162 the sustainability risk of HCM

Geosciences Journal Vol 19, No 3, p 547  560, September 2015 DOI 10.1007/s12303-014-0052-4 ⓒ The Association of Korean Geoscience Societies and Springer 2015 The sustainability risk of Ho Chi Minh City, Vietnam, due to saltwater intrusion } Faculty of Geology, HCMC University of Science, Ho Chi Minh, Vietnam Minh Thien Ngo Department of Earth System Sciences, Yonsei University, Seoul 120-749, Republic of Korea Department of Earth System Sciences, Yonsei University, Seoul 120-749, Republic of Korea Jae Min Lee Korea Basic Science Institute, Seoul center, Seoul 136-075, Republic of Korea Hyun A Lee Nam Chil Woo* Department of Earth System Sciences, Yonsei University, Seoul 120-749, Republic of Korea ABSTRACT: Groundwater is important for domestic, industrial, and agricultural uses in Ho Chi Minh City, Vietnam As the city has developed in a coastal environment, the issue of the fresh water supply must be solved for continuous development The aim of this study was to quantitatively estimate the risk of groundwater salinization in the Ho Chi Minh area due to saline water intrusion into the main coastal aquifer (the Upper Pliocene aquifer) based on field monitoring data, and to evaluate the sustainability of the city with respect to groundwater resources From the national monitoring database, water level data were obtained for the last 10 years (2000 to 2009), and a total of 33 hydrogeochemical and isotope data sets were obtained from the aquifer The sustainability of Ho Chi Minh City with respect to the groundwater supply was quantitatively evaluated at an aquifer scale using groundwater sustainability indicators (GWSIs) suggested by the UNESCO/IAEA/IAH Working Group The results indicated that groundwater in the southern region, part of the western region, and the area along the Saigon riverside was of poor quality, with very high total dissolved solids (>1,000 mg/L) and high concentrations of Cl and Fe, exceeding the World Health Organization’s drinking water guidelines The Br:Cl ratios and the δ2H and δ18O values of the samples indicated that the salinization of groundwater resulted mainly from mixing with seawater over a long period During 2004–2009, the saline boundary moved inland, with the farthest distance reaching ~3.2 km The long-term abstraction of groundwater, which has been much greater than its recharge capability, is probably causing the decline in water level (in 39% of the aquifer area), the degradation of groundwater quality (in 62% of the area), and the continuously expanding saline water intrusion (by 7.4% in years) Thus, for the sustainable development of Ho Chi Minh City, in addition to passive measures to regulate over-pumping and pollution controls, active measures should be considered to prevent further seawater intrusion and to increase groundwater recharge through artificial recharge or better management of aquifer recharge (MAR) Key words: saltwater intrusion, groundwater sustainability indicators (GWSIs), Upper Pliocene aquifer, Ho Chi Minh City, aquifer recharge INTRODUCTION Currently, approximately 700 million people worldwide live under conditions of water shortage, and by 2025, this *Corresponding author: ncwoo@yonsei.ac.kr number is expected to increase to billion (UNDP Human Development Report, 2006) Population increase results in increased demand on the water supply, and accordingly, water resource management to ensure national sustainable development has become an important subject of national policy around the world Moreover, half of the world's population lives within 60 km of the sea, and three-quarters of all large cities are located in coastal areas (http://www.unep.org/ urban_environment/issues/coastal_zones.asp) In coastal environments, where fresh water and saline water exist in a dynamic balance, increased exploitation of freshwater resources can disrupt the balance, causing saltwater intrusion into areas that previously contained freshwater Consequently, coastal areas under continuous development are facing sustainability risk due to the limited capacity of the freshwater supply Because coastal areas are open to the ocean and the surface water is readily mixed with saline water, most coastal cities depend on groundwater for their freshwater supply Therefore, from the perspective of water resources, the sustainable development of coastal cities is dependent on the availability of fresh groundwater resources in the area, and thus, groundwater aquifers become the targets of water-resource development and management for protection and preservation from contamination and over-exploitation Groundwater salinization by the intrusion of seawater, which results from the mass transport of saline waters into zones previously occupied by fresh groundwater, degrades water quality by raising the salinity of freshwater above acceptable levels for drinking or other purposes Numerous studies have examined the risk of seawater intrusion (Werner and Simmons, 2009; Morgan et al., 2012; Park et al., 2012; Sophiya and Syed, 2013) and have suggested effective solutions such as pumping control (Mantoglou, 2003), use of injection wells (Allow, 2011), and reverse osmosis desalination (Vinson et al., 2011) Ho Chi Minh City (previously known as Saigon) is located in the southeastern region of Vietnam and is the largest city in Vietnam, occupying an area of 2,095 km2, with more than 7.5 million inhabitants, and it functions as the country’s 548 Minh Thien Ngo, Jae Min Lee, Hyun A Lee, and Nam Chil Woo Fig (a) A map of the study area (b) A schematic hydrogeological cross section: blue and brown layers indicate aquifers and aquitards, respectively (Bui, 2010) The different shades indicate the rate of groundwater abundance Darker shades represent more abundant zones of groundwater Sustainability of Ho Chi Minh City due to saltwater intrusion economic center The city is bordered by neighboring provinces and the Can Gio Sea in the south (Fig 1a), and its topography is relatively higher (35 m above sea level) in the north and lower in the south (0.5–1.5 m above sea level) Two large rivers, the Saigon and the Dongnai, flow through the Ho Chi Minh area from the northeast to the Can Gio Sea in the south Both groundwater and river water have been used in the city since the early 19th century (Ho Chi Minh City, 2010a) However, the existing water supply is insufficient for the increased water demand, particularly during the dry season, when rivers are seriously affected by seawater intrusion (Dan et al., 2007) As for the groundwater resources in the area, several studies have reported groundwater storages (Nguyen, 1991; Tran, 1998), water level declines, water quality degradation, and land subsidence (Nguyen 1998, 2008, 2009; Nguyen and Tran, 2003; Dan et al., 2006, 2007) As it is located in a coastal environment, Ho Chi Minh City also needs to solve issues of freshwater supply for continuing development Therefore, the aim of this study was to quantitatively estimate the risk of groundwater salinization in the Ho Chi Minh area due to saltwater intrusion into the main coastal aquifer based on field monitoring data, and to evaluate the sustainability of the city with respect to groundwater resources 549 effectively protect and preserve the groundwater resource MATERIALS AND METHODS A total of 33 hydrogeochemical data sets from the aquifer (n22) were obtained from the National Monitoring Database for the Ho Chi Minh area, collected by the Division for Water Resources Planning and Investigation for the South of Vietnam Measured parameters included pH, total dissolved solids (TDS), Ca2+, Na+, K+, Mg2+, NH4+, Fe2+, Fe3+, HCO3‒, Cl‒, SO42‒, and NO3‒ An additional sampling campaign was undertaken in April 2012 A total of 17 samples were taken for Br and Cl concentrations and analyzed using ion chromatography (Model: Metrohm 883) at the hydrogeology laboratory of Yonsei University From 11 of the 17 samples, the stable isotopic ratios of hydrogen (δ2H) and oxygen (δ18O) were determined using a stable isotope ratio mass spectrometer (Prism II, VG Instruments) at the Korean Basic Science Institute The 11 selected samples were distributed throughout the research area from north to south, representing both fresh and saline water All stable isotopic compositions were reported in the standard notation in which δ = {(Rsample/Rstandard) – 1} × 1000 (‰), where Rsample and Rstandard represent the ratios of heavy to light isotopes in the sample and standard, respectively Based on water-level data collected from 2000 to 2009, STUDY AREA the long-term water level change was delineated using linear The Ho Chi Minh City area has six regional aquifers con- regression and the Mann-Kendall test The Mann-Kendall sisting of alluvial sediments (Nguyen and Tran, 2003) Each test, a rank nonparametric test developed by Mann (1945) and aquifer is separated from the others by clay layers that act Kendall (1975), has been recognized as useful for detecting as confining layers (Fig 1b; Bui, 2010) The confining lay- linear and non-linear trends (Hamed and Rao, 1998; Wu et ers vary widely in thickness, ranging from m to more than al., 2008; Shadmani et al., 2012) In this test, the null hypoth30 m in the study area, and are possibly absent in some local esis (H0) and the alternative hypotheses (H1) were the nonareas The aquifers were named according to the formation existence and existence of a trend in the time-series data, sequence, from the uppermost to the lowest, as follows: the respectively (Shadmani et al., 2012) If the standardized test Upper Pleistocene aquifer (qp3), with an average thickness statistic ZM-K had a positive value, it indicated an increasing of 23 m; the Upper‒Middle Pleistocene aquifer (qp2‒3), with trend, whereas a negative value indicated a decreasing trend an average thickness of 27 m; the Lower Pleistocene aquifer in the time series When |ZM-K| > Z1‒α/2, H0 was rejected, (qp1), with an average thickness of 27 m; the Upper Pliocene and a significant trend was considered to exist in the time aquifer (n22), with an average thickness of 38 m; the Lower series Z1‒α/2 was the critical value of Z from the standard Pliocene aquifer (n21), with an average thickness of 34 m; and normal table at the 5% significance level the Miocene aquifer (n13) with an average thickness of 25 m The sustainability of Ho Chi Minh City with respect to Currently, the Upper Pliocene aquifer (n22) is the main the groundwater supply was quantitatively evaluated using source of groundwater supply because of its relatively large groundwater sustainability indicators (GWSIs) suggested by thickness (38 m), high hydraulic conductivity (14 m/day), the United Nations Educational, Scientific and Cultural Orgaand good water quality (Bui, 2010) Groundwater pumping nization (UNESCO)/ International Atomic Energy Agency volume from this aquifer was estimated to be ~272,000 m3/ (IAEA)/International Association of Hydrogeologists (IAH) day, accounting for approximately 38% of the total pumping Working Group (Vrba and Lipponent, 2007) GWSIs are based volume for the city (717,000 m3/day) (Department of Natural on measurable and observable data, and provide informaResources and Environment of Ho Chi Minh City, 2009) tion about groundwater quantity and quality (contemporary This study only focused on the groundwater in the main states and trends) They focus on social (groundwater availabilaquifer (n22) due to the availability of long-term monitoring ity and use), economic (groundwater abstraction and prodata and its importance for water supply Our results provide tection), and environmental (groundwater vulnerability and information essential for designing a management plan to pollution) aspects of groundwater resources policy and man- 550 Minh Thien Ngo, Jae Min Lee, Hyun A Lee, and Nam Chil Woo agement These indicators have recently been successfully applied in different countries on various scales from the national to the regional, and finally, to the aquifer scale (Girman, 2007; Hirata et al., 2007; Pernia and Lamban, 2007; Lavapuro et al., 2008; Lamban et al., 2011) Originally, the Working Group proposed 10 indicators for estimating sustainability However, in the present study, the following three indicators were adopted based on the availability of the necessary data: (1) total groundwater abstraction/exploitable groundwater resources; (2) groundwater depletion; and (3) groundwater quality Additionally, taking into consideration that this is a coastal aquifer system, another indicator, (4) groundwater salinization, was developed and tested in this study RESULTS AND DISCUSSION 4.1 Groundwater Quality Table presents a summary of the chemical compositions of the 33 groundwater samples from the main aquifer (n22) The pH, TDS, Cl‒, and Fe concentrations exceeded the World Health Organization (WHO) guideline values for drinking water Table Statistical summary of the Upper-Pliocene aquifer (n22) data (N = 33) compared with the 2011 World Health Organization (WHO) guidelines for drinking water Parameter Sample depth pH TDS Na+ K+ Ca2+ Mg2+ NH4+ Fe2+ Fe3+ Al3+ HCO3‒ Cl‒ SO42‒ NO3‒ Units m mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L Min 60.5 3.10 27.06 3.49 0.10 1.40 0.60 0.01 0.01 0.01 0.01 1.00 4.25 1.00 0.01 Max 240.70 8.30 28700.70 8482.10 400.00 851.70 1185.60 19.26 194.66 3.22 1.40 170.86 16076.58 1753.09 6.31 Median 195.21 6.49 3309.92 892.77 44.60 124.00 136.92 1.00 7.92 0.34 0.05 70.49 1835.91 195.14 0.79 Standard deviation WHO guideline 45.71 1.44 6.5–8.5 7107.03 1000 2076.05 96.08 221.96 299.23 3.31 1.5 33.91 0.3 0.67 0.24 0.2 48.15 4011.58 250 460.61 250 1.15 50 % over WHO limits 33.3 33.3 9.1 45.5 3.0 42.4 18.2 0.0 Fig Piper diagram showing different water types and the saline boundary map of the main aquifer (n22) using the total dissolved solid (TDS) cutoff value of 1,000 mg/L Sustainability of Ho Chi Minh City due to saltwater intrusion 551 (WHO, 2011) in 33%, 33%, 42%, and 46% of the samples, respectively, indicating a significant degradation of the groundwater in the aquifer Based on a TDS cutoff of 1,000 mg/L (Freeze and Cherry, 1979), groundwater in the study area could be divided into two groups: fresh and saline (Fig 2) Groundwater in Region I, the central and the northern areas, exhibited TDS concentrations of