most important of which are sorption–desorption – leading to retardation, and
degradation – which causes dissipation. Degradation can be caused by chemical
as well as microbiological reactions and normally tends to diminish the toxicity of
pesticides; although occasionally, the metabolic degradation products used are
more toxic than the parental compounds (Cheng 1990). Measured dissipation
may also be due to volatilization (Ferrari et al. 2005) and the formation of bound
residues (Gevao et al. 2000). In Europe and North America, there is much data on
the impact of pesticides and on the level of exposure to environmental pesticides
due to agriculture, but for the mountainous areas of Southeast Asia, there have been
only a few studies published that deal with pesticide exposure.
For a typical soil in northern Thailand (Haplic Acrisol, following the FAO
classification), Ciglasch et al. (2005) investigated vertical water flux dynamics
and pesticide leaching patterns on a plot scale. On a 10-year-old litchi (Litchi
chinensis Sonn. or “lychee”) orchard, the authors equipped two excavated soil
trenches with tensiometer-controlled glass suction lysimeters. A lysimeter is a
vessel containing local soil placed with its top flush to the ground surface and is
typically used to study phases of the hydrological cycle, such as infiltration, run-off
and evapotranspiration, or the soluble constituents removed in drainage, among
other uses. The lysimeters were installed at a depth of 55 cm, indicating the
transition between the B1 and B2 horizons. The lysimeters were directly connected
to on-line solid-phase extraction devices comprising vacuum chambers and
cartridges filled with graphitized non-porous carbon. In 2001, nine insecticides
with varying physico-chemical properties were applied on the soil surface, and
leaching was monitored for 8 weeks. Total recovery of pesticides ranged between
trace levels and 1.3 %. Recovery values however, were negatively correlated with
respective sorption coefficients. After a heavy rain event of 80 mm occurred shortly
after application, the authors observed that between 0.001 % and 2 % of the applied
mass of pesticides leached instantaneously to a depth of 55 cm. These fractions
represented between 75 and 100 % of the total measured mass of leached pesticides,
indicating that preferential flow paths dominate the displacement of pesticides, at
least after heavy rain events.
Particularly in mountainous areas, lateral surface and subsurface flows may be key
processes in the loss of pesticides from agricultural fields to adjacent environmental
compartments. Under certain environmental conditions, lateral flow processes may
be more important than vertical flow processes, but there has been much less research
done on the lateral subsurface transport of pesticides. In general, it is known that
antecedent soil wetness, bedrock topography and soil depth control whether vertical
preferential and matrix flow reaching the bedrock participate in lateral flow (Buttle
and McDonald 2002). Kahl et al. (2008) summarized the various forms of lateral
preferential flow that may take place, including movement as a thin layer above
infractured bedrocks, run-off along micro-channels above the bedrock surface, pipe
flow at the base of the soil profile and flow through a self-organizing interconnection
of macropores and mesopores embedded in the soil matrix, among others.
The continuous introduction of new active ingredients and commercial pesticide
formulations in remote mountainous areas necessitates the permanent testing and
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M. Lamers et al.
degradation – which causes dissipation. Degradation can be caused by chemical
as well as microbiological reactions and normally tends to diminish the toxicity of
pesticides; although occasionally, the metabolic degradation products used are
more toxic than the parental compounds (Cheng 1990). Measured dissipation
may also be due to volatilization (Ferrari et al. 2005) and the formation of bound
residues (Gevao et al. 2000). In Europe and North America, there is much data on
the impact of pesticides and on the level of exposure to environmental pesticides
due to agriculture, but for the mountainous areas of Southeast Asia, there have been
only a few studies published that deal with pesticide exposure.
For a typical soil in northern Thailand (Haplic Acrisol, following the FAO
classification), Ciglasch et al. (2005) investigated vertical water flux dynamics
and pesticide leaching patterns on a plot scale. On a 10-year-old litchi (Litchi
chinensis Sonn. or “lychee”) orchard, the authors equipped two excavated soil
trenches with tensiometer-controlled glass suction lysimeters. A lysimeter is a
vessel containing local soil placed with its top flush to the ground surface and is
typically used to study phases of the hydrological cycle, such as infiltration, run-off
and evapotranspiration, or the soluble constituents removed in drainage, among
other uses. The lysimeters were installed at a depth of 55 cm, indicating the
transition between the B1 and B2 horizons. The lysimeters were directly connected
to on-line solid-phase extraction devices comprising vacuum chambers and
cartridges filled with graphitized non-porous carbon. In 2001, nine insecticides
with varying physico-chemical properties were applied on the soil surface, and
leaching was monitored for 8 weeks. Total recovery of pesticides ranged between
trace levels and 1.3 %. Recovery values however, were negatively correlated with
respective sorption coefficients. After a heavy rain event of 80 mm occurred shortly
after application, the authors observed that between 0.001 % and 2 % of the applied
mass of pesticides leached instantaneously to a depth of 55 cm. These fractions
represented between 75 and 100 % of the total measured mass of leached pesticides,
indicating that preferential flow paths dominate the displacement of pesticides, at
least after heavy rain events.
Particularly in mountainous areas, lateral surface and subsurface flows may be key
processes in the loss of pesticides from agricultural fields to adjacent environmental
compartments. Under certain environmental conditions, lateral flow processes may
be more important than vertical flow processes, but there has been much less research
done on the lateral subsurface transport of pesticides. In general, it is known that
antecedent soil wetness, bedrock topography and soil depth control whether vertical
preferential and matrix flow reaching the bedrock participate in lateral flow (Buttle
and McDonald 2002). Kahl et al. (2008) summarized the various forms of lateral
preferential flow that may take place, including movement as a thin layer above
infractured bedrocks, run-off along micro-channels above the bedrock surface, pipe
flow at the base of the soil profile and flow through a self-organizing interconnection
of macropores and mesopores embedded in the soil matrix, among others.
The continuous introduction of new active ingredients and commercial pesticide
formulations in remote mountainous areas necessitates the permanent testing and
158
M. Lamers et al.