Effects of the sea level rise on underground water resources in Ho Chi Minh area

Journal of Science and Technology 54 (4B) (2016) 260-269 EFFECTS OF THE SEA LEVEL RISE ON UNDERGROUND WATER RESOURCES IN HO CHI MINH AREA Ky Nguyen Viet1, Thong Ho Chi1, Oanh Tran Thi Phi1, Chan Ngo Duc2 1HCM city University of Technology, 268 Lý Thường Kiệt Street, Ward 14, Tân Bình District, Hochiminh city 2Division for Water Resources Planning and Investigation for the South of Viet Nam, Trầa Nao Street, An Phu Ward – 2nd District- Hochiminh city *Email: nvky@hcmut.edu.vn Received: 15 August 2016, Accepted for publication: 10 November 2016 ABTRACT Ho Chi Minh City has 7 aquifers with different distributions, ascending from the east, western-north to eastern-southeast with total potential reserves of about 1.65 million mP3P of fresh water/day, potential reserves of underground water brackish-salty approximately 2.25 million mP3P/day. This resource is invaluable for the development of the city today and the future. However, groundwater resources are at risk of depletion of reserves, quality under the impact of climate change and sea level rise. In this paper, the authors focus on evaluating the impact of rising sea levels to shift the boundaries of the aquifer salinity, which narrow the area of fresh water and diminishing reserves of fresh water . To assess, first based on climate change scenarios and sea level rise has been Vietnam announced in 2012, at the same time as the underground water is exploited more constant (the maximum amount of water extraction in 2015 basis), the authors conducted for running surface flow model to get the water level data at some point to put into models of groundwater flow. Thanks to model groundwater flow, the authors showed that the sea level rise significantly shift the boundaries of the aquifer salinity toward the inner city. The area contains fresh water and reduced water reserves only light compared with present reserves. Key words: underground water, impact of see level rising. 1. INTRODUCTION Underwater at Ho Chi Minh area was concerned by many researchers. In 2010, the Department of Science and Technology accepted subject “Geological and hydrogeological maps area of Ho Chi Minh City at scale 1:50,000” edited by Division for Water Resources Planning and Investigation for the South of Viet Nam. Accordingly, Ho Chi Minh city has 7 aquifers with total potential mining reserves reached 1.659.581 m3 fresh water per day, the total potential Effects of the sea level rise on underground water resources in Hochiminh area 261 reserve of salty 2,219,038 m3 per day during the dry season. This can be considered as the most complete study of the groundwater here. Regarding climate change and sea level rise, Ho Chi Minh City area is considered one of the ten cities most affected of this phenomenon. Significantly, those of climate change and sea level rise scenarios for Vietnam’s Ministry of Natural Resources and Environment 2012 including Hochiminh city. As most extreme sea level rise scenario, Ho Chi Minh city can loose more than 400 km2 area in the low area of Ho Chi Minh city as its location is downstream of the Saigon Dong Nai river system and under the hydropower cascade upstream, with relatively low topography compared to sea level, so very vulnerable to adverse change of climate change status such as flooding, salinization, shortage of running water and production activities, epidemic diseases.. The research on climate change and sea level rise also mention to the study of Le Manh Hung [1]. In these study, the authors showed signs of climate change such as rising temperatures, rainfall, extreme droughts, sea level rise, salinization etc in Ho Chi Minh City area and their impact to some form of disaster. However, the author only interested in the salinization of surface water. Furthermore, there have been some studies of climate change and sea level rise impacts on geological conditions - hydrogeology of Division for Geological Mapping for the South of Viet Nam and Division for Water Resources Planning and Investigation for the South of Viet Nam. In our study Ngo Duc Chan, Bui Tran Vuong also assessed the shift of the boundaries of the aquifer salinity between parallel aquifer not due to climate change and sea level rise, but due to the increased exploitation of underground water [2]. The team also ran models of groundwater flow flowing under to all the aquifer with climate change scenarios, such as temperature change, evaporation, precipitation to evaluate the change of water level of the aquifers. At the same time, on the basis of observational data of 400 wells in the system of national monitoring wells, monitoring wells of the city by the Department of Environmental Resources Management and exploitation wells by the Center for Domestic Water and Sanitation rural management from 2000 to 2011, the team also assessed shift of salinity boundaries (1 g/l) of the aquifers; thence, the team evaluated the shrinking or dilating of the container fresh water in each aquifer. Accordingly, within 10 years, area of fresh water at Pliocene aquifer shrink about 7 – 9 km2; it means saltwater area dilate about 7 – 9 km2. These studies did not consider rising sea level [3]. 2. RESEARCH METHODS Structurally, aquifers are divided into three regions: recharge zone, movement zone, discharge zone. The aquifers in Ho Chi Minh City has recharge zone (to coincide with movement zone) or in areas such as Cu Chi district – HCM city and Southeastern provinces. The inner city and the city's southeastern region mostly are movement zone and exit to the sea. With such structures, many factors affect recharge zone, movement zone and discharge zone of underground water such as climate (rainfall, evaporation); fragmented terrain and the terrain, elevation correlation between recharge zone and discharge zone, new tectonic activities ... However, in this paper, we are interested first of all to the change of precipitation due to climate change – an element involved in the shift of salinity boundaries of underwater. The second element is emphasized that sea level rise - a factor high-levelled correlated changes between recharge and discharge, thereby changing the flow velocity, it is the consequence of accelerating the process of salinisation into the aquifer. Ky Nguyen Viet, et al 262 With such views, the team has used the radical change scenarios, which means the highest emission scenario of rainfall change and sea level rise. The following provides some rainfall monitoring data at Tan Son Hoa station and change scenarios for the highest rainfall areas of Ho Chi Minh city (Figure 1, Table 1). Figure 1. The average monthly rainfall for many years in Tan Son Hoa station. Table 1. Climate change scenarios following rainfall. From these scenarios, the researchers will calculate the amount of rain for the area in Ho Chi Minh City under timelines as in Table 1. Sea level rise used according to Vung Tau station is shown in Figure 2. Climate change sea level rise scenarios shows at Table 2 and sea level rise calculated under A1FI scenario in Can Gio (Figure 3). Figure 2. Observational Document at Vung Tau station. Eff sys [4] riv sea par ch cal do ects of the s Figu To review tem, the res to conduct er system (F son from 15 ticipation o angesof the culation. Th cument at V ea level rise Figure re 4. Location and assess earch team a running o igure 4) for to 30 Septe f these reg East Sea. 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T roundwater. sed the obse to Ho Chi M an Gio sea l ear 2100 in H el rise to the Ky Nguyen added an a ng to data N entation at for the per ext year) ra tion is alwa ground wat n, some of d to compl e much mor rp in the en he increase These data rvation data inh City a evel is show o Chi Minh c year 2100. Viet, et al dditional BD and Tan Son iod from infall for ys much er, some the water ement of e rainfall d of the in such will also in Vung nd adjust ed in the ity. Effects of the sea level rise on underground water resources in Hochiminh area 265 Through Figure 5 we see sea level during the rainy season (May, 6, 7 and 8) seems to be lower than in the dry season. The water level in Can Gio rises over 1.0m in many months of the year compared with the current. This will change the relationship between recharge zone and discharge zone, thereby changing the flow speed, creating conditions for moving saltwater toward recharge zone. These data will be input for the model of groundwater flow. With a view approaching the problem as showed in the research methods part, using the model of groundwater flow from the preventing flooding project of Ho Chi Minh City [1], entering the data on the additional level from rainfall and water level data on rivers’ border under climate change scenarios and sea level rise, the team solve the problem by MT3DMs model for all aquifers in order to determine the saline boundary shift (1g / l ). In the framework of this paper, the research team presented only saline boundary at 3 moments: in 2020, 2050 and 2100 for the Pleistocene aquifer (qp3, qp2-3 and qp1) with tallest emission scenarios. These saline boundary reflect the impact of two factors: rainfall (recharge amount) changes and sea level rise. Also in this model, it is assumed that the groundwater mined is unchanged. Results showed that, qp3 aquifer - the shallowest aquifer lies under the impact of the two factors mentioned above most. Saline boundary encroach upon many mainland locations nearly 600m in 2100. However, there are also several positions, saline boundary ( 1g / l ) barely shifted against the current (Figures 7a, 7b and 7c). Figure 7a. Saline boundary of upper Pleistocene aquifer in 2100. Figure 7b. Saline boundary of upper Pleistocene aquifer in 2100.2050. Ky Nguyen Viet, et al 266 Figure 7c. Saline boundary of upper Pleistocene aquifer in 2100. Figure 8a. Saline boundary of Middleupper Pleistocene aquifer in 2020. Figure 8b. Saline boundary of Middle-upper Pleistocene aquifer in 2050. Effects of the sea level rise on underground water resources in Hochiminh area 267 Figure 8c. Saline boundary of Middle-upper Pleistocene aquifer in 2100. Pleistocene aquifer middle– upper (qp2-3) distribute beneath qp3 aquifer, many places have tight relations withhydraulic upper Pleistocene aquifer (qp3); therefore current saline boundary is also close to the saline boundary of qp3 aquifer though it is more into inland. This is also the current aquifer exploited by people because it is shallow, water get quality standard, and it is easy to exploit by haft- industrial wells. This aquifer is also has many saline boundary encroach deeply into the mainland, in some areas such as Binh Thanh, Binh Chanh, saline boundary shift reached nearly 1200 m (Figures 8a, 8b and 8c). Similarly to upper Pleistocene aquifer, in some places like South Cu Chi, Thu Duc, District 9 ... saline boundary nearly do not move. Saline boundary shift amplitude is not only middle-upper Pleistocene aquifer but also for all the Pleistocene aquifer- except sea level rise and climate change- could depend heavily on current exploiting flow. In this aquifer has more than 78,800 wells with exploitation flow about 278,000 m3/ngđ. Lower Pleistocene aquifer (qp1) distribute beneath qp2-3 aquifer, many places have tight relations with qp2-3 aquifer; therefore, saline boundary shape and distribution position is also nearly similar to the Pleistocene aquifer lying above. The amplitude of the saline boundary is quite high. Binh Thanh, District 4, 7, 6, 8 and a part of Binh Chanh, saline boundary can moved nearly to 1200m deep into the city. Similar to the aquifers located above, in multiple locations, saline boundary almost do not move because of main reasons of exploitation (Figures 9a, 9b and 9c). Figure 9a. Saline boundary of lower Pleistocene aquifer in 2020. Ky Nguyen Viet, et al 268 Figure 9b. Saline boundary of lower Pleistocene aquifer in 2050. Figure 9c. Saline boundary of lower Pleistocene aquifer in 2100. 4. CONCLUSION Climate change and sea level rise affect on HCM city area, especially hardly impact on the groundwater here. With a view: precipitation changes lead to change recharge amount; sea level rise is changing hydraulic slope of the flow - is element shifting saline boundary (1g/l) of the aquifers, the team has calculated rainfall in HCM city and sea level rise at Can Gio with climate change and sea level rise scenarios was published by the Ministry of Natural Resources and Environment in 2012. These data are input of groundwater flow models as well as solve the problem by shifting saline boundary MT3DMs model. To operate the model, the team accepted an supposition that the amount of groundwater exploitation does not change during calculating time( to the year 2100). The result shows that the Pleistocene aquifer was affected most, saline Effects of the sea level rise on underground water resources in Hochiminh area 269 boundary shifts from 600m to 1200m, an area of fresh water lost about 18km2 per Pleistocene aquifer. However, in many saline boundary position almost less change. This shows that, except the impact of the recharge amount and sea level rise, groundwater is mined from time to time also have a particular impact. The Pliocene aquifers are distributed deeply, so the impact of climate change and sea level rise are insignificant, the result of model running shows that saline boundary nearly does not move compared to the present time. In another word, this current movement is due to water exploiting in these aquifer Acknowlegement: The authors would like to say thank to University of Technology, Vietnam National University HCMC that supports finance to implement the project: "Studying the effects of sea level rise to groundwater resource of Ho Chi Minh City area ", MSDT: C2015-20-29 / VNU type C. REFERENCES 1. Le Manh Hung - The impact of climate change to natural disasters and measures to respond to the Ho Chi Minh City, WebSite of steering action programs CCA ARD sector, Tác động của biến đổi khí hậu /item/745/ Tác động của biến đổi khí hậu đến thiên tại và giải pháp ứng phó cho thành phố Hồ Chí Minh. 2. Ho Chi Thong, Dau Van Ngo and Nguyen Viet Ky - Study the impact of climate change and sea level rise on the saline boundary of groundwater aquifers - the case of Ho Chi Minh City, the International Conference " geological works and the Environment ", Hue, 2012. 3. Ngo Duc Chan - Report assessing recharge groundwater after building a network of regulation lakes, Anti-flooding Project, Ho Chi Minh City, 2013. 4. Giang Le Song - Building Computational models for integrated urban drainage calculations, Report on the Results of The Summary of Technological Themes available and HCM city, Vietnam National University, November, 2011.