Environmental Engineering Reference
In-Depth Information
morphology, wind speed and direction, and other weather variables. For instance, the pres-
ence of trees at the edge of agricultural field, in front of the water body, and in the direction
of wind can almost completely reduce the spray drift (Vischetti et al. 2008). It is estimated
that spray droplets smaller than 10 μm can be transported over several kilometers even at
low wind speeds during application. Their transport is favored by a continuous decrease in
the droplet size due to evaporation (Hüskes and Levsen 1997). Agricultural pesticides can be
transported by air through much larger distances and cause contamination in remote areas
(Blanchoud et al. 2002; Bradford et al. 2010). It was determined that a large fraction of abun-
dantly applied volatile pesticides reached the remote lakes up to 80 km downwind from the
pesticide source with a delay time of 1-2 weeks. Dry deposition to the lake surface was pre-
sumably involved as an entry route because the concentration pattern in lake water followed
the application rate pattern regardless of rainfall events (Bradford et al. 2010).
In order to remove excess water from slowly permeable soils or those with shallow water
tables, land drainage systems are designed. This artificial drainage is responsible for drain
flow, the transport of dissolved pesticides or pesticides adsorbed on sediment particles
when heavy rainfall and subsequent drainage occur shortly after application, thus limit-
ing the contact of these solutes with the soil. Surface waters receive nonpoint input of pesti-
cides via subsurface field drainage pipes (Kladivko et al. 1999; Gentry et al. 2000). Pesticide
losses in this way can be up to 1.9% of the amount applied to the soil but are generally less
than 0.1% (Kladivko et al. 1999). Research findings have shown that drain flow is caused
by preferential flow through soil macropores in tile-drained structured soils, especially in
low-permeable clay soils (Novak et al. 2001; Leu et al. 2004a; Gärdenäs et al. 2006; Köhne
et al. 2009). For soils with high clay content, high pesticide concentrations and losses are
expected, because such soils are more structured and more prone to pesticide transport
by preferential flow. In addition to clay content of the soil, important factors that influence
pesticide losses in drain flow are pesticide properties and weather conditions. For pesti-
cides with stronger sorption to soils and shorter half-life, losses were found to be smaller.
With the increase of the time between pesticide application and first rainfall and drainage,
pesticide losses are expected to decrease (Brown and van Beinum 2009).
Interflow is the lateral movement of water below the soil surface, instead of seeping
vertically to the groundwater, which occurs naturally in the absence of artificial drain-
age (Kahl et al. 2008). Interflow can enter surface water via bankside seepage and will
generally contain less pesticide residues than drain flow, as it has passed through the soil
matrix having the opportunity for sorption and degradation of the pesticide (Carter 2000).
It was found that the antecedent soil water content had a strong effect on triggering of
interflow. Namely, as little as 0.1 mm of the rainfall (the “new” water) triggered the pes-
ticide transport, and even two months after application, pesticide traces were detected in
water (Kahl et al. 2008). This was explained by mixing small amounts of “new” water with
larger amounts of “old” (antecedent) soil water, creating the lateral interflow. Studies have
revealed that some of the leading factors influencing the lateral interflow and pesticide
transport are antecedent soil moisture and antecedent rainfall (Ng et al. 1995).
9.2 Fate of Pesticides in Water
Pesticides by their nature are designed to destroy unwanted organisms. The key to their
selective toxic effect is that they act against certain organisms without adversely affecting
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