VNUJournalofScience,EarthSciences23(2007)170‐176 170 CharacteristicsofQuaternarysedimentaryfacies inrelationtowaterbearingcapacityofaquifers andaquicludesintheRedRiverDelta,Vietnam NguyenThanhLan*,TranNghi,DangMai,DinhXuanThanh CollegeofScience,VNU Received07March2007 Abstract.There are five Quaternary sedimentary sequences in the Red River Delta, Vietnam. The formingofeachsequenceisrelated to transgressionandregressionphases.Thesequences, which was formed in transgression period, composed mainly of fine grained size leading to the water bearingcapacityisvery lowandplaysaroleasaquicludes.Besides,inthesefinegrainsizelayers, thecontentsofarsenicandironarehigh,especiallyindarkclay,siltyclayrichinorganic material ofswampfaciesandbrown,darkbrown clay of flood facies sothattheyarestated as a sourceof 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 goodaquif e r s .Theseaquif e r s arethemainsupplysourcesforhumanactivitiesintheRedRiverDelta. Keywords:Aquiclude;Aquifer;RedRiverDelta;Sedimentaryfacies. 1.Introduction * LikemanycitiesandprovincesofVietnam, the provinces in the Red River Delta receive watersupplyfromgroundwaterinQuaternary 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 transitionalenvironment.IntheRedRiverDelta area, the facial association rule is examined in _______ * Correspondingauthor.Tel.:84‐4‐5587059. E‐mail:lannt@vnu.edu.vn three spatial directions: vertical, perpendicular andparallelwiththepresentcoastline. 2. Characteristics of Quaternary sedimentary sequences In Quaternary, the Red River Delta experienced glaciation stages: Gunz, Mindel, Riss,Wurm1,Wurm2,andcorrespondingwith 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 NguyenThanhLan etal./VNUJournalofScience,EarthSciences 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 coarsesandofriverchannelenvironmentinthe lower part and medium sand, silt and clay of floodplainintheupperpart. ‐ 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 proluvialandmountainousriverbedfacies. ‐ 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 lagoonalfacies. ‐ The fourth sequence was formed in Lowermost of Late Pleistocene to Early‐ Middle Holocene, corresponding with the fourth depositional cycle of Hai Hung Formation,composedofgreenishgrayclayand peatoflagoonandcoastalswampfacies. ‐ The fifth sequence was formed in Late Holocene and composed of sand, silt and clay of deltaic facies, corresponding with the fifth depositionalcycleofThaiBinhFormation. In spatial distribution,the sediments of the first and second sequences are rather widespreadintheplain.Thethirdsequencehas 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 siltyclayofflood plain, oxbow,coastalswamp andpeatfaciesinthedirectionfromtheplainto the modern coastline. The fourth sequence consistsofswampclay,lagoonalclayandpeat. Inthedirectiontowardsthecenteroftheplain, eachofthesesequencesappearsmoreandmore particular facies that forms a quite complete faciesassociation.Thus,intheevolutionprocess 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.QuaternarysedimentaryfacialcrosssectionintheRedRiverDelta[5]. NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 172 of the Quaternary sediments in the Red River Delta,thereisaclearchangeofsedimentaryfacies associations and corresponding with changing in lithological composition and sedimentation parameters such as: Md, Ro, So, Me, permeabilityandpaleogeographicenvironments in each period, which represent the water bearing properties as well as the quality of the groundwater in the region. Therefore, the important aquiferswithgoodqualit ycorrespon ds with the coarse grained sediment layers of alluvialandproluvialfacies(formedinthefirst stage of each cycle), while the aquicludes are fine sediments of deltaic, coastal swamp and marinefacies(formedinthe finalstageofeach cycle). Based on this point of view, the Quaternarysedim e ntsintheRedRiverDeltacan be divided into the following hydrogeological units: Holocene aquifer (Qh); Pleistocene aquifer(Qp),Pleistocene‐Holoceneaquiclude, Middle‐LatePleistoceneaquiclude. 3. Facial characteristics and water bearing properties of the aquifers and aquicludes in theRedRiverDelta 3.1.Holoceneaquifer(Qh) The Holocene aquifer is distributed at a shallow depth where the water fluctuates in a widerangefrom2‐4mupto36m, withthickness increasing towards the center of plain. It is composedofsand,silt,clay ofalluvialfaciesof 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‐ clay15‐70%.Thesediment indicatorsare:Md= 0.02‐0.11mm,So=1.21‐2.85,pH sediments =5.5‐8.2, Eh sediments =‐5÷‐10mV.Inthislayer,thespecific capacityofwellsisverylowandchangingfrom 2to3l/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 waterleveldecreases withthedistancefromthe river. Therefore, this aquifer is mainly rechargedbytherain waterand surfacewater, 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 qualityofthislayerisfresh, softtoslightlyhard withtotaldissolvedsolid(TDS)content<1mg/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 1.2‐11.7g/l,inparticularitsironcontentreaches 1.24to33.5mg/l. Fig.2.Laminatedfinesandinterclatedwithsiltof alluvialfaciesofHoloceneaquiferinVP2borehole, Hanoiarea(6.9‐7.2mdepth).CourtesyofESTNV‐2 Project(EnvironmentalScienceandTechnologyin NorthernVietnam)supportedbySDCand implementedbyCETASDandEawag. 3.2.Pleistoceneaquifer(Qp) ThePleistoceneaquiferiswidelydistributed in the Red River Delta and is overlain by the younger sedimentary layer which is the Pleistocene‐Holoceneaquiclude.Thesediments of this layer consist of 3 formations: Le Chi Formation, Hanoi Formation, and Vinh Phuc Formation (Fig. 3‐5). These sediments are intercalatedbyfinesedimentsofclayeymudof lagoon environment of Le Chi Formation, the NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 173 tidalflatsiltysandandlagoonalclayandlenses of lacustrine clay of Ha noi Formation, the laterizedlagoonalclayofVinhPhucFormation, 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‐clay40.8%.Thesedimentindicators areMd=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 capacityofboreholesdrilledintothisaquiferis higher than that into the Holocene aquifer (in the sedimentlayerofVinhPhucFormation:2‐9 l/ms, Hanoi and Le Chi formations: 26‐51 l/ms). Thegroundwaterinthisaquiferisfresh(TDS< lg/l),ofbicarbonatecalcium,bicarbonatesodium ‐ calcium type, with r Na /r Cl ratio = 0.98‐5.6, pH water =1.0‐8.1,TDS=0.1‐0.5g/landveryhigh iron content (2‐25 mg/l). However, the iron content in this aquifer is lower than that in the Holoceneaquifer. 3.3. Uppermost of Late Pleistocene‐Holocene aquiclude The Holocene and Pleistocene aquifers are separatedbythePleistoceneHoloc ene aquiclude. Fig.3.LatePleistocenecoarsegrainedsand ofriverchannelfaciesinVP2borehole,Hanoiarea (28‐28.3mdepth)CourtesyofESTNV‐2Project. Fig.4.LatePleistocenecoarsegrainedsand,gravelof riverchannelfaciesinVP2borehole,Hanoiarea (39.2‐41.2mdepth).CourtesyofESTNV‐2Project. Fig5.EarlyPleistocenepebble,cobbleoffluvialfacies inVP2borehole,Hanoiarea(55‐57mdepth). CourtesyofESTNV‐2Project. The Pleistocene‐Holocene aquiclude is composed mainly of greenish gray fine clayey sand of lago onal and coastal swamp facies of HaiHungFormationintheuppersequenceand thefinesedimentsofdeltaicandmarinefaciesof VinhPhucFormationinthelowersequence. The sedimentof Vinh Phuc Formation was weathered (Fig. 6) that created reddish brown toorange.Fe 2+ /Fe 3+ cont ent equals0.1‐0.05,TOC is changing from 0.23 to 0.26µg/g in deltaic sediments; and Fe 2+ /Fe 3+ = 1.7, TOC = 0.54 in NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 174 marinesediments.The r Na /r Cl ra tio ofthislayer is 0.66, pH water = 1‐8 and the water hardness is 1.5‐1.6mg/l. ThesedimentofHaiHungFormation,which corresponds to Early‐Middle Holocene, is composedmainlyofsiltyclayandfinesandmixed clayofdeltaicfacies.Thislayerischaracterized byoxidationenvironmentthatindicatedbylow value ofTOCandratioofFe 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,contentoftotalarsenic in sediment of this facies are very high (6‐33.0 mg/g)[1,2,4]. Regressionphase happened afterFlandrian transgression that formed sandy silt clay and siltyclayofdeltaicanddarkbrownclayofriver flood facies. The sediments what took in dark brownclayofriverfloodinPhapVanborehole havecontentofarsenicreachto2‐12um/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 aquiferbelow. ThethicknessofthePleistocene‐ Holoceneaquiclude varies from 6 m to 11.5 m. However,duetotheactionofoldriversystems, insomeplacesthereisnotraceofthefinegrain sediments, but there remain only the coarse grainedsedimentsofriverbedfacies,whichare of high storage and water bearing capacity. These areʺhydrogeological windowsʺ (Fig. 8). The hydrogeological windows are areas where the groundwater in the Holocene aquifer is interconnectedwiththePleistoceneaquifer. 3.4.Middle‐LowermostofLatePleistoceneaquiclude TheupperpartoftheHanoiFormationwith green gray to dark gray clay and silt of lagoon facies, lens of lacustrine clay formed during the Middle‐LatePleistocene 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, thisaquicludehassmallerextent than the Pleistocene ~ Holocene aquiclude above. Also for this reason, it is usually neglectedandattributedtothelocalaquicludein the Pleistocene aquifer. The pa rticle size distribution is: sand: 46%, silt‐clay: 60%. The sedimentindicatorsare:Md=0.15mm,So= 4.5, Eh sediments =50mV,pH sediments =7.5.Ther Na /r Cl ratio of the groundwater equals 0.8, pHwater= 7.5. The iron content in the water is relatively high (>10mg/l). 4.Resultsanddiscussion 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 lowerpart that is a good potential and quality layer. Besides, the upper part of each layer was covered mainlybysiltyclay;clayishsiltmixedfinesand 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 arehigh)andhavehighcontentoftotalarsenic. Besides, the upper part of Holocene aquiclude appeared brown and dark brown clay of flood facies what are in high content of arsenic. Arsenicabsorbedbyironhydroxidewhileiron hydroxide absorbing by clay mineral. Therefore, this layer was considered as a main source of arsenic contamination in the groundwater. NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 175 Fig.6.Siltyclay,clayspottedweatheringofmarinesedimentinVP1borehole, Hanoiarea(13‐13.74mdepth).CourtesyofESTNV‐2Project. Fig.7.Clay,siltyclayrichinorganicmaterialofswampfacies inVP2borehole,Hanoiarea(0.95‐1.0mdepth).CourtesyofESTNV‐2Project. Fig.8.HydrogeologicalwindowbetweenPleistoceneaquiferandHoloceneaquiferin HanoiareaaccordingtoNguyenVanDan[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 NguyenThanhLan etal./VNUJournalofScience,EarthSciences 23(2007)170‐176 176 Northern Vietnam) implemented by CETASD andEawagunderfinancialsupportofSDC. References [1] M. Berg,TranHongCon, NguyenThiChuyen, Pham Hung Viet, Arsenic contamination of ground and drinking water in Vietnam: A human health threat, Journal of Environmental Science&Technology13(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(inVietnamese). [3] Nguyen Van Dan, Nguyen Thi Dung, Current status of groundwater pollution in Hanoi area, JournalofHydrogeology24(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 GeologyandMineralsofVietnam,Hanoi,2005. [5] Tran Nghi,Relationshipbetweenthelithofacies and ground water of Quaternary sediments in the Red River Delta, Journal of Geology 226A (1995)11(inVietnamese). . VNUJournal of Science,EarthSciences23(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 NguyenThanhLan*,TranNghi,DangMai,DinhXuanThanh College of Science,VNU Received07March2007 Abstract.There. m. However,due to the action of old river systems, in someplacesthereisnotrace of the finegrain sediments, but there remain only the coarse grainedsediments of river bed facies, whichare 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. ‐