217
soil particles is the adsorptive partitioning coefficient (K d ), calculated as a ratio
between the amount of a pesticide adsorbed to the soil and the amount that remains
in the water. Higher K d values indicate the higher capability of the pesticide to be
adsorbed to the soil particles, and thus the leaching potential of such pesticides is
lower. This coefficient doesn’t take into account the organic matter content in the
soil which has a great capability to adsorb pesticides in the soil. This limitation was
overcome when the organic carbon coefficient (K oc ) was introduced, calculated as
the ratio between the K d and the percent of the organic carbon in the soil (Karickhoff
et al. 1979). Similar behavior to organic matter showed the clay particles in the soil.
They adsorb ionic pesticides and trap them on their way to the aquifer exposing
them to the processes of degradation and decomposition for a longer time (Talbert
and Fletchal 1965; Bailey and White 1970; Paim and Lagenbach 1996).
Worrall (2001) in their study of pesticide leaching conclude that when it comes
to highly water-soluble pesticides the behavior is different. Highly water-soluble
pesticides will leach very fast especially when high precipitations occur no matter
the amount of clay and organic matter content in the soil, the porosity of the soil, or
the bedrock. Weber et al. (1969) studied the adsorption of s-triazines from aqueous
solutions by organic soil colloids at different pH levels from 1 to 5.2 and concluded
that maximum adsorption occurs at pH levels close to the pK a values of the respective compound. Considering all these properties mentioned previously, Cohen et al.
(1984) set up the important physical and chemical characteristics of a pesticide
leacher: water solubility greater than 30 mg/kg, partition coefficient between soil
and water (K d ) less than 5 usually less than 1 or 2, soil organic partition coefficient
(K oc ) less than 300–500, Henry’s law constant (k H ) less than 10
−2
atm
−1
m
−3
mol,
negatively charged at ambient pH, hydrolysis half-life greater than 25 weeks, photolysis half-life greater than a week, and field dissipation half-life greater than
3 weeks. For the evaluation of pesticide leaching potential, various indices are proposed in the literature. These indices are index-based screening tools that use the
physical and chemical properties of the pesticide concerning soil properties. The
evaluation is made considering setting threshold values (Table 7.1).
7.1.2 Methods for Determination of Pesticides in Groundwater
The extraction and development of simultaneous multi-residue methods for pesticide determination are a great challenge since the differences in chemical and physical properties like different solubility and volatility and different degrees of polarity
and pK a values result in different behavior during the processes of extraction, clean
up, injection, and chromatographic separation. Due to these differences, there is no
single universal technique for pesticide identification. On the other hand, low detection limits for groundwater (100 μg/l) set by the EU Commission and the regulatory
agencies contribute to the complexity of their identification. According to Tomlin
(2003), more than 860 pesticide active substances are used in pesticide formulations
on the market, belonging to more than 100 substance classes. In general, thermally
7 Improving Quantitative Analysis of GC-MS for Tracking Potential Contaminants…
soil particles is the adsorptive partitioning coefficient (K d ), calculated as a ratio
between the amount of a pesticide adsorbed to the soil and the amount that remains
in the water. Higher K d values indicate the higher capability of the pesticide to be
adsorbed to the soil particles, and thus the leaching potential of such pesticides is
lower. This coefficient doesn’t take into account the organic matter content in the
soil which has a great capability to adsorb pesticides in the soil. This limitation was
overcome when the organic carbon coefficient (K oc ) was introduced, calculated as
the ratio between the K d and the percent of the organic carbon in the soil (Karickhoff
et al. 1979). Similar behavior to organic matter showed the clay particles in the soil.
They adsorb ionic pesticides and trap them on their way to the aquifer exposing
them to the processes of degradation and decomposition for a longer time (Talbert
and Fletchal 1965; Bailey and White 1970; Paim and Lagenbach 1996).
Worrall (2001) in their study of pesticide leaching conclude that when it comes
to highly water-soluble pesticides the behavior is different. Highly water-soluble
pesticides will leach very fast especially when high precipitations occur no matter
the amount of clay and organic matter content in the soil, the porosity of the soil, or
the bedrock. Weber et al. (1969) studied the adsorption of s-triazines from aqueous
solutions by organic soil colloids at different pH levels from 1 to 5.2 and concluded
that maximum adsorption occurs at pH levels close to the pK a values of the respective compound. Considering all these properties mentioned previously, Cohen et al.
(1984) set up the important physical and chemical characteristics of a pesticide
leacher: water solubility greater than 30 mg/kg, partition coefficient between soil
and water (K d ) less than 5 usually less than 1 or 2, soil organic partition coefficient
(K oc ) less than 300–500, Henry’s law constant (k H ) less than 10
−2
atm
−1
m
−3
mol,
negatively charged at ambient pH, hydrolysis half-life greater than 25 weeks, photolysis half-life greater than a week, and field dissipation half-life greater than
3 weeks. For the evaluation of pesticide leaching potential, various indices are proposed in the literature. These indices are index-based screening tools that use the
physical and chemical properties of the pesticide concerning soil properties. The
evaluation is made considering setting threshold values (Table 7.1).
7.1.2 Methods for Determination of Pesticides in Groundwater
The extraction and development of simultaneous multi-residue methods for pesticide determination are a great challenge since the differences in chemical and physical properties like different solubility and volatility and different degrees of polarity
and pK a values result in different behavior during the processes of extraction, clean
up, injection, and chromatographic separation. Due to these differences, there is no
single universal technique for pesticide identification. On the other hand, low detection limits for groundwater (100 μg/l) set by the EU Commission and the regulatory
agencies contribute to the complexity of their identification. According to Tomlin
(2003), more than 860 pesticide active substances are used in pesticide formulations
on the market, belonging to more than 100 substance classes. In general, thermally
7 Improving Quantitative Analysis of GC-MS for Tracking Potential Contaminants…