Environmental Engineering Reference
In-Depth Information
Whereas the point sources can be controlled, the diffuse sources are much more difficult
to handle because they are influenced by various interacting factors including soil type,
weather, pesticide properties, and agricultural management practices. Watersheds can be
significantly impacted by nonpoint source agricultural inputs. Nonpoint sources are related
to the movement of pesticides from the field where they are applied to surface water and
groundwater via leaching, surface runoff, spray drift, volatilization and atmospheric deposi-
tion, drain flow, groundwater flow, etc. (Carter 2000; Neumann et al. 2002). Owing to their
dispersed nature of distribution in the environment, transport from nonpoint sources is
difficult to predict, especially in large watersheds.
9.1.1 Pesticide Entry Routes into Water
There are numerous entry routes of pesticides into water. They can enter water either
directly, in applications such as mosquito control, or indirectly, such as from drainage
of agricultural lands. From agricultural applications, pesticides can contaminate natural
waters via agricultural runoff to surface waters, leaching into groundwater, or spray drift.
In urban applications, contamination can be caused by leaching from chemically treated
surfaces or seepage from accidental spills and leaks, runoff from roads and paved areas,
atmospheric fallout, and domestic sewage effluents. Also, in industrial areas, pesticides
can reach waters by direct industrial spillages, industrial wastewater effluents, and leach-
ing from industrial landfill sites. The predominant routes of entry arising from diffuse
applications of pesticide include surface runoff, spray drift, and field drainage. Less signif-
icant routes to surface water include seepage to groundwater (leaching), subsurface lateral
flow (interflow), and wet or dry deposition following long-range atmospheric transport.
Leaching or percolation through the soil is a process of pesticide transport directly to
underlying groundwater, and subsequently surface water (Fait et al. 2010; Gonzalez
et al. 2010; Leistra and Boesten 2010). It is assumed to be negligible as pesticides occur in
groundwater in generally low concentrations. Moreover, the slow movement of ground-
water favors sorption of pesticides to soil and their degradation, thus further decreasing
pesticide concentration (Röpke et al. 2004). Losses of applied pesticide by leaching are
typically less than 1% (Carter 2000). Residues that are strongly adsorbed to soil do not
leach, but they can be transported in a suspended form in leachate. Leaching is greatly
affected by soil irrigation and rainfall regime (Fait et al. 2010; Gonzalez et al. 2010).
When the infiltration capacity of the agricultural soil is exceeded during rainfall, dis-
solved pesticides and their residues adsorbed to soil particles can move across the treated
agricultural soil as surface runoff or overland flow. It is considered to be the most important
and major pathway of pesticides from agricultural fields as diffuse sources to surface water
(Senseman et al. 1997; Huber et al. 1998, 2000; Schulz et al. 1998; Liess et al. 1999; Bach et al.
2001, 2010). Pesticides that are strongly adsorbed to soil particles are essentially transported
via soil erosion (Wu et al. 2004). The abrasive power of surface runoff and the impact of rain-
drops detach soil particles and cause soil erosion. The partitioning of a pesticide between
the solution and the soil solid phase is influenced by factors such as organic carbon and clay
content of the soil (Holvoet et al. 2007). For water-soluble pesticides, losses via runoff are
considered far more important, because the amount of eroded soil lost from a field is usually
small compared with the runoff volume. Pesticide losses in agricultural runoff have been
quantified to assess the contamination potential. It was estimated that they are typically
less than 1.5% of the applied pesticide (Pereira and Hostettler 1993; Schottler et al. 1994) and
sometimes significantly lower, less than 0.1% (Liess et al. 1999; Wu et al. 2004). Although pes-
ticide losses seem small compared to the original concentration applied, their concentrations
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