Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 170

biologicalbed,theDT 50 valueformalathionwas17days,thatforglyphosatewas33days
and that for λ-cyhalothrin was 43 days (De Rofignac et al. 2008). Degradation of the
pirimiphos-methylandbenalaxylinthetopsoilwasmonitoredbyPatakioutasetal.(2002).
Theslopeofthesoilsurfaceandthedifferentsorptioncapacitiesofthecompoundsarethe
mainparametersthatinluencedthetransportationofthestudiedpesticides,pirimiphos-
methylandbenalaxylresiduesviasurfacewaterinsoil-watersystems(Patakioutasetal.
2002). Sorption isotherms of pirimiphos-methyl and benalaxyl on clay soil from experi-
mentalplotsobtainedfrombatchexperimentswerelinearovertherangeofconcentrations
studied(0.1-20mg/l);theslopesofthelog-transformedFreundlichisothermswere1.26for
pirimiphos-methyland1.0forbenalaxylwithcorrelationcoeficients0.97and0.99,respec-
tively (Patakioutas 2000; Patakioutas and Albanis 2002). A higher sorption capacity was
obtainedforpirimiphos-methyl;theK d valueswere274.4forpirimiphos-methyland41.5
forbenalaxyl(Patakioutas2000;PatakioutasandAlbanis2002).TheestimatedK oc values
fromthelog-transformedFreundlichequation(Equation6.8)

log S = logK 1 n logC

d

+

(6.8)

e

whereSisconcentrationofcompoundinsupernatant(mg/l 1 )were13,800forpirimiphos-
methyland2090forbenalaxyl(Patakioutas2000;PatakioutasandAlbanis2002).
Thedisappearanceratesindicatethatpotatoplantingdecreasedthehalf-livesofthetwo
pesticides compared with the control ields: for pirimiphos-methyl from 16.7 to 9.2 days
andforbenalaxylfrom26.7to12.6days(Patakioutasetal.2002).

6.4.3 Carbamate Pesticides

During the last 30 years, there has been an increasing number of reports correlating
the reduced eficacy of soil-applied pesticides with enhanced biodegradation induced
by application to the same soil. However, for carbamate pesticides, there are only a few
papers,forexample,forcarbofuran(2,3-dihydro-2,2-dimethyl-benzofuran-7-ylmethylcar-
bamate) (Felsot et al. 1981; Read 1986; Suett et al. 1993). The effect of the initial concen-
tration on the development and stability of its enhanced biodegradation in topsoil and
subsoil was described by Karpouzas et al. (2001). The results obtained by these authors
suggestthatnormalcarbofuranapplicationstotopsoilareunlikelytoactivatethesubsoil
microbiota for its accelerated degradation, but show that some subsoils already possess
an inherent ability for relatively rapid biodegradation of the insecticide. Application of
the VARLEACH model to simulate carbofuran movement through the soil proile indi-
catedthatapproximately0.01mg/kgofcarbofuranmayreachadepthof70cm400days
afterastandardieldapplication.Theresultsthereforeimplythatadaptationofthesubsoil
microlora (ca. 1 m depth) by normal ield rate applications of carbofuran is unlikely to
occur.Theresultsfurtherconirmedthatsomesubsoilsampleshaveaninherentcapacity
for rapid biodegradation of carbofuran (Karpouzas et al. 2001). The high levels of vari-
abilityobservedbetweenthereplicatesinsomeofthesubsoilsampleswereattributedto
theunevendistributionofalowpopulationofcarbofuran-degradingmicroorganismsin
thesubsurfacesoil(Karpouzasetal.2001).Ahmadetal.(2004)investigatedthebioavail-
abilityandbiodegradationofcarbaryl(1-naphthylmethylcarbamate)insoils.Carbarylis
weaklysorbedandgenerallyconsideredtobeeasilydegradableinsoils.Extractionstud-
ies revealed that 49% of the total carbaryl in soils (88 mg/kg) was not water-extractable
andalsonotbioavailable,asdemonstratedbyinoculationofthecontaminatedsoilwitha
carbaryl-degradingmixedbacterialculture.Inoculationofthecontaminatedsoilwiththe