Groundwater abstraction-induced land subsidence prediction: Bangkok and Jakarta case studies

Transcription

1 Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). IAHS Publ. no. 234, Groundwater abstraction-induced land subsidence prediction: Bangkok and Jakarta case studies RAYMOND N. YONG, EDUARDO TURCOTT Geotechnical Research Centre, McGill University, Montreal, Québec, Canada H3A 2K6 HARM MAATHUIS Groundwater Research Division, Saskatchewan Research Council, Saskatoon, Saskatchewan, Canada S7N 2X8 Abstract Studies conducted in Bangkok and Jakarta show the problems of predicting subsidence with a model developed for one region, for application to another region, without proper recognition of the differences in the geologic and hydrogeologic settings. The subsidence model developed for the Quaternary deposit underlying Bangkok accounts for the alternating layers of incompressible aquifers and compressible aquitards, i.e. measured subsidence results from subsidence occurring only in the aquitards. The model has been successful in correlating subsidence predictions with actual observations. Direct application of the Bangkok model to the Jakarta setting is inappropriate, principally because the Quaternary deposits underlying Jakarta are characterized by a complex sequence of marine and non-marine deposits. Individual aquifers and aquitards can be only be traced over very short distances. The Jakarta model considers subsidence as the result of the compression of the total complex substrate. INTRODUCTION In the evaluation and prediction of land subsidence resulting directly from abstraction of groundwater via pumping wells, some of the more important factors and pieces of information required include: (a) the geologic and hydrogeologic settings; (b) the yield, storage and recharge information and conditions associated with the water bearing substrate; (c) the various parameters and boundary conditions associated with the abstraction wells. Analytical/mathematical models can be successfully implemented if the physical situation is properly modelled, i.e. if the model accurately reflects the physical setting and boundary conditions. This study considers two major coastal cities in southeast Asia (Bangkok and Jakarta) where ground subsidence has occurred as a result of the phenomenon described above. Comparison of data for the two cities show common features concerning urbanization and metropolitan growth, well pumping and usage, distribution and control of well pumping. However the geologic and hydrogeological settings for both cities are

2 90 Raymond N. Yong et al. significantly different. Unless this is properly recognized, subsidence prediction models developed from one particular hydrogeologic setting for application to another completely different setting can lead to considerable error. In this study, the model developed for the analysis and prediction of land subsidence based upon the geologic and hydrogeologic settings for Bangkok is discussed in light of the different settings for Jakarta, and the requirements for model refinement or redevelopment are examined as necessary inputs for development of the more appropriate model for the Jakarta setting. DATA Bangkok and Jakarta geologic and hydrogeologic settings The Lower Central Plain (lower Chao Phraya basin), upon which the metropolis of Bangkok is founded, extends about 200 km northward from the Gulf of Thailand. The exact configuration of the basin floor is not well known, but is generally understood to consist of quartzite, gneiss, and granitic gneiss. The fault block tectonics which formed this basin during Tertiary time is apparently filled with clastic sediments from the Tertiary to Quaternary age, with depositional environments considered to be river plain and deltas. Occasional incursions of thin marine clay lenses have been noted in the sediments which show a total thickness ranging from about 400 m in the northern portion to more than 1800 m in the south. The geologic setting portrays a lithology which consists of thick sand-gravel layers separated by less thick clay layers. Hydrogeologists have noted nine individual and distinct aquifers consisting of sand and gravel beds, with strata of clay separating the aquifers as shown in Table 1. The aquifers are very permeable, and can yield water at the rate of 100 to 300 m 3 h" 1. The specific capacity of wells in the various aquifers is similar, and is in the range of 15 to 40 m 3 h' 1 m" 1 drawdown, depending on well design and construction. The transmissibility of the upper three aquifers ranges from 1500 to 3600 m 2 day" 1, and the storage coefficient of the aquifers is about 10" 4 for all water bearing formations, with an exceptionally high value of 3 X 10" 3 for the third aquifer. Lithological correlations based on existing well log data indicate the possibility of interconnection of the aquifers. There is considerable evidence that the clay layers are distributed in a random manner and that while many clay layers are pinched over a considerable lateral distance, the third, fourth and fifth aquifers are interconnected (Metcalf & Eddy, 1977). The geological and hydrogeological setting of the Jakarta basin consists of a 200 to 300 m thick sequence of Quaternary deposits which overlies Tertiary sediments. The top is considered to be the base of the groundwater basin. Although no formal stratigraphical framework exists, the Quaternary sequence can be subdivided into three major units, in ascending order (after JWRMS, 1994a): a sequence of Pleistocene marine and nonmarine sediments, a Late Pleistocene volcanic fan deposit, and Holocene marine and floodplain deposits. The Pleistocene marine and non-marine sediments were deposited in near-shore and deltaic environments. Dispersed between these sediments are deposits of volcanic origin. The unit is mainly composed of silts and clays, with thin interbeds of silty/clayey sands. According to Soefner et al. (1986), borehole logs indicate that total thickness of sandy layers constitute only 20 to 25 % of the entire thickness of the unit. Individual sandy layers are typically less than 5 m thick, and are composed of fine-

3 Groundwater abstraction-induced land subsidence prediction 91 Table 1 Aquifer description (after Metcalf & Eddy Inc., 1977). Aquifer Bangkok (upper), 30 m Bangkok (lower), 50 m Phra Pradaeng (100 m zone) Nakhon Luang (150 m zone) Nonthaburi (200 m zone) Sam Khok (250 m zone) Phaya Thai (350 m zone) Thon Buri (400 m zone) Pak Nam (550 m zone) Depth to top of aquifer (m) Total thickness (m) 20 to to to to to to to to < 1 to 30 < 1 to 50 < 1 to 70 <5to70 <5 to to to to Description Fine to coarse sand with gravel. Directly underlies Bangkok in most places. Aquifer missing in some areas or too fine-grained to be a source of supply. Predominantly fine to coarse sand with gravel and clay layers. In many places directly interconnected with 30 m aquifer. Fine to coarse white sand with gravel and clay layers. In some places directly interconnected with the 50 m aquifer. Occasionally missing in eastern and western parts of the basin. Fine to coarse sand and gravel interbedded with clay layers which are locally extensive. Individual sand layers up to 30 m thick. Fine to coarse sand and gravel layers interbedded with clay and silt. Sand and gravel layers interbedded with clay. Sand and gravel layers interbedded with clay. Sand and gravel layers interbedded with clay. Aquifer section may contain several distinct water bearing zones. Variable thick layers of sand and gravel interbedded with clay. Individual sand layers as little as 5 m thick. grained, silty sands. Because of the depositional environment, the lack of a stratigraphical framework and the often poor quality of the description of sediments by drillers, individual sand and silt/clay layers can only be traced over short distances (e.g. JWRMS, 1994a). The Late Pleistocene volcanic fan deposit was identified as a separate geological unit by JWRMS (1994a). However, its continuity beneath the coastal area of Jakarta remains a matter of speculation. The base of this unit is about 40 m below ground surface. The Holocene sediments are composed of marine silts and clays, overlain by floodplain deposits. Sands are encountered in channel deposits and in beach ridges. The sediments are between 10 and 20 m thick. The hydrogeological setting proposed by Soekardi (1982) and ILN (1987) consists of a six-layer model for the Quaternary deposits: an upper aquifer (0 to 40 m), an aquifer-aquitard system with the aquifer bounded at the top and bottom by an aquitard (40 to 150 m) and an aquifer-aquitard system (150 to 250 m). Soefner et al. (1986), JWRMS (1994b), Maathuis & Yong (1994) consider the Pleistocene sediments underlying the volcanic fan deposits as a single hydrogeological unit. At a regional scale, this unit can be described as a complex, but homogeneous, aquifer-aquitard system, with significant anisotropy. Although there is no physical basis, for practical reasons, Soefner et al. (1986), JWRMS (1994b), Maathuis & Yong (1994) subdivide

4 92 Raymond N. Yong et al. this hydrogeological unit into aquifer "zones/horizons" similar to those used by Soekardi (1982) and ILN (1987). Despite the fact that individual sand layers cannot be traced, lateral and perhaps vertical continuity must exist as there are no reports of wells going dry. The Late Pleistocene volcanic fan deposits can be considered as an individual aquifer, but its extent and continuity beneath the northern part of the basin remains to be confirmed. The Holocene deposits form an aquitard. In contrast to the Bangkok area, the transmissivities in the Jakarta area are much smaller. Soefner et al. (1987) estimate that the transmissivity of the entire Quaternary sequence ranges from about 250 m 2 day 4 in the coastal area of Jakarta to 500 m 2 day" 1 20 km from the shore. Based on a numerical model, JWRMS (1994b) suggests that these transmissivities may be up to a factor of two too high. The horizontal hydraulic conductivity of the sands typically is in the order of 1.5 m day" 1, and is seldom larger than 10 m day" 1. Values for the vertical hydraulic conductivity of clay/silt units are typically about 8.5 x 10" 5 m day" 1. Review of numerous results of geotechnical tests on core samples from depths less than 70 m indicate an average vertical hydraulic conductivity at reconsolidation pressure of about 1.3 x 10" 4 m day" 1 (Maathuis & Yong, 1994). No data are available on the vertical hydraulic conductivity of clay/silt layers at depth of more than 70 m. Values for the storativity of sands layers and the elastic storage coefficient of clay layers are also taken from the literature. Aquifer storativity values used for the Jakarta area range from 10" 3 to 10" 6, and those for the elastic storage coefficient of clay layers between 10" 2 and 10" 3 m" 1. Subsidence and groundwater abstraction The report by Cox (1968) provides an early intimation of the problem of subsidence in the Bangkok region resulting from groundwater abstraction. Quantitative assessment of land subsidence however was not conducted until early in From mid-1978 until 1982, the Royal Thai Survey Department (RTSD) conducted seven runs of first order surface levelling at half-year intervals to monitor benchmarks. These results, together with measurements undertaken by the Asian Institute of Technology at 27 observation stations in 1978 and at four additional stations in 1981 and onward, provide useful information regarding subsidence in the Bangkok area (AIT, 1982). The greatest amount of groundwater abstraction in the Bangkok metropolitan region was found to be about 1.39 Mm 3 day" 1 in The amount of abstraction in 1988 was estimated at about 1.2 Mm 3 day" 1 with about 1 Mm 3 day" 1 being withdrawn by "private" wells. The situation is somewhat similar in Jakarta where about 30% of the population of Jakarta is connected to the water supply distribution system, and groundwater is a major source of water supply for domestic, industrial and commercial purposes. Since the early 1900s groundwater has been used as a water supply source, but a dramatic increase in groundwater withdrawals occurred since the early 1970s, as a result of the rapid urbanization and industrialization of the Jakarta area. In turn, the withdrawals resulted in the creation of significant drawdown cones in the hydrogeological unit deeper than 40 m as the hydraulic heads in this zone declined by as much as 40 to 60 m. This changed the basin recharge-discharge pattern in that the coastal region of Jakarta, originally a discharge area, became a recharge area. The large drawdowns raised concerns regarding seawater intrusion and land subsidence. In contrast to reported seawater intrusions extending 10 to 15 km from the shoreline (e.g Soefner et al., 1986;

5 Groundwater abstraction-induced land subsidence prediction 93 Tjahjadi, 1991), recent studies suggest that seawater intrusion may only be a minor problem, if occurring at all (JWRMS, 1994c; Maathuis & Yong, 1994). Relative benchmark surveys conducted between 1974/1978 and 1989/1990 indicate subsidence in the order of 50 cm over large areas of northern Jakarta, and locally up to 100 cm. Reliable data on subsidence are currently not available. Relating the subsidence to groundwater withdrawals from depth greater than 40 m is in particular hampered by the lack of geotechnical/hydrogeological data below this depth, and by uncertainties in the distribution of wells and volumes withdrawn. Information on the distribution of and production from wells is only available for the period Deriving an idea of the spatial distribution of withdrawals is difficult as, for example, for 35% of the registered wells (i.e non domestic wells) in 1990 no completion data were reported. Furthermore, survey data reported by JWRMS (1994a) suggest that there are more unregistered wells than registered and that volumes withdrawn are higher than the reported data. Groundwater-abstraction and land subsidence modelling The two different geologic settings as typified by Bangkok and Jakarta are summarized as follows: - For Bangkok, one obtains a multiple aquifer-aquitard system which is somewhat well ordered. The aquifers are relatively incompressible, whereas the aquitards are compressible, i.e. can consolidate as a result of depletion of the water content in the aquitards. - The geologic setting for Jakarta is seen to be comprised of a complex mixture of aquifers with intercalated clay lenses. The water bearing strata cannot be readily demarcated into distinct aquifers, and the assumption that the entire substrate is water bearing requires a judicious evaluation of the various transmissibilitycompression coefficients. The multiple aquifer-aquitard subsidence physical model (i.e. "Bangkok model") shown in Fig. 1 provides the basis for the development of the analytical procedures for the Bangkok setting. The basic idea in the development of the model is similar to that of boundary integral techniques, i.e. one assumes that the characteristics of the hydraulic head are directly related to the abstraction or recharge of the aquifer, and that subsidence occurs principally because of the compressibility (consolidation) of the aquitards. The technique is one of boundary analysis as opposed to domain analysis. The details of the analytical model have been reported previously (Yong et al, 1989) and will not be repeated herein. In testing or applying the Bangkok model, unless one is cognizant of the particular incompressible aquifer-compressible aquitard nature of the problem, there exists the possibility that consolidation-subsidence calculations will be carried over the entire aquifer-aquitard sequences. This leads to considerable error, i.e. a magnified subsidence value is generally obtained. The three-step sequence shown in Fig. 1, in a simplistic sense, includes: - Withdrawal of water from the aquifer from the pumping well which creates a drawdown cone. - Drawdown in the aquifer which creates a negative pore pressure gradient in the overlying aquitard (Fig. 2).

6 94 Raymond N. Yong et al. Pumping well for groundwater withdrawal 3. Pore water from aquitard moves into aquifer due to hydraulic head drop in aquifer, resulting in consolidation of aquitard Fig. 1 Schematic drawing showing developed hydraulic heads in aquifer and aquitard. Radial coordinate system with r = distance trom well casing, t = time and z = depth h c (r,2,t) = hydraulic head in clay aquitard So(t) h a (r,t) = hydraulic head in aquifer s,(t) Fig. 2 Characteristics used for modelling "compressible" aquitard and incompressible aquifer. - Downward movement of pore water in the aquitard into the aquifer in response to the gradient, creating thereby a consolidation effect. The characteristic pressure heads in the aquifer h a and aquitard h c developed as a result of abstraction in the aquifer (Fig. 2) are given as:

7 Groundwater abstraction-induced land subsidence prediction 95 h a (r,t) = -Sfi) h c (r,z,t) = -Stf) 1 i r W) S 2 2(t) 1-2- s 2(0 s 2 2 (0 ' 1-2- S3 (0 S 2 (0 (1) and can be established by determining the appropriate S n (t) conditions that would satisfy the initial and boundary conditions. Note that for r > S 2 (t), h a becomes zero. Similarly, for z > S 3 (t), h c becomes zero. The solution of these relationships and method of application have been given by Yong et al. (1989). The underlying geology and hydrogeological setting for Jakarta differs markedly from that of Bangkok. Since there are no clear demarcations separating aquifers from aquitards, the multiple aquifer-aquitard Bangkok model is not appropriate. Considering the Jakarta underlying substrate to be entirely water bearing, the principal assumptions invoked for development of the governing relationships and solution for the Jakarta geological and hydrogeological settings (identified as the Jakarta model) include: flow in the water bearing substrate obeys Darcy's law; the confined water bearing substrate is visco-elastic, "homogeneous" and of approximate constant thickness; the storativity S is constant; the amount of water derived from storage due to an increment of drawdown As during an interval of time from T to r + AT consists of two parts; namely a volume of water instantaneously released from storage and a delayed yield from storage at any time t > r from the onset of pumping; a constant rate pumped from the fully penetrating wells. The governing relationship are developed in accord with visco-elastic theory and are shown in a polar coordinate format for a multiple-well pumping case as: d 2 s Ids W + dr 2 r dr Y f(r) = S* ^+I^lf,(r)e X p[jl^ ]dr (2) dt 7jk dt I a 2 ii where the output of the groundwater from the aquifer per unit area with a circular well field is given as W = T-QIA, (Q = flow rate, A = area) and where s = drawdown, r = radial coordinate, R = radius of the circular well field, S* = My w (^l3 + a{), -q = apparent viscosity of the "visco-elastic" material representation of the water bearing layer, a x and a 2 are the parameters that represent primary and secondary consolidation of the layer, k is the coefficient of permeability, and f(r) = 1 for 0 < r < R, and f(r) = 0 for r > R. The solution to equation (2) for the standard boundary conditions of zero drawdown at time equals zero and at infinite distance from the drawdown source, and drawdown gradients of zero t infinite r and r = 0, combines the Laplace transform, the convolution theorem and the Hankel transform to give the following: WR JJottRWl-r)^ (3) r+l/2? Equation (3) is not analytically integrable, but can be solved numerically (Yong et al., 1994).

8 96 Raymond N. Yong et al, DISCUSSION AND CONCLUDING REMARKS This simplistic aquifer-aquitard Bangkok model has been tested using input obtained through physical testing of the consolidation-compression of representative cores from the aquifers and aquitards. Reasonable correlations between "predicted" and measured subsidence have been obtained over a period of years as shown by Yong et al. (1991). Application of the Bangkok subsidence prediction model to the Jakarta setting is difficult, not only because of its more complex hydrogeological setting, but also because of the lack of reliable geotechnical data for depths greater than 40 m, uncertainty in both the distribution and amounts of withdrawals, and absence of reliable subsidence measurements. To test the capability of the Jakarta model, field results from Bangkok were used. The predictions obtained by the Jakarta model have been reported in Yong et al. (1994). They are seen to be less well correlated with the field data - in comparison to those obtained by the Bangkok model. This is not surprising since the Bangkok model deals with the resulting consolidation of the aquitard in relation to the hydraulic head drop in the aquifer - as opposed to the Jakarta model which considers the basic setting as an "average" compressible layer with water bearing characteristics. Comparison with measurements obtained from the well withdrawal studies are presently underway and will be reported at a later time. Acknowledgements The financial support for the study was provided by the International Development Research Centre (IDRC) of Canada. The authors are indebted to (a) Dr Aung Gyi of IDRC for his invaluable support and guidance, and (b) the research partners in both Bangkok and Jakarta for their input and assistance in the course of the studies. REFERENCES Asian Institute of Technology (AIT) (1982) Groundwater resources in Bangkok area: development and management study. Comprehensive Report , submitted to the Office of the National Environment Board. Cox, J. B. (1968) A review of engineering properties of the recent marine clays in Southeast Asia. Research Report no. 6, Asian Institute of Technology, Bangkok. ILN (Indec & Associates Ltd, Lavalin International Inc. & Nippon Koei Co. Ltd, Ministry of Public Works, Directorate General of Water Resources Development, Indonesia) (1987) Cisadane River basin feasibility study. Groundwater?/. JWRMS (1994a) Jabotabek Water Resources Management Study. Final Report, vol. 6, annex 10: Groundwater Resources. Ministry of Public Works, Directorate of Water Resources Development, Indonesia. JWRMS (1994b) Jabotabek Water Resources Management Study. Final Report, vol. 6, annex 11: Groundwater Models. Ministry of Public Works, Directorate of Water Resources Management, Indonesia. JWRMS (1994c) Jabotabek Water Resources Management Study. Final Report, vol. 7, annex 12: Groundwater Salinity. Ministry of Public Works, Directorate of Water Resources Development, Indonesia. Maathuis,H. & Yong, R. N. (1994) Development of groundwater management strategies in the coastal region of Jakarta, Indonesia. Year II Report (1993). Saskatchewan Research Council, Saskatoon, Canada. Metcalf & Eddy Inc. (1977) Report on groundwater monitoring, well constructionand future programs for MWWA. Report for Metropolitan Water Works Authority, Bangkok. Soefner, B., Hobler, M. & Schmidt, G. (1986) Jakarta groundwater study. Final report. Federal Institute of Geosciences and Natural Resources, Hannover & Directorate of Environmental Geology, Bandung. Soekardi, R. (1982) Aspek geologi terhadap perkembangan pantai dan tata airtanah daerah Jakarta. Sarjana-Thesis, Universitas Padjadjaran, Bandung. Tjahjadi, B. (1991) The Impact of Abstraction on Groundwater Quality and Monitoring in the Jakarta Region, Indonesia. Directorate of Environmental Geology, Bandung.

9 Groundwater abstraction-induced land subsidence prediction 97 Yong, R. N., Xu, D. M. & Mohamed, A. M. 0. (1989) Groundwater resource management model; Final report. Report for International Development Research Centre, Canada. Yong, R. N., Nutalaya.P., Mohamed, A. M. O. &Xu, D. M. (1991) Land subsidenceand flooding in Bangkok. In: Land Subsidence (ed. by A. I. Johnson) (Proc. Fourth Int. Symp. on Land Subsidence, Houston, May 1991), IAHS Publ. no Yong, R. N., Turcott, E. & Gu, D. (1994) Artificial recharge subsidence control, Bangkok, Thailand. Report for International Development Research Centre, Canada.