Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). 1AHS Publ. no. 234, 1995. 55 Land subsidence due to groundwater withdrawal in Hanoi, Vietnam TUNG Q. NGUYEN & DONALD C. HELM Nevada Bureau of Mines and Geology, University of Nevada-Reno, Mail Stop 178, Reno, Nevada 89557-0088, USA Abstract Groundwater has been exploited since the beginning of this century to supply water for the Hanoi urban area from two porous unconfined and confined aquifers. These aquifers occur in Quaternary unconsolidated sediments and are separated by a virtually impervious clay layer. Most of the water used in Hanoi is extracted from the confined aquifer which is underlain by bedrock of Tertiary age. Due to socio-economic growth, the groundwater extraction system has been broadened from 9 to 11 water plants in order to increase the amount of water development. As a consequence of this action, water levels within the aquifers have declined to about 35 m below the initial level. This leads to land subsidence, which has been observed since 1988 by levelling. It shows that the ground surface has declined more in the centre, where water extraction wells are located, than in the surrounding adjacent areas. In some places the magnitude of subsidence reaches about 10 to 15 cm - damaging houses, schools and factories. Land subsidence due to groundwater withdrawal is a new field of research in Vietnam. Future research should clarify our understanding of land subsidence mechanisms and predict subsidence for groundwater management and city planning. INTRODUCTION Hanoi is located in the centre of Bacbo plain at an elevation of about 5 to 12 m above mean sea level. The climate is characterized by a monsoon regime, which is divided into two seasons: a dry season from November to April and a rainy season from May to October. The mean annual precipitation is about 1500 mm and is unevenly distributed through the year: 84% during the rainy season and 16% during the dry season. The seasonal precipitation water serves as a natural replenishment of the unconfined aquifer but is restricted by impervious areas such as houses and streets, as well as evaporation from the surface, which can reach 1100-1300 mm annually. The major drainage system of the area is the Red River with its distributaries. Groundwater is the only source of water supply to Hanoi City. Since 1978, the process of water exploitation has increased, especially during the last decade due to socio-economic development. Along with this, urban hazards such as land subsidence, floods, and Assuring become increasingly serious and dangerous to the city causing great damage (Tran & Nguyen, 1990).
56 Tung Q. Nguyen & Donald C. Helm GEOLOGY The Hanoi area is underlain by unconsolidated sediments of Holocene to Pleistocene age Q t _ 4 about 45 to 120 m thick that overlie a basement of Tertiary age N. The basement is composed of gravelstone, sandstone and clay stone which have not been well investigated. Quaternary unconsolidated sediments of coarse-grained size in the deeper part and fine to small-grained size in the upper part are of primary interest. The deeper part consists of Lower to Middle Pleistocene Qj_ 2 pebble, gravel, coarse sand and alternating clay lens of alluvial origin that dip and generally thicken in the south-east direction. The upper part consists of Upper Pleistocene and Holocene Q 3 _ 4 soils of marine, lacustrine and alluvial genesis: fine and small-grained sands, clayey sand, sandy clay, clay, peat and silt. Clay of Upper Pleistocene Q 3 is the principal confining bed in the Hanoi area, which in some places bares at the surface. A stratigraphie column is generalized and illustrated in the Table 1. Table 1 General stratigraphy in Hanoi area. Geologic age Cenozoic Quaternary Q 3 Holocene- 0-30 Upper Pleistocene Q 3-4 Upper Pleistocene 0-15 Middle-Lower 40-70 Pleistocene Ql-2 Approximate thickness (m) Tertiary Pliocene-Miocene > 1000 N Description Fine and small-grained sands, clayey sand, sandy clay, soft clay, peat and silt Clay Pebble, gravel, coarse-grained sand and clay lens Gravelstone, sandstone and claystone HYDROLOGY Two aquifers, leaky unconfined and confined, are classified to correspond to Q 3 _ 4 and Q!_ 2 respectively in the stratigraphie column (Fig. 1). The unconfined aquifer is supplied by rainfall and lateral recharge from Red River. Evaporation is its main way of drainage. The specific yield of wells is less than 1.0 1 s" 1 m" 1. The water is mainly used for agriculture and houses and is extracted by sparsely distributed private wells. The confined aquifer is separated from the unconfined aquifer by a virtually impervious stratum of clay. Most supplies of industrial and municipal water in the Hanoi urban area are drawn from this aquifer, which gets recharge through leakage from the above unconfined aquifer and from lateral flow of groundwater. The specific yield ranges from 5 to 15 1 s" 1 m" 1.
Land subsidence due to groundwater withdrawal in Hanoi, Vietnam 57 =-^=~ Clay Sand Sandy clay Coarse sand, gravel, pebble : =^Zs : - Silt, peat Claystone, sandstone Fig. 1 Hydrogeological cross section of the Hanoi urban area. 2 km _J GROUNDWATER EXTRACTION AND LAND SUBSIDENCE Groundwater began to be exploited in the beginning of this century by the French on a small scale. The primary rate of pumpage was about 20 000 m 3 day" 1. This rate doubled by 1954 and increased further to 164 000 m 3 day" 1 by 1978. Since 1985, the groundwater extraction system has been broadened from 9 to 11 water plants with the assistance of the Finnish Government in order to increase the amount of water extraction. It reached a rate of 300 000 m 3 day" 1 in 1988 and about 400 000 m 3 day" 1 in 1991. However, this amount satisfies only about 60% of water demand. Therefore, it will be increased in the future (Rau et al., 1991). The heavy pumping has resulted in a substantial decline of piezometric head to form a large drawdown cone beneath the Hanoi urban area. The piezometric head in deep wells initially was 2-5 m below land surface, but by 1993 it had dropped to 37-40 m (Fig. 2). Meanwhile, the water table of the Q 3 _ 4 aquifer has declined by 2-6 m. The drawdowns of piezometric head as well as of the water table have caused a significant increase in effective stress within the aquifers themselves with an accompanying decrease of pore water pressure and increase in effective stress within the confining bed. Stress transfer results from the release of stored water in highly compressible clay layers and their subsequent compaction (Helm, 1984). Therefore, the occurrence of land subsidence can not be avoided. The first evidence of subsidence was observed during the 1988-1989
58 Tung Q. Nguyen & Donald C. Helm Fig. 2 Map of the Hanoi urban area showing the piezometric head in 1993. Contour lines show the piezometric head in m below sea level, interval 5 m. A: location of water plant. levelling of land surface over the Hanoi urban area. It showed that land surface has declined more in the centre, where water extraction wells are located, than in the adjacent areas. During this period subsidence reached about 10-15 cm and has formed a subsidence bowl that coincides with the cone of depression of piezometric head (Fig. 3). Land subsidence has damaged several houses, schools and factories in the central and southern parts of Hanoi. Related to land subsidence, fissures have been found in southwest part of the city, which were never observed before. Moreover, in the centre of the land subsidence area, surface water can not be drained in a timely manner during the rainy season, which adversely influences city traffic. In summary, land subsidence has brought a great damage to the city. Because of the time lag (due to low permeability)
Land subsidence due to groundwater withdrawal in Hanoi, Vietnam 59 Fig. 3 Map of the Hanoi urban area showing the subsidence bowl. Contour lines show subsidence in cm, interval 5 m. A : location of water plant. in pressure changes progressing vertically through the clay confining bed (Q 3 ), the highly compressible confining layer is not a hundred percent consolidated. Hence, even if water levels decline no further in the two aquifers, latest subsidence can be expected to continue for years (even decades) into the future. Increased drawdown in the future within the two aquifers will only increase the subsidence further. The 10-15 cm subsidence already observed may only be a fraction of what can be expected during the coming decades if steps are not taken quickly to plan wisely for the future. CONCLUSION Groundwater extraction in the Hanoi urban area is the main cause of land subsidence. In contrast to many other places throughout the world, land subsidence in Hanoi has not
60 Tung Q. Nguyen & Donald C. Helm yet been controlled. Therefore, land subsidence investigations are urgent by needed to clarify its mechanism and predict subsidence for effective and timely groundwater management and city planning. REFERENCES Helm, D. C. (1984) Field-based computational techniques tor predicting subsidence due to fluid withdrawal. In: Man- Induced Land Subsidence (ed. by T. L. Holzer), 1-22. Reviews in Engineering Geology 6, Geological Society of America. Rau, J. L., Nutalaya, P., Tran, H. V. & Nguyen, T. Q. (1991) The problem of land subsidence in the world and Vietnam. In: Proc. Second Conference on Geology of Indochina, vol. 1, 421-428. Tran, H. V. & Nguyen, T. Q. (1990) The Alteration of Geological Environment in the Hanoi Area in the Process of Urbanization (in Vietnamese). Centre for Applied Geophysics, National Centre for Scientific Research, Hanoi, Vietnam.