.5]
Lead
0.26
(0.33)
0.19
(0.3)
0.24
(0.37)
0.05[0.5]
Nickel
0.92
(2.18)
1.16
(2.54)
1.21
(2.43)
0.3[1.5]
TP
1.67
(1.76)
2.19
(2.5)
5.42
(10.8)
0.5[5]
TKN
2.2
(4.05)
3.83
(5.2)
7.77
(20.9)
0.5*[2]
Zinc
0.37
0.29
0.3
0.5[15]
