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Jurnal Sains dan Teknologi Lingkungan Volume 1, Nomor 1, Januari 2009, Halaman 16‐24 ISSN: 2085‐1227

Distribution of Water Resources Potential in Jambusari Area, District Cilacap, Central Java Using Geoelectrical Method∗

Sismanto, Edy Hartantyo, and Mochamad Nukman

University Gadjah Mada, Indonesia e-mail: [email protected]

Abstract

Government of District Cilacap in Central Java plans to develop some new tourism product especially in scientific perception on Jambusari area. This area is potential to be a scientific recreational area in the future. Jambusari is located in the border of district Cilacap-Banyumas at Cilacap-Wangon highway. Unfortunately, Jambusari is lack of shallow ground source. For supplying the water to Jambusari for sustainability of tourism, it needs geological information and distribution of water resources in subsurface. This information can be found by geoelectrical sounding survey on the surface. The 10-point measurements of geoelectrical sounding were performed in August 25 to 29, 2007 by McOhm Mark-2 model 2115. The results show that the water resources are located in the depth of (20-30) m for shallow water as a first aquifer that it is not so significant water assets and the second aquifer is (60-80) m in depth. The second aquifer is a potential water resource to be exploited. The water of both aquifers flow to westward as long as the syncline that it has a direction to northwest southeast. The distance of the central syncline is about 700 m from point 7 of geoelectrical measurement. Keywords: Jambusari Cilacap, geoelectrical, water resources, geology

1. Introduction

To develop some local potential and natural resources of District Cilacap, central Java province via

tourism, the local government especially Tourism Agency of Cilacap will plan to develop a

scientific recreational park in Jambusari. This new scientific tourism object is an integrated

environmental, sociological, technologic and recreational aspect. Jambusari area is located in the

border of Cilacap and Banyumas district in the Cilacap-Wangon highway. Due to the strategic

location and the topographic, this area is potential to be a scientific recreational park in the future.

Unfortunately, Jambusari is lack of water on the surface. For supplying the water to Jambusari for

sustainability of tourism needs, it desires geological information and distribution of water resources

in subsurface. This information can be found by geoelectrical (resistivity) sounding and geological

or geohydrological survey on the surface.

Climatology and Rainfall

Generally, the climate in Indonesia is dry and wet season included in central Java. In rainy season,

the rainfall is an important factor in triggering landslide and mass movements. So that it is

important to know the rainfall rate per year. According to the rainfall map of Cilacap area, the ∗The paper has been presented in poster session of International Conference of Mathematical and Natural Science, ITB, October 28-30, Bandung 2008.

Volume 1 Nomor 1 Januari 2009 Jurnal Sains dan Teknologi Lingkungan

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Jambusari area is about 1.00 mm – 2.00 mm per year. In the north-side area of Jambusari the

rainfall rate is higher than the south side (Dekan, 2007).

Stratigraphy

The geomorphology of Jambusari is generally coarse to fine undulating area. The slope varies from

0% to 50%. In the wavelike hill area is controlled by folded sediment and the several of height is

about (30-550) m above mean sea level. Regionally, Cilacap belong to zone Southern Mountain,

and part of them is in zone Tekukan Tengah and in zone Pegunungan Serayu Selatan (Asikin, et al.,

1992).

Regional stratigraphic of Jambusari consists of Halang Fm., which is intruded by relatively parallel

basalt sheet form to the fault system in the circumstance area. The basalt intrusion occurs after

regional north-south compression to form northeast fault system and east-west folds. Halang Fm.

consists of interbeded of sandstone, shale, clay and tuff by filling of breccia. The sediment

environment of this formation is considered as deep-sea environment which dominated by

turbidities current process (Asikin, et al., 1992). The youngest lithological unit is alluvium that it

consists of shale, sandstone, gravel that dominates in central Cilacap to south Cilacap.

Geological Structures

Generally, the north part of Cilacap area is controlled by folds, faults, and joints as shown in Figure

1. The fold system is more complicated than the fold system in the eastpart of Banyumas district.

The fold (anticline) system in the north part of Cilacap was sliced by faults. The faults in the study

area generally is strike slip fault that slice the proceed anticline.

Figure 1. Regional geological map of north part of Cilacap area (Asikin, et al., 1992). The red

points are geolectrical measurement point.

Sismanto; Edy Hartantyo; Mochamad Nukman Jurnal Sains dan Teknologi Lingkungan

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The major fold is trending east-west and other fold, it might an older old, is trending to the

northeast-southwest. A complex folded formation occurs at the east and the western part at the

geologiocal map of Banyumas Sheet, in the middle part is dominated by a single fold (Asikin, et al.,

1992).

The anticline along Prapagan to Gunung Wetan is bisected by faults system. The anticline across

Jeruklegi has a low limb dip at the north and a slightly forming a recumbent form. This structure

folded Tapak Formation of Early Pliocene age, which indicates the age of deformation is Middle-

Late Pliocene or Plio-Pliotocene (Asikin, et al., 1992).

The fault system at the study area is thrust, strike slip and normal fault. The thrust faults occur at the

west and eastern part of Geological Map Sheet, trending to the east –west. The south block is the

hanging wall. This thrust is bisected by strike slip fault. The fault system at the west is covered by

alluvium deposit and the trace obviously appear along K. Jambuhandap which the Halang Fm. is

crop out (Asikin, et al., 1992).

The tectonic and basin development at Cilacap Area is strongly related to the Eurasia and

Australian plate subduction since Cretaceous time (Early Tertiary age) (Asikin, et al., 1992).

2. Method

The survey were performed by geoelectrical (resistivity) method i.e., Schlumberger sounding

configuration. The lay out is about 500 m by McOhm resistivity meter. The resistivity method is a

non-destructive survey to know the resistivity distribution of the soil vertically. The procedure is by

injecting DC electrical current into the ground, and then measures the voltage on the surface by

double potential electrodes at in line position. The comprehensive theory and method are described

in Telford, et al., (1976). The processing and the modeling use Progress 3.0.

3. Results

Local Geological Information

The oldest rock in this area is sandy siltstone which mainly crop out at the eastern part of Camping

Park. The sandy sandstone is in form of (1-10) cm layer, interbedded siltstone and sandstone which

is intensively fractured. The porosity of this lithology is considered good and suitable for water

reservoir; whereas the siltstone acts as its cap rock. The physical properties of sandy siltstone are

conductive (low resistivity).


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The younger rock is laminated limestone interbedded with siltstone which conformably lies over

sandy siltstone. This laminated limestone is crop out at the north of Camping Park which is also

intensively fractured. The conductivity of this lithology is low, if there is no fluid filling the pores.

The laminated limestone is crop out at the north of highway which is the youngest rock in the area

which interbedded with finely laminated siltstone.

Regionally, the Camping Park of Jambusari is controlled by fold which was uplifted and bisected by

fault trending toward North. Locally, however,this fault is hardy to be found, which is probably

covered by alluvium deposit. The strike slip fault position is interpreted at the east of Point# 7 (500-

700 m). This slip fault is based on the fact of lithology offset. The slickenside is not crop out. The

soil is majorly mixed with finely sand sediment. The infiltration rate is low to medium.

Geoelectrical Results

There are six measurement points in Jambusari Camping Park i.e., point 1, 2, 4, 5, 6, and 7. While

in the north side (Kebun Karet) has two measurement points (10 and KK) and two point in the south

side (8 and 9). Positioning of measurement point was done by Garmin GPS device as shown at

Figure 2. The Progress 3.0 was used to process and to interpret the resistivity model. We state the

correlation among points 5, 1, 7, and 6 is block Guling Badak Southwest-Northeast, while points 4,

2, and 1 is block Guling Badak Nortwest-Southeast, and block Kebun Karet consists of point 10

and KK.

Figure 2. Situation map of the survey area and measurement points of geoelectrical sounding.

There are 3 groups for resistivity correlation i.e., point 5, 1, 7 and 6 as Guling Badak Southwest-Northeast block (blue line), point 4, 2, and 1 as Guling Badak Nortwest-Southeast block (red line),

and block Kebun Karet (point 10 and KK) in green line.


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Block of Guling Badak Southwest-Northeast Area

The result of processing and correlation by Progress 3.0 in block Guling Badak Southwest-

Northeast is shown in Figure 3. The average error of modeling is about 10%. We identify the layers

from the resistivity model. The surface water layer is about 3 m in depth with 3 Ohmmeter in

resistivity (grey color). A, B, C, and D are silty layers (white color), while the depth of shallow first

aquifer is 10 m by thickness about 4 m with 1 Ohmmeter (the green layer), and the depth of shallow

second aquifer is about 40 m with 3 Ohmmeter. These shallow aquifers are not so potential for

water resources due to the small dynamic water flow (Dekan, 2007). The blue one is the deep third

aquifer. The third aquifer as a deep-water aquifer is about 60 m in depth and 1 Ohmmeter. The

identification is based on the range of resistivity value for natural water and surface water in

sediment of Telford, et al., (1976).

Figure 3. Resistivity correlation result of Guling Badak Southwest-Northeast block shows that the

second water layer in the yellow one (the other note see in the text).

Block of Northwest-Southeast Guling Badak Area

Figure 4 is the result of Guling Badak Southwest-Northeast block. The average error of modeling is

about 15%. From the resistivity model, we interpret that the surface water layer is about 1.3 m in

depth with 3 Ohmmeter in resistivity. While the depth of shallow first aquifer is 12 m by thickness


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of 11 m with 13 Ohmmeter. The depth of shallow second aquifer is about 54 m with 6 Ohmmeter

and 45 m in thick. The third aquifer as a deep-water aquifer is about 100 m in depth and 14

Ohmmeter.

Figure 4. Resistivity correlation result of Guling Badak Nortwest-Southeast block shows that the

deep potensial water layer in the yelow one is (40-100)m in depth. Block of Kebun Karet

The result of Kebun Karet block is shown in Figure 5. The average error of modeling is about 10%.

From the resistivity model, the surface water layer is about 2 m in depth with 4 Ohmmeter in

resistivity. While the depth of shallow first aquifer is 19 m with 2 Ohmmeter. The depth of shallow

second aquifer is about 38 m with 4 Ohmmeter and 16 m in thick. The third aquifer as a deep-water

aquifer is about 79 m in depth and 3 Ohmmeter.

The vertical resistivity model at each point in Jambusari area can be prepared a lateral resisvity

distribution to figure out a conductivity area that the layer contains water on groundwater potential

distribution map.


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Figure 5. Resistivity correlation result of Kebun Karet block shows that the deep potensial water

layer in the yelow one is (40-80) m in depth.

Groundwater Potential Distribution

The groundwater potential distribution map of shallow first aquifer is shown in Figure 6. The depth

contour lines are obtained from the low resistivity model and measured from the surface in meter. It

shows that the groundwater at 13 m depth of shallow first aquifer is accumulated in syncline axis

and the flow to westward until 30 m. We guess that the water current flow as long as the syncline

axis that is going northwest ward. The shallow groundwater accumulation will be located on the red

block area as shown in Figure 6. However, according to Dekan, 2007 the both shallow aquifer is not

potential groundwater resources, because the debit is relatively too small to cover the people need.

To find the potential aquifer, we choose the deep aquifer that is the third aquifer located in 60-80 m

as shown in Figure 7 and the water came and flow from north to south area and northeast to

southwest area. Then, the water is accumulated as long as the syncline axis and the depth of water

accumulation in Jambusari Camping Park is about 70 m. Unfortunately, there is no resistivity data

in west side of syncline. If we have some resistivity data in west side, we will confirm the type of

syncline, symmetrical or asymmetrical syncline. However, from the resistivity model, we have


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already tried to estimate that the accumulation of water in the circumstance of syncline axis by

assumption that it is a symmetrical syncline.

Based on the result, we propose a location in red block area (Figure 7) to drill a well for testing

water accumulation and debit measurement as longs the syncline axis to 80 m depth.

Figure 6. Interpretation of the shallow first aquifer distribution and the subsurface groundwater flow. The red block area is predicted as water accumulated area in the circumstance of syncline

axis.

Figure 7. Interpretation of the deep aquifer (3rd aquifer) distribution, the subsurface groundwater

flow, and the prediction area of water accumulation in the circumstance of syncline axis by assumption that it is a symmetrical syncline.


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4. Conclusions

1. The depth shallow groundwater potential layer beneath of Jambusari Camping Park is (20-30)

m. However, the dynamic water content of shallow groundwater aquifer relatively is small.

2. The depth deep groundwater potential layer is (60-80) m in medium to coarse sandstone that it

contents a more groundwater and is suitable to exploit it.

3. For both aquifers, the groundwater is accumulated in the circumstance of the syncline axis and

then flow to westward as long as the syncline that it has a direction to northwest southeast. The

syncline position on the surface is in 700 m from point 7 of geoelectrical measurement.

Acknowledgment

Authors thank to Mr. Kafid, Mr. Widi, Mr. Usmardin, and Mr. Sriwid who are the undergraduate students at Universitas Gadjah Mada, for helping data acquisition and processing, and some field crews from Mining and Natural resources and tourism agencies of Cilacap for further information and discussion, and unanimous reviewer for reading and correcting this manuscript.

References

Asikin, S., Handoyo, A., Pratistho, B., and Gafoer, S. (1992). Geologi Lembar Banyumas, Jawa Tengah. Pusat Penelitian dan Pengembangan Geologi, Bandung. BMG Stasiun, Yogyakarta.

Dekan, J. (2007). Laporan Hasil Pengukuran Zona Aquifer Air Tanah di Bumi Perkemahan Jambusari Kabupaten Cilacap. PEMDAKAB Cilacap, Propinsi Jateng-CV Jasa Tirta. Jawa Tengah.

Prasetyo, L. B. (2004). Deforestasi dan Degradasi Lahan DAS Citanduy. Pusat Studi Pembangunan IPB, UNDP, Bogor.

Telford, W.M., Geldart, L.P., Sheriff, R.E., and Keys, D.A. (1976). Applied Geophysics, ed. 1, Cambridge University Press, Cambridge, London, New York, Melbourne.

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