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Groundwater study of a volcanic area near Bandung, Java, Indonesia
Journal of Hydrology, 28 (1976) 53--72
53
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
GROUNDWATER STUDY OF A VOLCANIC JAVA, INDONESIA
AREA NEAR BANDUNG,
BOGDAN PULAWSKI and HANS ~)BRO
Cowiconsult, Consulting Engineers and Planners Ltd., Copenhagen (Denmark) Nielsen & Rauschenberger, Consulting Engineers Ltd., Copenhagen (Denmark) (Received July 16, 1974; revised and accepted April 4, 1975)
ABSTRACT
Pulawski, B. and Obro, H., 1976. Groundwater study of a volcanic area near Bandung, Java, Indonesia. J. Hydrol., 28: 53--72. A hydrogeological investigation of the Bandung area, Java, Indonesia, is described. The investigation was carried out as part of a feasibility study directed towards improvement and development of the city's water supply. The area is situated in a tropic mountainous region, dominated by pyroclastic volcanic deposits and with abundant rainfall. The main activities of the investigation were compilation and evaluation of existing climatological and hydrogeological data, testing of four existing wells, a geo-electrical survey, drilling and testing of a new test well, study of water quality by analysis of samples from both springs and wells, and measurements of spring yields. The results of the investigation indicated presence of large groundwater resources within a distance of 15--20 km from the city. The feasibility study recommended that Bandung's water supply be based on these groundwater resources and this recommendation is being implemented. During the investigation some results concerning rainfall, infiltration, aquifers, geoelectrical surveying, and groundwater quality were obtained, which may be of general interest for hydrologists and geologists working in tropical volcanic and mountainous regions. These results are summarized in the conclusion of this paper.
INTRODUCTION T h i s p a p e r d e s c r i b e s h y d r o g e o l o g i c a l i n v e s t i g a t i o n s c a r r i e d o u t in c o n n e c tion with a water supply feasibility study for the city of Bandung, Indonesia, conducted from September 1972 to October 1973 under assignment from t h e A s i a n D e v e l o p m e n t B a n k as a p a r t o f t h e b a n k ' s t e c h n i c a l a s s i s t a n c e t o the Republic of Indonesia. B a n d u n g ( p o p u l a t i o n 1 , 2 5 0 , 0 0 0 ) is t h e t h i r d l a r g e s t c i t y in I n d o n e s i a a n d t h e p r o v i n c i a l c a p i t a l o f W e s t J a v a ( F i g . 1). T h e e x i s t i n g w a t e r s u p p l y ( 1 , 0 4 0 1/sec), m a i n l y b a s e d o n r i v e r a n d s p r i n g w a t e r , is i n a d e q u a t e . S o far, l a r g e - s c a l e g r o u n d w a t e r a b s t r a c t i o n h a s n o t b e e n p r a c t i s e d in I n d o n e s i a , b u t in t h e c a s e o f B a n d u n g t h e i n d i c a t i o n f r o m t h e o n s e t o f t h e p r o j e c t w a s t h a t t h i s was a d i s t i n c t p o s s i b i l i t y .
54 N /
J AVA
INDIAN
SEA
I
OCEAN SCALE 0
100
200
300
400 I~
Fig. 1. The island of Java.
The groundwater study was carried o u t in five months by a field team which besides studying available geological, hydrological and climatological data carried out several field investigations in order to create new data. Most important of these activities were: drilling and testing of a new 150 m deep test well with two observation wells to the same depth, redeveloping and test-pumping of four existing wells and surveying of the area by electrical resistivity methods. Chemical analyses of both spring and well water were also carried out. The authors hope that some of the results obtained during the study may be of general interest for hydrogeologists working in areas with similar geoo logical and climatological conditions. TOPOGRAPHY
Bandung is situated on the northern edge of the Bandung Plateau (Fig. 2). The plateau has the shape of a flat elliptical bowl at 600--725 m elevation and axes of 35 and 15 km in E--W and N--S directions, respectively. The plateau is surrounded by volcanic mountain ranges to the north and south reaching 2,000--2,400 m elevation. Notable among the peaks are Burangrang, Tangkuban Prahu and Bukit Tunggul to the north, and Puncak Besar to the south. Towards west the plateau is separated from the smaller Batujajar plain b y a low range of hills. The plateau is drained by the Citarum river, which flows westwards through an opening in the hills west of Bandung through the Batujajar plain, and further to the Java Sea.
55
LEGEND
Fig. 2. Topographic units around Bandung.
GEOLOGY General
According to Van Bemmelen (1949), the western part of Java consists of four geological units {Fig. 3) distinctly different in their tectonic and stratigraphic development. From the north these units are: (1) (2) (3) (4)
Plains of Jakarta. Bogor Zone. Bandung Zone. Southern Mountains.
The Bandung Zone is tectonically an intermontane depression infilled with products of volcanic activity at both the northern and southern borders of the zone. In the vicinity of Bandung, lacustrine deposits overlying the volcanic products are also present.
56
SOUTHERN MOUNTAINS
BANDUNG
ZONE
JAKARTA B O G O R ZONE PLAINS
N
1 i -2
] 0
25
50
FAULTS
"
150km
1 O0 ~
;~T~
C T ERRESTRIAL vOLCANIC FORMATIDN
,NTRA
MfOCENE
J
BATHOLITH
T ER TIARY ,YDLCANIC ,aND MARINE SEDIMENTS
Fig. 3. G e o l o g i c N - - S cross s e c t i o n o f western Java.
Stratigraphy The deposits surrounding and underlying the area are geologically rather young, the oldest of them being mid-Tertiary--Early Miocene -- and developed as reef limestones. The Upper Miocene, Pliocene and the Lower Quaternary consists mainly of volcanic deposits which, however, north of Bandung are intercalated by lignitic series and litoral sandstones. The hills which border the Bandung Plateau to the west (Fig. 3) and consist of volcanic breccia and igneous rocks, are probably Pliocene. The Middle and Upper Quaternary are entirely represented by products of terrestrial volcanic activity. The volcanic mountain ranges north and south of the Bandung Plateau developed during this period. During the early Holocene an intermontane lake was created and a series of lacustrine products was deposited in it. These deposits constitute the present Bandung Plateau, south and southeast of the town (Fig. 2). The Quaternary volcanism and its products have been divided by Van Bemmelen (1934) into Old Quaternary and Young Quaternary. This division, being based on morphologic consideration alone, does not coincide with the stratigraphic division of this period.
Detailed geology o f the study area The area subjected to detailed study by the team stretched a b o u t 20 km to the east and 20 km to the west of the town (Fig. 4). The area was limited towards north by the watershed in the mountains approximately 15 km from the city, and towards south by the river Citarum some 10 km away.
57
Geologically, this area could be divided into two units: (A) The Northern Mountains. (B) Bandung Plateau. However, it was found practical by the field team to introduce a third unit: (C) The city and vicinities immediately to the east and west of it. This third unit follows the contact zone between units A and B (Fig. 4). The geology of these three units is discussed in more detail below.
LEGEND LAKE DEPOSrTS RECE N ~ TUFFS AND ASHES
]
BASALTIC
[ YOUNG QdA-L f'dAR R ¢
LAVA
TUFFS AND
F~OWS
MdDF[ O~S
UNDIFFERENTIATED PRODUCTS
VOLCANIC
/
I
Fig. 4. Geology of the study area, simplified.
58 The Northern Mountains range in E--W direction and consist entirely of volcanic products. The Old Quaternary Volcanics form most of the range and are the sole constituents of the slopes just north and northeast of the town. Like most of the volcanic products in Java, they consist of pyroclastics including all-size materials from volcanic blocks to ashes. Common are poorlysorted tufts and ashes, whereas lavas, though present, are comparatively less frequent and usually appear as narrow valley flows. Both andesitic and basaltic products are present. An intense faulting which followed the Old Quaternary volcanism reached the magma chamber and gave rise to a new volcanic activity, the Young Quaternary Volcanism. This activity is mainly represented by products of the still active Tangkuban Prahu and by the y o u n g volcanic centres north and northeast of Ujungbrung (Fig. 4). Products from this period, in the study area largely originating from the big Tangkuban Prahu, consist of usually well-graded pyroclastics covering over great areas the older volcanics. Large amounts of this material saturated with water from the crater lake or from the heavy rains which usually accompany intense volcanic activity, have flowed as gigantic mudflows (lahars) down the slopes to the depressions north and south of the mountains (Purbohadiwidjojo and Surjo, 1968). The southern, delta-like flow towards the Bandung depression was of special interest for the study since the city is located in the southeastern part of the flow and most of the high-yielding wells appeared to be situated within the area covered by it. Most of the mountain area is covered by a 10--30 m thick layer of y o u n g tuff ashes. On Fig. 4 these deposits are after Van Bemmelen only shown within the Young Quaternary although they are also present within the Old Quaternary. The Bandung Plateau. This plain is a result of sedimentation in the abovementioned lake created in the Younger Quaternary by the Tangkuban Prahu lahar flows damming up the Citarum river. After having reached an elevation of a b o u t +725 m, the lake drained by a quick incision into the weak " d a m " material. This basin gave rise to some 60--100 m thick deposits of mainly fine andesitic t u f f sands and clays. Only a few layers (of some meters thickness) of coarse sand and gravels have been encountered. The material becomes finer towards the center of the basin, where the prevailing grain size seems also to diminish with depth. As the underlying prelacustrine deposits are also of volcanic origin, it is difficult to distinguish the lake sediments from the underlying products. The city area and vicinities. In the city area three series of deposits are present, viz. Old Quaternary volcanics, Young Quaternary volcanics and lake deposits.
59 The main part of the town is situated within the Young Volcanic delta but the suburbs expand over the Old Quaternary and lake deposits {Fig. 4). The Old Quaternary volcanic series is the main geological member in this area. It crops out just northeast of the town, covered only by the y o u n g ashes, whereas in the city and areas south and west of it, these deposits underly the Young Quaternary tuffs and mud flow as well as the lake deposits. In the city area, the depth to the old land surface is found to be some 50 m. To the west, towards the central thicker parts of the deltaic mud flow, the depth seems to increase. In general the difference in appearance between the younger and older volcanics is not great and careful study of the logs and samples is necessary to distinguish between them. They are both well-graded pyroclastics and exhibit same type of layering. The Young Quaternary volcanics, however, are on the whole somewhat coarser in this area, and probably less consolidated than the older, underlying products. These differences in grain size and consolidation give rise to changes in piezometric surface gradients from the western part of the area, where coarse unconsolidated y o u n g volcanics are present, to the east, where older deposits are at or close to the surface (Fig. 5). Characteristic for both these series is the great variation in distribution of sediments, both in lateral and vertical directions. Correlation of deposits between drillings based on drill-logs alone was seldom possible (Fig. 6), but the geo-electric survey carried out during the study within that area, gives a more consistent picture of subsurface conditions, suggesting that the lack of correlation between drill-logs is partly due to differing drilling methods, drillings crews and logging procedures. The survey showed that the volcanic products in the study area are grouped around two sets of resistivity values (Fig. 7): (a) 5--20 gt • m clays, ashes and mudflow (b) 40--80 ~2 • m coarse pyroclastics (sand and gravel fractions), both loose and consolidated To the south and southeast, the volcanic products disappear under lake deposits which in this area are less important from a hydrogeologic point of view. Aquifers
In the study area, aquifers differ genetically and structurally depending on whether they appear within the sedimentary lake deposits or in the volcanic area. In the former lake area, the aquifers consist of layers of mainly fine often silty sand, ranging from a few to 15--20 m in thickness, as well as some minor layers of gravel and coarse sand. Correlation of deposits over a distance of 2--4 km between boreholes was possible here. In the volcanic area, comprising the Northern Mountains and most of the city area and vicinities (units A and C from the foregoing section) the water
60
j~
~>
v
W ~U
Jwu~
b3 _j U J C Q ~ UJ ~Ou ~_
.JI
0
61
Z ©
1452 1556 ~T
1113
265
838
478 --l'-
1299
T
46o -030 0 55
uJ 75O
S 700
45
ao P
550 oo
5OO
i
i
55O
0
i
Fig.
1
~
2
Ctay Sand. graveL, stones Tuff Sandstone Tuff sondstone
3
~ 2~ P 165 -165
4
b
J
6km
i
Screen WeLL no ( a f t e r GSI ) Pumice Measured static water revel (m above ground) Calculated static ~ t e r Level -,,-
6. Typical geologic section of the city area. For location, see Fig. 4.
bearing deposits consist of the coarse fractions of tuffs. The grain size varies from silty fine sands to gravels, but the predominant fraction is medium to fine sand. Thickness of the aquifers within the volcanic area differs widely from less than 1 m to 2 0 - - 3 0 m or more (Fig. 6), and one may expect that they are interconnected both laterally and vertically, though in a complicated and n o t readily recognizable way. Considerable amounts of the volcanic products have become consolidated or welded (hot tuffs) into more or less hard tuff-sandstone. These also act as aquifers and together with the unconsolidated tuff-sands constitute the main water-bearing deposits in the study area (De Jongh, 1 9 2 1 ) .
62
.{).m
100. YOUNG VOLCANICS (W. OF TOWN)
..........
"%~- OLD VOLCANICS ( E OF TOWN)
10.
,
,
,
,
,
,
,,
10
100
I000 AB/2 (m)
Fig. 7. Typical resistivity sounding curves.
In the northern part of the volcanic area, shallow-seated aquifers have been developed in the youngest tuffs and ashes. The importance of water-bearing layers within these deposits grows with the proximity to the eruption centres as they become thicker (up to 30 m) and coarser towards the north. This aquifer is usually utilized by shallow, hand-dug wells. Where morphological conditions were favourable, springs tapping this aquifer developed. The possibility of effusive products like basalt and andesite flows acting as aquifers is rather limited. They are not frequent in the studied area and, when encountered, are usually compact. However, lava flows closer to Tangkuban Prahu appear to be more porous and fractured, and, in several cases, act as aquifers, giving rise to spring activity. GEOHYDROLOGY
Rainfall The mean annual rainfall in the Bandung basin varies from 1,700 mm in the centre of the Bandung Plateau southwest of the city to 3,600 mm on the watershed north of the city and to about 3,000 mm on the southern watershed. The main factor influencing the mean annual rainfall is the elevation. In both the mountain ranges north and south of Bandung the rainfall increases by 50--60 mm per 100-m increase in elevation. A frequency analysis of the annual values from four selected stations (two in the Northern Mountain~ and two on the southern range) showed that the ratio: (annual rainfall exceeded with 90% probability)/(mean annual rainfall)
63
0.9
+ 0.8
+ J~-NORTHERNMOUNTAINS • SOUTHERNMOUNTAINS 0.7 500
1000
2000 E LEVATI ON ( rn )
150(
Nso - MEAN ANNUAL RAINFALL N 90 = ANNUAL
RAINFALL WHICH
IS EXCEEDED
WITH
90 %
PROBABILITY
Fig. 8. Frequency analysis of rainfall at four stations in the Bandung area.
increases with elevation, indicating an increasing regularity of the annual rainfall with height (Fig. 8). The distribution of rainfall within the year is characterized by a wet and a dry season (Fig. 9).
Evapo transp ira tion Estimates of evapotranspiration were obtained in three ways: (1) Study of available literature on the local hydrological conditions. (2) Calculation using formulas relating evapotranspiration and climatic conditions. MEAN RAINFALL
(ram) 4OO
300
I
200
100
J
F
M
A
ll f M
J
J
A
Fig. 9. Mean rainfall in Bandung.
S
O
N
D
MONTH
64
(3) Water-budget calculations for river basins. The upper, mountainous part of the Cikapundung river basin (76 km 2) north of Bandung (Fig. 2) was especially studied since both rainfall and river discharge data were available for this area, and because the basin could be considered representative for the volcanic mountain range to the north. Bakker {1952) has calculated evapotranspiration for several river basins on Java by means of the water-budget method (Fig. 10). The Cikapundung river basin has an average elevation of a b o u t 1,400 m, and from Fig. 10 the evapotranspiration was estimated at a b o u t 1,050 mm/year. Using the formulas of Turc (1956) and Penman (FAO/UNDP, 1973) the following estimates of evapotranspiration in Cikapundung basin were obtained: Turc: Penman:
950 m m / y e a r 980 mm/year
Water-budget calculations for the basin indicated an evapotranspiration of a b o u t 1,000 mm/year. On the basis of these data it was concluded that 1,000 mm/year is a reasonable estimate for evapotranspiration in the Cikapundung river basin. The agreement between results from formulas and water-budget calculations is remarkably good. This indicates that application of both the Turc and Penman equations gives meaningful results under conditions such as those prevailing on Java, even though Turc's equation is empirical and was derived in a region with climatic conditions very different from those on Java.
ELEVATION (m)
2O0O
1500
IOO0
50O
0 900
1(~0
11~30
1200
13~30
14~0
1~0 1~30 1700 EVAPOTRANSPI RATION (mm/yeor) Fig. l 0. Evapotranspiration on Java (after Bakker, 1952),
65
Infiltration and recharge Bakker and Van Wijk (1951) studied the infiltration capacity of young volcanic deposits such as those in the volcanic mountains surrounding Bandung by analysis of detailed measurements of rainfall and r u n o f f from several small experimental watersheds on Java. Their results indicate t h a t 40--85% of the rainfall infiltrates the soil. Part of this water recharges the groundwater reservoir, while the rest of it is lost by evapotranspiration. In order to check whether these results were representative for the studied area, the groundwater recharge within the studied part of the Cikapundung river basin was estimated using a very simple mathematical model of the rainfall--run off process. The model was based on mean m o n t h l y values of rainfall, run off, and evapotranspiration. From these data m o n t h l y values of groundwater storage was calculated, and using dry-season data a "rating curve" was established giving the relation between groundwater discharge into Cikapundun~ river and total groundwater storage. The curve was then used to estimate groundwater discharge during the rainy season, and finally the recharge was estimated as the total annual groundwater discharge. The calculations showed t h a t the average recharge to the groundwater reservoir in the Cikapundung basin amounts to about 1,250 m m / y e a r or 50% of the total rainfall. This result is in general agreement with the figures given by Bakker and Van Wijk (1951). On basis of these and similar calculations for a river basin in the mountains to the south it was concluded that 30--50% of rainfall is a reasonable estimate of average recharge of aquifers in the volcanic mountain ranges surrounding the Bandung Plateau.
Existing wells About 100 deep wells have been drilled in the Bandung area since the beginning of this century. Hydrogeological and technical data on these wells were found in the files of Geological Survey of Indonesia and used for preparation of various hydrogeological maps and graphs concerning the study area. Most of the wells are 50--150 m deep and 6 in. in diameter. Usually they are free-flowing with specific capacities of 1--5 1 sec -1 m -1 drawdown. The average age of the wells is about 25 years. Four representative wells belonging to the municipality of Bandung were selected by the study team for closer examination. The wells were redeveloped by surging and bailing operations and the improvement was determined by means of free-flowing or pumping tests before and after redevelopment. The improvement of the specific capacity was 13--26% (from about 1--1.2 1 sec -1 m -1 ) and the final well loss constant C, determined by analysis of stepdrawdown tests after Jacob (1946) was 8- 10-7--56 • 10 -7 day2/m s, which is considerably higher than normal for a new and properly designed well (2.10-7--3 • 10-7). In order to check the validity of the existing hydrogeologic data, a new
66
test well with two observation wells was drilled and tested during the field study. The well, located west of the town (Fig. 5), was 150 m deep and of 10 in. diameter. The observation wells were drilled to the same depth and were of 4 in. dia. The distances from test well to observation wells were 70 and 130 m. In all three wells the major aquifers as presented in Table I were found. Only the t w o deeper aquifers were subject to more detailed testing which in this case could be carried o u t by free-flow. Fig. 11 shows the cone of depression observed during testing. It was found that of the total flow of 70 1/sec, a b o u t 13 1/sec came from aquifer II and 57 1/sec from aquifer III. Stepdrawdown test of the well indicated that even at maximum discharge the well loss was negligible. The hydraulic properties of the tested aquifers are given later in the article. Groundwater levels
The shallow water-bearing layers in the study area are water-table aquifers and artesian aquifers with the piezometric surface below the ground level (see test well above). Aquifers deeper than 40--50 m are artesian with the piezometric level rising above the ground. In several instances water levels up to 20 m above ground surface were encountered, especially within the city area and vicinities. The piezometric surface becomes higher with depth, indicating an upward groundwater flow. This appears from Fig. 6 (well No. 1113) and Fig. 12, which also gives a generalized picture of groundwater flow in the Bandung area. It can also be seen in the case of the test well where the piezometric level for the deepest aquifer (III), is about 6 m higher than for aquifer II. In order to get an indication of any possible drop in the piezometric level due to the continuous extraction of groundwater from the artesian aquifers in the city area since year 1900, three generalized water-level maps were constructed. The first using data from wells drilled during the period 1898--1940, the second using drill-log data from 1941--1961, and the third using data from 1963--1971. The last map is shown as Fig. 5. Although the facts of the upward flow c o m p o n e n t and the varying depth of the deep wells call for caution when interpreting these maps, the rather
TABLE I M a j o r aquifers f o u n d in test well Aquifer
Approx. depth
Deposit
Static w a t e r level
I II III
3 0 - - 50 6 0 - - 90 95--150
t u f f s a n d a n d gravel tuffsandstone tuff sandstone
2 rn b e l o w g r o u n d 14 m a b o v e g r o u n d 20 m a b o v e g r o u n d
67
OBS.WELL 1 ] 30 I O0"
OBS.WELL2 [---1
100
1.0 -
<
4 0 •
DISTANCE FROMTESTWELL 10000m
~
,/ 8 a o~= ~
1000
,
,->," .
/ ".
-
//
~ - ~ y / ' ~ - - c o N E oF DEPRESS,ON .~,,,..~
,~,~/X¢~ . ~z
~/~
~'-"
f / %
~"
50
~:)ULFER :
95-
I s o ,.
TRANSMISSIVITy: 668 m'/do,y _ STORAGE COEFFICIENT : 5.5x 10- 4
N=~TEMEAFTERFREEFLOWSTARTED
60
Fig. 11. C o n e o f d e p r e s s i o n d u r i n g a q u i f e r test (free-flowing test well).
constant average depth of the wells and the significantlY constant pressure pattern throughout the periods -- induced the authors to feel safe in concluding, from the absence of any major changes in water levels from one period to another, that the safe yield of the aquifer(s) has not been exceeded.
Hydraulic properties of aquifers Values of transmissivity (T) were estimated for about 60 wells from the specific capacity data. Average values of T were 600 m:/day in the central
T A B L E II Hydraulic p r o p e r t i e s of deep aquifers f o u n d in test well Aquifer
Tests
Transmissivity (m2/day)
60--90 m
preliminary (while drilling)
201 133
under ground
final (after completion)
130
0.7 • 10 -4
preliminary
715 963
5.6" 10 -4
final
668
5.5" 10 -4
95--150 m under ground
Storage coefficient
68
part of the city and areas west of it, and a b o u t 200 m2/day in areas to the east. These values were confirmed by the above mentioned tests on the four existing wells and the new test well. T values and other hydraulic properties of the deep aquifers encountered in the test well are listed in Table II. As might be expected {Fig. 12) all tests clearly indicated presence of a strong leakage and underflow. In conclusion, it may be stated that the aquifers in this area are generally thick b u t only moderately permeable. The hydraulic conductivity is usually within the range 5--30 m/day. S
N
PUNC..AK BESAR
BANDUNG PLATEAU
TANGKUBAN
PRAHU
Fig. 12. Deep groundwater flow in the Bandung area (schematic).
Springs The Geological Survey of Indonesia has registered a b o u t 500 springs in the Bandung area and surroundings, especially in the hilly and mountainous country northwest and northeast of the city. Most of them are small and seasonal only, but a few are relatively large with an average yield of 40--60 1/sec. A majority of the springs originate where the contact face between the
DISCHARGE
(t/~c) 60
40
20
J
F
M
A
M
J
J
A
S
O
N
D
MONTH
Fig. 13. Discharge from Ciwangun spring (12 km north of Bandung) during 1928.
69 y o u n g e s t t u f f s and the u n d e r l y i n g o l d e r volcanic p r o d u c t s has been intersected. In s o m e cases springs have arisen w h e r e y o u n g erosion activity has i n t e r s e c t e d an old b u r i e d river valley. T h e yield t e n d s to grow t o w a r d s T a n g k u b a n Prahu, due t o the fact t h a t the volcanic p r o d u c t s b e c o m e b o t h t h i c k e r and coarser t o w a r d s the e r u p t i o n centre. In general, the spring yield varies c o n s i d e r a b l y t h r o u g h o u t t h e year. T h e m a x i m u m discharge occurs in M a r c h - - M a y and m i n i m u m in N o v e m b e r - D e c e m b e r (Fig. 13).
Water quality Water samples f r o m s e v e n t e e n springs l o c a t e d 6 - - 1 0 k m n o r t h o f B a n d u n g and t w e n t y - f i v e d e e p wells within or near the city area were c o l l e c t e d and analyzed. S o m e typical results are given in Table III. As i n d i c a t i o n o f t h e degree o f corrosiveness t h e R y z n a r i n d e x ( R y z n a r , 1 9 4 4 ) was used. Generally, t h e spring water is soft t o m o d e r a t e l y hard (hardness 2--7 Germ a n degrees), low in dissolved solids, slightly acidic (pH = 6.0--6.5), corrosive ( R y z n a r i n d e x = 1 0 - - 1 2 ) , and has a substantial c o n t e n t o f aggressive CO2. T h e groundwater is low in dissolved solids, m o d e r a t e l y hard (hardness 5--8 G e r m a n degrees), neutral or o n l y slightly corrosive ( R y z n a r i n d e x = 7.7--8.5),
TABLE III Chemical analyses of spring water and groundwater Cigentur spring pH Residue on evaporation (mg/l) SiO2 (mg/1) Ca (rag/l) Na (mg/l) Mg (mg/1) K (mg/l) SO~ (mg/1) CI (rag/l) HC03 (mg/l) Aggressive CO2 (mg/1) Fe (mg/l) Mn (mg/1) F (mg/l) Total hardness (German degrees) Ryznar index *No test performed.
6.0 278 --* 11
Test well (aquifer 95--150 m) 7.3
22 7.2 88 4O 0.0 0.0 --
254 60 34 16 10 9.3 19 7.5 172 0 0.0 0.0 0.25
2.8 11.2
7.0 8.2
-
-
5.5 -
-
70 and contains small amounts or no aggressive CO2 at all. The bacteriological quality of both spring water and groundwater is good. On the basis of these data it was concluded that spring-water and groundwater resources in the Bandung area are of good quality and can be exploited with relatively small treatment costs. It is interesting to note the difference in quality between water from springs and deep wells. This difference can most probably be ascribed to the distance these two water types have flowed underground. This distance is relatively short for the springs discharging from the shallow aquifers, whereas it amounts to at least 10--15 km in the case of the deep wells near Bandung. It should also be noted that the chemistry of the analysed groundwater agrees remarkably well with the data one would expect from tropical, volcanic areas with abundant rainfall and vigorous plant growth. (Hem, 1970, p. 292; Davis and De Wiest, 1966, p. 343). RESULTS OF THE STUDY The investigations resulted in an inventory and evaluation of groundwater resources in the Bandung area and recommendations as to their utilization. From the point of view of future development of these resources, the area has been divided into six zones. Four of them comprised the study area as shown on Fig. 4, whereas the two other stretched over more distant areas not discussed in this paper. For hydrogeological and economical reasons, the zones most favourable for exploitation appeared to be the three following: (1) City and areas west of it. (2) Areas east of the city. (3) The Northern Mountains. In zones (1) and (2), the southerly groundwater flow within the deepseated aquifers {50--150 m) over a width of some 30 km {15 km to the west and east of the town) along the border between the Northern Mountains and the Bandung Plateau has been calculated as about 2,500 1/sec. Considering the present withdrawal rate of about 300 1/sec, an additional extraction of some 1,600 1/sec in that area was considered to be feasible. As the geological conditions are, furthermore, rather well known in these areas, priority to development of groundwater abstraction here has been recommended. In zone (3), comprising the hilly and mountainous areas to the north, a further 2,500 1/sec can economically be withdrawn. The annual production of groundwater in this area has been calculated at about 9,500 1/sec. Furthermore, it was found feasible to develop more of the numerous springs in the mountains. However, these should be supplemented by wells in order to make up for seasonal fluctuations in the spring yields and to provide for better utilization of the transmission system. The areas to the north can supply the town by gravitation which makes them especially attractive and which
71
to some degree outweighs the cost of the more detailed hydrogeological investigations still needed in this area. The Bandung Plateau, entirely covered by rice fields and to some extent irrigated by numerous artesian b a m b o o wells, was not found feasible for development. Even a moderate lowering of the piezometric surface would put most of these wells out of function. Besides, the extracted groundwater would have to be lifted some hundred meters to the supply areas. CONCLUSIONS
Based on the investigations of groundwater resources described and on a parallel evaluation of the surface water resources, an economic comparison was made between the possible sources of supply. Considering the discounted costs of treatment, pumping, pipes etc., the comparison showed that development of groundwater resources would be the most economical way of supplying Bandung with additional water. It was found that the increase of water demand in Bandung until 1990 could be covered by development of the deep aquifers inside and west of the city area, supplemented by development of both spring-water and groundwater resources north of Bandung. During the study, some results were obtained that may be of general interest for hydrogeologists working in tropical volcanic and mountainous regions. These are: (1) Statistical analysis of rainfall data indicated an increasing regularity of the annual rainfall with increasing elevation. (2) Infiltration capacity of y o u n g volcanic pyroclastics was remarkably large, a b o u t 50%. (3) Aquifers consisted mainly of tuff-sands which, if welded, appear as sandstones. Permeabilities were in the range of 5--30 m / d a y and the storage coefficient was 1 . 1 0 - 4 - - 6 • 10 -4. (4) Geo-electrical resistivity surveying proved to be a workable m e t h o d for a quick evaluation of the subsurface lithologic conditions. (5) Specific resistivities of water-bearing formations were within 40--80 gt. m. Sensibly higher resistivity may indicate presence of solid lava flows or compact, dry ash-tufts, both with practically no groundwater potential. (6) The quality of groundwater was good. Spring water discharging from shallow aquifers was aggressive, whereas water from deep wells was neutral. ACKNOWLEDGEMENTS
The authors are grateful to the Asian Development Bank for permission to publish the results presented in this paper. During the field work, the authors benefited greatly from the extraordinary helpfulness and kindness of the Indonesian authorities involved of which the Geological Survey of Indonesia deserves special thanks. The authors are also indebted to Dr. O. Berthelsen, Director of the Geological Survey of Denmark,
72
who acted as geological adviser for the project. Various problems and methods have been fruitfully discussed with Prof. Dr. C. Vofite, Jakarta, who has also provided the authors with valuable references on the subject. Mr. Aa. Christensen, Project Manager of the study, and Mr. Howard Thomas, both of Copenhagen, willingly assisted the authors with advice and comments.
REFERENCES Bakker, A.J., 1952. Aantekeningen over het Hydrologisch Karakter van Rivieren op Java. Biro Bendungan dan Hidrometri, Bandung, 114 pp. Bakker, A.J. and Van Wijk, Chr.L., 1951. Infiltration and Runoff under Various Conditions on Java. Biro Bendungan dan Hidrometri, Bandung, 22 pp. Davis, S.N. and De Wiest, R.J.M., 1966. Hydrogeology. Wiley, New York, N.Y., 463 pp. De Jongh, C.A., 1921. Verslag over het Artesische Bekken van Bandoeng en Tjimahi (unpublished). FAO/UNDP, 1973. Water Availability Appraisal. Indonesia, Bogor, 55 pp. Hem, J.D., 1970. Study and interpretation of the chemical characteristics of natural water. U.S. Geol. Surv., Water-Supply Pap. 1473 2nd ed., 363 pp. Jacob, C.E., 1946. Drawdown test to determine effective radius of artesian well. Proc. Am. Soc. Civ. Eng., 79(5): 629--646. Klompe, Th.H.F., 1956. The Geology of Bandung. (Lecture compiled by R.P. Koesoemadinal unpublished ). Purbohadiwidjojo, M.M. and Surjo, J., 1968. Volcanic Activity and Its Implications on Surface Drainage, Irrigation Seminar, Malang, July 9--23, I968, 9 pp. Ryznar, J.W., 1944. A new index for determining amount of calcium carbonate scale formed by a water. J. Am. Water Works Assoc., 36: 472--483. Turc, L., 1956. Le bilan d'eau des sols; relation entre les pr6cipitations, l'~vaporation et l'6coulement. Houille Blanche, Num~ro Spec., A-1956, pp. 216--217. Van Bemmelen, R.W., 1934. Geologische kaart van Java, Toelichting bij Blad 36, Scale 1:100,000, Dienst Mijnbouw Nederlandsch Indi~, Bandung, 95 pp. Van Bemmelen, R.W., 1949. Geology of Indonesia, Vol. I, Government Printing Office, The Hague, 732 pp.
53
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
GROUNDWATER STUDY OF A VOLCANIC JAVA, INDONESIA
AREA NEAR BANDUNG,
BOGDAN PULAWSKI and HANS ~)BRO
Cowiconsult, Consulting Engineers and Planners Ltd., Copenhagen (Denmark) Nielsen & Rauschenberger, Consulting Engineers Ltd., Copenhagen (Denmark) (Received July 16, 1974; revised and accepted April 4, 1975)
ABSTRACT
Pulawski, B. and Obro, H., 1976. Groundwater study of a volcanic area near Bandung, Java, Indonesia. J. Hydrol., 28: 53--72. A hydrogeological investigation of the Bandung area, Java, Indonesia, is described. The investigation was carried out as part of a feasibility study directed towards improvement and development of the city's water supply. The area is situated in a tropic mountainous region, dominated by pyroclastic volcanic deposits and with abundant rainfall. The main activities of the investigation were compilation and evaluation of existing climatological and hydrogeological data, testing of four existing wells, a geo-electrical survey, drilling and testing of a new test well, study of water quality by analysis of samples from both springs and wells, and measurements of spring yields. The results of the investigation indicated presence of large groundwater resources within a distance of 15--20 km from the city. The feasibility study recommended that Bandung's water supply be based on these groundwater resources and this recommendation is being implemented. During the investigation some results concerning rainfall, infiltration, aquifers, geoelectrical surveying, and groundwater quality were obtained, which may be of general interest for hydrologists and geologists working in tropical volcanic and mountainous regions. These results are summarized in the conclusion of this paper.
INTRODUCTION T h i s p a p e r d e s c r i b e s h y d r o g e o l o g i c a l i n v e s t i g a t i o n s c a r r i e d o u t in c o n n e c tion with a water supply feasibility study for the city of Bandung, Indonesia, conducted from September 1972 to October 1973 under assignment from t h e A s i a n D e v e l o p m e n t B a n k as a p a r t o f t h e b a n k ' s t e c h n i c a l a s s i s t a n c e t o the Republic of Indonesia. B a n d u n g ( p o p u l a t i o n 1 , 2 5 0 , 0 0 0 ) is t h e t h i r d l a r g e s t c i t y in I n d o n e s i a a n d t h e p r o v i n c i a l c a p i t a l o f W e s t J a v a ( F i g . 1). T h e e x i s t i n g w a t e r s u p p l y ( 1 , 0 4 0 1/sec), m a i n l y b a s e d o n r i v e r a n d s p r i n g w a t e r , is i n a d e q u a t e . S o far, l a r g e - s c a l e g r o u n d w a t e r a b s t r a c t i o n h a s n o t b e e n p r a c t i s e d in I n d o n e s i a , b u t in t h e c a s e o f B a n d u n g t h e i n d i c a t i o n f r o m t h e o n s e t o f t h e p r o j e c t w a s t h a t t h i s was a d i s t i n c t p o s s i b i l i t y .
54 N /
J AVA
INDIAN
SEA
I
OCEAN SCALE 0
100
200
300
400 I~
Fig. 1. The island of Java.
The groundwater study was carried o u t in five months by a field team which besides studying available geological, hydrological and climatological data carried out several field investigations in order to create new data. Most important of these activities were: drilling and testing of a new 150 m deep test well with two observation wells to the same depth, redeveloping and test-pumping of four existing wells and surveying of the area by electrical resistivity methods. Chemical analyses of both spring and well water were also carried out. The authors hope that some of the results obtained during the study may be of general interest for hydrogeologists working in areas with similar geoo logical and climatological conditions. TOPOGRAPHY
Bandung is situated on the northern edge of the Bandung Plateau (Fig. 2). The plateau has the shape of a flat elliptical bowl at 600--725 m elevation and axes of 35 and 15 km in E--W and N--S directions, respectively. The plateau is surrounded by volcanic mountain ranges to the north and south reaching 2,000--2,400 m elevation. Notable among the peaks are Burangrang, Tangkuban Prahu and Bukit Tunggul to the north, and Puncak Besar to the south. Towards west the plateau is separated from the smaller Batujajar plain b y a low range of hills. The plateau is drained by the Citarum river, which flows westwards through an opening in the hills west of Bandung through the Batujajar plain, and further to the Java Sea.
55
LEGEND
Fig. 2. Topographic units around Bandung.
GEOLOGY General
According to Van Bemmelen (1949), the western part of Java consists of four geological units {Fig. 3) distinctly different in their tectonic and stratigraphic development. From the north these units are: (1) (2) (3) (4)
Plains of Jakarta. Bogor Zone. Bandung Zone. Southern Mountains.
The Bandung Zone is tectonically an intermontane depression infilled with products of volcanic activity at both the northern and southern borders of the zone. In the vicinity of Bandung, lacustrine deposits overlying the volcanic products are also present.
56
SOUTHERN MOUNTAINS
BANDUNG
ZONE
JAKARTA B O G O R ZONE PLAINS
N
1 i -2
] 0
25
50
FAULTS
"
150km
1 O0 ~
;~T~
C T ERRESTRIAL vOLCANIC FORMATIDN
,NTRA
MfOCENE
J
BATHOLITH
T ER TIARY ,YDLCANIC ,aND MARINE SEDIMENTS
Fig. 3. G e o l o g i c N - - S cross s e c t i o n o f western Java.
Stratigraphy The deposits surrounding and underlying the area are geologically rather young, the oldest of them being mid-Tertiary--Early Miocene -- and developed as reef limestones. The Upper Miocene, Pliocene and the Lower Quaternary consists mainly of volcanic deposits which, however, north of Bandung are intercalated by lignitic series and litoral sandstones. The hills which border the Bandung Plateau to the west (Fig. 3) and consist of volcanic breccia and igneous rocks, are probably Pliocene. The Middle and Upper Quaternary are entirely represented by products of terrestrial volcanic activity. The volcanic mountain ranges north and south of the Bandung Plateau developed during this period. During the early Holocene an intermontane lake was created and a series of lacustrine products was deposited in it. These deposits constitute the present Bandung Plateau, south and southeast of the town (Fig. 2). The Quaternary volcanism and its products have been divided by Van Bemmelen (1934) into Old Quaternary and Young Quaternary. This division, being based on morphologic consideration alone, does not coincide with the stratigraphic division of this period.
Detailed geology o f the study area The area subjected to detailed study by the team stretched a b o u t 20 km to the east and 20 km to the west of the town (Fig. 4). The area was limited towards north by the watershed in the mountains approximately 15 km from the city, and towards south by the river Citarum some 10 km away.
57
Geologically, this area could be divided into two units: (A) The Northern Mountains. (B) Bandung Plateau. However, it was found practical by the field team to introduce a third unit: (C) The city and vicinities immediately to the east and west of it. This third unit follows the contact zone between units A and B (Fig. 4). The geology of these three units is discussed in more detail below.
LEGEND LAKE DEPOSrTS RECE N ~ TUFFS AND ASHES
]
BASALTIC
[ YOUNG QdA-L f'dAR R ¢
LAVA
TUFFS AND
F~OWS
MdDF[ O~S
UNDIFFERENTIATED PRODUCTS
VOLCANIC
/
I
Fig. 4. Geology of the study area, simplified.
58 The Northern Mountains range in E--W direction and consist entirely of volcanic products. The Old Quaternary Volcanics form most of the range and are the sole constituents of the slopes just north and northeast of the town. Like most of the volcanic products in Java, they consist of pyroclastics including all-size materials from volcanic blocks to ashes. Common are poorlysorted tufts and ashes, whereas lavas, though present, are comparatively less frequent and usually appear as narrow valley flows. Both andesitic and basaltic products are present. An intense faulting which followed the Old Quaternary volcanism reached the magma chamber and gave rise to a new volcanic activity, the Young Quaternary Volcanism. This activity is mainly represented by products of the still active Tangkuban Prahu and by the y o u n g volcanic centres north and northeast of Ujungbrung (Fig. 4). Products from this period, in the study area largely originating from the big Tangkuban Prahu, consist of usually well-graded pyroclastics covering over great areas the older volcanics. Large amounts of this material saturated with water from the crater lake or from the heavy rains which usually accompany intense volcanic activity, have flowed as gigantic mudflows (lahars) down the slopes to the depressions north and south of the mountains (Purbohadiwidjojo and Surjo, 1968). The southern, delta-like flow towards the Bandung depression was of special interest for the study since the city is located in the southeastern part of the flow and most of the high-yielding wells appeared to be situated within the area covered by it. Most of the mountain area is covered by a 10--30 m thick layer of y o u n g tuff ashes. On Fig. 4 these deposits are after Van Bemmelen only shown within the Young Quaternary although they are also present within the Old Quaternary. The Bandung Plateau. This plain is a result of sedimentation in the abovementioned lake created in the Younger Quaternary by the Tangkuban Prahu lahar flows damming up the Citarum river. After having reached an elevation of a b o u t +725 m, the lake drained by a quick incision into the weak " d a m " material. This basin gave rise to some 60--100 m thick deposits of mainly fine andesitic t u f f sands and clays. Only a few layers (of some meters thickness) of coarse sand and gravels have been encountered. The material becomes finer towards the center of the basin, where the prevailing grain size seems also to diminish with depth. As the underlying prelacustrine deposits are also of volcanic origin, it is difficult to distinguish the lake sediments from the underlying products. The city area and vicinities. In the city area three series of deposits are present, viz. Old Quaternary volcanics, Young Quaternary volcanics and lake deposits.
59 The main part of the town is situated within the Young Volcanic delta but the suburbs expand over the Old Quaternary and lake deposits {Fig. 4). The Old Quaternary volcanic series is the main geological member in this area. It crops out just northeast of the town, covered only by the y o u n g ashes, whereas in the city and areas south and west of it, these deposits underly the Young Quaternary tuffs and mud flow as well as the lake deposits. In the city area, the depth to the old land surface is found to be some 50 m. To the west, towards the central thicker parts of the deltaic mud flow, the depth seems to increase. In general the difference in appearance between the younger and older volcanics is not great and careful study of the logs and samples is necessary to distinguish between them. They are both well-graded pyroclastics and exhibit same type of layering. The Young Quaternary volcanics, however, are on the whole somewhat coarser in this area, and probably less consolidated than the older, underlying products. These differences in grain size and consolidation give rise to changes in piezometric surface gradients from the western part of the area, where coarse unconsolidated y o u n g volcanics are present, to the east, where older deposits are at or close to the surface (Fig. 5). Characteristic for both these series is the great variation in distribution of sediments, both in lateral and vertical directions. Correlation of deposits between drillings based on drill-logs alone was seldom possible (Fig. 6), but the geo-electric survey carried out during the study within that area, gives a more consistent picture of subsurface conditions, suggesting that the lack of correlation between drill-logs is partly due to differing drilling methods, drillings crews and logging procedures. The survey showed that the volcanic products in the study area are grouped around two sets of resistivity values (Fig. 7): (a) 5--20 gt • m clays, ashes and mudflow (b) 40--80 ~2 • m coarse pyroclastics (sand and gravel fractions), both loose and consolidated To the south and southeast, the volcanic products disappear under lake deposits which in this area are less important from a hydrogeologic point of view. Aquifers
In the study area, aquifers differ genetically and structurally depending on whether they appear within the sedimentary lake deposits or in the volcanic area. In the former lake area, the aquifers consist of layers of mainly fine often silty sand, ranging from a few to 15--20 m in thickness, as well as some minor layers of gravel and coarse sand. Correlation of deposits over a distance of 2--4 km between boreholes was possible here. In the volcanic area, comprising the Northern Mountains and most of the city area and vicinities (units A and C from the foregoing section) the water
60
j~
~>
v
W ~U
Jwu~
b3 _j U J C Q ~ UJ ~Ou ~_
.JI
0
61
Z ©
1452 1556 ~T
1113
265
838
478 --l'-
1299
T
46o -030 0 55
uJ 75O
S 700
45
ao P
550 oo
5OO
i
i
55O
0
i
Fig.
1
~
2
Ctay Sand. graveL, stones Tuff Sandstone Tuff sondstone
3
~ 2~ P 165 -165
4
b
J
6km
i
Screen WeLL no ( a f t e r GSI ) Pumice Measured static water revel (m above ground) Calculated static ~ t e r Level -,,-
6. Typical geologic section of the city area. For location, see Fig. 4.
bearing deposits consist of the coarse fractions of tuffs. The grain size varies from silty fine sands to gravels, but the predominant fraction is medium to fine sand. Thickness of the aquifers within the volcanic area differs widely from less than 1 m to 2 0 - - 3 0 m or more (Fig. 6), and one may expect that they are interconnected both laterally and vertically, though in a complicated and n o t readily recognizable way. Considerable amounts of the volcanic products have become consolidated or welded (hot tuffs) into more or less hard tuff-sandstone. These also act as aquifers and together with the unconsolidated tuff-sands constitute the main water-bearing deposits in the study area (De Jongh, 1 9 2 1 ) .
62
.{).m
100. YOUNG VOLCANICS (W. OF TOWN)
..........
"%~- OLD VOLCANICS ( E OF TOWN)
10.
,
,
,
,
,
,
,,
10
100
I000 AB/2 (m)
Fig. 7. Typical resistivity sounding curves.
In the northern part of the volcanic area, shallow-seated aquifers have been developed in the youngest tuffs and ashes. The importance of water-bearing layers within these deposits grows with the proximity to the eruption centres as they become thicker (up to 30 m) and coarser towards the north. This aquifer is usually utilized by shallow, hand-dug wells. Where morphological conditions were favourable, springs tapping this aquifer developed. The possibility of effusive products like basalt and andesite flows acting as aquifers is rather limited. They are not frequent in the studied area and, when encountered, are usually compact. However, lava flows closer to Tangkuban Prahu appear to be more porous and fractured, and, in several cases, act as aquifers, giving rise to spring activity. GEOHYDROLOGY
Rainfall The mean annual rainfall in the Bandung basin varies from 1,700 mm in the centre of the Bandung Plateau southwest of the city to 3,600 mm on the watershed north of the city and to about 3,000 mm on the southern watershed. The main factor influencing the mean annual rainfall is the elevation. In both the mountain ranges north and south of Bandung the rainfall increases by 50--60 mm per 100-m increase in elevation. A frequency analysis of the annual values from four selected stations (two in the Northern Mountain~ and two on the southern range) showed that the ratio: (annual rainfall exceeded with 90% probability)/(mean annual rainfall)
63
0.9
+ 0.8
+ J~-NORTHERNMOUNTAINS • SOUTHERNMOUNTAINS 0.7 500
1000
2000 E LEVATI ON ( rn )
150(
Nso - MEAN ANNUAL RAINFALL N 90 = ANNUAL
RAINFALL WHICH
IS EXCEEDED
WITH
90 %
PROBABILITY
Fig. 8. Frequency analysis of rainfall at four stations in the Bandung area.
increases with elevation, indicating an increasing regularity of the annual rainfall with height (Fig. 8). The distribution of rainfall within the year is characterized by a wet and a dry season (Fig. 9).
Evapo transp ira tion Estimates of evapotranspiration were obtained in three ways: (1) Study of available literature on the local hydrological conditions. (2) Calculation using formulas relating evapotranspiration and climatic conditions. MEAN RAINFALL
(ram) 4OO
300
I
200
100
J
F
M
A
ll f M
J
J
A
Fig. 9. Mean rainfall in Bandung.
S
O
N
D
MONTH
64
(3) Water-budget calculations for river basins. The upper, mountainous part of the Cikapundung river basin (76 km 2) north of Bandung (Fig. 2) was especially studied since both rainfall and river discharge data were available for this area, and because the basin could be considered representative for the volcanic mountain range to the north. Bakker {1952) has calculated evapotranspiration for several river basins on Java by means of the water-budget method (Fig. 10). The Cikapundung river basin has an average elevation of a b o u t 1,400 m, and from Fig. 10 the evapotranspiration was estimated at a b o u t 1,050 mm/year. Using the formulas of Turc (1956) and Penman (FAO/UNDP, 1973) the following estimates of evapotranspiration in Cikapundung basin were obtained: Turc: Penman:
950 m m / y e a r 980 mm/year
Water-budget calculations for the basin indicated an evapotranspiration of a b o u t 1,000 mm/year. On the basis of these data it was concluded that 1,000 mm/year is a reasonable estimate for evapotranspiration in the Cikapundung river basin. The agreement between results from formulas and water-budget calculations is remarkably good. This indicates that application of both the Turc and Penman equations gives meaningful results under conditions such as those prevailing on Java, even though Turc's equation is empirical and was derived in a region with climatic conditions very different from those on Java.
ELEVATION (m)
2O0O
1500
IOO0
50O
0 900
1(~0
11~30
1200
13~30
14~0
1~0 1~30 1700 EVAPOTRANSPI RATION (mm/yeor) Fig. l 0. Evapotranspiration on Java (after Bakker, 1952),
65
Infiltration and recharge Bakker and Van Wijk (1951) studied the infiltration capacity of young volcanic deposits such as those in the volcanic mountains surrounding Bandung by analysis of detailed measurements of rainfall and r u n o f f from several small experimental watersheds on Java. Their results indicate t h a t 40--85% of the rainfall infiltrates the soil. Part of this water recharges the groundwater reservoir, while the rest of it is lost by evapotranspiration. In order to check whether these results were representative for the studied area, the groundwater recharge within the studied part of the Cikapundung river basin was estimated using a very simple mathematical model of the rainfall--run off process. The model was based on mean m o n t h l y values of rainfall, run off, and evapotranspiration. From these data m o n t h l y values of groundwater storage was calculated, and using dry-season data a "rating curve" was established giving the relation between groundwater discharge into Cikapundun~ river and total groundwater storage. The curve was then used to estimate groundwater discharge during the rainy season, and finally the recharge was estimated as the total annual groundwater discharge. The calculations showed t h a t the average recharge to the groundwater reservoir in the Cikapundung basin amounts to about 1,250 m m / y e a r or 50% of the total rainfall. This result is in general agreement with the figures given by Bakker and Van Wijk (1951). On basis of these and similar calculations for a river basin in the mountains to the south it was concluded that 30--50% of rainfall is a reasonable estimate of average recharge of aquifers in the volcanic mountain ranges surrounding the Bandung Plateau.
Existing wells About 100 deep wells have been drilled in the Bandung area since the beginning of this century. Hydrogeological and technical data on these wells were found in the files of Geological Survey of Indonesia and used for preparation of various hydrogeological maps and graphs concerning the study area. Most of the wells are 50--150 m deep and 6 in. in diameter. Usually they are free-flowing with specific capacities of 1--5 1 sec -1 m -1 drawdown. The average age of the wells is about 25 years. Four representative wells belonging to the municipality of Bandung were selected by the study team for closer examination. The wells were redeveloped by surging and bailing operations and the improvement was determined by means of free-flowing or pumping tests before and after redevelopment. The improvement of the specific capacity was 13--26% (from about 1--1.2 1 sec -1 m -1 ) and the final well loss constant C, determined by analysis of stepdrawdown tests after Jacob (1946) was 8- 10-7--56 • 10 -7 day2/m s, which is considerably higher than normal for a new and properly designed well (2.10-7--3 • 10-7). In order to check the validity of the existing hydrogeologic data, a new
66
test well with two observation wells was drilled and tested during the field study. The well, located west of the town (Fig. 5), was 150 m deep and of 10 in. diameter. The observation wells were drilled to the same depth and were of 4 in. dia. The distances from test well to observation wells were 70 and 130 m. In all three wells the major aquifers as presented in Table I were found. Only the t w o deeper aquifers were subject to more detailed testing which in this case could be carried o u t by free-flow. Fig. 11 shows the cone of depression observed during testing. It was found that of the total flow of 70 1/sec, a b o u t 13 1/sec came from aquifer II and 57 1/sec from aquifer III. Stepdrawdown test of the well indicated that even at maximum discharge the well loss was negligible. The hydraulic properties of the tested aquifers are given later in the article. Groundwater levels
The shallow water-bearing layers in the study area are water-table aquifers and artesian aquifers with the piezometric surface below the ground level (see test well above). Aquifers deeper than 40--50 m are artesian with the piezometric level rising above the ground. In several instances water levels up to 20 m above ground surface were encountered, especially within the city area and vicinities. The piezometric surface becomes higher with depth, indicating an upward groundwater flow. This appears from Fig. 6 (well No. 1113) and Fig. 12, which also gives a generalized picture of groundwater flow in the Bandung area. It can also be seen in the case of the test well where the piezometric level for the deepest aquifer (III), is about 6 m higher than for aquifer II. In order to get an indication of any possible drop in the piezometric level due to the continuous extraction of groundwater from the artesian aquifers in the city area since year 1900, three generalized water-level maps were constructed. The first using data from wells drilled during the period 1898--1940, the second using drill-log data from 1941--1961, and the third using data from 1963--1971. The last map is shown as Fig. 5. Although the facts of the upward flow c o m p o n e n t and the varying depth of the deep wells call for caution when interpreting these maps, the rather
TABLE I M a j o r aquifers f o u n d in test well Aquifer
Approx. depth
Deposit
Static w a t e r level
I II III
3 0 - - 50 6 0 - - 90 95--150
t u f f s a n d a n d gravel tuffsandstone tuff sandstone
2 rn b e l o w g r o u n d 14 m a b o v e g r o u n d 20 m a b o v e g r o u n d
67
OBS.WELL 1 ] 30 I O0"
OBS.WELL2 [---1
100
1.0 -
<
4 0 •
DISTANCE FROMTESTWELL 10000m
~
,/ 8 a o~= ~
1000
,
,->," .
/ ".
-
//
~ - ~ y / ' ~ - - c o N E oF DEPRESS,ON .~,,,..~
,~,~/X¢~ . ~z
~/~
~'-"
f / %
~"
50
~:)ULFER :
95-
I s o ,.
TRANSMISSIVITy: 668 m'/do,y _ STORAGE COEFFICIENT : 5.5x 10- 4
N=~TEMEAFTERFREEFLOWSTARTED
60
Fig. 11. C o n e o f d e p r e s s i o n d u r i n g a q u i f e r test (free-flowing test well).
constant average depth of the wells and the significantlY constant pressure pattern throughout the periods -- induced the authors to feel safe in concluding, from the absence of any major changes in water levels from one period to another, that the safe yield of the aquifer(s) has not been exceeded.
Hydraulic properties of aquifers Values of transmissivity (T) were estimated for about 60 wells from the specific capacity data. Average values of T were 600 m:/day in the central
T A B L E II Hydraulic p r o p e r t i e s of deep aquifers f o u n d in test well Aquifer
Tests
Transmissivity (m2/day)
60--90 m
preliminary (while drilling)
201 133
under ground
final (after completion)
130
0.7 • 10 -4
preliminary
715 963
5.6" 10 -4
final
668
5.5" 10 -4
95--150 m under ground
Storage coefficient
68
part of the city and areas west of it, and a b o u t 200 m2/day in areas to the east. These values were confirmed by the above mentioned tests on the four existing wells and the new test well. T values and other hydraulic properties of the deep aquifers encountered in the test well are listed in Table II. As might be expected {Fig. 12) all tests clearly indicated presence of a strong leakage and underflow. In conclusion, it may be stated that the aquifers in this area are generally thick b u t only moderately permeable. The hydraulic conductivity is usually within the range 5--30 m/day. S
N
PUNC..AK BESAR
BANDUNG PLATEAU
TANGKUBAN
PRAHU
Fig. 12. Deep groundwater flow in the Bandung area (schematic).
Springs The Geological Survey of Indonesia has registered a b o u t 500 springs in the Bandung area and surroundings, especially in the hilly and mountainous country northwest and northeast of the city. Most of them are small and seasonal only, but a few are relatively large with an average yield of 40--60 1/sec. A majority of the springs originate where the contact face between the
DISCHARGE
(t/~c) 60
40
20
J
F
M
A
M
J
J
A
S
O
N
D
MONTH
Fig. 13. Discharge from Ciwangun spring (12 km north of Bandung) during 1928.
69 y o u n g e s t t u f f s and the u n d e r l y i n g o l d e r volcanic p r o d u c t s has been intersected. In s o m e cases springs have arisen w h e r e y o u n g erosion activity has i n t e r s e c t e d an old b u r i e d river valley. T h e yield t e n d s to grow t o w a r d s T a n g k u b a n Prahu, due t o the fact t h a t the volcanic p r o d u c t s b e c o m e b o t h t h i c k e r and coarser t o w a r d s the e r u p t i o n centre. In general, the spring yield varies c o n s i d e r a b l y t h r o u g h o u t t h e year. T h e m a x i m u m discharge occurs in M a r c h - - M a y and m i n i m u m in N o v e m b e r - D e c e m b e r (Fig. 13).
Water quality Water samples f r o m s e v e n t e e n springs l o c a t e d 6 - - 1 0 k m n o r t h o f B a n d u n g and t w e n t y - f i v e d e e p wells within or near the city area were c o l l e c t e d and analyzed. S o m e typical results are given in Table III. As i n d i c a t i o n o f t h e degree o f corrosiveness t h e R y z n a r i n d e x ( R y z n a r , 1 9 4 4 ) was used. Generally, t h e spring water is soft t o m o d e r a t e l y hard (hardness 2--7 Germ a n degrees), low in dissolved solids, slightly acidic (pH = 6.0--6.5), corrosive ( R y z n a r i n d e x = 1 0 - - 1 2 ) , and has a substantial c o n t e n t o f aggressive CO2. T h e groundwater is low in dissolved solids, m o d e r a t e l y hard (hardness 5--8 G e r m a n degrees), neutral or o n l y slightly corrosive ( R y z n a r i n d e x = 7.7--8.5),
TABLE III Chemical analyses of spring water and groundwater Cigentur spring pH Residue on evaporation (mg/l) SiO2 (mg/1) Ca (rag/l) Na (mg/l) Mg (mg/1) K (mg/l) SO~ (mg/1) CI (rag/l) HC03 (mg/l) Aggressive CO2 (mg/1) Fe (mg/l) Mn (mg/1) F (mg/l) Total hardness (German degrees) Ryznar index *No test performed.
6.0 278 --* 11
Test well (aquifer 95--150 m) 7.3
22 7.2 88 4O 0.0 0.0 --
254 60 34 16 10 9.3 19 7.5 172 0 0.0 0.0 0.25
2.8 11.2
7.0 8.2
-
-
5.5 -
-
70 and contains small amounts or no aggressive CO2 at all. The bacteriological quality of both spring water and groundwater is good. On the basis of these data it was concluded that spring-water and groundwater resources in the Bandung area are of good quality and can be exploited with relatively small treatment costs. It is interesting to note the difference in quality between water from springs and deep wells. This difference can most probably be ascribed to the distance these two water types have flowed underground. This distance is relatively short for the springs discharging from the shallow aquifers, whereas it amounts to at least 10--15 km in the case of the deep wells near Bandung. It should also be noted that the chemistry of the analysed groundwater agrees remarkably well with the data one would expect from tropical, volcanic areas with abundant rainfall and vigorous plant growth. (Hem, 1970, p. 292; Davis and De Wiest, 1966, p. 343). RESULTS OF THE STUDY The investigations resulted in an inventory and evaluation of groundwater resources in the Bandung area and recommendations as to their utilization. From the point of view of future development of these resources, the area has been divided into six zones. Four of them comprised the study area as shown on Fig. 4, whereas the two other stretched over more distant areas not discussed in this paper. For hydrogeological and economical reasons, the zones most favourable for exploitation appeared to be the three following: (1) City and areas west of it. (2) Areas east of the city. (3) The Northern Mountains. In zones (1) and (2), the southerly groundwater flow within the deepseated aquifers {50--150 m) over a width of some 30 km {15 km to the west and east of the town) along the border between the Northern Mountains and the Bandung Plateau has been calculated as about 2,500 1/sec. Considering the present withdrawal rate of about 300 1/sec, an additional extraction of some 1,600 1/sec in that area was considered to be feasible. As the geological conditions are, furthermore, rather well known in these areas, priority to development of groundwater abstraction here has been recommended. In zone (3), comprising the hilly and mountainous areas to the north, a further 2,500 1/sec can economically be withdrawn. The annual production of groundwater in this area has been calculated at about 9,500 1/sec. Furthermore, it was found feasible to develop more of the numerous springs in the mountains. However, these should be supplemented by wells in order to make up for seasonal fluctuations in the spring yields and to provide for better utilization of the transmission system. The areas to the north can supply the town by gravitation which makes them especially attractive and which
71
to some degree outweighs the cost of the more detailed hydrogeological investigations still needed in this area. The Bandung Plateau, entirely covered by rice fields and to some extent irrigated by numerous artesian b a m b o o wells, was not found feasible for development. Even a moderate lowering of the piezometric surface would put most of these wells out of function. Besides, the extracted groundwater would have to be lifted some hundred meters to the supply areas. CONCLUSIONS
Based on the investigations of groundwater resources described and on a parallel evaluation of the surface water resources, an economic comparison was made between the possible sources of supply. Considering the discounted costs of treatment, pumping, pipes etc., the comparison showed that development of groundwater resources would be the most economical way of supplying Bandung with additional water. It was found that the increase of water demand in Bandung until 1990 could be covered by development of the deep aquifers inside and west of the city area, supplemented by development of both spring-water and groundwater resources north of Bandung. During the study, some results were obtained that may be of general interest for hydrogeologists working in tropical volcanic and mountainous regions. These are: (1) Statistical analysis of rainfall data indicated an increasing regularity of the annual rainfall with increasing elevation. (2) Infiltration capacity of y o u n g volcanic pyroclastics was remarkably large, a b o u t 50%. (3) Aquifers consisted mainly of tuff-sands which, if welded, appear as sandstones. Permeabilities were in the range of 5--30 m / d a y and the storage coefficient was 1 . 1 0 - 4 - - 6 • 10 -4. (4) Geo-electrical resistivity surveying proved to be a workable m e t h o d for a quick evaluation of the subsurface lithologic conditions. (5) Specific resistivities of water-bearing formations were within 40--80 gt. m. Sensibly higher resistivity may indicate presence of solid lava flows or compact, dry ash-tufts, both with practically no groundwater potential. (6) The quality of groundwater was good. Spring water discharging from shallow aquifers was aggressive, whereas water from deep wells was neutral. ACKNOWLEDGEMENTS
The authors are grateful to the Asian Development Bank for permission to publish the results presented in this paper. During the field work, the authors benefited greatly from the extraordinary helpfulness and kindness of the Indonesian authorities involved of which the Geological Survey of Indonesia deserves special thanks. The authors are also indebted to Dr. O. Berthelsen, Director of the Geological Survey of Denmark,
72
who acted as geological adviser for the project. Various problems and methods have been fruitfully discussed with Prof. Dr. C. Vofite, Jakarta, who has also provided the authors with valuable references on the subject. Mr. Aa. Christensen, Project Manager of the study, and Mr. Howard Thomas, both of Copenhagen, willingly assisted the authors with advice and comments.
REFERENCES Bakker, A.J., 1952. Aantekeningen over het Hydrologisch Karakter van Rivieren op Java. Biro Bendungan dan Hidrometri, Bandung, 114 pp. Bakker, A.J. and Van Wijk, Chr.L., 1951. Infiltration and Runoff under Various Conditions on Java. Biro Bendungan dan Hidrometri, Bandung, 22 pp. Davis, S.N. and De Wiest, R.J.M., 1966. Hydrogeology. Wiley, New York, N.Y., 463 pp. De Jongh, C.A., 1921. Verslag over het Artesische Bekken van Bandoeng en Tjimahi (unpublished). FAO/UNDP, 1973. Water Availability Appraisal. Indonesia, Bogor, 55 pp. Hem, J.D., 1970. Study and interpretation of the chemical characteristics of natural water. U.S. Geol. Surv., Water-Supply Pap. 1473 2nd ed., 363 pp. Jacob, C.E., 1946. Drawdown test to determine effective radius of artesian well. Proc. Am. Soc. Civ. Eng., 79(5): 629--646. Klompe, Th.H.F., 1956. The Geology of Bandung. (Lecture compiled by R.P. Koesoemadinal unpublished ). Purbohadiwidjojo, M.M. and Surjo, J., 1968. Volcanic Activity and Its Implications on Surface Drainage, Irrigation Seminar, Malang, July 9--23, I968, 9 pp. Ryznar, J.W., 1944. A new index for determining amount of calcium carbonate scale formed by a water. J. Am. Water Works Assoc., 36: 472--483. Turc, L., 1956. Le bilan d'eau des sols; relation entre les pr6cipitations, l'~vaporation et l'6coulement. Houille Blanche, Num~ro Spec., A-1956, pp. 216--217. Van Bemmelen, R.W., 1934. Geologische kaart van Java, Toelichting bij Blad 36, Scale 1:100,000, Dienst Mijnbouw Nederlandsch Indi~, Bandung, 95 pp. Van Bemmelen, R.W., 1949. Geology of Indonesia, Vol. I, Government Printing Office, The Hague, 732 pp.