Pesticide Residues in Vegetation - Pesticides: Evaluation of Environmental Pollution - page 302

toxicity of pesticides to wildlife is predominantly an aquatic problem in comparison with
terrestrial ecosystems (Cooper 1991).
In terrestrial ecosystems, soil pollutants can be taken up by plants and ultimately passed
into the human food chain, where they can be very persistent and have a high bioaccumu-
lation potential along the food chain (Hayes and Laws 1991).
There are a number of different pathways by which pesticides may enter vegetation.
Deposition from the bulk atmosphere is the main process of movement of the pesticides
from the troposphere to the terrestrial surface, where they are likely stored in terrestrial
vegetation and soil (Syversen and Beckam 2004). In addition, they can be taken up by roots,
may be volatilized from the soil followed directly by foliar uptake from the gaseous phase,
or resuspended in the soil particles by varied mechanisms such as the actions of wind and
rain, which may result in their subsequent capture by vegetation (Smith and Jones 2000).
Only a small portion of POPs deposited by rain reaches the stream water due to the veg-
etation buffer effects. The dominating retention process in the surface runoff buffer zones
is the trapping of sediments and sediment-bound nutrients (Syversen and Beckam 2004).
The levels of pesticides in/on vegetation are influenced by different factors, which can
be divided into three groups: first, the characteristics of the pesticide, mainly related to its
stability in general and to its metabolites; second, environmental aspects, primarily tem-
perature, precipitation, humidity, and wind, whose influence is lower; and third, the plant
species implicated, structure of its cuticle, and its stage and rate of growth. Of all these
factors, the stability of the pesticide, which depends on its physical and chemical charac-
teristics, and the plant species and the rate of growth are the most important (Edwards
1975). However, for a specific pesticide, its adsorption, mobility, and degradation in a
determinate plant species are highly influenced by climatic conditions.
In general, low temperature and high water content may reduce the degradation of
pesticides in plants (Syversen and Beckam 2004). In that sense, there are several stud-
ies that demonstrate that in high mountain environments and in cold places, in general,
the accumulation and deposition of persistent organic compounds, such as pesticides, are
higher. Several studies have shown that regions with cold climates are susceptible to be
enriched with POPs (e.g., Vighi 2006; Wang et al. 2006). Compounds prone to enrichment
in cold environments are typically those with subcooled liquid vapor pressures between
0.01 and 1.0 Pa at 25°C (including many pesticides, highly volatiles). It is hypothesized that
cold temperatures promote the deposition and accumulation of pesticides and other POPs,
which tend to evaporate in warmer areas and be transported by air currents and deposited
in colder alpine and arctic areas. Simonich and Hites (1995) have shown that the distribu-
tion of relatively volatile organochlorine compounds (such as hexachlorobenzene (HCB))
is dependent on the latitude and demonstrates the global distillation effect. By contrast,
less volatile compounds (such as endosulfan) are not as effectively distilled and tend to
remain in the region of use (Simonich and Hites 1995).
Natural vegetation has an important role in determining the general environmental fate
of these chemicals, especially the levels of air concentrations, and there are many stud-
ies that use vegetation as a passive sampler to monitor remote areas (e.g., Calamari et al.
1991; Jensen et al. 1992). In many researches, plant samples have been used to qualitatively
indicate the atmospheric contamination levels, since plants accumulate and circulate the
residues of the pollutants, so they can offer indirect evidence of the atmospheric transport
of POPs to remote areas (Wang et al. 2006). Also, they can be used to discover the sources
of organic pollutants, to find out contamination within different places, such as cities or
countries, and to determine the global contamination of organic pollutants (Holoubek
et al. 1999). For instance, in Paris, leaves from plane trees ( Platanus vulgaris ) have been used