Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 96

Overall, monitoring data for pesticide degradation products are still relatively sparse.
Table 4.1 summarizes monitoring data, where pesticide degradation products were
detected and quantified in groundwater and surface water. Most studies include only a
few known degradation products of some pesticides. In some cases, these degradation
products are obvious to be included due to their toxicity on target or nontarget pests, such
as aldicarb, sulfoxide, and sulfone (see Table 4.1).
In some monitoring studies, degradation products occurred in groundwater in which
the parent compound was no longer detected (Spalding et al. 2003). These findings
reveal that degradation products of some pesticides are important contaminants of
groundwater.
With the exception of some insecticides such as aldicarb, endosulfan, and methiocarb,
the large majority of detections of currently used pesticides have been of the soil-applied
herbicides (Sancho et al. 2004), as it can be observed in Table 4.1.
Triazine herbicides, particularly atrazine and its degradation products, are among the most
monitored and the most commonly detected pesticides in groundwater. They have moderate
soil mobility and relatively high soil persistence. The degradation of atrazine through hydro-
lysis, UV radiations, and microbial activity leads mainly to the formation of dealkylated and
hydroxylated species. As a result of the widespread use of atrazine and the persistence of its
degradation products, these have been frequently detected in surface waters and groundwa-
ter. The determination of atrazine and its degradation products in water is of huge interest as
these compounds are suspected to be carcinogenic. Di Corcia and coworkers (1997) carried
out a survey of atrazine and its degradation products by analyzing three river (Tiber) water
samples collected between June and August 1995. Except for deethyl-deisopropyl-atrazine
(DEDIA) ( Figure 4.7 and Table 4.1), all the other atrazine metabolites were present in the three
samples analyzed. Therefore, these data confirm that even deisopropyl-hydroxy-atrazine
(DIHA) and deethyl-hydroxy-atrazine (DEHA) (Figure 4.7) can contaminate surface waters.
Concentration levels of atrazine measured in the Tiber river were only three to four times
lower than that measured 8 years before, and the metabolite concentrations were comparable
with that of the parent compound. These results give further evidence of the persistence of
atrazine and its metabolites in environmental compartments.
Data show that some degradation products of atrazine have a higher leaching potential
than the active substance (Barrett 1996; Wauchope 1996). Deisopropyl-atrazine (DIA) and
deethyl-atrazine (DEA) are the degradation products of atrazine more frequently detected,
with concentrations that can reach a maximum of 20.9 mg/L for DEA and 7.4 mg/L for
DIA in groundwater (Spalding et al. 2003). All monitoring studies (Table 4.1) confirm that
atrazine degradation products contribute to the total triazine residues in groundwater and
surface water. In the same study, the total residue of these degradation products and atra-
zine constitute two or three times the residue of atrazine alone (Di Corcia et al. 1997).
Metribuzin is the most frequently applied herbicide in potato crops in various coun-
tries and its dissipation is usually fast with DT50 values ranging from 30 to 60 days in
both laboratory and field studies (Tomlin 2006). The primary degradation products of
metribuzin are desaminometribuzin (DA), desaminodiketometribuzin (DADK), and dik-
etometribuzin (DK). Low sorption capacity of both metribuzin and its metabolites has
been reported in lysimeters studies (Bowman 1991), concluding that metribuzin and its
metabolites are considered more mobile than atrazine. Kjaer and coworkers (2005) evalu-
ated the leaching of metribuzin and its primary metabolites DA, DADK, and DK at a sandy
test site in Denmark. Soil water and groundwater were sampled monthly over a 4-year
period. Results showed that while metribuzin and DA were not detected in any of the ana-
lyzed groundwater samples, monitoring did reveal marked groundwater contamination