Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 156

anorganochlorinepesticidethathasbeenusedextensivelysinceWorldWarIItocontrol
hundreds of insect pests associated with agricultural practices: as a mosquito larvicide,
asaresidualsprayfortheeradicationofmalaria,andasadelousingdustfortyphuscon-
trol.ThepersistenceofDDTintheenvironmenthasbeenattributedtoitslowvolatility,
its low water solubility, and the presence of chlorine substituents in the molecule. The
major transformation produces compounds such as 1,1-dichloro-2,2-bis(p-chlorophenyl)
ethane (DDD) and 2,2-bis(p-chlorophenyl)-1,1-dichlorethylene (DDE) that are more toxic
and recalcitrant than the parent compound. In 1972, the use of DDT was banned in the
UnitedStates;whileindevelopedcountries,ithasbeenprogressivelyrestrictedorphased
out.However,inanumberoftropical,lessrichcountries,DDTisstillbeingusedtoday
andsomeoftheOCPsarestilldetectedintheecosystemsofvariouscountries.Fromthe
monitoringresultsobtainedsofar,thepresenceofpersistentOCPsisstillwidespread,that
is,inCentralandNorthEastAsia,Korea,China,Romania,Greece,andtheUnitedStates
(UnitedNationsEnvironmentProgramme2002;Leeetal.2001;Fuetal.2003;Zhuetal.
2005b;Covacietal.2001;Galanopoulouetal.2005;Ouyangetal.2003).
ThefateandbehaviorofOCPsintheenvironmentaregovernedbytheirphysicochemi-
calproperties.MostOCPsaresemivolatile,havelowwatersolubility,andhavehighvalues
oflogK ow andlogK oc (see Table6.4 ).
Especiallyforthehydrophobicpesticides,theirmobilityandthereforetheriskoftheir
leaching into groundwater have been correlated with weak sorption on the soil matrix,
as quantiied by K oc , the ratio of adsorbed to solution-phase pesticide normalized with
respect to organic matter content (Arias-Estévez et al. 2008). However, pesticides with
K oc ≥1000havealsobeenobservedingroundwateranddrainagewater(Elliottetal.2000),
presumablyastheresultofleaching:transporttogroundwatermaybecausedbyheavy
rainfallshortlyafterapplicationofthepesticidetowetsoilswithpreferentiallowpaths
(Arias-Estévezetal.2008).
After withdrawal of DDT from use in 1960, it was replaced with 1,1,1-trichloro-2,2-
bis(4-metoxyphenylo)-ethane,thatis,methoxychlor.Itstransformationsarepresentedin
Figure6.2 .
ThemostimportantreactionsofthemareO-dealkylation,dehydrochlorination,dechlo-
rination, hydroxylation in the aromatic ring, and coupling reactions of metabolites con-
taininghydroxylgroup.Transformationsoccurringintheethylfragmentofthemolecule
areanalogoustotransformationsofDDT.Inabioticsystems,noO-dealkylationreactions,
forexample,duringphotochemicalreactions,werereported.
Faster dissipation and degradation of 1,1,1-trichloro-2,2-bis(4-chlorophenyl)-ethane
(p,pʹ-DDT)intropicalregionshasbeenreportedpreviously(UnitedNationsEnvironment
Programme 2002; Larsson et al. 1995; Carvalho et al. 1998; Allsopp and Johnston 2000).
Since p,pʹ-DDT and its degradation products are known to have high K ow values (see
Table 6.4), they are more easily associated with the organic matter of the soil than with
the mineral content, as observed by the linear correlation between log K ow and log K oc
(NoegrohatiandHammers1992).AgoodcorrelationbetweenthelevelofOCPs(chlordanes,
DDTanditsrelatedcompounds)andthetotalorganiccarbonofthesoilwasreportedby
Leeetal.(2001).However,furtherresearchisrequiredtoelucidatetheexactmechanisms
thatgoverntheirfateanddistributioninthetropicalenvironment.Thefateandbehavior
of organochlorine pesticides in the Indonesian tropical climate, which is characterized
byheavyrainfallintherainyseasonandlowrainfallinthedryseason,weredescribed
(Noegrohatietal.2008).SinceOCPshaveahighafinityforsoil,aieldstudyonthedis-
sipation and degradation pattern of soil-applied 1,1,1-trichloro-2,2-bis(4-chlorophenyl)-
ethane (p,pʹ-DDT) and 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethylene (p,pʹ-DDE) as model