Environmental Engineering Reference
In-Depth Information
the runoff (Sharpley 1985). Higher intensity will cause more runoff because of the greater
total amount of water going through the surface. Furthermore, higher soil moisture con-
tent will increase the runoff (Sharpley 1985) because it reduces the infiltration of water.
Some agricultural management practices, such as tillage systems and crop rotation, can
also affect the movement of pesticides (Clemente et al. 1993). For example, atrazine and
metalochlor were found to persist longer in tillage ridges than in tillage valleys because of
lower moisture content of the ridges (Gaynor et al. 1987, 1998).
A number of models and equations have been developed for estimating different
parameters in a runoff event. Runoff depth can be calculated using the US Soil Conservation
Service (USSCS) Curve Number Method (USSCS 1972) (Equation 2.13):
(
)
2
Q P
=
(
−
0
.
2S
)
P
+
0
.
8S
,
P
>
0
.
2S
(2.13)
where Q is storm runoff depth (mm); P is storm rainfall (mm); S is the potential maximum
retention after run off begins (mm), in which S=(25400/CN) -254, where CN is the runoff
curve number.
Soil loss can be simulated using the Modified Universal Soil Loss Equation (MUSLE,
Wischmeier and Smith 1978) (Equation 2.14):
=
(
) (
)
(
)
Xt
11 8 A Vt qt
.
*
*
0
.
56 Ke LS Ct SP
*
*
*
*
(2.14)
where A is field area (ha); Vt, runoff volume (m
3
) given by 100 AQt; qt, peak runoff rate (m
3
/s);
Ke, standard soil erosion factor; LS, topographic factor; Ct, coyer factor; SP, supporting factor.
Pesticide partitioning in runoff can be estimated using the Equation 2.15 (Clemente 1991):
Pr Pt PXt PQt
=
−
-
(2.15)
where Pr is total pesticide remaining in the top 10 mm soil layer after the rainstorm (g/ha);
Pt, pesticide level in the surface 10 mm (g/ha); PXt, is solid phase pesticide loss in runoff
(g/ha); PQt, loss of dissolved pesticide in runoff (g/ha).
2.3.6 Leaching Through the Soil Profile
Pesticides have the potential to migrate to groundwater under certain conditions and can
lead to groundwater contamination. Therefore, understanding the mobility and transport
of pesticides is critical to the assessment of environmental distribution and risks associ-
ated with the use of pesticides. Leaching of pesticides through soils is undoubtedly the
most important route by which these chemicals percolate into the groundwater. Studies
examining pesticide leaching have been conducted in field scale or in the laboratory using
packed soil columns or undisturbed soil cores (Banks et al. 1979; Van Genuchten and
Cleary 1979; White 1985; Flury 1996). The results from these studies show that pesticides
leach out below the root zone and the leaching process is controlled by various factors,
such as physicochemical and biological properties of pesticides and soils and the timing
and amount of rainfall events following pesticide application.
In a study conducted by Kookana (1995) using sandy soils, it was found that the intrinsic
mobility of a pesticide through leaching is inversely related to its sorption on soil. Low
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