Review https://doi.org/10.1007/s12303-019-0021-z pISSN 1226-4806 eISSN 1598-7477 Geosciences Journal GJ A hydrogeological and geochemical review of groundwater issues in southern Vietnam Quang Khai Ha 1 , Kangjoo Kim 1 * , Nam Long Phan 1 , Thanh Huy Phung 1 , Jinwon Lee 1 , Viet Ky Nguyen 2 , and Chu Nam Phan 3 Department of Environmental Engineering, Kunsan National University, Gunsan, Jeonbuk 54150, Republic of Korea Faculty of Geological and Petroleum Engineering, Ho Chi Minh City University of Technology, Ho Chi Minh City, Vietnam Division for Water Resources Planning and Investigation of the South of Vietnam, Ho Chi Minh City, Vietnam ABSTRACT: Southern Vietnam is known as one of the most vulnerable areas to climate change in the world due to its low elevation and flat topography, heavy groundwater use, and dense population. This paper introduces hydrogeological conditions and critically reviews groundwater issues that southern Vietnam faces by summarizing previous literatures. Issues that this paper covers include groundwater use and associated phenomenon, acid sulfate soils, the occurrence of saline groundwaters, seawater intrusion, land sub- sidence, groundwater acidification, and groundwater contamination by nitrate, ammonium, and trace metals such as As, Fe, Mn, Al, Cd, and Pb. This paper shows that most of these issues are inter-related and essentially linked with the unique hydrogeological setting of the study area. However, many of these problems have also been triggered or aggravated by human activities that use water resources and land. Issues such as those of As contamination and groundwater acidification are connected with the aquifer characteristics that are generally confined and rich in organics and pyrite. However, groundwater salinization, acidification, groundwater level declining, land subsidence, and high heavy metal concentrations are in large part caused by human activities such as well installation and heavy groundwater abstraction. While previous studies have proposed artificial recharge as a promising means of mitigating groundwater issues in the study area, the development of techniques that help minimizing aquifer disturbances is also likely required. Key words: acid sulfate soil, arsenic, sea level change, saline groundwater, acidification, artificial recharge Manuscript received April 14, 2019; Manuscript accepted May 14, 2019 1. INTRODUCTION The study area covers 62,000 km 2 and includes Ho Chi Minh City (HCMC) and the Mekong Delta in Vietnam (Fig. 1a). The Mekong Delta is 59,000 km 2 in area, 39,000 km 2 of which belongs to Vietnam. As the average elevation of the Mekong Delta is only approximately 1 m above mean sea level, the area is known as one of the most vulnerable areas to climate change and the sea level rise (To, 2014). The lowland area of southern Vietnam has been known as the ‘rice bowl’ of Vietnam because it contributes 55% of total rice production (GSO, 2017). The area supports a population of 34.5 million people and contributes 45.2% of Vietnam’s annual budget revenue (GSO, 2017). Groundwater is the main freshwater resource in the study area. However, the groundwater has faced serious challenges due to a series of environmental issues including climate change, sea level rise, dam construction, increased use of groundwater, and contamination. According to a report from the Ministry of Natural Resources and Environment in Vietnam (MONRE, 2016), climate change is already apparent in Vietnam. The annual temperature has increased by an average of 0.62 °C across Vietnam, and annual rainfall increased by 19.8% in southern Vietnam from 1958–2014 (MONRE, 2016). Rainfall is growing more intensive during the rainy season but is declining in the dry season. According to extreme projections on national climate change (MONRE, 2016), temperatures will increase by up to 4 °C and precipitation may increase by 20% by 2100 in southern Vietnam. Sea levels are projected to increase by up to 100 cm. Due to low land elevation, the study area is frequently flooded during the rainy season and the land is widely salinized in coastal zones. The area is likely to experience more floods, droughts, salinity intrusion, and tidal inundation in the future (Wassmann et al., 2004; Smajgl et al., *Corresponding author: Kangjoo Kim Department of Environmental Engineering, Kunsan National University, 558 Daehak-ro, Gunsan 54150, Republic of Korea Tel: +82-63-469-4766, Fax: +82-63-469-4964, E-mail: kangjoo@kunsan.ac.kr The Association of Korean Geoscience Societies and Springer 2019