Pesticide Transport And Enrichment In Water-Soil Environment

This research was performed as a case study of central Jianghan Plain targetinggroundwater,the main source of drinking water supply for local people,as well aswater like rain,lake water,and river water from Han River or its tributary,DongjingRiver,to investigate the general hydrochemical characteristics,pesticide residue indifferent aquatic environments and contributing factors influencing their residue levelsin water-soil interface,the interaction between groundwater and rivers and also theadsorption-desorption mechanism among organic substances and those pesticides.Interms of pesticides,the most commonly employed organochlorine pesticides (OCPs)and organophosphorus pesticides (OPPs) were analyzed.Groundwater was classifiedinto two categories,phreatic groundwater and shallow confined groundwater accordingto the hydrogeological conditions of Jianghan Plain and the depths of the sampledwells.Most groundwater was grouped into HCO₃-Ca-Mg and phreatic groundwater andsurface water (HCO₃-SO₄-Ca-Mg type) had a more diversified hydrochemistry.Mn,Asand NO₃^- contents were found to exceed the allowable limits for drinking water ofWHO guidelines.Abnormally high nitrate content,up to 150-190 mg/1 was found onlyin phreatic groundwater,which suggested that the nitrate pollution might be caused byagricultural activities.Both water and soil samples were collected to assess the levels of pesticide residuesand find out controlling factors.SPE-GC/ECD and sohxelt extraction-GC/ECDmethods were adopted to analyze OCPs in water and soil samples,respectively;SPE-GC/PFPD and shaking extraction-GC/PFPD methods were employed to analyzeOPPs in water and soil samples,respectively.Endrin,Endosulfan2,o,p'-DDT andEndrin aldehyde were not detected at all in all water samples.Highest totalconcentrations of OCPs were found in Han River water.For OCPs,four Han River samples were above the EU limit (500 ng/l),up to 1075 ng/l.Total OCPs in phreaticgroundwater ranged from 17.0 to 58.7 ng/l and from 1.9 to 636.0 ng/l in confinedgroundwater.α-HCH,β-HCH,γ-HCH,δ-HCH,HCB,Aldrin,and Heptachlor epoxidewere frequently detected and heptachlor epoxide was found in highest frequency withlow concentration of about lng/1.High concentration of OCPs in the river revealed anegative contribution of pesticides to water pollution as a result of direct use ofpesticides in bodies of water,transport of pesticides into bodies of water in rainwateror irrigation water and the input pathways of OCPs into the river include discharge ofdomestic sewage and industrial wastewater,runoff from nonpoint sources,and directdumping of wastes into the river.Most river samples and more than half of thegroundwater samples contained two or more pesticides.Endosulfan sulfate have higherconcentrations than other OCPs,and are several hundred ppb higher than EU standard,the highest up to 753 ng/l.In phreatic water samples,there is no detection ofheptachlor,heptachlor epoxide,dieldrin,Endosulfanl,p,p'-DDE,endosulfan sulfate,p,p'-DDT.However,no p,p'-DDT or methoxychlor was detected in river water andmethoxychlor only in confined groundwater.HCHs occurrence was also detected inmuch larger amount in Han River than in groundwater and their concentrationsdecreased along the river,down to 125 ng/l in No.35 at downstream,due to surfacerunoff or point pollution somewhere along the river,especially at wet period,the soilerosion was stronger and more pesticides were taken to the river or downstream.InNo.35 sample and those samples like No.31,32,33,34 that were close to it,waspredominated inα-HCH,probably due to the recent agriculture activities or longdistance transport from other areas,the higher concentration ofα-isomer thanγ-isomerin this area and also in all the river samples may establish the use of lindane in the past.DDTs were found at a little bit lower concentration than HCHs,probably because thewater solubility of HCHs is higher that of DDTs.ppDDE showed higher percentage inriver samples produced from DDT under aerobic conditions.In the few groundwatersamples containing DDTs,there was only DDD detection,produced under anaerobicconditions.The relative concentrations of the parent DDT and its metabolites indicatedthe residue came from aged use but No.35 showed recent use of DDT.The residue oftotal OCPs in the soil profile didn't follow the same trend in all three boreholes butdepth from 10cm to 50cm should be the most active path of leaching and there was fewoccurence at 100cm for most OCPs.DDTs was the most frequently found compoundin sediment (＞90%);also the result shows that DDTs were the most dominantcompounds in the soils and HCHs was the second highest and then Aldrin.In soilsamples,concentrations ofα-,β-,γ- andδ-HCH are in consequence ofδ-HCH＞β-HCH＞γ-HCH＞α-HCH,agreeing with the fact that the degradation rates ofthese compounds areα-HCH＞γ-HCH＞δ-HCH＞β-HCH,which leads to thedominant role ofδ-HCH andβ-HCH.In soil samples,the ratios of p,p-DDT/ΣDDTsand DDD/DDE implied recent use of DDT pesticides.Metabolites found is dominated by pp-DDE,suggesting that the input of DDT compounds via the weatheredagricultural soils and was retained under anaerobic conditions within the sedimentfrom old sources.o,p'-DDT and p,p'-DDT also play a dominant role,opDDT shows asmaller percentage than ppDDT,which may be an indication of higher evaporationthan ppDDT.Most OCPs were not detected in those corresponding wells with depthsof 17 m,indicating that the leaching path can't be very long and degradation happenedalong the path for them.However,if the spray dose is very high or pesticide use isinappropriate,it could also lead to leaching or leakage to the water in deep wells.Far lower concentrations of OPPs were observed than that of OCPs in Han Riverand the highest total concentration of OPPs in Han River was 604.2 ng/l.But still totalOPPs concentrations in surface water were much higher than those in mostgroundwater samples and some shallow phreatic groundwater also showed highresidue.In terms of groundwater,most of the OPPs concentrations were below 100ng/l,and thimet,dimethoate,diazinon and quinalphos have relatively higheroccurrence.Highest average concentration of each OPP was mostly in the order of lake(rain)＞Han River＞shallow gw＞confined gw.Methamidophos and dimethoate hadhigher average concentration than other OPPs,and frequently exceed the limit of0.1μg/l,due to a large amount of application loads in study area.Their maximumconcentration in water samples were 137.9 ng/l and 344.0 ng/l,respectively.Except formethamidophos,malathion,isocarbophos and quinalphos,the highest concentration ofindividual OPP appeared in the rain but the was not detected in adjacent well,whichimplied that the transport in atmosphere after pesticide spraying leads to highconcentration of OPPs in the rain.The relationships between OPP residues in the soiland the detection in corresponding groundwater,and theirvertical concentrationdistribution,change from place to place without any regular pattern.This means thepesticide use may vary at these regions and OPPs were more unstable while leachingand influenced by soil properties.Still,the intense zone of OPPs leaching andadsorption is above 50 cm underground from the changing trends of them along thesoil profiles.Methamidophos and dimethoate were frequently detected at most siteswith relatively high concentrations,consistent with the findings in water environment.Parathion-methyl and malathion were also detected in soils with frequent occurrencebut the absence in groundwater samples showed they broke down too fast to reach thegroundwater.The rest of OPPs were found in traces or less frequently detected.Dichlorvos and phoxim were rarely detected in soil samples but had some occurrencein groundwater.The contributing factors for their residues were pH,contents of soil organic matter,moisture,clay (silt) mineral and redox condition.When pesticides infiltrate intogroundwater,the contamination can last for many years and spread over a large areabefore dilution.A major challenge facing modern agriculture,therefore,is to controlpests and protect crop yields without allowing pesticides to contaminate surrounding water sources.Some measures could be taken to prevent the pesticide pollution togroundwater like addition of manure to increase organic matter content,or somesmectites to get more clay in order to retain more pesticides in soils before they reachgroundwater.In addtion,while application,pesticide applicators should tailor pesticideapplications to the particular site conditions like not spraying on coarse,permeablesoils.However,some other factors should also be considered,the construction of thelocal wells and application management like rate or time to spray;it would causeintense leaching if spraying just before the rainfall,and the specific soil structure likeworm hole makes preferential flow leading to leaching potential to groundwater.Besides,the properties of pesticides are also very noteworthy and less persistent oneswith relatively short half-life or strong Koc should be the priority choices for the localfarmers.The leaching experiment conducted with an undisturbed sandy soil column fromJianghan Plain demonstrated the high mobility of atrazine as well as the potential ofglyphosate and AMPA leaching.The amount of herbicides leached through the 15cmlong soil column,in the 160 hours of the experiment,amounted to almost 100 % of theatrazine in the feedwater,but less than 16% of the glyphosate in the stock solution wasrecovered as glyphosate and AMPA.The greater leaching of atrazine through the soilcolumn might be due to its lower adsorption and greater persistence compared withglyphosate.However,the risk of contamination is different for different soil types andis related to the soil's organic carbon content,structure,and Fe and A1 contents of soils.The glyphosate metabolite,AMPA,which was detected in the effluent,may also pose arisk of groundwater contamination.Thus,not only the parent molecules,but also thedegradation products should be monitored in groundwater.The results of leachingexperiment using undisturbed soil column showed that both atrazine and glyphosatecan be transported through soils,with atrazine showing a higher contaminationpotential in the study area.At a watershed scale,surface water could be contaminatedby subsurface flow or overland flow.Furthermore,the risk of contaminatinggroundwater used for drinking water in Jianghan Plain is significant wheregroundwater has been exploited as a the major source of water supply in recent years.Therefore,pesticide management has to be enforced to preserve the quality of theaquatic environment in this region of intense agricultural activities for centuries.The potential adsorption mechanism of OPPs to soil organic matters were discussedbased on the characterization of DOM fractions and humic substances.Elemental andspectroscopic characterizations revealed that the FAs were less aromatic with highacidic groups compared to HAs.The results suggest that the soil DOM had a lowerdegree of humification but a greater degree of oxidation.FAs have less affinity forbinding hydrophobic compounds like malathion,parathion,phoxim,parathion-methyl,isocarbophos,sulfotep,quinalphos and diazinon.Hydrophobic OPPs interact mainlywith the more aromatic HAs,while the higher content of oxygenated functional groups of FAs can depress their binding capacity.Complex formation of DOM with organicchemicals or competition for sorption sites could lead to reduced residue in soil but onthe other hand,DOM in soils can sorb to soil surfaces,resulting in an overallenhancement in sorption and retardation.Affinity was also influenced by its chemicalcharacteristics.In addition,the soil DOM could modify the hydrophobic-hydrophiliccharacteristics of soil surfaces,increasing the number of sites available for the lesswater-soluble pesticides and decreasing the number of sites available for the morehydrophilic compounds.