PDF 2003 Ciremai Iagi

8/6/2019 PDF 2003 Ciremai Iagi

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Topic: Environment and hazard mitigation

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Influence of Detail Geological Parameters to Variation of Groundwater Flow Pattern and Spring Discharges

Case Study: East part of Mt. Ciremai,Kabupaten Kuningan, West Java

Deny Juanda P.* Department of Geology, Institut Teknologi Bandung,

[email protected]

D. Erwin Irawan ** Department of Geological Engineering, Institut Teknologi dan Sains Bandung,

[email protected]

Abstract

The decreasing spring discharge has been considered as geological hazards. Many governmental and nongevermental elements has been issuing the mitigation of this hazard. A case study has been carried out on eastslope of Mt. Ciremai. It is a strato-type volcano with elevation of 3072 masl, at Kabupaten Kuningan, West JavaProvince.

This research use 2 methods, described as follows: 1). Surface mapping of volcanic aquifer system on 1 : 25.000map scale. 2). Hydrodynamic analysis, consists of regional flow analysis at Linggarjati and Cibulan spring area;and detail flow at Cibulan spring area.

Fracture zone controls the high discharge of springs in volcanic terrain. There are 2 genetics of fractures:1). Large conduits in between large boulder in Laharic breccia deposits; 2). Cooling joints control the highspring discharge.

The change of rock distribution can form a slope break in elevation range of 500 - 650 masl. The slope break controls the hydrodynamic of groundwater. Therefore the largest occur at elevation range of 500 650 masl. Thevolcanic deposit distribution forms morphological feature of ridges that also functions as groundwater flow path.Rock distribution controls the groundwater movement. The radial distribution of volcanic deposit controls theradial pattern of groundwater flow.

Keywords : geological parameter, volcanic aquifer systems, macro fracture system, groundwater flow pattern


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Skala 1 : 1.500.000

Daerah penelitian

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1. INTRODUCTION

Due to the vast growth of population and industry, the groundwater resources has been decreasingrapidly. The decreasing spring discharge has been considered as geological hazards. Manygovernmental and non gevermental elements has been issuing the mitigation of this hazard.

Concerning the spring management issues, identifying the geological control should be the first step inorder to determine the groundwater flow system. According to Mandel (1981) 1, volcanic aquifersystem shows a distinct aquifer system, which is complicated by the rapid changing of the deposit. Thepyroclastic and lava material are interchanging in close distance.

Some previous research has been done in identifying the recharge-discharge system on volcanicaquifer system, as follows: Asseggaf and Puradimaja (1998) 2; Irawan et.al (2000) 3, Irawan andPuradimaja (2002) 4. All the research were using physical-chemical properties analysis, combined withsurface and subsurface geological observations. The general result is that the groundwater flow involcanic area is controlled by the distribution of volcanic aquifer.

A case study has been carried out on east slope of Mt. Ciremai. It is a strato-type volcano withelevation of 3072 masl, situated 20 km south of Cirebon, Kecamatan Cilimus Jalaksana, KabupatenKuningan, West Java Province ( Figure 1 ). Its diameter from the peak to the foot slope is about 10 km.The location was selected because of the large amount of groundwater which are forming spring beltwith no less than 300 springs; discharged over 1500 l/sec of water (IWACO-WASECO, 1989 5). Thescientific interest is to determine the geological control to groundwater flow and spring discharge.

Figure 1 Location of study area

2. THE METHODS

This research uses 2 methods, described as follows:

(1) . Surface mapping of volcanic aquifer system on 1 : 25.000 map scale. This method was carriedout in order to recognize the geometry of the aquifer and the hydraulic properties of soil(unconfined aquifer) from 10 field permeability measurements. The observations were taken onvolcanic rock exposures and spring locations.

(2) . Hydrodynamic analysis. This method is done in order to recognize the groundwater flow system.The hydrodynamic analysis consists of 2 measurements:a. Regional flow has been done at Linggarjati area and Cibulan area.b. Detail flow has been done at Cibulan spring area.


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3. HYDROGEOLOGICAL CONDITIONS

3.1 The spring and aquifer zonation

All of the observed-aquifer is unconfined aquifer. Based on the elevation, the springs at East slope of Mt. Ciremai (Cilimus-Jalaksana area) can be divided into 3 zones: Zone 1 100-250 masl; Zone 2 250-650 masl; and Zone 3 650-1250 masl. Zone 2 has the largest spring occurences. The upper slope zone,above 1250 masl, there are no spring that can be identified. This zone is labeled as Zone 4 No springzone . Each zones can be grouped based on the aquifers system ( Figure 1 ):

Zone 1 and Zone 2 (100-650 masl). The zone consist of Laharic breccia deposits . The aquifersystem is composed of small to large andecite to dacite boulders with large conduits in between.The conduits set up a good permeability in form of fracture system. The aquifer dischargedfracture spring with total observed spring discharge of 1063 l/sec. The estimated thickness of theLaharic breccia layers is more than 100 m.

Zone 3 Lava flows fracture aquifer system (650-1250 masl). The aquifer system dischargesfracture spring with total observed spring discharge of 80 l/sec Zone 4 No spring zone (1250 3100 masl) . The elevation above 1250 masl is a no spring zone.

The groundwater emerges only in form of seepages.

The slopes angle changes from peak to slope, 19 0 - 4 0 at elevation range of 500 650 masl. Thechange of slopes angle forms slope break, which also can control the hydrodynamics of groundwater.As a result, the largest spring emergence occurs at elevation range of 500 650 masl.

The thickness of the 3 aquifer systems can not be determined, because there are not any subsurfacedata. The main source of data, IWACO-WASECO, 1989 6, has many data located at the footslopes,

only at Laharic breccia distribution area).

3.2 Geological control on spring

All fracture system form springs with large discharge, 10 l/sec to nearly 1000 l/sec. With such largedischarge, it has very high energy that impossible for a porous aquifer system to be kept intact. Fromthe observations, it can be identified only one spring type, fracture spring. The genetic of the fracturecan be grouped in to 2 types:

1. Fracture on lava flowThe fractures are made from cooling joints. The cooling joints forms narrow openings in rock,

however, the frequencies of the joints is very high. The high conduits can transmit aconsiderable amount of water. The genetic of the fracture is shown on Figure 3 .

2. Fracture on laharic brecciaThe fracture are made from large conduits that forming between large boulders. The otherfractures is also formed by the erosional surface between laharic breccia layes, as shown onFigure 4 .


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Figure 1 Volcanic deposit distribution. Laharic breccia deposit is deposited at the lowest elevation (100 - 650 masl). At the upper part,series of lava flow are deposited at elevation range of 650 1250 masl. Next , series of pyroclastic (flow and fall) is deposited, at elevation of

1250 3100 masl. The changing of rock type, from lava flows to pyroclastic breccia, forms a slope break. The slope is changing from 19 0 to 4 0 at the elevation range of 500 650 masl.

Laharic depositLava flowsPyroclastic flowsPyroclastic fall

Laharic depositLava flows

Pyroclastic flows

Pyroclastic flows

Pyroclastic fall


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Figure 3 Fractures on lava flows and laharic breccia. There are 2 spring systems at this location.The upper spring occurs in lava flows aquifer. The spring emerges from fractures from cooling joints.

The lower spring occurs in laharic breccia, emerges from large conduits between large boulders.

Figure 4 Fractures on laharic breccia. The laharic breccia layer form thick layer which is consist of at

least 4 layers. The spring emerge at the erosional surface in between 2 layers of laharic breccia.


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3.3 Field permeability test

From field permeability test (Chow et.al., 1964 6; Miyazaki, 1993 7), it can be concluded that all typesof soil can functioned as potential recharge materials. The conclusion is confirmed by the permeable

soils that varies upon rock type. Soil derived from lahar shows the largest permeability values of 1.26 -2.53 cm/min, followed by residual soil from pyroclastic breccias 1.5 cm/min, and residual soil fromlava flow 0.5 1.2 cm/min. The high field permeability value (Linsley & Franzini, 1978 8) indicatesthat the soil material is very potential to infiltrate rain water into the aquifer.

3.2 Groundwater flow

A. Regional flow

Regional isophreatic map based on spring elevation and water table measurements on 2 areas, area 1Linggarjati area and area 2 Cibulan spring area, shows 2 groundwater flow directions, SW-NE andNW-SE. Based on the condition, the overall groundwater flow is appeared to be radial.

Result on Area 1 shows SW-NE major flow direction with 0.4 of hydraulic gradient, while result onArea 2 presents NW-SE flow with gradient of 0.3. The groundwater flow on both areas is controlledby undulating morphology of strato volcano. This condition is found especially on the slope of riverstreams which consisted of many small depression springs or seepage zones.

B. Detailed flow

One of detail groundwater flow is completed at Cibulan spring area. The spring is located at the endpart of a ridge, which is discharging overall 400 l/sec of groundwater ( figure 5A ). Based on theisophreatic map, the groundwater flows westward which is match the topographical contour, as shownin figure 5B . The topography forms a ridge which consist of 10 m thick of laharic breccia ( figure 5C ).


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Figure 5 The 3 figures shows the situation of Cibulan spring area, theisophreatic map, and the secton of groundwater flow.

A

B

C


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5. CONCLUSIONS

Fracture zone controls the high discharge of springs in volcanic terrain. There are 2 genetics of fractures:

1. Large conduits in between large boulder in Laharic breccia deposits2. Joints in form of cooling jointsThe large inter-boulders conduits and cooling joints control the high discharge from spring.

The change of rock distribution can form a slope break in elevation range of 500 - 650 masl. Theslope break controls the hydrodynamic of groundwater. Therefore the largest spring emergencesoccur at elevation 500 - 650 of elevation. The volcanic deposit distribution forms morphologicalfeature of ridges that also functions as groundwater flow path. Rock distribution controls thegroundwater movement. The radial distribution of volcanic deposit control the radial pattern of groundwater flow.

References

1 Mandel S. (1981). Groundwater Resources: Investigation and Development. Academic Press,pp. 217

2 Asseggaf, A. & Puradimaja, D.J. (1998). Identifikasi Kawasan G. Salak G. Gede sebagai ZonaResapan dan Luahan Daerah Ciawi Bogor Kabupaten Bogor Jawa Barat . Prosiding PITIAGI XXVII, pp. 4-136 - 4-142

3 Irawan, D.E., Puradimaja, D.E., Yuwono, S. & Syaifullah, T.A. (2000)., Pemetaan EndapanBahan Volkanik dalam Upaya Identifikasi Akifer pada Sistem Gunungapi. Studi Kasus:Daerah Pasir Jambu- Situwangi Soreang, Kabupaten Bandung, Jawa Barat , Jurnal BuletinGeologi, Vol 3, Tahun 2000

4 Irawan, D.E., Puradimaja, D.J. (2002). Geological Mapping and Groundwater Physical-Chemical Properties Characterization. An Approach to Spring Recharge AreaConservation , Proceeding of International Conference on Urban Hydrology, KualaLumpur, pp.

5 IWACO- WASECO. (1989). West Java Provincial Water Sources Master Plan for WaterSupply: Kabupaten K uningan. Vol A, Directorate of Water Supply, Ministry of Public Works

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Chow, VT (ed). (1964). Handbook of Applied Hydrology. McGraw-Hill, pp. 12.1-12.307 Miyazaki, T. (1993). Water Flow in Soils. Dekker, pp. 29 45

8 Linsley, R.K. & Franzini, J.B. (1978). Water Resources Engineering. McGraw Hill

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