Page 307 - Soil and water contamination, 2nd edition

294 Soil and Water Contamination

resulting concentration patterns of the heavy metals Zn, Cd, and Pb in the topsoil in the
vicinity of the incinerator. The heavy metals have dispersed predominantly north-eastwards,
due to the prevailing south-western winds. The maximum concentrations of heavy metals
occur at a distance from 1.5 to 2.0 kilometres away from the incinerator, where the plume
from the chimney most often reaches the soil surface. It is conspicuous that the maximum
concentrations do not coincide for the different metals. The maximum levels of Cd are found
nearer the source than Zn and Pb. This is probably due to different transport ranges of the
different particle size fractions to which the metals bind. Zinc and lead vaporise during
incineration and condense mainly onto the smallest airborne particles that have the largest
specific surface. Cadmium does not vaporise and, as a consequence, remains also associated
to the somewhat larger particle fractions. These larger particles are transported over shorter
distances than the smaller particles (De Fré et al., 1992).
At the local scale, deposition is affected by aerodynamic surface roughness controlled
by local surface topography and land cover (Bachhuber et al., 1987; Draaijers, 1993).
The presence of hills and transitions in the height and structure of vegetation cover causes
airflow perturbations which alter the rates of dry and wet atmospheric deposition . Draaijers
(1993) studied the effect of canopy and forest edge structure on deposition amounts in the
Netherlands, using the so-called throughfall method. Throughfall refers to the precipitation
water dripping from the leaves, needles or branches and falling through the canopy gaps.
By measuring the amount and composition of the throughfall water using gutters of 5 m
2
long and a total collection area of 0.054 m , the total deposition (i.e. wet + dry + occult
deposition ) flux could be estimated. Strong correlations were found between net throughfall
-
2-
+
-
+
fluxes of SO , NO , NH , Na , and Cl and the roughness and leaf area of forest canopies.
4 3 4
Draaijers (1993) also observed an exponential increase of the net through fluxes towards the
forest edge, although there was a considerable scatter around this general trend (Figure 16.7).
Because the sample support (i.e. gutter length) was of the same order of magnitude of one
tree crown, this scatter was largely attributed to short-range variability associated with the
leaf area and the occurrence of branches and canopy gaps above the gutters. The width of the
zone with enhanced net throughfall fluxes approximated 5 edge heights. The net throughfall
+
2-
-
increase of SO , NO , NH near the forest edges (on average, a factor of 2) was smaller
4 3 4
-
+
than that of Na and Cl (on average, a factor of 5). These differences were attributed to
-
2-
the sources for these ions in throughfall. In the Netherlands, net throughfall of SO , NO ,
3
4
+
NH occurs predominantly through dry deposition of gases and particles smaller than 1 μm,
4
+
-
whereas net throughfall of Na and Cl mostly occurs by dry deposition of sea salt particles
larger than 1 μm. In the Draaijers study, differences between forests in net throughfall
increase in forest edges were attributed to differences in forest density and edge aspect. Net
throughfall fluxed was found to be positively correlated to forest density; forest edges exposed
to prevailing wind directions (south and south-west) exhibited enhanced dry deposition.
Draaijers et al. (1994) and Weathers et al. (2001) have discussed the relevance of these
forest edge effects for atmospheric deposition in complex and patchy forested landscapes. As
forest edges can function as significant traps both for airborne nutrients and for pollutants
from adjacent agricultural or urban landscapes, an important factor in quantifying
contaminant inputs to the soil surface through atmospheric deposition is the landscape
structure and fragmentation. In the Netherlands, almost 80 percent of all forest complexes
are smaller than 5 ha, and more than 70 percent of all individual forest stands are smaller
than 1.5 ha. Assuming a forest edge of five edge heights, at least 50 percent of the total forest
area in the Netherlands is affected by edge effects (Draaijers, 1993).
The atmospheric deposition of acidifying components (NO , NH , SO ) has not only
x 3 2
caused soil pH to fall, but has also caused secondary pollution through the loss of Ca
from the soil profile and the mobilisation of toxic aluminium. These effects are especially
noticeable in poorly buffered soils that are low in bicarbonate, soil organic matter, cation
10/1/2013 6:45:31 PM
Soil and Water.indd 306 10/1/2013 6:45:31 PM
Soil and Water.indd 306