Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 155

P s andP w arethepotentiallyavailableadsorbed-phaseanddissolved-phasepesticidelev-
elsasshownin Figure6.1 (Arias-Estévezetal.2008).
Kinetic studies have revealed several interactions between sorption and degradation
(Arias-Estévezetal.2008).Itiscommonlyacceptedthatsorbedchemicalsarelessaccessible
tomicroorganismsandthatsorptionaccordinglylimitstheirdegradationaswellastheir
transport(Arias-Estévezetal.2008).Kinetically,thesorptionofmostorganicpesticidesis
a two-step process: an initial fast step that accounts for the greater part of total sorption
isfollowedbyamuchslowersteptendingtowardinalequilibrium(Arias-Estévezetal.
2008;Pignatello1998).Thismeansthatbatchequilibriumpartitioningcoeficientsbasedon
freshlytreatedsamplesunderslurryconditionscanseriouslyoverestimatetheavailability
ofagedpesticide:biologicalavailabilityandbiodegradationrateofpesticidesinsoilwill
oftendecreasemarkedlywithincreasingtimesinceapplication(Arias-Estévezetal.2008;
Barriusoetal.1997;Kristensenetal.2001;Parketal.2003,2004).Furthermore,withlonger
contacttimesbetweenthesoilandthechemicals,thefractionofstronglyboundresidues
increasesattheexpenseofextractableresidues(Arias-Estévezetal.2008;Boivinetal.2004).
Soil is a matrix that is not generally monitored on a regular basis, and there is a gap
in knowledge at the national and global levels regarding the pesticide residue levels.
Regarding Europe, recent discussions have taken place to consider regulation of persis-
tenceofsoilresiduesbeyondtheguidelinesgivenintheDirective91/414/EEC(European
Economic Community 1991). In this regard, stronger emphasis should be given to soil
monitoring programs such as Monitoring the State of European Soils (MOSES; http://
projects-2004.jrc.cec.eu.int/ ) and Environmental Indicators for Sustainable Agriculture
(ELISA)(ELISA; http://www.ecnc.nl/CompletedProjects/Elisa_119.html ) .
Rapidityofdegradationofpesticidesinsoilsdependsinessentialdegreeonthechemical
andbiologicalpropertiesofthesoils.Eventhoughsoilsareverydiverse,thereexistgeneral
regularitiesandcharacteristicsofdegradationandfragmentationofpesticidesinsoils.
Thegeneralregularitiesareasfollows.
• Morepolarpesticidesdegradeinsoilsfasterthannonpolarpesticides;
• AnionicPesticidesdegradeinsoilseasilythancationicpesticides;
• Aromaticpesticidesaremoredurablethanaliphaticpesticides;
• Anincreaseintemperatureusuallyacceleratesthedegradationandfragmentation
ofpesticides;
• Adecreaseinthemoisturecontentofthesoilusuallydecreasesthedegradation
andfragmentationrateofpesticides(butexcessiveincreaseinthemoisturecon-
tentofthesoilcausestheformationofoxygen-freesoil);
• AchangeinthepHofsoiltowardthealkalinedirection(increaseofpHvalues)
causesaccelerationofchemicalprocesseswithoutenzymesandviceversa;acidi-
icationofthesoil(decreaseofpHvalues)usuallyincreasesthedurabilityofpes-
ticidesinsoil;
• Fe 3+ , Cu 2+ , and other cations of metals and aluminum oxide are also inorganic
catalystsoftransformationprocesses.
6.4.1 Organochlorine Insecticides
Organochlorinepesticides(OCPs)constitutethemostimportantgroupofpesticides.The
best-knownOCPintheworldis1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane(DDT).Itis