Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 150

ofpesticideleachingthroughsoilduetosorptionandpartitioningofpesticidebetween
thevaporandliquidphases;FC,theieldcapacityofsoil(fraction);BD,thebulkdensity
ofthesoil(kg/m 3 );AC,theair-illedporosity(fraction)(P-FC);P,theporosity(1-BD/PD);
PD,theparticledensity(kg/m 3 );OC,theorganiccarbon(fraction);K oc ,thesorptioncoef-
icient of pesticide (m 3 /kg); t 1/2 , the degradation half-life (year); K H , the dimensionless
Henry'sconstant;q,thegroundwaterrecharge(m/year);L,thedepthtothegroundwater
orthedepth(m)atwhichAFistobecalculated.
AF serves as an index for mass emission from the vadose zone and its value ranges
between0and1.Avalueof1indicatesthatallofthesurface-appliedpesticideislikelyto
leachtothegroundwater,whereasavalueof0suggeststhatnoneoftheappliedpesticide
will leach to the groundwater (Rao et al. 1985). For each soil, depth-weighted values of
soilpropertiesarecalculatedbasedonthevaluesavailablefordifferentlayers(e.g.,0-20,
20-40cm,etc.).
Forevaluatingtheimpactofmanagementpracticesonpesticideleaching,theground-
water loading effect of agricultural management systems (GLEAMS) is a widely used
ield-scalemodel,whichassumesthataieldhashomogeneouslanduse,soils,andprecipi-
tation(Konstantinouetal.2008).Effectivepredictionofpesticidefateusingmathematical
modelsrequiresgoodprocessdescriptionsinthemodelsandgoodchoiceofparameter
values by the user. Garratt et al. (2002) examined the ability of seven pesticide leaching
models(LEACHP,MACRO,PELMO,PESTLA,PLM,PRZM,andVARLEACH)todescribe
anarableieldenvironment.Themodelswereevaluatedintermsoftheirabilitytorepro-
duceielddataofsoilwatercontentandpesticideresidues(aclonifenandethoprophos)in
the soil water and groundwater. The models varied in their ability to predict soil water
contentinsummerandtheirabilitytosimulatethepersistenceofthepesticidesinthesoil
(Garrattetal.2002).
Torecapitulate,thefactorsinluencingtheresidualcharacteristicsareasfollows
• Aknowledgeofthephysicochemicalpropertiesoftheanalytes,thatis,structure,
vaporpressure(v.p.),acid-basecharacter(pK a ),octanol-waterpartitioncoeficient
(K ow ,expressedinthelogarithmicformlogP),andsoil-waterpartitioncoeficient
(K oc );solubilityinwaterallowsthefateandbehaviorofpesticidesintheenviron-
menttobepredicted;
• Persistence, which is usually expressed in terms of half-life, that is, the time
requiredforonehalfofthepesticidetodecomposetoformproductsotherthan
theparentcompound;
• Aknowledgeofthephysicalandchemicalcharacteristicsofthesoilsystem,such
asmoisturecontent,organicmatter,claycontents,pH,etc.;
• Chemicaltransformationsinwhichlivingorganismsparticipate:onthesorption,
desorption,anddegradationofpesticides(bioticandbiochemicaltransformations,
physical,chemical,andphotochemicalprocesses)andtheiraccesstogroundwater
and surface waters (all pesticides in groundwater and most residues present in
surfacewatersenterviathesoil);
• Photochemicaltransformationofpesticides,whicharesensitivetosolarradiation
(intherange290-450nm)andareinsoilandonplantsurfaces;
• Theformulation(e.g.,granulesorsuspendedpowderorliquid)andtechniqueof
applicationofpesticidesinagriculture (e.g.,precisionbandsprayingcanreduce
thedose,whichcanbeaveryeffectivewaytominimizetransportandemission
andalsotoavoidabuildupofresistanceintargetorganisms).