VNUJournalofScience,EarthSciences23(2007)170‐176 170 CharacteristicsofQuaternarysedimentaryfacies inrelationtowaterbearingcapacityofaquifers andaquicludesintheRedRiverDelta,Vietnam NguyenThanhLan*,TranNghi,DangMai,DinhXuanThanh CollegeofScience,VNU Received07March2007 Abstract.There are five Quaternary sedimentary sequences in the Red River Delta, Vietnam. The formingofeachsequenceisrelated to transgressionandregressionphases.Thesequences, which was formed in transgression period, composed mainly of fine grained size leading to the water bearingcapacityisvery lowandplaysaroleasaquicludes.Besides,inthesefinegrainsizelayers, thecontentsofarsenicandironarehigh,especiallyindarkclay,siltyclayrichinorganic material ofswampfaciesandbrown,darkbrown clay of flood facies sothattheyarestated  as a sourceof arsenic contamination in the groundwater. For the sequences formed in regression period composed mainly of gravel, cobbles, pebbles, and coarse sand of fluvial, river channel facies are goodaquif e r s .Theseaquif e r s arethemainsupplysourcesforhumanactivitiesintheRedRiverDelta. Keywords:Aquiclude;Aquifer;RedRiverDelta;Sedimentaryfacies. 1.Introduction *  LikemanycitiesandprovincesofVietnam, the provinces in the Red River Delta receive watersupplyfromgroundwaterinQuaternary sediments. The characteristics, potential and variation trend of this water resource depends much on the distribution of sedimentary facies in the stratigraphic column. The depositional environment and material composition express different paleogeographic environments such as marine, continental environment or transitionalenvironment.IntheRedRiverDelta area, the facial association rule is examined in _______ * Correspondingauthor.Tel.:84‐4‐5587059. E‐mail:lannt@vnu.edu.vn three spatial directions: vertical, perpendicular andparallelwiththepresentcoastline. 2. Characteristics of Quaternary sedimentary sequences In Quaternary, the Red River Delta experienced glaciation stages: Gunz, Mindel, Riss,Wurm1,Wurm2,andcorrespondingwith them transgression phases. These stages have left behind 5 sedimentary sequences corresponding with 5 sedimentary  cycles in Quaternary each began. The coarse grain size sediments were formed in regressions phase and fine grain size sediments were formed in transgression phases. Accordingly, the NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 171 Quaternary sediments in the Red River Delta are identified by the following sedimentary sequences: ‐ The first sequence was formed in Early Pleistocene and corresponded with Le Chi Formation. The sediments of this sequence are composed mainly of cobbles, pebbles, and coarsesandofriverchannelenvironmentinthe lower part  and medium sand, silt and clay of floodplainintheupperpart. ‐ The second sequence that corresponds with the second depositional cycle, formed in Middle to Lowermost Late Pleistocene. This sequence corresponds with Hanoi Formation and consisting of cobbles, pebbles, gravel of proluvialandmountainousriverbedfacies. ‐ The third sequence corresponds with the third depositional cycle and was formed in Uppermost of Late Pleistocene. It corresponds with Vinh Phuc Formation which contains coarse to medium grained sand of plain river bed facies and chan gin g up war ds into finer sediments co nsisting mainly silty clay mixed with fine sand of flood plain  facies and clay of lagoonalfacies. ‐ The fourth sequence was formed in Lowermost of Late Pleistocene to Early‐ Middle Holocene, corresponding with the fourth depositional cycle of Hai Hung Formation,composedofgreenishgrayclayand peatoflagoonandcoastalswampfacies. ‐ The fifth sequence was formed in Late Holocene and composed of sand, silt and clay of deltaic facies, corresponding with the fifth depositionalcycleofThaiBinhFormation. In spatial distribution,the sediments of the first and second sequences are rather widespreadintheplain.Thethirdsequencehas a continuous change of facies from sand, silt  and clay of alluvial facies to clayey silt mixed with sand of deltaic facies and silty clay of lagoonal facies in the central part passing into siltyclayofflood plain, oxbow,coastalswamp andpeatfaciesinthedirectionfromtheplainto the modern coastline. The fourth sequence consistsofswampclay,lagoonalclayandpeat. Inthedirectiontowardsthecenteroftheplain, eachofthesesequencesappearsmoreandmore particular facies that forms a quite complete faciesassociation.Thus,intheevolutionprocess 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Depth (m) BH59-64 605 BH-11 105 To Lich River Red River BH2-HN 156 BH3-HN 180 BH4-HN ta.amQ tb 2 3 am lbQ hh 2 1-2 amQ vp III 2 aQlc I Nvb 2 2 ~ . ~ . ~ . ~ ~ . ~ . ~ ~ . ~ . ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ ~ . ~ . ~ . ~~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . ~ . aQ b IV 3 apQ hn 1 2-3a a.amQ tb 2 3 a.am Q tb 2 3 a.amQ tb 2 3 aQ tb 2 3 amQ vp 1 3a a,am Q vp 1 3a Fig.1.QuaternarysedimentaryfacialcrosssectionintheRedRiverDelta[5]. NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 172 of the Quaternary sediments in the Red River Delta,thereisaclearchangeofsedimentaryfacies associations and corresponding with changing in lithological composition and sedimentation parameters such as: Md, Ro,  So, Me,  permeabilityandpaleogeographicenvironments in each period, which represent the water bearing properties as well as the quality of the groundwater in the region. Therefore, the important aquiferswithgoodqualit ycorrespon ds with the coarse grained sediment layers of alluvialandproluvialfacies(formedinthefirst stage of each cycle), while the aquicludes are fine sediments of deltaic, coastal swamp and marinefacies(formedinthe finalstageofeach cycle). Based on this point of view, the Quaternarysedim e ntsintheRedRiverDeltacan be divided into the following hydrogeological units: Holocene aquifer (Qh); Pleistocene aquifer(Qp),Pleistocene‐Holoceneaquiclude, Middle‐LatePleistoceneaquiclude. 3. Facial characteristics and water bearing properties of the aquifers and aquicludes in theRedRiverDelta 3.1.Holoceneaquifer(Qh) The Holocene aquifer is distributed at a shallow depth where the water fluctuates in a widerangefrom2‐4mupto36m, withthickness increasing towards the center of plain. It is composedofsand,silt,clay ofalluvialfaciesof Thai Binh Formation (Fig. 2) in the upper part and lens of sand, silt, clay of Hai Hung Formation in the lower part which are low water bearing capacity layers. The particle size distribution is: gravel 2%, sand 30‐85%, silt‐ clay15‐70%.Thesediment indicatorsare:Md= 0.02‐0.11mm,So=1.21‐2.85,pH sediments  =5.5‐8.2, Eh sediments =‐5÷‐10mV.Inthislayer,thespecific capacityofwellsisverylowandchangingfrom 2to3l/sm. Especially, the groundwater regime is affected directly by river and rain water percolating through the overlying Thai Binh Formation. The fluctuation amplitude of the waterleveldecreases withthedistancefromthe river. Therefore, this aquifer is mainly rechargedbytherain waterand surfacewater, especially in some big rivers in the Red River system. The groundwater is discharged mainly through the evaporation process and percolation to the Pleistocene aquifer. Water qualityofthislayerisfresh, softtoslightlyhard withtotaldissolvedsolid(TDS)content<1mg/l and bicarbonate calcium, bicarbonate sodium calcium type. According to the results of chemical analysis, the groundwater in the Holocene aquifer has a r Na /r Cl  ratio of 1.56, a hardness of 2‐9, a pH of 7.5, a TDS content of 1.2‐11.7g/l,inparticularitsironcontentreaches 1.24to33.5mg/l.  Fig.2.Laminatedfinesandinterclatedwithsiltof alluvialfaciesofHoloceneaquiferinVP2borehole, Hanoiarea(6.9‐7.2mdepth).CourtesyofESTNV‐2 Project(EnvironmentalScienceandTechnologyin NorthernVietnam)supportedbySDCand implementedbyCETASDandEawag. 3.2.Pleistoceneaquifer(Qp) ThePleistoceneaquiferiswidelydistributed in the Red River Delta and is overlain by the younger sedimentary layer which is the Pleistocene‐Holoceneaquiclude.Thesediments of this layer consist of 3 formations: Le Chi Formation, Hanoi Formation, and Vinh Phuc Formation (Fig. 3‐5). These  sediments are intercalatedbyfinesedimentsofclayeymudof lagoon environment of Le Chi Formation, the NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 173 tidalflatsiltysandandlagoonalclayandlenses of lacustrine clay of Ha noi Formation, the laterizedlagoonalclayofVinhPhucFormation, the alternating greenish gray clay and peat bearing swamp clayey mud of Hanoi Formation which serve as an aquiclude and aquitard. The particle size distribution is as follows: cobbles, pebbles, gravel 2.5%, sand 56.7%,silt‐clay40.8%.Thesedimentindicators areMd=0.25,So=3.2,pH sediments =6.7.However, these layers are very thin and were formed as small lenses due to erosion and sheet washing during regression phases before the formation of the overlying coarse sediments. The specific capacityofboreholesdrilledintothisaquiferis higher than that into the Holocene aquifer (in the sedimentlayerofVinhPhucFormation:2‐9 l/ms, Hanoi and Le Chi formations: 26‐51 l/ms). Thegroundwaterinthisaquiferisfresh(TDS< lg/l),ofbicarbonatecalcium,bicarbonatesodium ‐ calcium type, with r Na /r Cl  ratio = 0.98‐5.6, pH water =1.0‐8.1,TDS=0.1‐0.5g/landveryhigh iron content (2‐25 mg/l). However, the iron content in this aquifer is lower than that in the Holoceneaquifer. 3.3. Uppermost of Late Pleistocene‐Holocene aquiclude The Holocene and Pleistocene aquifers are separatedbythePleistoceneHoloc ene aquiclude.  Fig.3.LatePleistocenecoarsegrainedsand ofriverchannelfaciesinVP2borehole,Hanoiarea (28‐28.3mdepth)CourtesyofESTNV‐2Project.  Fig.4.LatePleistocenecoarsegrainedsand,gravelof riverchannelfaciesinVP2borehole,Hanoiarea (39.2‐41.2mdepth).CourtesyofESTNV‐2Project.  Fig5.EarlyPleistocenepebble,cobbleoffluvialfacies inVP2borehole,Hanoiarea(55‐57mdepth). CourtesyofESTNV‐2Project. The Pleistocene‐Holocene aquiclude is composed mainly of greenish gray fine clayey sand of lago onal and coastal swamp facies of HaiHungFormationintheuppersequenceand thefinesedimentsofdeltaicandmarinefaciesof VinhPhucFormationinthelowersequence. The sedimentof Vinh Phuc Formation was  weathered (Fig. 6) that created reddish brown toorange.Fe 2+ /Fe 3+ cont ent equals0.1‐0.05,TOC is changing from 0.23 to 0.26µg/g in deltaic sediments; and Fe 2+ /Fe 3+  = 1.7, TOC = 0.54 in NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 174 marinesediments.The r Na /r Cl ra tio ofthislayer is 0.66, pH water  = 1‐8 and the water hardness is 1.5‐1.6mg/l. ThesedimentofHaiHungFormation,which corresponds to Early‐Middle Holocene, is composedmainlyofsiltyclayandfinesandmixed clayofdeltaicfacies.Thislayerischaracterized byoxidationenvironmentthatindicatedbylow value ofTOCandratioofFe 2+ /Fe 3+ . Besides, silty clay and clay rich in organic material of swamp facies that formed in early Flandrian transgression (10.000 to 6.000 year BP, Fig. 7) are in high values of TOC and Fe 2+ /Fe 3+ , which indicated for reduction environment.Moreover,contentoftotalarsenic in sediment of this facies are very high (6‐33.0 mg/g)[1,2,4]. Regressionphase happened afterFlandrian transgression that formed sandy silt clay and siltyclayofdeltaicanddarkbrownclayofriver flood facies. The sediments  what took in dark brownclayofriverfloodinPhapVanborehole havecontentofarsenicreachto2‐12um/g. The combination of them created as thick sedimentary layer playing a role of  relatively thick aquiclude that separating the overlying Holocene aquifer above and the Pleistocene aquiferbelow. ThethicknessofthePleistocene‐ Holoceneaquiclude varies from 6 m to 11.5 m. However,duetotheactionofoldriversystems, insomeplacesthereisnotraceofthefinegrain sediments, but there remain only the coarse grainedsedimentsofriverbedfacies,whichare of high  storage and water bearing capacity. These areʺhydrogeological windowsʺ (Fig. 8). The hydrogeological windows are areas where the groundwater in the Holocene aquifer is interconnectedwiththePleistoceneaquifer. 3.4.Middle‐LowermostofLatePleistoceneaquiclude TheupperpartoftheHanoiFormationwith green gray to dark gray clay and silt of lagoon facies, lens of lacustrine clay formed during the Middle‐LatePleistocene transgression is called Middle‐Late Pleistocene aquiclude. However, as it is eroded during the regression phase and its dis tribution area is small  and is associated with silty sand of tidal flat facies  and alluvial cobbles,pebbles, thisaquicludehassmallerextent than the Pleistocene ~ Holocene aquiclude above. Also for this reason, it is usually neglectedandattributedtothelocalaquicludein the Pleistocene aquifer.  The pa rticle size distribution is: sand: 46%, silt‐clay: 60%. The sedimentindicatorsare:Md=0.15mm,So= 4.5, Eh sediments =50mV,pH sediments =7.5.Ther Na /r Cl ratio of the groundwater equals 0.8, pHwater= 7.5. The iron content in the water is relatively high (>10mg/l). 4.Resultsanddiscussion The relationship between sedimentary facies and aquifers, aquicludes is very strong. The primary and secondary sedimentary sequences are composed mainly of coarse‐ grained sand, cobbles and pebbles of mountainous river facies in the lowerpart that is a good potential and quality layer. Besides, the upper part of each layer was covered mainlybysiltyclay;clayishsiltmixedfinesand of flood and deltaic facies is a bad aquiclude. The Pleistocene‐Holocene aquiclude, which contained mainly  of fine grained size such as clay, silt, silty clay, clayey silt and rich in organic material is good regional aquiclude. These sediments are mainly formed in reduction environment (Fe 2+ /Fe 3+  > 1 and TOC arehigh)andhavehighcontentoftotalarsenic. Besides, the upper part of Holocene aquiclude appeared brown and dark brown clay of flood facies what are in high content of arsenic. Arsenicabsorbedbyironhydroxidewhileiron hydroxide absorbing by clay mineral. Therefore, this layer  was considered as a main source of arsenic contamination in the groundwater. NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 175  Fig.6.Siltyclay,clayspottedweatheringofmarinesedimentinVP1borehole, Hanoiarea(13‐13.74mdepth).CourtesyofESTNV‐2Project.  Fig.7.Clay,siltyclayrichinorganicmaterialofswampfacies inVP2borehole,Hanoiarea(0.95‐1.0mdepth).CourtesyofESTNV‐2Project.  Fig.8.HydrogeologicalwindowbetweenPleistoceneaquiferandHoloceneaquiferin HanoiareaaccordingtoNguyenVanDan[3]. Acknowledgements This paper is completed within the framework of Fundamental Research Program funded by Vietnam Ministry of Science and Technology. The pictures in this paper were published by courtesy of ESTNV‐2 Project (Environmental Science and Technology in Top Botto m Top Bottom NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 176 Northern Vietnam) implemented by CETASD andEawagunderfinancialsupportofSDC.  References [1] M. Berg,TranHongCon, NguyenThiChuyen, Pham Hung Viet, Arsenic contamination of ground and drinking water in Vietnam: A human health threat, Journal of Environmental Science&Technology13(2001)2621. [2] Nguyen Trong Uyen, Tran Hong Con et al., Actual situation of heavy metals contamination in groundwater  of Hanoi area, VNU Journal of Science, Natural Sciences and Technology No. 1 / XVII(2001)32(inVietnamese). [3] Nguyen Van Dan, Nguyen Thi Dung, Current status of groundwater pollution in Hanoi area, JournalofHydrogeology24(2004)18. [4] Pham Hung Viet et al., Preliminary survey for evaluation of arsenic level in groundwater and supply water in Hanoi area, in Current situation of arsen ic pollution in  Vietnam, Department of GeologyandMineralsofVietnam,Hanoi,2005. [5] Tran Nghi,Relationshipbetweenthelithofacies and ground water of Quaternary sediments in the Red River Delta, Journal of Geology 226A (1995)11(inVietnamese). . VNUJournal of Science,EarthSciences23(2007)170‐176 170 Characteristics of Quaternary sedimentary facies  in relation to water bearing capacity of aquifers  and aquicludes in the Red River Delta, Vietnam NguyenThanhLan*,TranNghi,DangMai,DinhXuanThanh College of Science,VNU Received07March2007 Abstract.There. m. However,due to the action of old river systems, in someplacesthereisnotrace of the finegrain sediments, but there remain only the coarse grainedsediments of river bed facies, whichare of high  storage and water bearing capacity.  These. plain river bed facies and chan gin g up war ds into finer sediments co nsisting mainly silty clay mixed with fine sand of flood plain  facies and clay of lagoonal facies.  ‐