Environmental Engineering Reference
In-Depth Information
pH, which is lower at a depth of about 1 m, indicating acidification, influenced most
probably by the sludge accumulated in the ponds.
Increased organic content of soils is directly related to increased CEC, leading to
improved soil quality and improved capacity for absorption and immobilization of metals
and micro pollutants. This link is reflected in the results and illustrates one of the positive
consequences of long-term irrigation. Nyamangara et al. (2001) have obtained similar
results with respect to Firle farm. However, results with respect to metals concentrations
in the pastures soils (Table 9.3) show that actually, the highest metals concentrations are
found in the second depth layer (0.3 m-0.6 m), while the highest OC concentrations, are
found in the top 30 cm, as could be expected.
3.2.3 Metals
Metals, and more specifically, toxic metals are immobilized in soils by precipitation and
formation of metal complexes. This process might be enhanced by the presence of
sulfides, which results in the formation of precipitates of metal sulfides. Also,
immobilization of Fe, Zn, Ni and other metals could be achieved by precipitation under
increased pH conditions in alkaline media. Decreasing the pH of the soil creates
conditions of dissolution of such precipitates and leads to a reduction in the
concentrations of sulfide ions, thus leading to increased solubility of metal ions and their
leaching to ground water.
The results of this study with respect to the metal concentrations in soil (Table 9.3)
show that at the different locations, the metals' concentrations are comparable to the
control point. In general, the concentrations of all tested parameters are far below the
recommended limits. This reflects the fact that long - term irrigation has not influenced
adversely the soil characteristics. A possible explanation could be that the metals have
been flushed from the soil by the continuous percolation of water from the surface to the
groundwater due to the high hydraulic load and the acidic conditions at the most
frequently irrigated sites.
Fe concentrations are much higher compared to the rest of the tested metals and could
be associated with the natural soil conditions. Comparing BH2, which has never been
irrigated to the control point, we could notice relatively similar depth profile distributions
of metals.
With respect to Pb there is a distinctive pattern of increased concentrations with depth
at BH1, BH3, BH4 and BH5. The highest Pb concentration was observed at a depth of
0.3-0.6 m at BH1, BH3 and BH5. At BH4, which is the most frequently irrigated site, the
highest Pb concentration has been measured at the lowest depth profile (0.6 m-1.2 m).
Cd has not been detected at BH1, which is wet throughout the year. In respect to Zn
the highest concentrations have been found in the second depth profile for BH1 and BH5
only. Ni soil concentrations did not show variation along the depth profile. Comparing
the spatial variation of metal concentrations in the soil, BH5 shows slightly higher values
with the maximum concentrations at a depth of 0.3 m-0.6 m, with respect to Pb, Zn and
Cu.
Based on literature data and the discussion made in the previous sections, it could be
expected that metal concentrations in the farm soils should be much higher than the
actual observations during this study. Such results could be explained with the high
