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Source and release mechanism of arsenic in aquifers of the Mekong Delta, Vietnam
Journal of Contaminant Hydrology 103 (2009) 58–69
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Journal of Contaminant Hydrology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j c o n h yd
Source and release mechanism of arsenic in aquifers of the Mekong Delta, Vietnam Kim Phuong Nguyen a,⁎,1, Ryuichi Itoi b a
Department of Geo-environment, Faculty of Geology and Petroleum Engineering, HCMC University of Technology, 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam b Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Nishiku, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 28 February 2008 Received in revised form 29 August 2008 Accepted 11 September 2008 Available online 23 September 2008 Keywords: Arsenic Redox condition Iron hydroxides Mekong Delta Vietnam
a b s t r a c t In order to elucidate the arsenic source and its release mechanism into groundwater in the Mekong Delta, Vietnam, groundwater samples were collected from wells at different depths (20 to 440 m) and core samples (from 20 to 265 m depth) were analyzed. Based on the analytical results for groundwater and core samples, the As source in groundwater is considered to be pyrite (FeS2) in acid sulfate soil (ASS) under oxidizing conditions and hydrous ferric oxide (Fe(OH)3) under reducing conditions. Geochemical modeling demonstrated that As (III) is the dominant species and the presence of As-bearing sulfides, Fe-bearing sulfides and oxides phases may locally act as potential sinks for As. From variation between Fe and As concentrations in groundwater samples, the release mechanism of As is: dissolution of Fe(OH)3 containing As under reducing conditions and oxidative decomposition of FeS2 containing As under oxidizing conditions. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Arsenic is present as a groundwater contaminant in many Asian countries such as Bangladesh, Vietnam, Taiwan and China (Nickson et al., 1998; Chowdhury et al., 1999; Berg et al., 2001; Acharyya, 2004; McArthur et al., 2004; Stanger et al., 2005; Agusa et al., 2006) due to natural conditions. In general, the presence of inorganic arsenic in unconsolidated sedimentary systems is closely associated with geochemical reactions such as oxidation–reduction (Nickson et al., 2000), precipitation–dissolution, adsorption and desorption (Korte and Fernando, 1991; Wilkie and Hering, 1996; Lin and Puls, 2003). Influences of pH and Eh on arsenic solubility are governed by the dissolution of iron oxyhydroxide (FeOOH) and concurrent release of co-precipitated As5+ into solution, then the reduction of As5+ to As3+ (Pierce and Moore, 1982;
Anawar et al., 2003). In Vietnam a large number of wells have suffered from high arsenic concentrations in the Red River and the Mekong Delta (Berg et al., 2001; Stanger et al., 2005; Agusa et al., 2006). Recently, Stanger et al. (2005) reported on As contamination in areas along the Lower Mekong River, including the Mekong Delta in Vietnam, and proposed possible processes that cause a high concentration of As. The arsenic problem in the Mekong Delta has also been pointed out by Iwata et al. (2004) and Berg et al. (2007). However, neither sources of As nor release mechanisms of As to groundwater in the Mekong Delta have been considered. In this study, in order to elucidate the source of As and its release mechanisms, we focused on the vertical profiles and spatial distributions of dissolved and sedimentary As concentrations in the Mekong Delta. Based on the data, we discussed factors controlling the behaviour of As in this area. 2. Methodology
⁎ Corresponding author. Tel./fax: +81 92 802 3345. E-mail addresses: [email protected], [email protected] (K.P. Nguyen). 1 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Nishiku, Fukuoka 819-0395, Japan. 0169-7722/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2008.09.005
2.1. Study area The Mekong Delta, located in the southernmost part of Vietnam (Fig. 1), has a low elevation (in the range of 0–4 m
K.P. Nguyen, R. Itoi / Journal of Contaminant Hydrology 103 (2009) 58–69
Fig. 1. Geological map and location of groundwater samples in the Mekong Delta.
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above sea level) and the delta is vulnerable to flooding in the upstream and tidewater area from the lower sea. There are high alluvial banks and natural dikes with unconsolidated Holocene deposits along rivers with presence of peat and shell fragments (Nguyen, 1993). These deposits are exposed at the surface northeast of Cao Lanh and acid sulfate soil (ASS), which is enriched in pyrite, is formed (Minh et al., 1997; Husson et al., 2000). To the northeast of the Tien River, the Dong Thap-Cao Lanh region is located in a low-lying marshy plain, which potentially contains aluminous soil and/or ASS. The unconsolidated sediments of sand aquifer can be divided into three main geological units: 1) Drained alluvial swamp deposits overlying estuarine clayey sands; 2) Holocene fluvial and/or marine deposits, which overlie the Pleistocene deposits and consist primarily of silty sand, clayey sand, sand, silt and clay; and 3) Pleistocene fluvial, fluvio-marine and marine sand deposits, which consist primarily of clay, sand mixed with grit and gravel. 2.2. Sample collection and chemical analysis To understand the distribution of the arsenic concentration in the Mekong Delta, groundwaters were collected at 13 locations: Cao Lanh (CL), Lai Vung (LV), An Phong (AP), Tan My (TM), Hong Ngu (HN), Tan Hong (TH), Chau Phu (CP), Tan Chau (TC), Can Dang (CD), Long Xuyen (LX), Binh Minh (BM), Mang Thit (MT) and Tieu Can (TC). Fig. 1 shows a location map of groundwater samples collected along with the geological settings. The collection of groundwater samples was done in March and September 2006 (Fig. 1). Wells deeper than 100 m are mostly tube wells that are cased to the depth of the aquifer whereas shallow wells in CL, HN and TH are not cased for local use. 47 groundwater samples are classified into 8 groups as CL, LV, AP, TM, HN-TH, CP-TC, CD-LX, BM-MT by the proximity of their locations. Field measurements of pH, EC, temperature and oxidation-redox potential (ORP) were measured immediately after continuous pumping of groundwater for about 10 min from the wells. Samples for the chemical analysis of As, Fe and Mn were filtered with 0.45 µm membrane filter, acidified with 1 mL of 12 M HCl and preserved in 100 mL polypropylene bottles. Another 250 mL sample was used to analyze major ions. The pH, EC and ORP were measured onsite using a HORIBA D-54 meter. ORP was measured using a Pt combination ORP electrode. Prior to the measurement, accuracy of the equip-
ment was checked by ORP standard solution (89.0 mV at 25 °C). Total alkalinity as HCO3 was measured by titration using methyl orange and bromcresol green indicators in the laboratory. The presence of F−, Cl−, NO3 −, SO4 2−, NH4 +, Na+, K+, Ca+ and Mg2+ ions was determined by ion chromatography (Dionex ICS-90). Dissolved Mn and Fe were detected by inductively coupled plasma - atomic emission spectrometry (ICP-AES) (Vista-MPX). Inorganic arsenic was volatilized as arsine with sodium borohydride–hydrochloric acid solution using hydride generation equipment then analyzed on the HG-AAS (SOLAAR S4 with detection limit 1 µg/l). As(III) and As (V) can be determined separately by a combination of the AsH3 generation-AAS and separation of As(III) and As(V) by liquid chromatography-ICP-AES. In addition to the groundwater samples, core samples of the borehole LK 204 near Chau Phu (Fig. 1) were collected over 10 m intervals for depths ranging from 20 m to 265 m. The major elements (Na, K, Mg, Ca, P, Fe, Mn, Al, S and total As) are identified using wavelength-dispersive XRF (Rigaku RIX 3100). Core samples were ground by vibration mill in order to make pellets and determine the loss on ignition (LOI). X-ray diffraction (XRD-Rigaku RINT 2100) was used to identify constituent minerals in the core samples. 2.3. Geochemical modeling and mineral saturation index The geochemical code, PHREEQC that includes the thermodynamic database WATEQ4F.DAT was adopted to calculate the distribution of aqueous species of As (speciation) (Parkhurst and Appelo, 1999; Appelo and Postma, 2005). The program solves mass balance and mass action (chemical reaction) equations and evaluates the saturation index (SI) of minerals in groundwater. The positive and negative SI values represent the thermodynamic potential for precipitation and dissolution, respectively. In the model, fifteen chemical parameters, including pH, Eh, temperature, Ca, Mg, Na, K, NH4, Cl, alkalinity as HCO3, SO4, As, Fe and Mn were used. The free energies of formation for arsenic species in aqueous solutions at 25 °C (Ferguson and Gavis, 1972) and the mass action constants for the reactions among the selected species are given in Table 1. 3. Results 3.1. Chemical properties of groundwater Chemical composition of the groundwaters is summarized in Table 2. Fig. 2 shows the Piper diagram plotted for 47
Table 1 Thermodynamic data for selected As species Species
ΔG0f (kcal./mol)
Reference⁎
Chemical reaction
Log K
Thermodynamic equation
Reference⁎
As(V)
H2AsO4− HAsO42−
As(III) As(V)/As(III)
H3AsO3 H2AsO3−
−181.0 −171.5 −154.4 −141.8
(1) (1) (1) (1)
H2AsO4− ⇆ HAsO42− + H+ H2AsO4− ⇆ AsO43− + H+ H3AsO3 ⇆ H2AsO3− + H+ H3AsO3 + H2O ⇆ H2AsO4− + 3H+ + 2e−
−6.76 −11.5 −9.23 −21.14
pH = 6.76 pH = 11.5 pH = 9.23 3pH + 2pe = 21.14
(2) (2) (2) (2)
⁎ Reference: (1) = Ferguson and Gavis (1972). (2) = Appelo and Postma (2005).
Table 2 Chemical composition of groundwater ID
Lai Vung
LV 1 LV 2 LV 3 LV4 LV5 LV 6 LV7 CL1 CL2 CL3 CL4 CL5 CL6 CL7 CL8 AP 1 AP 2 AP 3 AP 4 AP 5 AP 6 AP 7 TM 1 TM 2 HN 1 TH 1 TH 2 TH 3 TH 4 TH 5 TH 6 TH 7 TH 8 TC 1 TC 2 TC 3 CP 1 CP 2 CD 1 CD 2 LX BM 1 BM 2 BM 3 MT 1 MT 2 TC
Cao Lanh
An Phong
Tan My Hong Ngu Tan Hong
Tan Chau
Chau Phu Can Dang Long Xuyen Binh Minh
Mang Thit Tieu Can
Depth
Temp
(m)
(°C)
154 268 336 250 90 154 100 61 63 68 60 90 70 100 70 21 86 160 289 25 76 140 350 350 60 39 20 43 48 20 25 30 35 15 27 58 50 33 49 103 50 115 190 310 85 161 440
30.2 30.6 37.1 34.2 29.9 31.2 29.4 29.4 29.5 28.9 28 30.1 29.4 29.7 29.5 28.9 29.7 29.4 28.9 29 29.2 28.9 34.9 33.7 29.6 29.2 29.1 29.8 29.6 29.2 29.8 30 29.2 30.8 29.6 29.5 29 28.9 29.2 29 28.5 29.5 31.7 31 30 30.4 30.7
7.03 9.12 7.99 7.56 7.1 8.51 7.14 6.45 6.4 6.46 6.53 6.49 6.62 6.64 6.51 7.13 6.46 7.79 7.62 6.88 7.05 8.26 6.94 7.31 6.41 6.46 6.27 5.77 6.07 6.37 5.98 5.72 6.22 6.48 6.85 7.25 6.98 6.95 7.78 6.78 7.36 8.19 7.12 9.01 6.07 6.19 9.38
EC
ORP
(mS/cm)
mV
4.77 0.93 1.18 1.5 5.41 4.85 4.04 1.76 2.53 2.08 2.2 4.33 1.73 1.46 1.22 0.58 6.87 14.7 1.08 0.66 7.29 14.9 0.52 0.82 8.07 1.89 0.67 1.58 0.75 0.38 0.67 1.05 0.6 0.47 0.55 1.85 6.05 4.55 4.42 2.55 2.92 2.35 1.69 2.67 16.6 22.6 3.86
− 94 − 76 − 58 − 21 − 143 − 39 − 144 − 21 73 62 − 20 6 23 42 66 − 19 − 81 − 83 − 38 − 198 − 134 − 103 − 89 − 70 38 − 19 − 59 124 63 22 71 74 20 − 94 − 118 − 130 − 65 − 142 − 42 − 40 − 112 − 171 − 165 − 246 − 71 − 86 − 260
Fe
Mn
0.85 0.02 0.04 0.11 7.0 0.21 6.74 0.51 0.25 0.30 0.5 0.5 0.15 0.30 0.25 23.2 0.41 0.04 2.18 17.5 8.0 0.42 0.21 0.37 0.42 1.04 13.8 0.25 0.56 0.5 0.81 1.27 1.33 2.6 15.5 9.44 5.08 43.4 2.41 3.62 4.78 0.41 28.1 bdl 12.3 13.5 bdl
Bdl 0.01 bdl 0.02 0.57 0.10 0.42 4.66 6.33 4.24 1.89 4.92 1.60 2.07 5.27 0.01 0.47 0.85 0.28 0.42 2.76 0.60 0.01 0.01 2.74 1.31 2.93 1.11 0.91 1.93 0.47 0.92 0.70 0.157 0.275 0.089 0.5 bdl 0.05 0.05 bdl 0.89 0.02 0.05 0.01 1.05 0.01
As
As(III)
As(V)
μg/l
μg/l
μg/l
1 1 2.4 5.7 136.5 11.5 95.3 7.70 2.26 31.8 3.36 13.0 1.4 2.3 2.6 741 1.4 1 3.5 284.5 1.3 1.4 8.8 5.1 1.4 1.1 45 53.5 50.6 22.5 2.9 5.8 15.5 1.4 219 59.7 16.7 38.8 9.1 1 12.7 1 16.9 1 1 1 1
n.a. n.a. n.a. n.a. 121.5 n.a. 82.9 n.a. n.a. 10.2 n.a. n.a. n.a. n.a. n.a. 629.8 n.a. n.a. n.a. 256.1 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 16 n.a. n.a. n.a. n.a. n.a. n.a. 192.7 n.a. n.a. 35.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
n.a. n.a. n.a. n.a. 15 n.a. 12.4 n.a. n.a. 21.6 n.a. n.a. n.a. n.a. n.a. 111.2 n.a. n.a. n.a. 28.4 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 37.5 n.a. n.a. n.a. n.a. n.a. n.a. 26.3 n.a. n.a. 3.1 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
HCO3−
F−
Cl−
SO42−
Na+
NH4+
K+
Mg 2+
Ca 2+
88 403 442 392 382 124 527 222 199 167 260 163 197 193 175 324 20 35 316 354 56 57 279 309 362 291 100 99 153 211 132 111 205 244 302 416 478 441 697 44 434 507 486 419 14 65 567
bdl 0.62 0.52 0.45 0.11 bdl 0.22 0.48 0.40 0.22 0.43 0.37 0.69 0.57 0.47 bdl bdl bdl 4.85 0.11 bdl bdl 0.32 0.24 0.37 0.36 0.29 0.12 0.2 0.35 0.22 0.21 0.24 bdl bdl bdl 0.15 bdl 0.44 bdl 1.65 bdl bdl 0.62 bdl bdl 0.91
1515 94 138 246 1110 1513 894 385 656 523 530 1239 392 317 264 25 2556 4611 189 28 2437 4798 17 90 2677 354 109 327 163 20 149 273 45 11 31 354 1886 1300 1136 769 620 495 302 534 5442 7640 814
122 28 68.2 81.4 0.2 103 121 8.3 20.5 20.6 3.0 0.4 7.6 8.4 10.6 bdl bdl 4.3 31.2 bdl bdl 2.0 31.2 38.2 29.2 97.2 37.7 100 26.6 9.2 24.0 29.4 40 31.8 bdl bdl 51 bdl bdl 0.48 154 20 1.7 144 17.6 773 241
832 228 262 316 537 832 462 149 21 189 242 663 168 142 113 31 997 2600 207 33 883 2567 99 153 963 236 71 163 89 30 80 121 79 26 22 291 1108 845 790 190 549 409 186 619 2584 4187 927
3.8 0.63 0.45 0.67 24.7 6.92 18.3 8.98 0.66 1.15 2.93 1.43 0.66 0.12 0.54 bdl 1.94 8.49 0.45 25.4 2.4 10.2 0.5 0.3 3.89 0.56 0.48 0.54 0.21 0.31 bdl 0.48 0.76 8.5 21 8.15 4.80 6.53 3.8 0.7 1.15 0.76 11.37 1.240 11.27 12.23 1.85
15.9 1.6 2.1 3 11 15.5 9.8 6.2 4.4 4.7 2. 5 4.2 2.1 2.4 4 4.5 13.3 78.2 3.2 3.9 13.6 84.3 4.1 3.7 6.3 4 2.1 7.4 4.6 4.3 3.6 8 6.3 1.5 2.15 14 6.3 6 13.5 7.46 8.5 10.4 10.5 5.5 23.6 40.4 14
134 3 5 7.0 89 143 119 50 93 69 55 39 49 43 38 12 199 411 8 17 202 423 6 7 216 52 20 53 26 21 22 29 21 13 18 42 122 80 99 101 35 61 79 13 446 620 23
34 3 15 17 124 30 141 86 145 111 15 121 78 64 47 50 215 111 23 103 238 105 17 24 404 66 59 49 25 19 20 28 23 49 48 47 144 75 81 157 65 55 59 5 548 614 2
61
bdl: below detection limit. n.a: Not analyzed. Concentrations in mg/L except as noted.
pH
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Location
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groundwater samples, indicating that groundwater is typically a sodium-bicarbonate and chloride type. The pH values range from slightly acidic to alkaline (5.77–9.38). Low pH values for samples TH3, TH6 and TH7 which are characterized by relatively high sulfate concentration probably resulted from both oxidation of pyrite and dissolution of gypsum. On the other hand, the high pH values for samples LV2, LV6, AP7, BM1, BM3 and TC can be affected by mixing with seawater. EC values in groundwater range from 377 to 22600 µS/cm. The major ions Na+, Cl− and HCO3 − have concentration ranges of 21–4187 mg/L, 17–8280 mg/L and 10–697 mg/L, respectively. The difference in the Na+ and Cl− concentration and high EC values is probably due to differences in mixing of fresh groundwater and seawater. High sodium and chloride concentrations are found in samples near the coastal area (MT) and in the Pleistocene aquifer (LX, CD, CP, AP3, AP4). 3.2. Distribution of As in groundwater and its speciation
Fig. 2. Chemical composition plotted on Piper diagram.
Fig. 3 shows distribution of arsenic corresponding to location of groundwater samples in the Fig. 1. Total dissolved As concentrations range from 1 to 741 µg/L (Table 2).
Fig. 3. Distribution of arsenic in groundwater in the Mekong Delta.
K.P. Nguyen, R. Itoi / Journal of Contaminant Hydrology 103 (2009) 58–69
In some wells, the As concentration varied with depth. Fig. 4 shows vertical profiles of concentrations of total As and Fe. In this figure, groundwater samples are divided into two groups based on ORP values: 1) CL and HN-TH, for which positive ORP values are predominant, and 2) LV, AP-TM, TC-CP, and CD-LX, for
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which ORP values are negative. In general, as CL and HN-TH samples have positive ORP values, Fe concentrations should be low because of precipitation of ferric hydroxide if organic ligands are absent. Moreover, As concentrations of these samples are lower than 55 µg/L. On the other hand, high As
Fig. 4. Concentration profiles of: (a) total As; (b) total Fe; (c) NH+4 and (d) SO2− 4 vs depth. ORP values of CL and HN-TH samples are positive while those of LV, AP-TM, CP-TC and CD-LX samples are negative.
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and Fe concentrations are detected in LV, AP and TC, which have negative ORP values. Arsenic concentrations exceeding 100 µg/L are present at depths less than 100 m in LV, AP and TC. As shown in Table 2, As concentrations more than 10 µg/L were not found at depths more than 100 m except for sample BM2. Fig. 4 shows that the depth where maximum As concentration of AP, LV, CP and TC appeared generally coincides with that of total maximum Fe concentration. These facts indicate that the dissolution and fixation of As occurs depend on dissolution and precipitation of Fe controlled by the change in redox conditions with depth. Arsenic in aqueous solution is ordinarily present as arsenious acid (H3AsO3 0, H2AsO3 −, As(III)) and arsenic acid (H2AsO4 −, HAsO4 2−, As(V)). According to Ferguson and Gavis (1972), the toxicity of As(III) is stronger than that of As(V). Therefore, the speciation of As is important in environmental chemistry. As shown in Table 2, we analyzed As(III) and As(V) separately. Both As(V) and As(III) form protolytes that may release protons stepwise in the same way as carbonic acid. The Eh versus pH diagram for samples in this study is shown in Fig. 5. The most thermodynamically stable arsenic species calculated for the groundwaters are HAsO4 −, H2AsO4 2− and H3AsO3 0 depending on Eh and pH. While some samples (LV2, LV3, LV4, LV6, CL2, CL3, CL8, AP4, AP7, and CD1) are plotted on the stability field for H2AsO4 − and HAsO4 2−, which is present under oxidizing conditions, most of the samples are plotted on the stability field of H3AsO3 0, which is present under reducing conditions. 3.3. Characterization of redox conditions in groundwater ORP values of the groundwater range from −260 mV to 124 mV (Table 2). Generally, chemical analysis results indicate that groundwater in this area is under reducing conditions
Fig. 5. Stability diagram of Eh–pH for dissolved As species at 25 °C. Boundaries indicate equal activities of both species.
Fig. 6. Eh–pH diagram for iron species (after Deutsch, 1997).
because of negative values of ORP and presence of reduced components such as NH4+ and Fe2+, except CL and HN-TH, which have positive ORP values (Table 2, Fig. 4). Under oxidizing conditions, hydrous Fe(III) oxide such as Fe(OH)3 can remove both arsenious and arsenic acids from solution. Oppositely, when the Fe(OH)3 dissolves under reducing conditions, the As is released into solution. In order to confirm the precipitation–dissolution conditions for Fe, Eh and pH values of groundwater samples are plotted on the stability diagram of Fe (Fig. 6). The result shows that most of the samples fall in the stability field of Fe2+ (dissolved state) or Fe(OH)3 (precipitated solid state). Higher Fe concentrations are expected for be contained in the groundwater samples whose plots are located in the stability field of Fe2+. In Fig. 6, many points are located near the boundary between Fe2+ and Fe(OH)3, indicating that Fe2+ is easily oxidized to precipitate Fe(OH)3, and Fe(OH)3 can be dissolved to form Fe2+ by only slight change in redox condition. Only samples from TM are plotted in the field of pyrite. In this area, pyrite may be present because of the occurrence of ASS. Sulfate is present in most of the samples (average 53 mg/L, maximum 773 mg/L). SO4 2− concentrations in HN-TH are up to 40 mg/L except for two samples of about 100 mg/L. High SO4 2− concentrations in this area may be caused by both oxidation of pyrite and dissolution of gypsum because aquifer in this area formed in the Moc Hoa Formation. Sources of SO4 2− in LX, LV, and BM-MT samples are results of interaction between groundwater and marine deposits or mixing of fresh water and seawater. The salinity in the Mekong Delta significantly increases from north to south. Therefore, groundwaters in BM, MT and Tieu Can with high Na+, Cl−, and SO4 2− are affected by seawater intrusion. However, there is no correlation between ORP values and SO4 2− concentrations (Table 2 and Fig. 4).
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Table 3 Concentration of major elements (%) and arsenic (ppm) in core samples at various depths Depth
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
S
As
(m)
%
%
%
%
%
%
%
%
%
%
ppm
30 51.5 57.2 82.5 90 98.2 101 103 109 115.5 127.5 133.7 137 143 155 166.5 182 194 203 209 215.5 225.5 245 252.5 265
54.62 63.81 70.22 88.97 79.8 82.94 73.43 64.75 87.51 85.19 76.25 57.65 86.94 92.14 66.64 81.89 80.15 54.7 69.69 56.99 84.33 74.79 79.14 76.87 73.07
27.7 18.04 14.16 4.29 8.23 7.59 11 17.63 4.62 5.4 10.48 10.43 6.06 2.88 14.38 9.54 10.56 33.78 19.84 29.41 9.07 14.17 10.96 12.87 13.61
13.61 4 3.99 1.11 3.85 2.71 6.04 4.27 2.96 3.66 3.93 12.62 1.56 5.08 8.33 1.63 1.94 18.98 1.04 1.53 0.62 1.31 0.57 0.85 0.87
0.4 0.02 0.02 0.01 0.17 0.09 0.05 0.04 0.14 0.18 0.11 0.78 0.03 0.03 0.11 0.01 0.01 0.6 0 0 0.01 0.01 0.01 0.01 0.01
1.31 1.48 1.27 0.52 1.07 0.97 1.01 1.41 0.55 0.7 1.31 1.6 0.73 0.42 0.96 0.71 0.67 1.06 0.34 0.4 0.31 0.43 0.31 0.4 0.36
0.21 0.51 0.49 1.16 0.32 0.22 0.87 0.38 0.22 0.24 0.32 0.9 0.18 0.14 0.28 0.19 0.25 0.34 0.08 0.13 0.06 0.19 0.11 0.11 0.09
0.34 0.24 0.22 0.24 0.44 0.45 0.37 0.21 0.38 0.43 0.42 0.35 0.43 0.23 0.25 0.22 0.14 0.36 0.19 0.19 0.17 0.53 0.5 0.33 0.27
2.31 1.47 0.97 1.16 2.14 1.65 1.52 2.01 1.44 1.49 1.7 1.54 1.49 0.8 1.41 0.91 0.19 1.36 2.19 1.49 2.09 3.69 5.37 4.48 3.01
0.03 0.03 0.03 0.03 0.04 0.04 0.44 0.06 0.04 0.05 0.08 0.17 0.05 0.03 0.08 0.02 0.02 0.07 0.02 0.02 0.01 0.01 0.01 0.01 0.01
0.111 0.011 0.016 0.028 0.023 0.015 0.249 0.024 0.031 0.054 0.106 0.100 0.039 0.047 0.022 0.016 0.012 0.008 0.006 0.007 0.006 0.010 0.006 0.010 0.011
37 12 8 7 8 6 18 14 7 9 15 24 9 11 16 8 6 45 7 4 5 7 7 5 4
Minerals
Smectite, illite, feldspar, goethite, hematite Smectite, illite Smectite, illite, kaolinite, feldspar, goethite Illite, kaolinite, feldspar, hematite Smectite, illite, kaolinite, feldspar Smectite, illite, feldspar, hematite. Smectite, illite, kaolinite, Feldspar, goethite Illite, kaolinite, feldspar, goethite Smectite, kaolinite, feldspar, hematite Illite, kaolinite, feldspar, goethite, pyrite. Illite, feldspar, goethite, hematite, pyrite Illite, feldspar, hematite Illite, kaolinite, feldspar, goethite Illite, kaolinite, feldspar, hematite Smectite, illite, kaolinite, feldspar, goethite Illite, feldspar, goethite, hematite Illite, hematite Illite, feldspar, goethite Illite, kaolinite, hematite Illite, kaolinite, feldspar, pyrite Illite, hematite Smectite, illite, feldspar, hematite Feldspar, hematite Feldspar, hematite, pyrite Feldspar, hematite, pyrite.
3.4. Mineral composition for core samples
3.5. Mineral saturation index
Table 3 shows major constituents and mineral composition obtained by XRF and XRD analyses for core samples of the borehole LK204. Total As content in core samples is 4–45 mg/kg. XRF analysis for the core samples shows appreciable Fe concentrations in the formation at depths N30 m and iron oxide minerals such as goethite and hematite were identified by XRD (amorphous hydrous oxide cannot be detected). Fig. 7 shows positive correlation of As with Fe and Mn oxides in the Mekong Delta core samples. These results indicate that hydrous ferric oxides occur with As or Fe(oxyhydr)oxides and/or hydrous Mn oxides are the principal As carrier phases. Pyrite is also identified at hundreds meters depths more than 100 m (115.5 m, 127.5 m, 209 m, 252.5 m and 265 m), although As contents in those samples are near baseline values (b10 mg/kg) except for the sample from 127.5 m.
Table 4 summarizes the calculated SI values of various mineral phases in groundwater. Fig. 8 also shows the relationship between ORP of groundwater and SI values for pyrite, Fe hydroxide, goethite and hematite. The samples in the oxidizing group represented with positive value of ORP are under subsaturated condition with respect to pyrite but less subsaturated in the reducing group (Fig. 8a). This implies that they might have been oxidized, and released As to the groundwater. The dissolution of sulfate minerals, gypsum and jarosite, contributes Fe and SO42− to groundwater. The samples are slightly subsaturated with respect to Fe hydroxide for both reducing and oxidizing conditions (Fig. 8b). In general, As can be adsorbed onto the surfaces of Fe hydroxide formed under oxidizing and reducing conditions, respectively. Therefore, the dissolution of Fe hydroxides occurs in most aquifers where the
Fig. 7. Relationship between contents of As and Fe2O3 or MnO in core samples.
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Table 4 Saturation index of different minerals in groundwater, computed using PHREEQC Sample ID
Positive ORP values
Negative ORP values
HN TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8 CL2 CL3 CL5 CL6 CL7 CL8 CL1 CL4 LV1 LV2 LV3 LV4 LV5 LV6 LV7 AP1 AP2 AP3 AP4 AP5 AP6 AP7 TM1 TM2 TC1 TC2 TC3 CP1 CP2 CD1 CD2
Mineral phases equilibrium constants (log K) Fe(OH)3
Goethite
Hematite
Gypsum
Jarosite
Pyrite
4.981a
−1.0a
−4.008a
−4.58a
−9.21a
−18.479a
−3.59 −3.85 −3.83 −1.28 −1.50 −3.38 −4.34 −4.65 −3.97 −3.09 −3.18 −4.75 −3.67 −2.75 −2.81 −4.84 NA −3.49 0.19 −1.28 −1.50 −3.38 1.06 −3.21 −0.68 −5.34 −2.50 −0.38 −4.32 −3.24 −0.44 −4.04 −2.45 −3.56 −3.07 −2.40 −2.53 −2.93 −0.14 −2.65
2.46 2.19 2.21 5.02 4.71 2.68 1.72 1.42 2.07 2.96 2.85 1.32 2.37 3.3 3.24 1.21 NA 2.59 6.27 5.02 4.71 2.68 7.17 2.84 5.35 0.72 3.55 5.65 1.71 2.80 5.59 2.19 3.74 2.53 2.98 3.65 3.50 3.10 5.90 3.39
6.95 6.40 6.44 12.11 11.46 7.40 5.48 4.87 6.17 7.95 7.72 4.67 6.77 8.63 8.51 4.44 NA 7.21 14.58 −12.11 11.46 7.40 16.38 7.70 12.72 3.47 9.13 13.33 5.54 7.63 13.21 6.44 9.53 7.10 7.98 NA 9.03 8.22 13.83 8.80
−2.02 −1.83 −2.12 −2.49 −2.39 −4.42 −2.73 −2.56 −2.45 −2.25 −2.32 −4.07 −2.83 −2.83 −2.82 −2.76 −3.93 −2.24 −3.63 −2.49 −2.39 −4.42 −2.38 −1.62 NA NA −3.54 −2.61 NA NA −3.90 −2.64 −2.47 −2.23 NA −5.61 −2.08 NA NA −3.88
−18.04 −17.49 −17.77 −14.14 −13.46 −23.14 −18.44 −18.15 −17.4 −16.48 −16.88 −25.15 −19.91 −17.00 −16.30 −22.37 NA −17.54 −14.40 −14.14 −13.46 −23.14 −8.43 −17.31 NA NA −19.65 −11.40 NA NA −15.55 −19.69 −16.06 −17.71 NA NA −15.88 NA NA −19.43
−41.00 −26.18 −13.17 −45.99 −45.96 −12.79 −41.20 −37.28 −32.00 −48.9 −47.14 −39.15 −41.83 −46.00 −49.26 −28.71 NA −18.39 −59.77 −45.99 −45.96 −12.79 −56.20 −7.37 NA NA −37.63 −40.64 NA NA −39.96 −20.94 −30.48 −8.93 NA NA −24.08 NA NA −30.58
NA: Not available. a : Numbers represent logarithms of equilibrium constant from Parkhurst and Appelo (1999).
groundwater samples were taken, which is prerequisite for reductive dissolution of Fe hydroxides as the releasing mechanism of As in groundwater. On the other hand, goethite and hematite are under supersaturated condition in all the samples (Fig. 8c, d), which is consistent with the presence of these minerals in core samples (Table 3). Precipitation of goethite and hematite could be a significant adsorbent for As in the aquifer where the borehole was drilled for core samples. 4. Discussion 4.1. Geological condition of the Mekong Delta Geologically, the Mekong Delta is occupied by Holocene sediments mainly composed of Holocene alluvial sediments unconformably overlying the Late Pleistocene sediments. The Late Pleistocene strata are mainly composed of stiff, slightly
oxidized, yellowish gray silt to fine-medium sand with scattered quartz pebbles and laterites (Fig. 1). The Holocene deposits are incised river silts and coastal sand dunes up to 30 m thick (Stanger et al., 2005). During the Late Pleistocene low-stand setting, the delta was developed with sediments transported from the upper stream. Medium sand and gravel beds covered the delta in upland terraces in the west and north (Fig. 1). In the Early–Mid Holocene, sea level continued to rise and caused transgression so that almost the entire area was inundated and occupied by tidal mangrove forests. Therefore the delta was divided into two parts: an upper delta plain dominated by fluvial processes, and the lower delta plain characterized by a welldeveloped beach-ridge system and mainly influenced by marine processes (Nguyen et al., 2000). The subsurface of the deposits is rich in organic matter under reducing conditions. Annual flooding in the Mekong Delta or inundation of swampland may contribute to reducing conditions in the aquifers in this area.
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Fig. 8. Relationship between ORP and SI for (a) Pyrite; (b) Fe(OH)3; (c) Goethite and (d) Hematite.
4.2. Source and release mechanism of As into groundwater Natural arsenic originates from the Upper Mekong basin and the Mekong Delta was supplied by the sediment containing arsenic. The present Mekong Delta was submerged due to marine transgression five to six thousand years ago. During this time, Fe included in sand from the upper Mekong River and SO4 from seawater combined with each other to form pyrite under reducing conditions. The soils that include pyrite are called “potential acid sulfate soils (ASS)”, and are present in most areas in the Mekong Delta. During regression, caused oxidation of pyrite occurs according to the generalized chemical reaction (Akira, 2006):
grayish brown sulfuric horizon with yellow brown and pale yellow mottles that represent goethite and jarosite, respectively, in the vicinity of HN, TM and Cao Lanh (Fig. 9), while
FeS2 þ O2 þ ðH2 OÞ→KFe3 ðSO4 Þ2 ðOHÞ6 þ H2 SO4 The areas whose upper soil horizon has undergone such chemical reaction are defined as acid sulfate soil groups. Fig. 9 shows the distribution of ASS, which covers approximately 1.8 million hectares, in the Mekong Delta. In the western delta, Brinkman et al. (1993) found that there are two types of sulfidic materials. The older ones, situated in the northwest of the area, occur at several meters depth and have high contents of pyrite and finely distributed organic matter. The younger ones are the most extensive. They are generally less than 1.5–3 m depth and have low contents of organic matter with pyrite mostly found in root remnants. On the other hand, in the northeastern Mekong Delta, soils are characterized by a
Fig. 9. Distribution of soils in the Mekong Delta.
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soils have a sulfuric horizon without jarosite or goethite mottles in Long An (Husson et al., 2000). Husson et al. (2000) also concluded that the sulfurous horizon with jarosite mottles had a pH 2.8–3.0, high sulfate concentrations, and Eh varied from 590 to 670 mV. Gustafsson and Tin (1994) reported the enrichment of As (6–41 mg/kg) in ASS due to the uptake of As by sulfides. If these sulfides containing As encounter oxidizing conditions, they are decomposed to ferric ions and sulfate ions, but ferric ions are rapidly hydrolyzed to form hydrous ferric oxide. Under this condition As is present as As(V) (arsenic acid) and is easily adsorbed by the hydrous ferric oxide. Therefore, it is reasonably concluded that there are two different As sources in the aquifers in the Mekong Delta, pyrite and hydrous ferric oxide. The redox conditions of groundwater samples can vary with depth or locality. In TH, all the water samples are under oxidizing conditions in the range of 20–48 m depth. In the case of CL, the water is under reducing conditions. For groundwater samples with negative ORP values and As concentrations higher than 10 µg/L (e.g. LV5, LV7, AP1, AP5, TC2, TC3, CP1, CP2, LX and BM2), the Fe and Mn concentrations are relatively high, suggesting that adsorbed As is released into groundwater by dissolution of hydrous ferric oxides or hydrous manganese oxide under reducing conditions. The dissolution of Fe hydroxides would control dissolved Fe concentrations in the reducing groundwaters. For groundwater samples with positive ORP values and high As concentrations (e.g. CL3, TH3, TH4, TH5 and TH8), the Fe and NH4 + concentrations are low. It is suggested that As may be released by decomposition of pyrite including As due to oxidation of Fe2+ and S2 2−. The Fe3+ ions hydrolyze rapidly to precipitate Fe(OH)3 and S2 2− ions are oxidized to SO42− ions, so the As may be present as As(V). 5. Conclusions Groundwater samples collected from wells in the Mekong Delta were chemically analyzed for major ions, arsenic and iron for understanding geochemical conditions in the aquifers. Core samples were also analyzed with XRF and XRD for identifying the distribution of As in addition with mineral species and contents of components. For groundwater, As concentrations higher than 100 µg/L were identified in the samples from wells shallower than 100 m. Samples from wells deeper than 150 m generally show low As concentrations. The diagenesis of As in the Mekong Delta sediment and groundwater is strongly coupled with Fe cycling and redox conditions. Reductive dissolution of iron hydroxides is a controlling mechanism that causes high arsenic concentrations in reduced groundwater samples (negative ORP values, high concentrations of NH4 +, total Mn and Fe). Low arsenic concentrations, however, in oxidized groundwater samples (CL and HN-TH) are the results of oxidative decomposition of sulfide-bearing minerals such as pyrite. Acknowledgements The support provided by N. T. Viet, N. K. Quyen, and Mr. N. V. Chinh, staff of the Division of Hydrogeology and Engineering Geology, as well as Rie Unoki and Nakao Kazuto, Dept. of Earth Resource Engineering, Kyushu University, in the
fieldwork is gratefully acknowledged. The authors would like to thank Dr. Takeshi Komai of Advanced Industrial Science and Technology for cooperation in the analysis of arsenic species. We also thank Dr. N. V. Ky and the staff of the Department of Geo-environment, Ho Chi Minh City University of Technology for their assistance. References Acharyya, S.K., 2004. Arsenic levels in groundwater from Quaternary Alluvium in the Ganga Plain and the Bengal Basin, Indian Subcontinent: insights into influence of stratigraphy. Gondwana Research 8, 55–66. Agusa, T., Kunito, T., Fujihara, J., Kubota, R., Tu, B.M., Pham, T.K.T., Iwata, H., Subramanian, A., Pham, H.V., Tanabe, S., 2006. Contamination by arsenic and other trace elements in tube-well water and its risk assessment to humans in Hanoi, Vietnam. Environmental Pollution 139, 95–106. Akira, Y., (2006). Department of Environment and Natural Resources Management, Can Tho University, Vietnam. Web site: Mekong Delta in Vietnam (http://cantho.cool.ne.jp). Anawar, H.M., Akai, J., Komaki, K., Terao, H., Yoshioka, T., Ishizuka, T., Safiullah, S., Kato, K., 2003. Geochemical occurrence of arsenic in groundwater of Bangladesh: sources and mobilization process. Journal of Geochemical Exploration 77, 109–131. Appelo, C.A.J., Postma, D., 2005. Geochemistry, Groundwater and Pollution, 2nd ed. A.A. Balkema Publishers, Amsterdam, Netherlands. Berg, M., Tran, H.C., Nguyen, T.C., Pham, H.V., Schertenleib, R., Giger, W., 2001. Arsenic contamination of groundwater and drinking water in Vietnam: a human health threat. Environmental Science and Technology 35, 2621–2626. Berg, M., Stengel, C., Pham, T.K.T., Pham, H.V., Sampson, M.L., Leng, M., Samreth, S., Fredericks, D., 2007. Magnitude of arsenic pollution in the Mekong and Red River Deltas—Cambodia and Vietnam. Science of the Total Environment 372, 413–425. Brinkman, R., Ve, N.B., Tinh, T.K., Hau, D.P., Van Mensvoort, M.E.F., 1993. Sulfidic materials in the western Mekong Delta, Vietnam. Catena 20, 317–331. Chowdhury, T.R., Basu, G.K., Mandal, B.K., Biswas, B.K., Samanta, G., Chowdhury, U.K., Chanda, C.R., Lodh, D., Lal Roy, S., Saha, K.C., Roy, S., Kabir, S., Quamruzzaman, Q., Chakraborti, D., 1999. Arsenic poisoning in the Ganges Delta. Nature 401, 545–546. Deutsch, W.J., 1997. Groundwater Geochemistry, Fundamentals and Applications to Contamination. Leis Publishers, Boca Raton, Florida, America. Gustafsson, J.P., Tin, N.T., 1994. Arsenic and selenium in some Vietnamese acid sulfate soil. The Science of Total Environment 151, 153–158. Ferguson, J.F., Gavis, J., 1972. A review of the arsenic cycle in natural waters. Water Research 6, 1259–1274. Husson, O., Verburg, P.H., Mai Thanh Phung, Van Mensvoort, M.E.F., 2000. Spatial variability of acid sulphate soils in the Plain of Reeds, Vietnam. Geoderma 97, 1–19. Iwata, H., Agusa, T., Inoue, S., Kubota, R., Nguyen, H.M., Tu, B.M., Nguyen, P.C.T., Kajiwars, N., Kunisue, T., Subramanian, A., Tanabe, S., Pham, H.V., Bui, C.T., 2004. Contamination of trace elements in groundwater and persistent organochlorines in sediments from Mekong Delta, South Vietnam. Proc. Int. Symposium on the Development of Water Resource Management System in Mekong Watershed, Dec. 2004, Hanoi, Vietnam, pp. 24–30. Korte, N.E., Fernando, Q., 1991. A review of arsenic (III) in groundwater. Critical Reviews in Environmental Control 21, 1–39. Lin, Z., Puls, R.W., 2003. Potential indicators for the assessment of arsenic natural attenuation in the subsurface. Advances in Environmental Research 7, 825–834. McArthur, J.M., Banerjee, D.M., Hodson-Edwards, K.A., Mishra, R., Purohit, R., Ravenscroft, P., Cronin, A., Howarth, R.J., Chatterjee, A., Talukder, T., Lowry, D., Houghton, S., Chadha, D.K., 2004. Natural organic matter in sedimentary basins and its relation to arsenic in anoxic groundwater: the example of West Bengal and its worldwide implications. Applied Geochemistry 19, 1255–1293. Minh, L.Q., Tuong, T.P., van Mensvoort, M.E.F., Bouma, J., 1997. Contamination of surface water as affected by land use in acid sulphate soils in the Mekong Delta, Vietnam. Agriculture, Ecosystems & Environment 1, 19–27. Nguyen, H.C., 1993. Geo-pedological Study of the Mekong Delta. Southeast Asian Studies, vol. 31, No. 2. Kyoto University. Nguyen, V.L., Ta, T.K.O., Tateishi, M., 2000. Late Holocene depositional environments and coastal evolution of the Mekong River Delta, Southern Vietnam. Journal of Asian Earth Sciences. 18, 427–439. Nickson, R.T., McArthur, J.M., Burgess, W.G., Ahmed, K.M., Ravenscroft, P., Rahman, M., 1998. Arsenic poisoning of Bangladesh groundwater. Nature 395, 338. Nickson, R.T., McArthur, J.M., Burgess, W.G., Ahmed, K.M., Ravenscroft, P., 2000. Mechanism of arsenic release to groundwater, Bangladesh and West Bengal. Applied Geochemistry 15, 40–3413.
K.P. Nguyen, R. Itoi / Journal of Contaminant Hydrology 103 (2009) 58–69 Parkhurst, D.L., Appelo, C.A.J., 1999. User's guide to PHREEQC (version 2)—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. Water Resources Investigations Report. U.S. Geological Survey, pp. 99–4259. Pierce, M.L., Moore, C.B., 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxides. Water Research 16, 1247–1353.
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Stanger, G., To, V.T., Le, T.M.N., Tran, T.T., 2005. Arsenic in groundwaters of the Lower Mekong. Environmental Geochemistry and Health 27, 341–357. Wilkie, J.A., Hering, J.G., 1996. Adsorption of arsenic onto hydrous ferric oxide: effects of adsorbate/adsorbent ratios and co-occurring solutes. Colloids and Surfaces 107, 97–110.
Contents lists available at ScienceDirect
Journal of Contaminant Hydrology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j c o n h yd
Source and release mechanism of arsenic in aquifers of the Mekong Delta, Vietnam Kim Phuong Nguyen a,⁎,1, Ryuichi Itoi b a
Department of Geo-environment, Faculty of Geology and Petroleum Engineering, HCMC University of Technology, 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam b Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Nishiku, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 28 February 2008 Received in revised form 29 August 2008 Accepted 11 September 2008 Available online 23 September 2008 Keywords: Arsenic Redox condition Iron hydroxides Mekong Delta Vietnam
a b s t r a c t In order to elucidate the arsenic source and its release mechanism into groundwater in the Mekong Delta, Vietnam, groundwater samples were collected from wells at different depths (20 to 440 m) and core samples (from 20 to 265 m depth) were analyzed. Based on the analytical results for groundwater and core samples, the As source in groundwater is considered to be pyrite (FeS2) in acid sulfate soil (ASS) under oxidizing conditions and hydrous ferric oxide (Fe(OH)3) under reducing conditions. Geochemical modeling demonstrated that As (III) is the dominant species and the presence of As-bearing sulfides, Fe-bearing sulfides and oxides phases may locally act as potential sinks for As. From variation between Fe and As concentrations in groundwater samples, the release mechanism of As is: dissolution of Fe(OH)3 containing As under reducing conditions and oxidative decomposition of FeS2 containing As under oxidizing conditions. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Arsenic is present as a groundwater contaminant in many Asian countries such as Bangladesh, Vietnam, Taiwan and China (Nickson et al., 1998; Chowdhury et al., 1999; Berg et al., 2001; Acharyya, 2004; McArthur et al., 2004; Stanger et al., 2005; Agusa et al., 2006) due to natural conditions. In general, the presence of inorganic arsenic in unconsolidated sedimentary systems is closely associated with geochemical reactions such as oxidation–reduction (Nickson et al., 2000), precipitation–dissolution, adsorption and desorption (Korte and Fernando, 1991; Wilkie and Hering, 1996; Lin and Puls, 2003). Influences of pH and Eh on arsenic solubility are governed by the dissolution of iron oxyhydroxide (FeOOH) and concurrent release of co-precipitated As5+ into solution, then the reduction of As5+ to As3+ (Pierce and Moore, 1982;
Anawar et al., 2003). In Vietnam a large number of wells have suffered from high arsenic concentrations in the Red River and the Mekong Delta (Berg et al., 2001; Stanger et al., 2005; Agusa et al., 2006). Recently, Stanger et al. (2005) reported on As contamination in areas along the Lower Mekong River, including the Mekong Delta in Vietnam, and proposed possible processes that cause a high concentration of As. The arsenic problem in the Mekong Delta has also been pointed out by Iwata et al. (2004) and Berg et al. (2007). However, neither sources of As nor release mechanisms of As to groundwater in the Mekong Delta have been considered. In this study, in order to elucidate the source of As and its release mechanisms, we focused on the vertical profiles and spatial distributions of dissolved and sedimentary As concentrations in the Mekong Delta. Based on the data, we discussed factors controlling the behaviour of As in this area. 2. Methodology
⁎ Corresponding author. Tel./fax: +81 92 802 3345. E-mail addresses: [email protected], [email protected] (K.P. Nguyen). 1 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Nishiku, Fukuoka 819-0395, Japan. 0169-7722/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2008.09.005
2.1. Study area The Mekong Delta, located in the southernmost part of Vietnam (Fig. 1), has a low elevation (in the range of 0–4 m
K.P. Nguyen, R. Itoi / Journal of Contaminant Hydrology 103 (2009) 58–69
Fig. 1. Geological map and location of groundwater samples in the Mekong Delta.
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above sea level) and the delta is vulnerable to flooding in the upstream and tidewater area from the lower sea. There are high alluvial banks and natural dikes with unconsolidated Holocene deposits along rivers with presence of peat and shell fragments (Nguyen, 1993). These deposits are exposed at the surface northeast of Cao Lanh and acid sulfate soil (ASS), which is enriched in pyrite, is formed (Minh et al., 1997; Husson et al., 2000). To the northeast of the Tien River, the Dong Thap-Cao Lanh region is located in a low-lying marshy plain, which potentially contains aluminous soil and/or ASS. The unconsolidated sediments of sand aquifer can be divided into three main geological units: 1) Drained alluvial swamp deposits overlying estuarine clayey sands; 2) Holocene fluvial and/or marine deposits, which overlie the Pleistocene deposits and consist primarily of silty sand, clayey sand, sand, silt and clay; and 3) Pleistocene fluvial, fluvio-marine and marine sand deposits, which consist primarily of clay, sand mixed with grit and gravel. 2.2. Sample collection and chemical analysis To understand the distribution of the arsenic concentration in the Mekong Delta, groundwaters were collected at 13 locations: Cao Lanh (CL), Lai Vung (LV), An Phong (AP), Tan My (TM), Hong Ngu (HN), Tan Hong (TH), Chau Phu (CP), Tan Chau (TC), Can Dang (CD), Long Xuyen (LX), Binh Minh (BM), Mang Thit (MT) and Tieu Can (TC). Fig. 1 shows a location map of groundwater samples collected along with the geological settings. The collection of groundwater samples was done in March and September 2006 (Fig. 1). Wells deeper than 100 m are mostly tube wells that are cased to the depth of the aquifer whereas shallow wells in CL, HN and TH are not cased for local use. 47 groundwater samples are classified into 8 groups as CL, LV, AP, TM, HN-TH, CP-TC, CD-LX, BM-MT by the proximity of their locations. Field measurements of pH, EC, temperature and oxidation-redox potential (ORP) were measured immediately after continuous pumping of groundwater for about 10 min from the wells. Samples for the chemical analysis of As, Fe and Mn were filtered with 0.45 µm membrane filter, acidified with 1 mL of 12 M HCl and preserved in 100 mL polypropylene bottles. Another 250 mL sample was used to analyze major ions. The pH, EC and ORP were measured onsite using a HORIBA D-54 meter. ORP was measured using a Pt combination ORP electrode. Prior to the measurement, accuracy of the equip-
ment was checked by ORP standard solution (89.0 mV at 25 °C). Total alkalinity as HCO3 was measured by titration using methyl orange and bromcresol green indicators in the laboratory. The presence of F−, Cl−, NO3 −, SO4 2−, NH4 +, Na+, K+, Ca+ and Mg2+ ions was determined by ion chromatography (Dionex ICS-90). Dissolved Mn and Fe were detected by inductively coupled plasma - atomic emission spectrometry (ICP-AES) (Vista-MPX). Inorganic arsenic was volatilized as arsine with sodium borohydride–hydrochloric acid solution using hydride generation equipment then analyzed on the HG-AAS (SOLAAR S4 with detection limit 1 µg/l). As(III) and As (V) can be determined separately by a combination of the AsH3 generation-AAS and separation of As(III) and As(V) by liquid chromatography-ICP-AES. In addition to the groundwater samples, core samples of the borehole LK 204 near Chau Phu (Fig. 1) were collected over 10 m intervals for depths ranging from 20 m to 265 m. The major elements (Na, K, Mg, Ca, P, Fe, Mn, Al, S and total As) are identified using wavelength-dispersive XRF (Rigaku RIX 3100). Core samples were ground by vibration mill in order to make pellets and determine the loss on ignition (LOI). X-ray diffraction (XRD-Rigaku RINT 2100) was used to identify constituent minerals in the core samples. 2.3. Geochemical modeling and mineral saturation index The geochemical code, PHREEQC that includes the thermodynamic database WATEQ4F.DAT was adopted to calculate the distribution of aqueous species of As (speciation) (Parkhurst and Appelo, 1999; Appelo and Postma, 2005). The program solves mass balance and mass action (chemical reaction) equations and evaluates the saturation index (SI) of minerals in groundwater. The positive and negative SI values represent the thermodynamic potential for precipitation and dissolution, respectively. In the model, fifteen chemical parameters, including pH, Eh, temperature, Ca, Mg, Na, K, NH4, Cl, alkalinity as HCO3, SO4, As, Fe and Mn were used. The free energies of formation for arsenic species in aqueous solutions at 25 °C (Ferguson and Gavis, 1972) and the mass action constants for the reactions among the selected species are given in Table 1. 3. Results 3.1. Chemical properties of groundwater Chemical composition of the groundwaters is summarized in Table 2. Fig. 2 shows the Piper diagram plotted for 47
Table 1 Thermodynamic data for selected As species Species
ΔG0f (kcal./mol)
Reference⁎
Chemical reaction
Log K
Thermodynamic equation
Reference⁎
As(V)
H2AsO4− HAsO42−
As(III) As(V)/As(III)
H3AsO3 H2AsO3−
−181.0 −171.5 −154.4 −141.8
(1) (1) (1) (1)
H2AsO4− ⇆ HAsO42− + H+ H2AsO4− ⇆ AsO43− + H+ H3AsO3 ⇆ H2AsO3− + H+ H3AsO3 + H2O ⇆ H2AsO4− + 3H+ + 2e−
−6.76 −11.5 −9.23 −21.14
pH = 6.76 pH = 11.5 pH = 9.23 3pH + 2pe = 21.14
(2) (2) (2) (2)
⁎ Reference: (1) = Ferguson and Gavis (1972). (2) = Appelo and Postma (2005).
Table 2 Chemical composition of groundwater ID
Lai Vung
LV 1 LV 2 LV 3 LV4 LV5 LV 6 LV7 CL1 CL2 CL3 CL4 CL5 CL6 CL7 CL8 AP 1 AP 2 AP 3 AP 4 AP 5 AP 6 AP 7 TM 1 TM 2 HN 1 TH 1 TH 2 TH 3 TH 4 TH 5 TH 6 TH 7 TH 8 TC 1 TC 2 TC 3 CP 1 CP 2 CD 1 CD 2 LX BM 1 BM 2 BM 3 MT 1 MT 2 TC
Cao Lanh
An Phong
Tan My Hong Ngu Tan Hong
Tan Chau
Chau Phu Can Dang Long Xuyen Binh Minh
Mang Thit Tieu Can
Depth
Temp
(m)
(°C)
154 268 336 250 90 154 100 61 63 68 60 90 70 100 70 21 86 160 289 25 76 140 350 350 60 39 20 43 48 20 25 30 35 15 27 58 50 33 49 103 50 115 190 310 85 161 440
30.2 30.6 37.1 34.2 29.9 31.2 29.4 29.4 29.5 28.9 28 30.1 29.4 29.7 29.5 28.9 29.7 29.4 28.9 29 29.2 28.9 34.9 33.7 29.6 29.2 29.1 29.8 29.6 29.2 29.8 30 29.2 30.8 29.6 29.5 29 28.9 29.2 29 28.5 29.5 31.7 31 30 30.4 30.7
7.03 9.12 7.99 7.56 7.1 8.51 7.14 6.45 6.4 6.46 6.53 6.49 6.62 6.64 6.51 7.13 6.46 7.79 7.62 6.88 7.05 8.26 6.94 7.31 6.41 6.46 6.27 5.77 6.07 6.37 5.98 5.72 6.22 6.48 6.85 7.25 6.98 6.95 7.78 6.78 7.36 8.19 7.12 9.01 6.07 6.19 9.38
EC
ORP
(mS/cm)
mV
4.77 0.93 1.18 1.5 5.41 4.85 4.04 1.76 2.53 2.08 2.2 4.33 1.73 1.46 1.22 0.58 6.87 14.7 1.08 0.66 7.29 14.9 0.52 0.82 8.07 1.89 0.67 1.58 0.75 0.38 0.67 1.05 0.6 0.47 0.55 1.85 6.05 4.55 4.42 2.55 2.92 2.35 1.69 2.67 16.6 22.6 3.86
− 94 − 76 − 58 − 21 − 143 − 39 − 144 − 21 73 62 − 20 6 23 42 66 − 19 − 81 − 83 − 38 − 198 − 134 − 103 − 89 − 70 38 − 19 − 59 124 63 22 71 74 20 − 94 − 118 − 130 − 65 − 142 − 42 − 40 − 112 − 171 − 165 − 246 − 71 − 86 − 260
Fe
Mn
0.85 0.02 0.04 0.11 7.0 0.21 6.74 0.51 0.25 0.30 0.5 0.5 0.15 0.30 0.25 23.2 0.41 0.04 2.18 17.5 8.0 0.42 0.21 0.37 0.42 1.04 13.8 0.25 0.56 0.5 0.81 1.27 1.33 2.6 15.5 9.44 5.08 43.4 2.41 3.62 4.78 0.41 28.1 bdl 12.3 13.5 bdl
Bdl 0.01 bdl 0.02 0.57 0.10 0.42 4.66 6.33 4.24 1.89 4.92 1.60 2.07 5.27 0.01 0.47 0.85 0.28 0.42 2.76 0.60 0.01 0.01 2.74 1.31 2.93 1.11 0.91 1.93 0.47 0.92 0.70 0.157 0.275 0.089 0.5 bdl 0.05 0.05 bdl 0.89 0.02 0.05 0.01 1.05 0.01
As
As(III)
As(V)
μg/l
μg/l
μg/l
1 1 2.4 5.7 136.5 11.5 95.3 7.70 2.26 31.8 3.36 13.0 1.4 2.3 2.6 741 1.4 1 3.5 284.5 1.3 1.4 8.8 5.1 1.4 1.1 45 53.5 50.6 22.5 2.9 5.8 15.5 1.4 219 59.7 16.7 38.8 9.1 1 12.7 1 16.9 1 1 1 1
n.a. n.a. n.a. n.a. 121.5 n.a. 82.9 n.a. n.a. 10.2 n.a. n.a. n.a. n.a. n.a. 629.8 n.a. n.a. n.a. 256.1 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 16 n.a. n.a. n.a. n.a. n.a. n.a. 192.7 n.a. n.a. 35.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
n.a. n.a. n.a. n.a. 15 n.a. 12.4 n.a. n.a. 21.6 n.a. n.a. n.a. n.a. n.a. 111.2 n.a. n.a. n.a. 28.4 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 37.5 n.a. n.a. n.a. n.a. n.a. n.a. 26.3 n.a. n.a. 3.1 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
HCO3−
F−
Cl−
SO42−
Na+
NH4+
K+
Mg 2+
Ca 2+
88 403 442 392 382 124 527 222 199 167 260 163 197 193 175 324 20 35 316 354 56 57 279 309 362 291 100 99 153 211 132 111 205 244 302 416 478 441 697 44 434 507 486 419 14 65 567
bdl 0.62 0.52 0.45 0.11 bdl 0.22 0.48 0.40 0.22 0.43 0.37 0.69 0.57 0.47 bdl bdl bdl 4.85 0.11 bdl bdl 0.32 0.24 0.37 0.36 0.29 0.12 0.2 0.35 0.22 0.21 0.24 bdl bdl bdl 0.15 bdl 0.44 bdl 1.65 bdl bdl 0.62 bdl bdl 0.91
1515 94 138 246 1110 1513 894 385 656 523 530 1239 392 317 264 25 2556 4611 189 28 2437 4798 17 90 2677 354 109 327 163 20 149 273 45 11 31 354 1886 1300 1136 769 620 495 302 534 5442 7640 814
122 28 68.2 81.4 0.2 103 121 8.3 20.5 20.6 3.0 0.4 7.6 8.4 10.6 bdl bdl 4.3 31.2 bdl bdl 2.0 31.2 38.2 29.2 97.2 37.7 100 26.6 9.2 24.0 29.4 40 31.8 bdl bdl 51 bdl bdl 0.48 154 20 1.7 144 17.6 773 241
832 228 262 316 537 832 462 149 21 189 242 663 168 142 113 31 997 2600 207 33 883 2567 99 153 963 236 71 163 89 30 80 121 79 26 22 291 1108 845 790 190 549 409 186 619 2584 4187 927
3.8 0.63 0.45 0.67 24.7 6.92 18.3 8.98 0.66 1.15 2.93 1.43 0.66 0.12 0.54 bdl 1.94 8.49 0.45 25.4 2.4 10.2 0.5 0.3 3.89 0.56 0.48 0.54 0.21 0.31 bdl 0.48 0.76 8.5 21 8.15 4.80 6.53 3.8 0.7 1.15 0.76 11.37 1.240 11.27 12.23 1.85
15.9 1.6 2.1 3 11 15.5 9.8 6.2 4.4 4.7 2. 5 4.2 2.1 2.4 4 4.5 13.3 78.2 3.2 3.9 13.6 84.3 4.1 3.7 6.3 4 2.1 7.4 4.6 4.3 3.6 8 6.3 1.5 2.15 14 6.3 6 13.5 7.46 8.5 10.4 10.5 5.5 23.6 40.4 14
134 3 5 7.0 89 143 119 50 93 69 55 39 49 43 38 12 199 411 8 17 202 423 6 7 216 52 20 53 26 21 22 29 21 13 18 42 122 80 99 101 35 61 79 13 446 620 23
34 3 15 17 124 30 141 86 145 111 15 121 78 64 47 50 215 111 23 103 238 105 17 24 404 66 59 49 25 19 20 28 23 49 48 47 144 75 81 157 65 55 59 5 548 614 2
61
bdl: below detection limit. n.a: Not analyzed. Concentrations in mg/L except as noted.
pH
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Location
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groundwater samples, indicating that groundwater is typically a sodium-bicarbonate and chloride type. The pH values range from slightly acidic to alkaline (5.77–9.38). Low pH values for samples TH3, TH6 and TH7 which are characterized by relatively high sulfate concentration probably resulted from both oxidation of pyrite and dissolution of gypsum. On the other hand, the high pH values for samples LV2, LV6, AP7, BM1, BM3 and TC can be affected by mixing with seawater. EC values in groundwater range from 377 to 22600 µS/cm. The major ions Na+, Cl− and HCO3 − have concentration ranges of 21–4187 mg/L, 17–8280 mg/L and 10–697 mg/L, respectively. The difference in the Na+ and Cl− concentration and high EC values is probably due to differences in mixing of fresh groundwater and seawater. High sodium and chloride concentrations are found in samples near the coastal area (MT) and in the Pleistocene aquifer (LX, CD, CP, AP3, AP4). 3.2. Distribution of As in groundwater and its speciation
Fig. 2. Chemical composition plotted on Piper diagram.
Fig. 3 shows distribution of arsenic corresponding to location of groundwater samples in the Fig. 1. Total dissolved As concentrations range from 1 to 741 µg/L (Table 2).
Fig. 3. Distribution of arsenic in groundwater in the Mekong Delta.
K.P. Nguyen, R. Itoi / Journal of Contaminant Hydrology 103 (2009) 58–69
In some wells, the As concentration varied with depth. Fig. 4 shows vertical profiles of concentrations of total As and Fe. In this figure, groundwater samples are divided into two groups based on ORP values: 1) CL and HN-TH, for which positive ORP values are predominant, and 2) LV, AP-TM, TC-CP, and CD-LX, for
63
which ORP values are negative. In general, as CL and HN-TH samples have positive ORP values, Fe concentrations should be low because of precipitation of ferric hydroxide if organic ligands are absent. Moreover, As concentrations of these samples are lower than 55 µg/L. On the other hand, high As
Fig. 4. Concentration profiles of: (a) total As; (b) total Fe; (c) NH+4 and (d) SO2− 4 vs depth. ORP values of CL and HN-TH samples are positive while those of LV, AP-TM, CP-TC and CD-LX samples are negative.
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and Fe concentrations are detected in LV, AP and TC, which have negative ORP values. Arsenic concentrations exceeding 100 µg/L are present at depths less than 100 m in LV, AP and TC. As shown in Table 2, As concentrations more than 10 µg/L were not found at depths more than 100 m except for sample BM2. Fig. 4 shows that the depth where maximum As concentration of AP, LV, CP and TC appeared generally coincides with that of total maximum Fe concentration. These facts indicate that the dissolution and fixation of As occurs depend on dissolution and precipitation of Fe controlled by the change in redox conditions with depth. Arsenic in aqueous solution is ordinarily present as arsenious acid (H3AsO3 0, H2AsO3 −, As(III)) and arsenic acid (H2AsO4 −, HAsO4 2−, As(V)). According to Ferguson and Gavis (1972), the toxicity of As(III) is stronger than that of As(V). Therefore, the speciation of As is important in environmental chemistry. As shown in Table 2, we analyzed As(III) and As(V) separately. Both As(V) and As(III) form protolytes that may release protons stepwise in the same way as carbonic acid. The Eh versus pH diagram for samples in this study is shown in Fig. 5. The most thermodynamically stable arsenic species calculated for the groundwaters are HAsO4 −, H2AsO4 2− and H3AsO3 0 depending on Eh and pH. While some samples (LV2, LV3, LV4, LV6, CL2, CL3, CL8, AP4, AP7, and CD1) are plotted on the stability field for H2AsO4 − and HAsO4 2−, which is present under oxidizing conditions, most of the samples are plotted on the stability field of H3AsO3 0, which is present under reducing conditions. 3.3. Characterization of redox conditions in groundwater ORP values of the groundwater range from −260 mV to 124 mV (Table 2). Generally, chemical analysis results indicate that groundwater in this area is under reducing conditions
Fig. 5. Stability diagram of Eh–pH for dissolved As species at 25 °C. Boundaries indicate equal activities of both species.
Fig. 6. Eh–pH diagram for iron species (after Deutsch, 1997).
because of negative values of ORP and presence of reduced components such as NH4+ and Fe2+, except CL and HN-TH, which have positive ORP values (Table 2, Fig. 4). Under oxidizing conditions, hydrous Fe(III) oxide such as Fe(OH)3 can remove both arsenious and arsenic acids from solution. Oppositely, when the Fe(OH)3 dissolves under reducing conditions, the As is released into solution. In order to confirm the precipitation–dissolution conditions for Fe, Eh and pH values of groundwater samples are plotted on the stability diagram of Fe (Fig. 6). The result shows that most of the samples fall in the stability field of Fe2+ (dissolved state) or Fe(OH)3 (precipitated solid state). Higher Fe concentrations are expected for be contained in the groundwater samples whose plots are located in the stability field of Fe2+. In Fig. 6, many points are located near the boundary between Fe2+ and Fe(OH)3, indicating that Fe2+ is easily oxidized to precipitate Fe(OH)3, and Fe(OH)3 can be dissolved to form Fe2+ by only slight change in redox condition. Only samples from TM are plotted in the field of pyrite. In this area, pyrite may be present because of the occurrence of ASS. Sulfate is present in most of the samples (average 53 mg/L, maximum 773 mg/L). SO4 2− concentrations in HN-TH are up to 40 mg/L except for two samples of about 100 mg/L. High SO4 2− concentrations in this area may be caused by both oxidation of pyrite and dissolution of gypsum because aquifer in this area formed in the Moc Hoa Formation. Sources of SO4 2− in LX, LV, and BM-MT samples are results of interaction between groundwater and marine deposits or mixing of fresh water and seawater. The salinity in the Mekong Delta significantly increases from north to south. Therefore, groundwaters in BM, MT and Tieu Can with high Na+, Cl−, and SO4 2− are affected by seawater intrusion. However, there is no correlation between ORP values and SO4 2− concentrations (Table 2 and Fig. 4).
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Table 3 Concentration of major elements (%) and arsenic (ppm) in core samples at various depths Depth
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
S
As
(m)
%
%
%
%
%
%
%
%
%
%
ppm
30 51.5 57.2 82.5 90 98.2 101 103 109 115.5 127.5 133.7 137 143 155 166.5 182 194 203 209 215.5 225.5 245 252.5 265
54.62 63.81 70.22 88.97 79.8 82.94 73.43 64.75 87.51 85.19 76.25 57.65 86.94 92.14 66.64 81.89 80.15 54.7 69.69 56.99 84.33 74.79 79.14 76.87 73.07
27.7 18.04 14.16 4.29 8.23 7.59 11 17.63 4.62 5.4 10.48 10.43 6.06 2.88 14.38 9.54 10.56 33.78 19.84 29.41 9.07 14.17 10.96 12.87 13.61
13.61 4 3.99 1.11 3.85 2.71 6.04 4.27 2.96 3.66 3.93 12.62 1.56 5.08 8.33 1.63 1.94 18.98 1.04 1.53 0.62 1.31 0.57 0.85 0.87
0.4 0.02 0.02 0.01 0.17 0.09 0.05 0.04 0.14 0.18 0.11 0.78 0.03 0.03 0.11 0.01 0.01 0.6 0 0 0.01 0.01 0.01 0.01 0.01
1.31 1.48 1.27 0.52 1.07 0.97 1.01 1.41 0.55 0.7 1.31 1.6 0.73 0.42 0.96 0.71 0.67 1.06 0.34 0.4 0.31 0.43 0.31 0.4 0.36
0.21 0.51 0.49 1.16 0.32 0.22 0.87 0.38 0.22 0.24 0.32 0.9 0.18 0.14 0.28 0.19 0.25 0.34 0.08 0.13 0.06 0.19 0.11 0.11 0.09
0.34 0.24 0.22 0.24 0.44 0.45 0.37 0.21 0.38 0.43 0.42 0.35 0.43 0.23 0.25 0.22 0.14 0.36 0.19 0.19 0.17 0.53 0.5 0.33 0.27
2.31 1.47 0.97 1.16 2.14 1.65 1.52 2.01 1.44 1.49 1.7 1.54 1.49 0.8 1.41 0.91 0.19 1.36 2.19 1.49 2.09 3.69 5.37 4.48 3.01
0.03 0.03 0.03 0.03 0.04 0.04 0.44 0.06 0.04 0.05 0.08 0.17 0.05 0.03 0.08 0.02 0.02 0.07 0.02 0.02 0.01 0.01 0.01 0.01 0.01
0.111 0.011 0.016 0.028 0.023 0.015 0.249 0.024 0.031 0.054 0.106 0.100 0.039 0.047 0.022 0.016 0.012 0.008 0.006 0.007 0.006 0.010 0.006 0.010 0.011
37 12 8 7 8 6 18 14 7 9 15 24 9 11 16 8 6 45 7 4 5 7 7 5 4
Minerals
Smectite, illite, feldspar, goethite, hematite Smectite, illite Smectite, illite, kaolinite, feldspar, goethite Illite, kaolinite, feldspar, hematite Smectite, illite, kaolinite, feldspar Smectite, illite, feldspar, hematite. Smectite, illite, kaolinite, Feldspar, goethite Illite, kaolinite, feldspar, goethite Smectite, kaolinite, feldspar, hematite Illite, kaolinite, feldspar, goethite, pyrite. Illite, feldspar, goethite, hematite, pyrite Illite, feldspar, hematite Illite, kaolinite, feldspar, goethite Illite, kaolinite, feldspar, hematite Smectite, illite, kaolinite, feldspar, goethite Illite, feldspar, goethite, hematite Illite, hematite Illite, feldspar, goethite Illite, kaolinite, hematite Illite, kaolinite, feldspar, pyrite Illite, hematite Smectite, illite, feldspar, hematite Feldspar, hematite Feldspar, hematite, pyrite Feldspar, hematite, pyrite.
3.4. Mineral composition for core samples
3.5. Mineral saturation index
Table 3 shows major constituents and mineral composition obtained by XRF and XRD analyses for core samples of the borehole LK204. Total As content in core samples is 4–45 mg/kg. XRF analysis for the core samples shows appreciable Fe concentrations in the formation at depths N30 m and iron oxide minerals such as goethite and hematite were identified by XRD (amorphous hydrous oxide cannot be detected). Fig. 7 shows positive correlation of As with Fe and Mn oxides in the Mekong Delta core samples. These results indicate that hydrous ferric oxides occur with As or Fe(oxyhydr)oxides and/or hydrous Mn oxides are the principal As carrier phases. Pyrite is also identified at hundreds meters depths more than 100 m (115.5 m, 127.5 m, 209 m, 252.5 m and 265 m), although As contents in those samples are near baseline values (b10 mg/kg) except for the sample from 127.5 m.
Table 4 summarizes the calculated SI values of various mineral phases in groundwater. Fig. 8 also shows the relationship between ORP of groundwater and SI values for pyrite, Fe hydroxide, goethite and hematite. The samples in the oxidizing group represented with positive value of ORP are under subsaturated condition with respect to pyrite but less subsaturated in the reducing group (Fig. 8a). This implies that they might have been oxidized, and released As to the groundwater. The dissolution of sulfate minerals, gypsum and jarosite, contributes Fe and SO42− to groundwater. The samples are slightly subsaturated with respect to Fe hydroxide for both reducing and oxidizing conditions (Fig. 8b). In general, As can be adsorbed onto the surfaces of Fe hydroxide formed under oxidizing and reducing conditions, respectively. Therefore, the dissolution of Fe hydroxides occurs in most aquifers where the
Fig. 7. Relationship between contents of As and Fe2O3 or MnO in core samples.
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Table 4 Saturation index of different minerals in groundwater, computed using PHREEQC Sample ID
Positive ORP values
Negative ORP values
HN TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8 CL2 CL3 CL5 CL6 CL7 CL8 CL1 CL4 LV1 LV2 LV3 LV4 LV5 LV6 LV7 AP1 AP2 AP3 AP4 AP5 AP6 AP7 TM1 TM2 TC1 TC2 TC3 CP1 CP2 CD1 CD2
Mineral phases equilibrium constants (log K) Fe(OH)3
Goethite
Hematite
Gypsum
Jarosite
Pyrite
4.981a
−1.0a
−4.008a
−4.58a
−9.21a
−18.479a
−3.59 −3.85 −3.83 −1.28 −1.50 −3.38 −4.34 −4.65 −3.97 −3.09 −3.18 −4.75 −3.67 −2.75 −2.81 −4.84 NA −3.49 0.19 −1.28 −1.50 −3.38 1.06 −3.21 −0.68 −5.34 −2.50 −0.38 −4.32 −3.24 −0.44 −4.04 −2.45 −3.56 −3.07 −2.40 −2.53 −2.93 −0.14 −2.65
2.46 2.19 2.21 5.02 4.71 2.68 1.72 1.42 2.07 2.96 2.85 1.32 2.37 3.3 3.24 1.21 NA 2.59 6.27 5.02 4.71 2.68 7.17 2.84 5.35 0.72 3.55 5.65 1.71 2.80 5.59 2.19 3.74 2.53 2.98 3.65 3.50 3.10 5.90 3.39
6.95 6.40 6.44 12.11 11.46 7.40 5.48 4.87 6.17 7.95 7.72 4.67 6.77 8.63 8.51 4.44 NA 7.21 14.58 −12.11 11.46 7.40 16.38 7.70 12.72 3.47 9.13 13.33 5.54 7.63 13.21 6.44 9.53 7.10 7.98 NA 9.03 8.22 13.83 8.80
−2.02 −1.83 −2.12 −2.49 −2.39 −4.42 −2.73 −2.56 −2.45 −2.25 −2.32 −4.07 −2.83 −2.83 −2.82 −2.76 −3.93 −2.24 −3.63 −2.49 −2.39 −4.42 −2.38 −1.62 NA NA −3.54 −2.61 NA NA −3.90 −2.64 −2.47 −2.23 NA −5.61 −2.08 NA NA −3.88
−18.04 −17.49 −17.77 −14.14 −13.46 −23.14 −18.44 −18.15 −17.4 −16.48 −16.88 −25.15 −19.91 −17.00 −16.30 −22.37 NA −17.54 −14.40 −14.14 −13.46 −23.14 −8.43 −17.31 NA NA −19.65 −11.40 NA NA −15.55 −19.69 −16.06 −17.71 NA NA −15.88 NA NA −19.43
−41.00 −26.18 −13.17 −45.99 −45.96 −12.79 −41.20 −37.28 −32.00 −48.9 −47.14 −39.15 −41.83 −46.00 −49.26 −28.71 NA −18.39 −59.77 −45.99 −45.96 −12.79 −56.20 −7.37 NA NA −37.63 −40.64 NA NA −39.96 −20.94 −30.48 −8.93 NA NA −24.08 NA NA −30.58
NA: Not available. a : Numbers represent logarithms of equilibrium constant from Parkhurst and Appelo (1999).
groundwater samples were taken, which is prerequisite for reductive dissolution of Fe hydroxides as the releasing mechanism of As in groundwater. On the other hand, goethite and hematite are under supersaturated condition in all the samples (Fig. 8c, d), which is consistent with the presence of these minerals in core samples (Table 3). Precipitation of goethite and hematite could be a significant adsorbent for As in the aquifer where the borehole was drilled for core samples. 4. Discussion 4.1. Geological condition of the Mekong Delta Geologically, the Mekong Delta is occupied by Holocene sediments mainly composed of Holocene alluvial sediments unconformably overlying the Late Pleistocene sediments. The Late Pleistocene strata are mainly composed of stiff, slightly
oxidized, yellowish gray silt to fine-medium sand with scattered quartz pebbles and laterites (Fig. 1). The Holocene deposits are incised river silts and coastal sand dunes up to 30 m thick (Stanger et al., 2005). During the Late Pleistocene low-stand setting, the delta was developed with sediments transported from the upper stream. Medium sand and gravel beds covered the delta in upland terraces in the west and north (Fig. 1). In the Early–Mid Holocene, sea level continued to rise and caused transgression so that almost the entire area was inundated and occupied by tidal mangrove forests. Therefore the delta was divided into two parts: an upper delta plain dominated by fluvial processes, and the lower delta plain characterized by a welldeveloped beach-ridge system and mainly influenced by marine processes (Nguyen et al., 2000). The subsurface of the deposits is rich in organic matter under reducing conditions. Annual flooding in the Mekong Delta or inundation of swampland may contribute to reducing conditions in the aquifers in this area.
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Fig. 8. Relationship between ORP and SI for (a) Pyrite; (b) Fe(OH)3; (c) Goethite and (d) Hematite.
4.2. Source and release mechanism of As into groundwater Natural arsenic originates from the Upper Mekong basin and the Mekong Delta was supplied by the sediment containing arsenic. The present Mekong Delta was submerged due to marine transgression five to six thousand years ago. During this time, Fe included in sand from the upper Mekong River and SO4 from seawater combined with each other to form pyrite under reducing conditions. The soils that include pyrite are called “potential acid sulfate soils (ASS)”, and are present in most areas in the Mekong Delta. During regression, caused oxidation of pyrite occurs according to the generalized chemical reaction (Akira, 2006):
grayish brown sulfuric horizon with yellow brown and pale yellow mottles that represent goethite and jarosite, respectively, in the vicinity of HN, TM and Cao Lanh (Fig. 9), while
FeS2 þ O2 þ ðH2 OÞ→KFe3 ðSO4 Þ2 ðOHÞ6 þ H2 SO4 The areas whose upper soil horizon has undergone such chemical reaction are defined as acid sulfate soil groups. Fig. 9 shows the distribution of ASS, which covers approximately 1.8 million hectares, in the Mekong Delta. In the western delta, Brinkman et al. (1993) found that there are two types of sulfidic materials. The older ones, situated in the northwest of the area, occur at several meters depth and have high contents of pyrite and finely distributed organic matter. The younger ones are the most extensive. They are generally less than 1.5–3 m depth and have low contents of organic matter with pyrite mostly found in root remnants. On the other hand, in the northeastern Mekong Delta, soils are characterized by a
Fig. 9. Distribution of soils in the Mekong Delta.
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soils have a sulfuric horizon without jarosite or goethite mottles in Long An (Husson et al., 2000). Husson et al. (2000) also concluded that the sulfurous horizon with jarosite mottles had a pH 2.8–3.0, high sulfate concentrations, and Eh varied from 590 to 670 mV. Gustafsson and Tin (1994) reported the enrichment of As (6–41 mg/kg) in ASS due to the uptake of As by sulfides. If these sulfides containing As encounter oxidizing conditions, they are decomposed to ferric ions and sulfate ions, but ferric ions are rapidly hydrolyzed to form hydrous ferric oxide. Under this condition As is present as As(V) (arsenic acid) and is easily adsorbed by the hydrous ferric oxide. Therefore, it is reasonably concluded that there are two different As sources in the aquifers in the Mekong Delta, pyrite and hydrous ferric oxide. The redox conditions of groundwater samples can vary with depth or locality. In TH, all the water samples are under oxidizing conditions in the range of 20–48 m depth. In the case of CL, the water is under reducing conditions. For groundwater samples with negative ORP values and As concentrations higher than 10 µg/L (e.g. LV5, LV7, AP1, AP5, TC2, TC3, CP1, CP2, LX and BM2), the Fe and Mn concentrations are relatively high, suggesting that adsorbed As is released into groundwater by dissolution of hydrous ferric oxides or hydrous manganese oxide under reducing conditions. The dissolution of Fe hydroxides would control dissolved Fe concentrations in the reducing groundwaters. For groundwater samples with positive ORP values and high As concentrations (e.g. CL3, TH3, TH4, TH5 and TH8), the Fe and NH4 + concentrations are low. It is suggested that As may be released by decomposition of pyrite including As due to oxidation of Fe2+ and S2 2−. The Fe3+ ions hydrolyze rapidly to precipitate Fe(OH)3 and S2 2− ions are oxidized to SO42− ions, so the As may be present as As(V). 5. Conclusions Groundwater samples collected from wells in the Mekong Delta were chemically analyzed for major ions, arsenic and iron for understanding geochemical conditions in the aquifers. Core samples were also analyzed with XRF and XRD for identifying the distribution of As in addition with mineral species and contents of components. For groundwater, As concentrations higher than 100 µg/L were identified in the samples from wells shallower than 100 m. Samples from wells deeper than 150 m generally show low As concentrations. The diagenesis of As in the Mekong Delta sediment and groundwater is strongly coupled with Fe cycling and redox conditions. Reductive dissolution of iron hydroxides is a controlling mechanism that causes high arsenic concentrations in reduced groundwater samples (negative ORP values, high concentrations of NH4 +, total Mn and Fe). Low arsenic concentrations, however, in oxidized groundwater samples (CL and HN-TH) are the results of oxidative decomposition of sulfide-bearing minerals such as pyrite. Acknowledgements The support provided by N. T. Viet, N. K. Quyen, and Mr. N. V. Chinh, staff of the Division of Hydrogeology and Engineering Geology, as well as Rie Unoki and Nakao Kazuto, Dept. of Earth Resource Engineering, Kyushu University, in the
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