Transportofsuspendedsolidsfromakarstictoanalluvialaquifer: theroleofthekarst/alluviuminterface N.Massei a, * ,M.Lacroix a ,H.Q.Wang a ,B.J.Mahler b ,J.P.Dupont a a Laboratoire de Ge Âologie, UMR CNRS 6143, Universite  de Rouen, 76821 Mont-Saint-Aignan, Cedex, France b US Geological Survey, 8027 Exchange Drive, Austin, TX 78754-4733, USA Received31October2000;revised26November2001;accepted29November2001 Abstract Thisstudyfocusesonthecoupledtransportofdissolvedconstituentsandparticulates,fromtheirin®ltrationonakarstplateau totheirdischargefromakarstspringandtheirarrivalatawellinanalluvialplain.Particulatemarkerswereidenti®edandthe transportofsolidswascharacterisedinsituinporousandkarsticmedia,basedonparticlesizeanalyses,SEM,andtraces. Transportfromthesinkholetothespringappearedtobedominatedby¯owthroughkarst:particulatetransportwasapparently conservativebetweenthetwosites,andtherewaslittledifferenceintheoverallcharacteroftheparticlesizedistributionofthe particulatesin®ltratingthesinkholeandofthosedischargingfromthespring.Qualitatively,themineralogyofthein®ltrating anddischargingmaterialwassimilar,althoughatthespringanautochthonouscontributionfromtheaquiferwasnotedchalk particles eroded from the parent rock by weathering). In contrast, transport between the spring and the well appears to be affected by the overlying alluvium: particles in the water from the well, showed evidence of considerable size-sorting. Additionally,SEMimagesofthewellsamplesshowedthepresenceofparticlesoriginatingfromtheoverlyingalluvialsystem; theseparticleswerenotfoundinsamplesfromthesinkholeorthespring.Thedifferencesbetweentheparticulatesdischarging fromthespringandthewellindicatethatthewaterpumpedfromthealluvialplainiscomingfromthekarstaquiferviathevery transmissive,complexgeologicinterfacebetweentheunderlyingchalkformationandthegravelatthebaseoftheoverlying alluvialsystem. q 2002ElsevierScienceB.V.Allrightsreserved. Keywords:Geologicinterface;Alluvium;Karst;Turbidity;Particletransport;Sizedistribution;Contaminantvector 1. Introduction Suspendedsedimentplaysanimportantroleinthe contamination of karst aquifers and potable water becauseoftheabilityofbacteriatosorbontoparticu- lates.Severalstudieshavedemonstratedtheincreased survivalofmicroorganismswhentheyareassociated with particulates Pommepuy et al., 1992); particu- latesarethuspotentialvectorsofmicrobialcontam- ination. The transport of particulates and the associatedriskstopublichealthinareaswheredrink- ingwatercomesfromkarstaquifershascomeunder scrutinyoverthelastseveralyears. Suspended material can be separated into two groups: colloids less than 1 mm in diameter) and particlesfromseveraltotensofmicronsindiameter). More attention has been paid to colloids than larger particulatematerial,ascolloidsaremobileinporous media e.g. Kretzschmar et al., 1997; Niehren and Kinzelbach,1998;Noelletal.,1998;VandeWeerd andLeijnse,1997)andtheirlargespeci®csurfacearea JournalofHydrology2602002)88±101 www.elsevier.com/locate/jhydrol 0022-1694/02/$-seefrontmatter q 2002ElsevierScienceB.V.Allrightsreserved. PII:S0022-169401)00608-4 * Correspondingauthor.Fax: 133-332-3514-7022. E-mail address: nicolas.massei@univ-rouen.frM.N.).