Abstract. Introduction

Transcription

1 HYDROGEOLOGCAL NVESTGATONS N THE GREATER JAKARTA AREA/NDONESA Djaeni, A. 1 >; Hohler, M. 2 ); Schmidt, G.2); Soekardi, P.1); Soefner, B.2) 1) Direktorat Geologi Tata Lingkungan, Jl. Diponegoro 57, Bandung/ndonesia. 2) Bundesanstalt fuer Geowissenschaften und Rohstoffe. Stilleweg 2, Hannover/ W.-Germany Abstract The paper summarizes results of a study of the groundwater situation in the surroundings of the ndonesian capital, Jakarta. The deep aquifer system is heavily overexploited and the groundwater head has dropped from 5-15 m above sea level in 1900 to m below sea level in some areas in Land subsidence and saltwater encroachment are the consequences of changes in the groundwater head. The hydrogeology, the hydraulic GOnditions, and the hydrochemistry of the groundwater system are described and a three-dimensional groundwater model is used in order to simulate possible future groundwater abstraction patterns. ntroduction The groundwater situation in the area surrounding Jakarta was investigated between 1983 and 1985 within the scope of an ndonesian/federal German long-term technical cooperation programme aimed at the upgrading of technical skills in sciences related to groundwater, for the staff of the Directorate of Environmental Geology, Bandung. The Jakarta area was regarded as a model for complex groundwater investigations in urban areas where conservation measures are already necessary due to the overexploitation of the aquifer system. 165

2 2 Historical development of groundwater abstraction Jakarta's population of about 7.5 mio in 1985 is expected to increase to more than 12 mio in 2005 (Figure 1). r ~ t 'redlcted t 4 f-16 Groundwater Sludy Jakarta f- ~ Population in DK Jakarta, f-14 Qrowth rat o f-13 r-12 Growth rate b ~ i-ll : ~ ~:: :: --= Jakarta moater plan c b :fj 9 i 2 - f/_ ~ 8 0 t9tt eo o ~ 4 3 f] 2 Al1l o 90 19oo o 40 so eo 9o 2ooo: Groundwater Abstraction [ 10 6 m'lyear ]'"'" / D Shallow aquifer -Dup aquifer /11 ~ ---rr::' Sourc:u :,. Water aupply ma1ter plan JCA,1983 l,.,. HAG 100 ( Ojaeni,1995) and JCA 1993 i Treated Spring and Surface Water [ 10 8 m'lyear],. BoQor aprlng Upply l_l.:~ ll / ',L ',L l,f j- G.S. 5/85 -rr L >oo !! oo Figure r. Population, groundwater abstraction and public water supply in Jakarta ( ) 166

3 The supply of water is a pressing problem. The actual total water demand is about 450 mio m 3 /year but piped water mainly from surface water, serves only about 25 % of the population of the city district. The groundwater contribution to the actual supply is about 250 mio m 3 / year and is mainly abstracted from innumerable shallow wells (80 %) and more than 2000 deep wells (20 %). Between 1900 and 1945, groundwater abstraction was below 10 mio m 3 /year but since that time, it has steadily increased in step with the growth in population and industrial development. The pressure head of the deep groundwater system in northern and central Jakarta in 1900 was at 5 to 15 m above sea level and wells were generally flowing. Head levels in these areas dr_opped continuously by m/year until the early 1970's (Figure 2). n 1980 in the main part of northern and central Jakarta head levels of the confined aquifer system were generally below sea level and had reached - 10 m to - 20 m in areas where the industrial development was intensified. Land subsidence became evident in central Jakarta after JAKARTA GROUNDWATER STUDY Piezometric Head of Groundwater On metre above/below sea level) Source : DEG -Well Files lml Figure 2. Time dependent regional head changes in Northern Jakarta

4 3 Results of the Jakarta Groundwater Study During the three years of the cooperation programme, information on the hydrogeological and hydraulic conditions of the investigation area were collected, compiled, and evaluated from available archive data and reports and from recent geological and geoelectrical field surveys, as well as from surveys of head and quality conditions of the shallow aquifer and the deep aquifer system. Drilling, testing, and installation of 25 monitoring wells were also completed, as well as special investigations concerning hydrochemistry and isotope distribution. Figure 3. the aquifer system - boundaries and flow regime under natural condit.ions The boundaries of the system (Figure 3) were established at the Java Sea, the Cisadane River, and the Bekasi River forming a triangle of 1800 km 2, which includes the Jakarta City District. The bottom of the system is formed by Miocene sediments which also crop out at the southern boundary of the system. The basin fill that consists of marine Pliocene and Quaternary fan and delta sediments is between 0 and more than 300 m thick. The thickness of single sandy aquifer layers intercalated with a predominantly silty/clayey sequence is only between 1 and 5 m and is only about 20 % of the total fill. Fine sand and silt is a very frequent component of aquifers. 168

5 The horizontal permeability (Kh) of tested layers was found to be between 0.1 m/day and 40 m/day. Transmissivity estimates (T) for the entire Quaternary sequence of 250 m thlckness are some 250 m 2 /day near the coast and increase to about 500 m 2 /day near the hinge line about 20 km to the south. The vertical permeability (Kv) is estimated as varying between 1/100 and 1/5000 of the horizontal permeability. The mean transmissivity of the Pliocene sequence cropping out in the western study area is below that of the Quaternary sequence. Storage coefficients in the deep aquifer system are between o-4 and a) Natural Sta~ be for deptftion RECHARGE Mscendino groundwater GROUNDWATER FLOW SYSTEM DSCHARGE oscl-n.dino groundwater (Recharge of the sholow aquifer durlno the rainy season e:cist5) l b).artificial Stooe durlnq ~pletion AE:CHAG desc~dino RECHARGE groundwater a) Groundwater table(uflconfined)- rainy season b) Gro111dwater table ( a.nconfined) - dry season cl Piezometric head ( conflneod ) - rainy season d) Piezometric head ( confinr-d) - dry season NB - Natural base!'~ of flow (rivf'r, ocean, e-tc.) / r -o- + "A~, l"t' AB Artificial base lryf'l of flow (welt field) Figure 4. Principles of groundwater flow under natural and depleted conditions Figure 4 demonstrates the principles of groundwater flow before and during depletion. Under natural flow conditions the recharge area of the deep system was situated in the southern hilly area at topographical elevations of between 25 and 200 m. Discharge from the confined aquifer system to the natural base level in the flat coastal area occurred mainly by upward leakage, evapotranspiration and outflow to the surface water system. Today, recharge to the deep aquifer system, 169

6 other than horizontal inflow, may occur throughout the city area by downward leakage, as head levels of the confined system have dropped regionally beyond the water table of the unconfined shallow aquifer. The shallow aquifer is still fully replenished during years of normal rainfall (1700 mm/year near the coast, 2900 mm/year near Depok). Discharge from the deep aquifer system in 1985 was almost exclusively due to pumping from deep water wells. Actual groundwater abstraction of about 47 mio m 3 /year from deep wells cannot be compensated for by the horizontal inflow across the hinge line (estimated to be some 15 mio m 3 /year) and by vertical leakage from the shallow aquifer. n response to overexploitation, water levels in the confined system have dropped by 1-7 m/year during the last 3 years and are locally at 20 to 30 m below sea level (Figures 5 and 6). -hh-,--/---'-,-1 -!;:0- TontoU~t lao- 'onlouro olpluonntrl,htodln rnolr t tooltnl W.o<UQUrtapltJOmllrl' htadporoqllartln "'''" t al nl ~ Plnomtlrlchud ~ bt'low-201!1 Figure 5. Tentative map of piezometric head, aquifer depth m. Observation period 6/84-4/85 170

7 s LAND SURFACE Gambir Koto N -200m--~~----~---f~--~~--~----~\-r+.~------~----~--~-200m :to ~:~'J'r~,o s 5 ~C:r:~:,,... Llneo of Equal / 0 Plr-zomt-trlc He-ad leo! HU: 1100 f1j 1102.ro OZ U 1101 S!/K& 10/85 Figure 6. Groundwater head conditions along a S-N section. Observation period 6/84-4/85 ao TME before PEROD; 1920 befdrl! ~ ~100 ~ ;:: 'g.!!.. SJ.!O.., E. ():~-":.,~"' ln lll Mean chloride content in moll Figure 7. Mean chloride content of groundwater from various time periods related to aquifer depth in the coastal area 171

8 Despite the heavy drawdown of head levels in the coastal basin, the significance of seawater encroachment is still lo.w due to the low permeability of the aquifer system. But groundwater salinity has steadily increased (Figure 7). Brackish groundwater is found in aquifers between 0 and 100 m as far as 5 km inland. ncreased salinization existing in deep aquifers below 200 m partly might be due to connate salt in marine Tertiary sediments which form the base of the system. Hydrocarbon gas is known and has been known to escape from several artesian wells that tap aquifer layers below 200 m. Chemical characteristics of the deep groundwater follow a rather uniform scheme: a predominance of bicarbonate is found nearly everywhere. The total mineralization increases from south to north with sodium finally becoming the predominant cat-ion. A predominance of chloride is found near the coast between 0 anrl 100 m and below 200 m (Figure 8). C1landak Kebayoron Kora A Zone of low ground~o~~orer solinify Zone of intermediate Groundwater salinity ~roundwoter salinity high.jy variable Co No lolg HCO~ JAKARTA GROUNDWATER STUDY HYDROCHEMCAL CROSS- SECTON No HCO) Pr1dommon1 Wolar Tvpe l!n 1arm1 of major ion porconrogu) T nlollva bol n of Eleclrical Conduo;Jivlly of Ground.. olar EC in micro mho~ em r'"'- 1' '" :;-w.- 1 Source of W<>hf Sample Pulrlon ol..::run :; ~::h :a/cm Eucl pulrlon of Wall numbor OfUn 1,10kno~n (C n f111lho1c' Figure 8. Hydrochemical cross-section of the confined aquifer system ( ) 172

9 Results of isotope investigations (C T) confirm the regional groundwater flow pattern. C14-ages are some 500 years in the recharge area and between and years in the coastal area. The water quality in the shallow aquifer which still supplies about 60 % of the Jakarta population with drinking water, is highly endangered by human pollution in the densely populated city area. 4 Groundwater model investigations Aquifer parameters compiled from hydrogeological data were used to establish a two-dimensional groundwater model (Figure 9) in order to simulate the groundwater flow and to calibrate the model under natural flow conditions. Regional parameter data were transferred into a three-dimensional model after successful calibration. The model was successfully run under natural and pumped conditions but groundwater abstraction from the deep aquifer system had to be increased from a registered 30 mio m 3 /year to 47 mio m 3 /year in order to simulate the 1985-situation satisfactorily. Figure 10 demonstrates the groundwater head situation at various reference nodes, taking into consideration the groundwater abstraction between 1900 and 1985 and presents three alternatives of head development in response to the three different groundwater abstraction patterns shown in Figure 11. Reducing or stopping groundwater abstraction in the northern part of Jakarta causes the water levels to rise by 20 to 25 m at the respective reference nodes until Consequently, the velocity of saltwater encroachment will be reduced due to the reduced flow gradient. An increase of abs.traction in southern Jakarta causes an additional drop of the water levels by 2 to 10 m during the same period but will probably not affect the quality of groundwater in the deep aquifer. A negative effect on the head conditions in the shallow aquifer is to be expected in periods of reduced replenishment from infiltration. 173

10 Horizontal Pormoablllty Kh tm/day l K 3o " "'oo Oburved and Calculated Hoado tmj ==~ ;: a;.::.. Nl u A.wMo w,._,..uyl.t.o. Figure 9. Cross-section of the two-dimensional groundwater model-permeability of the aquifer system and groundwater heads under natural flow conditions

11 c:::a=~:=t:=;;=:~=orn=l 0 =:Lc=Ei _ 85 6 L_..1_ -_!_1 _j_l -L 1900 YEAR ~ii ' '-----"-A --'----"---l~~gg MODEL SOUTH \ ~-to Depth d25 m below sea level _ 10 Calculated Groundwater Heads [m above sea eveii Figure 10. Development of groundwater head at selected refe.rence nodes and for different abstraction configurations, Cl C C ZONE G.S. 4/86 ll V v 20- "-~ =47 47 Abstraction.1Q,. 45 [%] Q reference nodes <a- ej NORTH -SOUTH ~ abstraction area Figure 11. Tentative configurations of groundwater abs.traction in 1985, 1995 and

12 Acknowledgement We are grateful to the Director of the Directorate of Environmental Geology Drs. E. J. Patty for permission to present this paper at the SWM Meeting 1986, in Delft. We also take this opportunity to express our thanks to all our ndonesian and German colleagues who have contributed to the Jakarta investigations. References German Hydrogeological Advisory Group in ndonesia ( ) Jakarta Groundwater Study, Working Papers 69, 75-83, , 114, 116, 117, unpublished reports. Bundesanstalt fiir Geowissenschaften und Rohstoffe, Hannover and Directorate of Environmen.tal Geology, Bandung. Japan nternational Cooperation Agency (1983). Masterplan for the Jakarta Water Supply Development Project - ntern. Report, unpublished, Ministry of Public Works, Jakarta 176