Formation and chemistry of the groundwater resource in the Mekong river delta, South Vietnam

The origin and chemistry of the groundwater in the middle Pleistocene (qp2 3 ), lower Pleistocene (qp1 ), upper Pliocene (n2 2 ), lower Pliocene (n2 1 ), and the Miocene (n1 3 ) in the Mekong river delta (MKRD) were investigated by using isotopic and geochemical techniques. The origin of the groundwater was evaluated based on the composition of the water stable isotopes (d2 H and d18O) in the local precipitation, in water from the rivers system, and in the groundwater samples. The hydraulic interaction between the surface water and the groundwater as well as between the aquifers was assessed by a statistical treatment for the mean and standard deviation of the d18O signature and based on the 14C-ages of the water samples taken from different aquifers. The salinization of groundwater in the deep aquifers was investigated using the d18O signature combined with the geochemical composition of the water samples. It was revealed that the groundwater in the deep aquifers in the MKRD could be divided into two groups. The first group is fresh and represents the regional precipitation with the long traveling time ranging from older than 100 years to older than 40 ka BP (kilo years Before Present). The second group is the regional precipitation that is recharged from the remote areas mixed with the seawater. Statistical treatment with the mean d18O using the Mann-Whitney test showed that the water from the Mekong river system did not or very weakly recharged the deep aquifers.

Geosciences | Geology Formation and chemistry of the groundwater resource in the Mekong river delta, South Vietnam Van Canh Doan1, Duc Nhan Dang1*, Kien Chinh Nguyen2 Hydrogeology Association of Vietnam Center for Nuclear techniques, Vietnam Atomic Energy Institute Received 11 August 2017; accepted 18 January 2018 Abstract: The origin and chemistry of the groundwater in the middle Pleistocene (qp23), lower Pleistocene (qp1), upper Pliocene (n22), lower Pliocene (n21), and the Miocene (n13) in the Mekong river delta (MKRD) were investigated by using isotopic and geochemical techniques The origin of the groundwater was evaluated based on the composition of the water stable isotopes (d2H and d18O) in the local precipitation, in water from the rivers system, and in the groundwater samples The hydraulic interaction between the surface water and the groundwater as well as between the aquifers was assessed by a statistical treatment for the mean and standard deviation of the d18O signature and based on the 14C-ages of the water samples taken from different aquifers The salinization of groundwater in the deep aquifers was investigated using the d18O signature combined with the geochemical composition of the water samples It was revealed that the groundwater in the deep aquifers in the MKRD could be divided into two groups The first group is fresh and represents the regional precipitation with the long traveling time ranging from older than 100 years to older than 40 ka BP (kilo years Before Present) The second group is the regional precipitation that is recharged from the remote areas mixed with the seawater Statistical treatment with the mean d18O using the Mann-Whitney test showed that the water from the Mekong river system did not or very weakly recharged the deep aquifers The groundwater in the deep aquifers in that region was likely to be connate, so that fresh groundwater resource in the region seemed to be limited The chemistry of the groundwater in the study region is controlled by the incongruent dissolution of the Mg-calcite as well as sulfate and iron oxy-hydroxide reductions by organic matters presented in the aquifer sediment The groundwater in the deep aquifers in the MKRD from some locations was saline, but the salinity in most aquifers was thought to result from the migration of saline water entrapped in the marine sediment pores to the fresh water in the aquifers Meanwhile, in other locations, the salinity was suggested to result from the salt intrusion due to the over-abstraction rate, as it was evident from the d18O vs [Cl-] relationship, or due to the up conning of saline water from the deeper aquifers to the upper ones Particular measures must be developed for the better management of groundwater in the MKRD to ensure a sustainable resource of freshwater being supplied to the local population in future Keywords: groundwater 14C-age, Mekong river delta, saline pore water migration, salt intrusion, Vietnam, water stable isotopic composition Classification number: 4.2 Introduction Water is a vital natural resource for every nation The life on Earth is possible only due to the existence of water on it Although Vietnam is located in a tropical region, however, during the last decades, it has severely suffered from water shortage During the dry season (from November till April), most of the reservoirs in the central part of the country were dried-up Therefore, the farmers had to take their cattle far away from their villages for water In the MKRD, the salt intrusion was observed very deep in the inland through the river system The drinking water for the population in the MKRD is mostly abstracted from the groundwater sources The demand for clean water has steadily increased, leading to the abstraction yield of groundwater in the region to be higher year by year At present, the rate of groundwater abstraction from the MKRD is around 2.106 m3 a day [1] There were two contrary hypotheses of groundwater resource formation in the MKRD Based on the results of a study conducted during the 1980s, Nguyen Kim Cuong and his co-workers concluded that the artesian groundwater in the MKRD is of the paleo-type, having been closely buried since the Delta was formed, and that this water has no recharge [2] However, Vu Van Nghi and Wesling [3] hypothesized that the origin of the groundwater in the MKRD was due to the *Corresponding author: Email: dangducnhan50@gmail.com March 2018 • Vol.60 Number Vietnam Journal of Science, Technology and Engineering 57 Geosciences | Geology local precipitation, and that groundwater in the region was continuously recharged from the surface sources at a very slow rate Louvat and Ho Huu Dung [4] had conducted a study on the groundwater in the MKRD using water stable isotopes and dating the water by the radiocarbon method to determine the possible recharge areas for the aquifers in the region Results of the study led them to the conclusion that the groundwater in the study area is recharged from areas at an altitude of around 170 m above the mean sea level The recharge areas could be on the northeast highland, i.e the Tay Nguyen plateau and/or the Cambodian territory [4] The aim of this study is to investigate in detail the origin and salinization of water from the most important aquifers in the MKRD, based on the isotopic and chemical composition of different types of water, namely, local precipitation, river water, and groundwater The use of the water stable isotopic content in conjunction with the water ionic content might help in the identification of processes, such as evaporation and salinization/desalination or mixing that affect the water bodies [5] However, the use of radioactive isotopes of the minerals dissolved in water, e.g 14C in the total dissolved inorganic carbon (TDIC) could offer an idea about the movement direction of groundwater as well as the interaction between the aquifers in the study region [6] Study area The study was conducted in the MKRD and Fig depicts a map of the study region The total area of the delta is around 54,250 km2, in which around 31,650,000 habitants have settled, among whom, almost 65% are farmers [7] Fig A map of the MKRD that comprises the Long An, Tien Giang, Dong Thap, Vinh Long, Ben Tre, Tra Vinh, Soc Trang, Hau Giang, Can Tho, An Giang, Kien Giang, Bac Lieu, and Ca Mau 58 Vietnam Journal of Science, Technology and Engineering Geology of the region The MKRD is one of the five geologic structural blocks that created the terrestrial Vietnam, namely, the northeast and northwest blocks in the northern part, Truong Son and Kon Tum blocks in the central part, and the Nam Bo block in the southernmost part The Nam Bo block was covered with a very thick (> 6,000 m) sequence of Cenozoic formations that have de-formed and become basins for sedimentation [8, 9] The Neogene and Upper Paleogene are the main deposits found in the Nam Bo geologic block that belong to the deltaic and marine sediment [8] Hydrology of the region The hydrologic regime of the MKRD is dominated by the Mekong river system that consists of the Tien (Mekong) river and the Hau (Bassac) river These rivers systems are the chief contributors of freshwater to the region for irrigation Annually, the Mekong river system discharges a total of about 500 billion cubic meters of freshwater through the Mekong river delta [10] Complicating the natural stream system of the Mekong delta is a closely integrated man-made network of approximately 4,000 km of canals and inland waterways These are subject to the tidal incursions of brackish water from Bien Dong, which might extend far inland during the dry season (February to April) on the Mekong river system Hydrogeology of the region To date, eight aquifers in the MKRD have been identified as shown in Fig and Fig along transect northeast toward southwest, NE-SW (Binh Phuoc - Ca Mau) and transect west toward east, W-E (Dong Thap - Tra Vinh), respectively [11, 12] The aquifers found are Holocene (qh), upper, middle, and lower Pleistocene symbolized, respectively, as qp3, qp23, and qp1, upper Pliocene (n22), lower Pliocene (n21), Neogene (n13), and bedrock (Mtz) From those aquifers, fresh water can be found only from the qp23 and qp1, n22, n21, n13, Mtz, and occasionally in the qp3 aquifers, but the Holocene aquifers contain either brackish or saline water that is not suitable for drinking purposes Fig The eight aquifers separated by the N-Q geologic structure along the transect NE-SW (Binh Phuoc - Ca Mau) in the MKRD [11, 12] March 2018 • Vol.60 Number Geosciences | Geology To take groundwater samples, a sampling procedure recommended by the IAEA was implemented [13] Briefly, it was as follows: Fig The eight aquifers separated by the N-Q geologic structure along the transect W-E (Dong Thap - Tra Vinh) in the MKRD [11, 12] The Holocene aquifers (qh) cover most of the plain with a thickness of up to 55 meters in the central part The thickness of the upper Pleistocene aquifer (qp3) is up to 33 m, from 50-55 m to 81 m on the land surface, following the middle Pleistocene (qp23) of a thickness up to 141 m Between the Holocene and the Pleistocene aquifers, there are aquitards of various thicknesses All the Pleistocene aquifers are confined The upper and lower Pliocene aquifers (n22, n21) are confined, and their thickness is up to 100-145 and 90-120 m, respectively, in between the Tien and the Hau rivers, and in the Ca Mau peninsula The fractured basement aquifer (Mtz) is confined and the groundwater is only tapped by a few boreholes in the Kien Giang province [12] Sampling sites and sampling procedure Groundwater was sampled from five aquifers as follows: qp23, qp1, n22, n21, and n13 The samples were taken along two transects corresponding to the NE toward SW and W toward S as shown in Fig and Fig Fig Sampling locations along the transect NE toward SW (Binh Phuoc - Ca Mau) First, the stagnant water in the wells was completely flushed out by pumping out till the pH and the temperature of the water were unchanged Alkalinity, including concentrations of bicarbonate ([HCO3-]), carbonate ([CO32-]), and free carbon dioxide ([CO2]), in the groundwater samples was then determined by titration using a HCl 0.1N solution The content of total dissolved inorganic carbon (TDIC) in the samples was calculated by summing up the concentrations of [HCO3-], [CO32-], and [CO2] [14] The TDIC in the groundwater used to determine the content of carbon-13 and carbon-14 needed for the radiocarbon dating was precipitated in the form of BaCO3 at pH = 10 by using a saturated BaCl2 solution, following a procedure described in [13] Rain water was collected monthly using a device constructed following an IAEA recommendation [15] The device was installed on the roof of the premises of the Center for Nuclear Techniques in Ho Chi Minh (HCM) city The surface water was sampled at a depth of 0.5 m from the surface and around m from the bank of the Tien and Hau rivers For chemical analyses, around 100 ml of groundwater samples were first filtered through 0.45 mm mesh filters to remove the suspended matters and then they were split into two parts One part was acidified with 2-3 drops of HNO3 (65%, PA grade, Merck, Germany) to make pH of the samples 1-2 These samples were subjected to analysis for cations, whereas the other part was kept without acidification for anion analysis For the tritium determination, one liter of water was sampled into a HDPE bottle with a tight cap to avoid isotopic Fig Sampling locations along the transect W toward E (Dong Thap - Tra Vinh) March 2018 • Vol.60 Number Vietnam Journal of Science, Technology and Engineering 59 Geosciences | Geology exchange with the atmospheric moisture The samples were transferred to the laboratory in Hanoi for further treatment and measurement for tritium activity For the tritium dating, the water samples were first subjected to distillation to remove the minerals dissolved until the electric conductivity was less than 10 mS cm-1 Around 500 ml of the distilled water samples were then subjected to Sample treatment and analytical procedure the electrolytic enrichment for tritium at 4°C till around 10 The ionic content of the water samples was quantified by ml was attained [17, 18] The tritium-enriched water samples ion chromatography using a DIONEX 600 from the Institute were purified again by distillation and then mixed with lowfor Nuclear Science and Technology (INST in Hanoi) Stable tritium Ultimagold scintillation cocktail (Hewlett-Packard, HP isotopes (2H, 18O, and 13C) were analyzed at the INST with Supplier) in vials of 20 ml capacity for counting the 3H activity an Isotope Ratio Mass-Spectrometer (IR MS, MicroMass, on a low background HP Liquid Scintillation Counter TriCarb The eight aquifers the N-Q geologic along the structure Fig UK), separated The eight by aquifers separated by structure the N-Q geologic along TR 3770 Thethe H activity in the water samples was expressed JV, with anMKRD Elemental (Eurovector, ct NE -SW (B inh Phuoc NE -equipped Ca Mau) in the [11, Analyzer 12] transect -SW (B inh Phuoc - Ca Mau) in the MKRD [11, 12] in the Tritium Unit (TU, TU = 0.118 Bq L-1) The limit of Italy) To determine the deuterium composition, the samples were pyrolyzed on a Ni catalyst at 1,050°C to form hydrogen, detection for H by the procedure was estimated to be as low followed by the purification on a chromatographic column as 0.4 TU The accuracy of the determination was checked by before entering the ion source of the IS MS The water oxygen the participation in the inter-comparison exercises organized was first converted into CO2 gas by decomposing the water by the IAEA Isotope Hydrology Section in the years 2004 samples on glassy carbon at 1,250°C The formed CO2 was and 2008 [19, 20] In the 2004 exercise, the Hanoi laboratory subjected to a chromatographic purification before entering the (No.74) produced results having Z-scores of -1.25 and 0.59 for the samples of 1.74 TU and 5.43 TU, respectively In the ion source of the IR MS 2008 exercise, the laboratory (No.27) produced results with Upon the arrival at the laboratory, the barium carbonates Z-scores of 0.42 and 1.57 for the samples of 4.07 TU and 1.54 were carefully washed off with hot deionized water to remove TU, respectively the alkaline excesses and then dried under a vacuum The dried The carbon-14 activity in the TDIC used for dating the carbonate samples with an amount of around 100 mg were groundwater samples was measured in the Center for Nuclear then wrapped in tin capsules and subjected to decomposition techniques in HCM city It was there that the BaCO3 was first ° at 1,250 C with a CuO2 catalyst in the Elemental Analyzer The converted into CO2 by decomposing it with concentrated H3PO4 formed CO2 was then allowed to pass through a chromatographic (PA grade, Merck supplier), followed by the benzene synthesis column to remove any contamination before entering the ion [21] The benzene obtained was mixed with the Ultimagold source of the IR MS to determine the carbon-13 content The The eight aquifers separated by the N-Q geologic structure along the scintillation cocktail (HP Supplier) and then counted for the Fig 3.stable The eight aquifers separated the N-Qcontent geologic along the water isotopic composition andbycarbon-13 in structure 14 ct W -E (Dong Thap -T Vinh) in the MKRD [11, 12] C activity on the HP LSC TriCarb TR 3770 The 14C content transect W -E (Dong Thap -Tdelta Vinh) in the(δ) MKRD [11, 12] TDIC were expressed in the notation as follows: in the samples was expressed in percent of the modern carbon (pMC) This is a relative measurement of the 14C activity in R H, sample R H, sample (1) δ2 H = ( - 1) 1000 the samples and (1) those in a standard supplied by the National δ H =( - 1) 1000 R H, std R H, std Institute of Standards and Technology (NIST, USA) The 14C R O, sample R O, sample (2) NIST standard(2)used in this study was oxalic acid II (ox-II) δ18 O = ( - 1) 1000 δ18 O = ( - 1) 1000 R O, std made from the molasses of French beet planted in 1977, which R O, std has a 14C activity of 0.2147 Bq/g C and d13C = -25‰ [22] R C, sample 13 R C, sample (3) δ C =( - 1) 1000 (3) transit time (the age) estimation δ13 C = ( - 1) 1000 Groundwater R C, std R 2 2 18 18 18 18 13 13 13 13 C, std age of the groundwater is defined as the transit time , R O, sample , R , O,Rstd , R , C,Rsample, and isotopic H, std R H, sample R C,the , RR O, C,stdstd, are , and R ratios are theThe isotopic ratios where: O, sample sample H, std C, std between the infiltration zone and the discharge point [5] The 18 16 13 12 H, O/ O, andof C/ respectively The value ofthe 1C samples 18 16 and standards, O/ O,ratios and 13of C/12 standards, Theof value ofthedetermination is to evaluate the flow direction of C 1samples 13 the age H/ H, 18O/16and O, and C/12Crespectively samples aim are H/ theH,isotopic and standards, respectively The value of the delta notation is water in an aquifer The groundwater age calculation was made 14 13 expressed in per mil (‰) The standard used in the analyses based on the data of C activity and d C in the TDIC The 14 of the water stable isotopes is the Vienna Standard of Mean principle of the C-dating method was the law of radioactive Ocean Water (VSMOW), but that for 13C in TDIC is the Vienna decay, and in the case of dating the groundwater, it was expressed by Equation (4) Pee Dee Belemnite (VPDB) [16] R H, sample, R 18 2 13 18 18 13 18 13 13 14 a The precision of d2H was ±2‰, and that of d18O and d13C 14 (4) (4) t  8268ln 14 in (a BP) was ±0.2‰ A quality assurance and quality control program a sample was applied for the ionic content determination by analyzing 14 where:(DIONEX) t denotes the age,14tindenotes years the Before Present (BP), of a groundwater the standard solutions supplied by the IC supplier age, in years Before Present (BP), of a sample where: 14 14 groundwater sample estimated by the C activity in TDIC; the half-life The standard deviation of the analytical results was more estimated bythan the C activity in TDIC; the number 8268 is the quotient of the 14 14 14 14 number 8268 is the quotient of the half-life of the C-isotope ±3% from the certified value for a respectiveof constituent the C-isotope (5,730 a) to ln2; a is the relative initial content of C in TDIC in before entering the saturated zone in pMC; and 60 Vietnam Journal of Science, Technology and Engineering 14 a sample is the relative 14 C content in pMC in TDIC of the sample March 2018 • Vol.60 Number 14 a0  δ13 C DIC δ13 Ccc (5) 14 14 t  8268ln 14 a in0 a sample (4) (a BP) 14 14 t  8268ln 14 14 a in0 a sample Geosciences | Geology (4) (a BP) where: t denotes the age, in years Before Present (BP), of a groundwater sample t denotes Present (BP), of a groundwater sample where:of14the estimated by the 14C activity in TDIC; the number 8268 is the quotient half-lifethe age, in years Before 14 a in 14 14 14 14 by 14 estimated the C activity in TDIC; the number 8268 is the quotient of taken the half-life (4) t  8268ln (a BP) 1‰ to 2‰, and the average value of 1.5‰ was in the 14 14 of the C-isotope(5,730 (5,730a)a) to to ln2; ln2;14 a in isis the initial content of ofC14in a therelative relative initial content C TDIC in 14 14 14 a sample (4) t  8268ln 14 in (a BP) 13 a C-isotope (5,730 a) to ln2; is the relative initial content of C in TDIC of the 14 14 TDIC beforezone entering the saturated zone in pMC; and is in correction for in The d CCO2,org in Equation (5) was taken as a sample is the relative C acontent before entering the saturated in pMC; anda 14 sample sample 14 14 14 t denotes the age, in years Before Present (BP), of athe groundwater sample where: high as zone -23‰, it was carbon dioxide 14 a sample is before entering the saturated in as pMC; andcharacterized the by relative C content in the relative C content in pMC in TDIC of the sample 14 sample pMC in TDIC of the 14 t denotes the age, in years Before Present (BP), of a groundwater sample where: estimated by the C activity in TDIC; the number 8268 is the quotient of the half-life generated from the C3 plants in the tropical areas Details for pMC in TDIC of the sample 14 14 Eq 4of the half-life 14 estimated The by theestimation C activityofinthe TDIC; 8268 isof quotient age the of number groundwater bythe the C-isotope the (5,730 a) to ln2; the relativecan initial content C in[25] TDIC the procedure of 14 a in0 iscalculation be found in of Fontes 14 14 14 C-isotope (5,730 a) for to 13ln2; the relative initial contentthe of CThe in TDIC of the requires before entering corrections the a in ,isbecause 14 14 13 data ofzone the in H pMC; activity werea sample usedis just to confirmC whether the relative content in before entering the saturated and δ C δ Ccc 14 14 a in  the (5) zone, theDIC carbon in the bicarbonate would participate a sample beforesaturated entering is the relative 14 C14the content in 13 C DIC sample 1314zone δ13 Ccc was old or modern 0  in pMC; and studiedδwater δ13 Csaturated  δ C pMC in TDIC of thea insample CO CO /DIC  13 (5) acc inwith 14 inTDIC the isotopic exchange the carbon in the biogenic CO2 (4)  8268ln (a BP) δ CCO δ13 Ccc   CO /DIC pMC in oft the sample 14 2, org Statistical treatment of isotopic data that would be releaseda sample from the plant root respiration on one 14 13 hand,a inand, on 9483 the other hand, a new portion of bicarbonate get an δoriginal signal, samples having water C DIC δ13isotopic Ccc 14To age, Present (BP), of a groundwater here: 14t 14denotes (4) t  8268ln the (a BP) 13 a in0  13 (5) 14 (6) sample  (in  years  23Before 89δ)13‰ CO be / DICformed 13 a C  δ C could there due to the oxidation of organic matters 9483 14 sample DIC cc δdisturbed   CO by isotope Tin  /DIC evaporation or by mixing with (5)δ CCOdata (6) stable  23 89C)cc‰ timated by the 14C activity in aTDIC; 8268 is the quotient of the half-life 13 the number 13 CO / DIC  (  CCOinorganic δ Ccccarbonates   CO /DIC presented in the T eliminated The mean of d18O and its standard or the dissolution ofδthe seawater were enotes 14 the age, in years Before Present (BP), 14 14 of0 a groundwater sample a the C-isotope (5,730 a) to ln2; is the relative initial content of C in TDIC 14 14 18 in unsaturated All these processes could modify the value(8) deviation were calculated for each group of samples, i.e for the 14 number is the of the(R half-life the C activity in TDIC; =8268 6.57d Oaquotient –in 3.36 = 0.875) d Hzone 18 (4) t relative 8268lninitial (a of BP) 14 14 14 14 14 pMC; H =water 6.57d O –groundwater 3.36 (Rfrom = 0.875) (8)1, din a otope a) to the ln2; is the content C in TDIC of To the correction, an isotope mixing model referred 9483and a is the relative C content fore (5,730 entering saturated zone in and in a the river’s each aquifer qp23, qp 10 sample sample ‰  (   23 89 ) (6) CO / DIC 14 9483 ng the saturated zone and 14 a sample is the relative C content in T to0 inaspMC; complete exchange with CO in the unsaturated zone 10 Precipitation 2H = 7.37d (  18O +5.15 23.892) ‰ n2 , n2 ,(6) and n1 MC in 14 TDIC of -10 the sample CO /dDIC T Precipitation t denotes theproposed age, inbyyears Present (BP), of a groundwater here: Tien River was Gonfiantini for a closed system and was sample C of the sample R²Before = 0.966 [23] d2H = 7.37d18O + 5.15 -20 -10 180.966 of the groups, aRiver 14 Tien To compare the means non-parametric test,(8) R² = Hau River H = 6.57d O – 3.36 (R = 0.875) d applied as follows: C activity in TDIC; the number 8268 is the 2quotient of-20 the half-life timated by the -30 18 Hau River the comparison O – 3.36 (R = 0.875) (8) d14H2 0= 6.57d namely, the Mann-Whitney test, which allows -40 -30 14 14 d H = is 6.57d18O relative - 3.36 the 14C-isotope to ln2; initial content of -40C 10in TDIC δ13 (5,730 C δ13 Ca) cc δ13 C DICa inR² δ13=the C0.875 1413 of of different any preliminary 18 - 3.36 size without a in0  13 -50 DIC (5) independent series cc 10 Precipitation 6.57d 18OO (5) a  (5) d2dH H= =7.37d + 5.15 δ CCO-60 δ Cincc   CO 14 -50 -10 14 13 013/DIC Precipitation R² = 0.875 a efore entering the saturated zone in pMC; and is the relative C content in δ C  δ C   18 Tien River hypothesis was used In our case, the sample size was large sample R² = 0.966 CO 2, org cc d H =CO -70 7.37d -60 -20 /DIC O + 5.15 -10 Tien River -70 R² = 0.966 -80 Hau River enough to consider that the variable built from the test, z, -30 -20 MC in TDIC of the-12sample -10 -8 -6 -4 -2 Hau River -80 -40 13-30 13 13 9483 would a -6normal C , d C , and d C are the content of carbon-13, where: d d-42H =distribution 6.57d -12 -10 follow -8 -2 18O -03.36 Therefore, the difference 18O, ‰ vs ‰ (6)  CO / DIC  (   23.89) -40 ccVSMOW org dDIC -50 2H = 6.57d18O - 3.36 T R² = 0.875 d 18 of means between the groups would be significant, if z>1.96 respectively, in calcareous materials, -60 -50 in TDIC d O, ‰ vs VSMOW 9483in a water sample, R² = 0.875 (6)  CO2 / DICwater  (  line  23.89 )‰ -70 -60 13 13 Fig Regional meteoric (RMWL) for HCM city and the water line in dioxide that(8)originates from the (critical value at a 5% limit), and very significant, if z>2.58 18 the biogenic δT(RC2 carbon δ C 6.57d = 0.875) d2H =and 14 O DIC cc -80 -70in the Mekong Fig Regional meteoric water (RMWL) for HCM city and the water line a in0 –3.36 (5) for the Tien and decomposition Hau rivers delta.ε valueline 13 of organic 13 river matters; fractionation (critical -80 -12 -10 -8at a 1% -6 limit) -4 -2 10 δ CCO2, org δ Ccc   CO2/DIC cc/DC is the for the Tien and Hau rivers in the Mekong river delta 13 -10 Precipitation -6 -4 -2 2reaction coefficient for C in the18-8isotopic exchange between 2+-12 d18O, ‰ vs VSMOW d2H = 7.37d18O 5.15 –vs.3.36 (R = 0.875) (8) d H = 6.57d -10 Results 18O,O River = 0.966 dTien ‰ carbonR²dioxide and TDIC and isVSMOW temperature dependent [24] -20 Hau River Fig 6.asRegional water line (RMWL) for HCM city and water line -30 10 Figure depicts the water line for groundwater from five aquifers follows: meteoric Theline isotopic composition of deuterium (d2H),the oxygen-18 -40 Fig Regional meteoric water line (RMWL) for HCM cityTien and and the Hau water for Figure the rivers in the Mekong river delta 9483 Precipitation d32H = 6.57d18O - 3.36 depicts the water line for groundwater from five aquifers as follows: 18 18 (6) (d O)(6) , qp1, n2for , nthe throughout the two transects qp2-50 d HMKRD 7.37d O +sampled 5.15riveralong as well as the tritium concentration in water, and the , and 1and ‰ nCO ( rivers =23 89 )and in the Mekong delta.the R²Hau = 0.875 -10Tien 2 / DIC -60 T n13 throughout MKRD and along the two qpTien R² = 0.966 , qpRiver 1, n2 , n2 , andcomposition ofthe carbon-13 (d13sampled C) and the content of transects carbon-14 -70 -20 Hau River -80 in TDIC are presented in Table along with the sampling where: T is the temperature of water sample in Kelvin -30 -12 -10 -8 -6 -4 -2 18 -40 locations The content the major ionic Figure depicts the line for of groundwater from constituents five aquifersdissolved as follows: d2H =area, 6.57d –6.57d 3.36 (Rto2 be = 0.875) (8) water 18O - 3.36 13 = d18O, ‰ vs VSMOW H dd2O C was found ranging from In the study the five aquifers as follows: Figure depicts the water line for groundwater from -50 cc in water are presented in Table , qp , n , n and n throughout the MKRD and sampled along the two transects qp , 2 R² = 0.875 2, org 2 2, org 2 d2H, ‰ vs VSMOW ‰ vs VSMOW d2H, ‰ vs VSMOW d2H, ‰ vs VSMOW d2H, 2, org d2H, ‰ vs VSMOW 2 d2H, ‰ vs VSMOW d2H, ‰ vs VSMOW 2, org -60 10line , n21, andfor n13HCM throughout thethe MKRD qpwater onal meteoric city and water and line sampled along the two transects , qp 1, n2 (RMWL) 14 Precipitation -70 Table Sampling water and Hau rivers in the Mekong river delta dlocations, 2H = 7.37d18O + 5.15 isotopic compositions and C-age -10 -80 MKRD Tien River R² = 0.966 -20 -12 -10 -8 -6 -4 -2 River Mau Middle Pleistocene aquifer (qp2 ), Transect NE-SW:Hau HCM-Ca -30 d18O, ‰ vs.five VSMOW e depicts the water-40 line for groundwater from aquifers as follows: Co-ordinate 18O - 3.36 Sample code m H, TU d18O, ‰ d2H, ‰ 6.57d d H =the , n21, and n13 throughout two Depth, transects Y -50 the MKRD andXsampled along R² = 0.875 g Regional -60 meteoric water line (RMWL) for HCM city and the water V36 18676015 1193936 40 −6.2 −39.8 0.4 line V99 18495854 968776 130 −6.4 −44.8 −