EFFECTS OF THE SEA LEVEL RISE ON UNDERGROUND WATER RESOURCES IN HO CHI MINH AREA

Transcription

1 Journal of Science and Technology 54 (4B) (2016) EFFECTS OF THE SEA LEVEL RISE ON UNDERGROUND WATER RESOURCES IN HO CHI MINH AREA Ky Nguyen Viet 1, Thong Ho Chi 1, Oanh Tran Thi Phi 1, Chan Ngo Duc 2 1 HCM city University of Technology, 268 Lý Thường Kiệt Street, Ward 14, Tân Bình District, Hochiminh city 2 Division for Water Resources Planning and Investigation for the South of Viet Nam, Trầa Nao Street, An Phu Ward 2 nd District- Hochiminh city * 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 m 3 fresh water per day, the total potential

2 Effects of the sea level rise on underground water resources in Hochiminh area reserve of salty 2,219,038 m 3 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 km 2 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 km 2 ; it means saltwater area dilate about 7 9 km 2. 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. 261

3 Ky Nguyen Viet, et al 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. 262

4 Effects of the sea level rise on underground water resources in Hochiminh area Table 2. Seaa level rise under scenarios of o Southern. Figuree 3. Sea level rise calculated under A1FI scenario in Can Gio Figure 4. Location maps of 22 section of model accordingg to the sea level rise scenarios. To review and assess the impact of sea level rise on water levels of Saigon - Dong Nai river system, the research team used the F28 software written and developed d byy Dr. Le Song Giang [4] to conduct a running of model scenarios based on sea level rise in the 22 sections on this river system (Figure 4) for two seasons: the dry season - from 15 to 30 March 2010 and the rainy season from 15 to 30 September 2010 the time is 01 hours. In the calculation model we consider participation of these regulates of irrigation lakes at upstream and thee characteristic tidal changesof the East Sea. Results showed that water levels on o the section is at the time of calculation. These results will be integrated with the sea level rise scenario, observational document at Vung Tau stations to calculate Can Gio sea level. 263

5 Ky Nguyen Viet, et al The model of groundwater flow was built earlier by GMSS software iss added an additional quantity of data on levels and boundary data on river water levels according to data NBD and running models for different times of the 21st century. 3. RESULTS AND DISCUSSION As mentioned above,, based on climate change scenarioss and documentation at Tan Son Hoa station, the research team reconstructed rainfall figures for each monthh for the period from the present to 2100 for Ho Chi Minh (Figure 5). The chart in Figure 5, we see, in the dry season (December to April next year) rainfall for almost a century, less variable, the ratio between precipitation and evaporation is always much less than 1. Thus, these months havee no recharge rain water for underground water, some aquifers have water beneath 3m will bee affected tooo much by the evaporation, some of the water tank as Dau Tieng and Tri An lake with water levels stronglyy decrease led to complement of groundwater levels decline significantly. In the rainy season, the closer it iss in the end of years of the century, the much more rainfall increase, except May. We can see thee cylindricall top of the chart is sharp in the end of the century and the average rainfall increase from 20 to 50mm per p month. The increasee in such monthly rainfall also greatly affect thee recharge water to the groundwater. g. These data will also be used as input data for groundwater flow model. For sea level rise, as mentioned above, the research team used u the observation dataa in Vung Tau stations combined with sea level rise scenarios to convert to Ho Chi Minh City and adjust according to the script as F28 model. The results obtained in Can C Gio sea level is showed in the Figure 6. Figure 5. Rainfall under climate change scenarios to the year y 2100 in Ho Chi Minh city. c Figure 6. Figures of Cann Gio sea levell under sea level rise to the year

6 Effects of the sea level rise on underground water resources in Hochiminh area 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 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 Figure 7b. Saline boundary of upper Pleistocene aquifer in

7 Ky Nguyen Viet, et al Figure 7c. Saline boundary of upper Pleistocene aquifer in Figure 8a. Saline boundary of Middleupper Pleistocene aquifer in Figure 8b. Saline boundary of Middle-upper Pleistocene aquifer in

8 Effects of the sea level rise on underground water resources in Hochiminh area Figure 8c. Saline boundary of Middle-upper Pleistocene aquifer in 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 m 3 /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

9 Ky Nguyen Viet, et al Figure 9b. Saline boundary of lower Pleistocene aquifer in Figure 9c. Saline boundary of lower Pleistocene aquifer in 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 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 268

10 Effects of the sea level rise on underground water resources in Hochiminh area boundary shifts from 600m to 1200m, an area of fresh water lost about 18km 2 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: C / 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, Ngo Duc Chan - Report assessing recharge groundwater after building a network of regulation lakes, Anti-flooding Project, Ho Chi Minh City, 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,