Caritas And Crs Hydro Geological Report - 07.02.23

Meulaboh - Aceh Barat / Republic of Indonesia

Hydrogeology of the Johan Pahlawan subdistrict Interpretation of the geophysical survey (2005) for the assessment of deep ground water reserves

Final Report Project Management, hydrogeological interpretation: Dr. Willi Finger Hydrogeologist Lavaterstrasse 66 CH-8002 Zürich / Switzerland Seismic field data acquisition, seismic data processing and seismic interpretation: GeoExpert AG Geophysical Prospecting Ifangstrasse 12b, P.O. Box 451 CH-8603 Schwerzenbach / Switzerland Geoelectrical surveying, field data acquisition, data processing and interpretation: Terratec Geophysical Services Schillerstrasse 3 D-79423 Heitersheim / Germany

The contents expressed in the report at hand are solely the opinion of author.

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Table of Content 1. Introduction___________________________________________5 1.1 General..................................................................................................................... ...................5 1.2 The aims of this report..................................................................................... ..........................5 1.3 Data base................................................................................................................. ....................5 1.4 Acknowledgments..................................................................................................... ..................6

2. General setting_________________________________________6 2.1 Location and extent of the study area............................................................... ........................6 2.2 Climate, Rainfall.............................................................................................................. ...........7 2.3 Geological situation.......................................................................................... ..........................7 2.3.1 Regional setting.................................................................................................. ...................7 2.3.2 The Meulaboh Embayment........................................................................................ ............7 2.3.3 Topography............................................................................................................... .............8 2.3.4 Geological units, Lithology....................................................................... ............................8 2.4 Hydrogeological situation...................................................................................................... .....9 2.4.1 Aquifers, aquitardes and aquicludes................................................................................ .......9 2.4.2 Water provenance in the existing wells.................................................................... ..............9 2.4.3 Hydrochemistry............................................................................................ .......................10 2.4.4 Water quality problems...................................................................................... ..................10 2.5 Water resources of the Meulaboh region.......................................................................... .......11 2.6 The water supply of Meulaboh County since the 1920s................................... ......................11

3. The geophysical survey 2005_____________________________11 4. New definition of hydrogeological units____________________12 4.1 Method................................................................................................................................... ....12 4.2 Unit I, including the shallow unconfined aquifer............................................. ......................12 4.3 Unit II, including the middle aquifer(s)....................................................... ...........................13 4.4 The units IIIa and IIIb, including the deep aquifers................................................... ...........13

5. A hydrogeological interpretation along the profiles__________13 5.1 The seismic profile S1 (enclosure S1)....................................................................................... 13 5.2 The seismic profiles S2 - S6 - S7 (enclosure S2b)....................................................... .............14 5.3 The seismic profiles S3 - S5 0-600 m (enclosure S3b)................................................ .............15 5.4 The seismic profiles S5 600 m to the end and S4 (enclosures S3b and S4)........................................................................................................ ..........16

6. A regional geologic interpretation on base of the geophysical survey and aerial photos__________________________________17 6.1 Structural geology and river valleys................................................................................ ........17 6.1.1 Fault zone directions................................................................................................. ...........17 6.1.2 The relation between the fault zones, river valleys and coastlines.......................................18 6.1.3 The present and older meanders of the Kreung Meureubo.......................................... .........18 2

6.2 Sedimentology and stratigraphy..................................................................................... .........18 6.2.1 General...................................................................................................... ..........................18 6.2.2 Progradational deposition patterns.......................................................................... .............18 6.2.3 Cyclic sedimentation sequences...................................................................................... .....19 6.2.4 Old reef structures............................................................................................................. ...19 6.2.5 Old coastlines.......................................................................................... ............................19 6.2.5 Progradation rate and accumulation rate............................................................... ...............20 6.3 A hydrogeological subdivision of the Johan Pahlawan sub-district......................................20

7 Conclusion regarding the hydrogeology and the water withdrawals_____________________________________________21 7.1 Water balance, recharge aereas and exploitable water reserves for the Meulaboh County21 7.2 Water quality......................................................................................................................... ....21 7.4 Summarizing the hydrogeological units and their potential for water withdrawals and additional recommendations when drilling........................................................ ..........................22 7.4.1 The hydrogeological unit I....................................................................................... ............22 7.4.2 Unit II............................................................................................................ ......................23 7.4.3 Unit III.................................................................................................................... .............24 7.4.4 Additional recommendations when drilling for groundwater................................... ............25

8. References____________________________________________26

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List of Enclosures REGIONAL HYDROGEOLOGICAL INTERPRETATION

A

A hydrogeological subdivision of the Johan Pahlawan sub-district

RESULTS OF THE SEISMIC SURVEY WITH A HYDROGEOLOGICAL INTERPRETATION OF THE SECTIONS R

Situation map of the seismic and geoelectrical measurements, scale 1:20’000, Date 15th Sept. 2005

S1

Profile S1: Uninterpreted and interpreted reflection seismic depth section

S2a S2b

Profiles S2, S6 & S7: Uninterpreted reflection seismic depth section Profiles S2, S6 & S7: Interpreted reflection seismic depth section

S3a S3b

Profiles S3 & S5: Uninterpreted reflection seismic depth section Profiles S3 & S5: Interpreted reflection seismic depth section

S4

Profile S4: Uninterpreted and interpreted reflection seismic depth section

RESULTS OF THE GEOELECTRICAL TOMOGRAPHY SURVEY T1

Line T1: Interpreted resistivity imaging results

T2

Line T2: Interpreted resistivity imaging results

T3

Line T3: Interpreted resistivity imaging results

T4

Line T4: Interpreted resistivity imaging results

T5

Line T5: Interpreted resistivity imaging results

T6

Line T6: Interpreted resistivity imaging results

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1.

Introduction

1.1

General

In the aftermath of the catastrophic events caused by the Tsunami of December 26th 2004 Swiss Water experts were sent by the Swiss Development Agency (SDC) to Meulaboh in the Johan Pahlawan sub district in Aceh (District Aceh Barat). The aim of the mission was to find and develop new and sustainable sources of drinking water. In coordination with Caritas Switzerland and Swiss Solidarity, the expert team conducted a study on the geological conditions and the water supply system in Meulaboh. First, all the previously researched data in the area was analyzed, in order to get more information on the characteristics of the groundwater bearing layers and the quantity and the quality of available groundwater. In addition, existing bore holes were analyzed. Secondly, a geophysical survey was implemented in July 2005. It was expected to map the subsurface structures in general, and to identify, in particular, suitable formations as potential targets for a drilling campaign. The data processing of the survey revealed the geometry of the groundwater bearing layers in the subsoil of the sub district Johan Pahlawan. The data is presented in the Caritas Switzerland report of October 2005 [5].

1.2

The aims of this report

The purpose of this study is to understand the hydrogeology of the area where many thousands of tsunami victims are being relocated. It will help in identifying where to drill production wells, which can supply good quality water to the town's water supply. This report combines the results of the CARITAS geophysical survey and the data collection concerning the geology and hydrogeology of the region especially the results of former drilled boreholes. This report describes the hydrostratigraphy of the study area (Johan Pahlawan sub district) and the hydraulic characteristics of the aquifers and the confining units. The hydrogeological framework of the area is presented in profiles and maps.

1.3

Data base

Few reports have been published which describe or discuss the aspects of the water resources of the Johan Pahlawan sub district. The major sources of information are listed in chapter 8. The main information on the regional geology was gathered from the geologic map (1:250'000) [1] and the recent publication on the geology of Sumatra by Barber et al. [2]. A description of the water supply of the region of Meulaboh is given in the 1993 study by IWACO (consultants for water & environment) [9, p. 54/55 and 74/75]. Excellent data concerning the hydrostratigraphy and hydraulic characteristics were gathered from the BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) report [3]. The following borehole data were evaluated: 5

   

1.4

Old "Dutch" boreholes of the 1920s PDAM boreholes of the 1980s PDAM boreholes of 2003 Recently drilled boreholes by following organizations: SDC, Solidarité, Spanish Red Cross and CRS

Acknowledgments

We are greatly indebted to following people who were involved in discussions and provided us valuable information:              

Mr. Saifullah (Director PDAM, Perusahan Daerah Air Minum, Public Water Utility) Mr. Zuardi (Technical director PDAM) Mr. Baharuddin (Drilling enterprise in Meulaboh) Ross Tomlinson (CRS, Catholic Relief Services) Jamie Ashe (CRS) Rimbawan Prathidira (IAGI, International Geosynthetic Installers Association and Spanish Red Cross) Graham Ride (World Vision Groundwater Consultant) Jaques Vuillemin (Solidarité) Ramon Scoble (UNICEF Watsan consultant) Diether Ploethner (BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Germany) Ralph Pahlmann (THW, Technisches Hilfswerk Deutschland) Lambok M. Hutasoit (ITB, Institute of Technology Bandung) Mr. Djaendi (GRDC, Geological Research and Development Centre, Bandung) Ariadi Subandro (IAGI, Association of Indonesian Geologists, Jakarta)

For logistic support great thanks is due to:  

Daniel Beyeler (SDC, Swiss Development Cooperation) Unggul Sudjarat (SDC and Caritas Switzerland)

2.

General setting

2.1

Location and extent of the study area

Extended study area The area relevant for the regional hydrogeology is the so-called "Embayment of Meulaboh". It is a more than 3000 km2 big costal plain, which has a flat relief. Specific study area The old part of the city of Meulaboh was built on the remnants of a patch reef, which juts out 1 km from the land into the Indian Ocean. During the past few decades the city extended to the flat swamp area. The agglomeration of Meulaboh comprises the city and some surrounding villages and belongs to the Kecamantan (subdistrict) Johan Pahlawan. This subdistrict is the specific study area. The population of the sub-district in 2004 was 70’000 people. 6

2.2

Climate, Rainfall

The climate of the District Aceh Barat is tropical with high relative humidity (80-90 %) and little variation in mean daily temperature (25-27 °C) throughout the year. Water resources in the district Aceh Barat are abundant due to high rainfall levels. According to the BGR report [3] the west coast is the wettest region of the whole Aceh province. Following the IWACO report [9] the mean annual rainfall in the surroundings of the city of Meulaboh is 3500 mm, rising to 4500-5000 mm in the nearby mountains. According to Binnie & Partners 1986 [3] the mean annual potential evapotranspiration in the Meulaboh region amounts to 1340 mm.

2.3

Geological situation

2.3.1 Regional setting The following description is taken from the 2006 geological study by order of CARITAS Switzerland [13]: Sumatra is located at an active shelf edge where the Indian oceanic plate plunges north-northeast beneath the continental Burma microplate (part of Eurasian plate) at a rate of 60 mm/year. In this context two massive transcurrent fault systems developed, the Sumatran Fault System and the Mentawai Fault. Transcurrent fault systems are vertical faults that displace parts of the plate along the fracture plane horizontally. Due to the different motion velocities, high stresses develop and provoke ‘connecting faults’ like the Batee Fault. From the BGR report [3, p.7]: Sumatra is part of the Sundaland continental plate, which includes a big part of South-East Asia. To the west, the oceanic crust floor of the Indian Ocean belongs to the Indian-Australian Plate. This plate is being subducted along the western margin of the Sundaland Plate. Here the Sunda Trench is located just off the west coast of Sumatra. Magma generation associated with this subduction has given rise to the NW-SE directed Sumatran volcanic arc. Dextral fault movements parallel to the plate margin have released stresses, which result from the oblique approach and subduction of the incoming oceanic crust. This includes the December 2004 earthquake that was triggered when an average drop of about 7 m occurred along a 340 km long fault; eventually this rapid tectonic movement caused the destructive tsunami.

2.3.2 The Meulaboh Embayment The "Embayment of Meulaboh" (called Meulaboh Embayment below) is easily recognizable on maps (see for example Cameron et al., 1983: The geology of Takengon quadrangle, Sumatra. Explanatory note and geological map 1:250'000 [1]). After Karig et al, 1978 [2] westwards-throwing movements on the Anu-Batee Fault formed this embayment. It represents a so-called progradating delta sedimentary complex. Progradation is the basinward (oceanward) shift of shorelines and the simultaneous vertical increasing of a sedimentary complex. The reason for these events is (simplified) the sinking of the delta area relative to the mountain range and the transport of eroded rock material from the mountains to the foredeep. The subsidence however did not occur constantly, but was interrupted by short and sudden phases of uplifting. As a consequence of these variations in the vertical movements 7

relative sea-level changes occurred, i.e. phases of transgression alternated with phases of regression. In a general view the deltaic sediments progradated from northeast to southwest. The major parts of the complex are the rocks of the Tutut Formation (QTt in the geologic map [1]). The laying down of this more than 1000 m thick sedimentary layers began sometime in the Pliocene, i.e. 4-5 million years before today. During a long lasting subsidence phase, fluviatile outwash conglomerates, sandstones and lignitic clays, as well as lacustrine sediments and beach sands were accumulated, with reefs situated offshore. A strong uplift marked the end of the Tutut Formation sedimentation. This uplift was followed by enhanced volcanism and, near the coasts, by new subsidence movements and temporary marine transgression, during which the fluvial to paralic Meulaboh Formation was laid down.

2.3.3 Topography The costal plain of Meulaboh Embayment is 100 km wide and 50 km long. The area of elevations near sea level generally extends for about 1-2 kilometers.

2.3.4 Geological units, Lithology The Meulaboh Embayment consists of a southwestward thickening wedge of sediments composed principally of unconsolidated gravel, sand, silts, and clays with variable amounts of peat, lignite and corals. From land surface to about 1000 m below sea level the sediments are unconsolidated to poorly lithified. Beneath this sediment there is Tertiary bedrock. The layers of the deposits are virtually flat with a dip of less than one degree to the southwest. The deepest poorly lithified sediments were deposited about 2.6 million years ago during the Early Piocene period, and some of the shallowest sediments were laid down within the past 10'000 years (Holocene). Alluvium (Holocene) The most recent sediments of the Holocene (Qh on the geologic map [1]) can be divided in the continental and the paralic alluvium. Continental Alluvium The continental alluvium consists of fluvial gravels, sands and clays in graben-like depression fillings along the river valleys. Paralic Alluvium: In the coastal zone the alluvium consist of beach deposits forming sandy ridges, coral debris, carbonatic mollusc and foraminifera sands and clays. A reef builds up the promontory of Meulaboh. A similar structure exists some 10 km to the north in Lhok Bubon (Samatiga). Meulaboh Formation (Pleistocene) The reworked gravels, sands and clays of the younger Pleistocene are named as the Meulaboh Formation and marked with Qpm on the geologic map [1]. Their thickness varies between 20 and 50 m. Northwest of the River Kreung Meureubo the formation covers up to 15 km inland; southeast of the river it only covers 5 km. Tutut-Formation (Plio-Pleistocene) The Tutut Formation (QTt on the geologic map) consists of several hundred meters of poorly lithified conglomerates and sandstones, lignitic mudstones, lignites and seat earths. The accumulation occurred during the Pio-Pleistocene in a fluviatile to paralic environment. Coal 8

layers of a minimum thickness of about 240 m are known in the NNE of Meulaboh (more details in the BGR report [3]).

2.4

Hydrogeological situation

2.4.1 Aquifers, aquitardes and aquicludes The water supply in the Meulaboh Embayment depends, in addition to the surface water, on groundwater tapped from unconsolidated or semi-consolidated permeable sediments (mainly gravels, sands and silty sands as well as conglomerates and sandstones) or fissured rocks, so called aquifers. In contrast to the aquifers, the less permeable confining sediments like silts, clays and mudstones can be designated as aquitardes (semi-confining) and aquicludes (impermeable). Each of the geologic formations listed in section 2.3.4 contain one or more aquifers: 1. In shallow depths there is the unconfined aquifer running in the alluvium, in the fluvial gravels and sands as well as in the beach sands and the reef related sediments. The aquifer is tapped by a large numbers of wells down to 5 m. 2. The deeper coarse layers of the Meulaboh Formation represent the next semiconfined aquifer with low hydraulic gradients. The water of these sandy layers can rise in drilled boreholes. 3. The confined aquifers in the Tutut Formation have a high hydraulic gradient. The groundwater is constrained to semi-consolidated conglomerates and sands or fissured lithified rocks. Quoted from the BGR report [3], “apart from the uppermost aquifer in shallow depths the Meulaboh Embayment consists of a confined multi-layer porous aquifer system that often sustains artesian flow and has a moderate groundwater potential”.

2.4.2 Water provenance in the existing wells Because of the moderate potential of the individual aquifers, in the deep wells drilled in the past all the found prominent coarse layers were equipped with screened casings: 



The water tapped in the "old Dutch wells" on the Meulaboh promontory drilled in the 1920s comes from a) the coarse Holocene sediments related to the reef, and b) the aquifers of the Meulaboh and Tutut Formations (see PT Dasa Catego Birki, 2006:IRKI (2006): Technical and Work Plan Proposal for Water Supply Meulaboh [10]). The well yields are up to 10 l/s. The water is still flowing from these wells. According to the 1983 report of the German Hydrogeological Advisory Group [8] the PDAM wells drilled in the 1980s produce water from both a) the semi confined aquifer of the Meulaboh Formation and b) the confined aquifers of the Tutut Formation. The total production from 5 deep wells was 25 L/s (design capacity 40 L/s following the IWACO report [9]).

The "small deep wells" drilled by organizations like SDC, NCA, Spanish Red Cross and CRS are believed to tap the uppermost part of the Tutut Formation represented by semiconsolidated sands and gravels in depths of 85 to 100 m, whereas the well Solidarité in Gampa 2 is believed to tap water of good quality from the Meulaboh Formation. The wells drilled by a drilling company from Medan in February 2005 near the road to Samatiga found salty water in the aquifer of the Meulaboh Formation in depths around 50 m.

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2.4.3 Hydrochemistry The 2005 report of the BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) [5] contains an excellent description of the hydrochemistry of the local groundwater. The main geochemical processes in the water can be summarized as follows:  





The genuine composition of the groundwater has been altered by geochemical interactions. Groundwater has been frequently depleted of dissolved oxygen, resulting in concentrations as low as 0.1 mg/L O2. The oxygen in the groundwater is consumed first during microbial decomposition of organic matter (i.e. in peat, lignite or coal). Following reaction sequence results: a) Microbial nitrate reduction to the final products nitrogen (N) and dioxide (CO2). b) Mobilization of iron and manganese. c) Sulfate decomposition to hydrogen sulfide (H2S) by oxidation of organic carbon. Soluble iron (Fe II) precipitates as solid sulfides.

The microbial alteration as a common process in the aquifer system of the Meulaboh Embayment is proved by the enrichment in bicarbonate (HCO 3), ammonia (NH3 and NH4), iron (Fe) and manganese (Mn) concentrations and the depletion in sulfate (SO4) and nitrate (NO3) concentrations in most of the groundwater samples either from shallow, medium-deep, or deep wells. The comparatively low concentrations of calcium (Ca) and magnesium (Mg) in the groundwater is explained in [5] by the fact that the water has passed along its flow path from the recharge to the discharge area through clayey aquitards. For more detailed description of the hydrochemistry of the local groundwater it is recommended to read the BGR report by D. Ploethner and B. Siemon [3, chapter 4, p. 19-26].

2.4.4 Water quality problems In the study area the top aquifer is shallow and widespread but known to be contaminated. There is also a range of problems in locating useful aquifers below the top aquifer and in obtaining suitable quality water for domestic use. A major problem in many areas is the high iron content of the groundwater. Another problem is that some aquifers in the coastal zone contain brackish or salty groundwater. The groundwater in the coastal zone is brackish to saline in many places. This water originates from the time during which the land was below sea level. The land level of Sumatra rose relatively quickly by uplifting; due to the low permeability of the clayey sediments the saline groundwater was not flushed out by fresh water. Fresh groundwater is found near the coast in the sandy beach deposits and more inland, in alluvial fans along the rivers. The tsunami contaminated surface waters in many areas near human settlements as the contents of septic tanks were mixed with seawater and other surface materials. Streams and rivers are generally anticipated to have been flushed clean, but the effect of saltwater and other materials intrusion into groundwater systems is of concern. The water obtained from many of the old wells was of insufficient quality, related to the content of iron, manganese and ammonium.

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2.5

Water resources of the Meulaboh region

For the cities and villages of the Meulaboh Embayment there are two main raw water sources available, groundwater and surface water (see IWACO report [9]). Springs are not available in the area. An option to consider in the future is the rainwater collection.

2.6

The water supply of Meulaboh County since the 1920s

The introduction of piped water in Meulaboh dates back to the 1920s and had been the base of the water supply until the 1990s. Dutch engineers constructed two wells in 1923 and 1926 respectively. From 1982 to 1984 six wells were constructed by the PDAM. These wells produced groundwater from maximal depths of 175 m. According to the IWACO study [9] the city of Meulaboh had a water supply system in the 1980s, taking the water from 5 deep wells 4 km north of the town. The total production was 25 L/s (designed capacity 40 L/s). The concept was based on ground water, iron removal treatment, and clear water pumping, with 24 hours per day operation. The main components were constructed in 1982 and in 1987. The quality of the abstracted ground water was below standard for two reasons: a) Algae found in some of the wells b) Iron contamination above the acceptable limit To reduce the problem of iron, iron treatment facilities with a capacity of 50 L/s were constructed in 1987. Unfortunately, for reasons related to operation and maintenance, this treatment plant reduced the iron concentration by only 20 percent. The use of the treatment plant therefore ceased some time ago. At the beginning of the 1990s the system was simply by-passed. Water was pumped directly from the wells into the system. In the years 1996/97 the PDAM Meulaboh installed a surface water system. Since this time, an average amount of 80L/s of water was taken from the river at the water intake station near the Kreung Meureubo and conducted to the treatment plant, where 65 L/s arrived. Sixty litres per second were fed into the distribution net, and 50L/s of piped water reached the consumers in approximately 3000 houses. The 1993 IWACO study [9] calculated the water demand for the Meulaboh County in the year 2004 at 150 L/s. Neither by the former groundwater system (25 L /s) nor by the river water system (50 L /s) can this demand be covered. The remaining 75 L/s are taken from the shallow aquifer by dug wells.

3.

The geophysical survey 2005

In the month of July 2005 a joint application of reflection seismic profiling (by GeoExpert AG, CH-8603 Schwerzenbach, Switzerland) and geoelectrical tomography imaging (by Terratec Geophysical Services, D-79423 Heitersheim, Germany) was carried out. The geophysical survey is described in detail in the CARITAS/CRS report of October 2005 [4]. Materials brought to Indonesia for the survey included computers, geophones, registration and measuring instruments, cables as well as a large weight dropper. All together almost two tons of materials were taken. Six specialists from Europe were present, supplemented by

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local auxiliary workers. The team was also aided by a geologist of the CARITAS Switzerland project team (W. Finger). Seven seismic profiles with a total length of 6.65 km and 1188 field records were measured. The six geoelectrical lines had a total length of 6.762 km. Thirteen geoelectrical deep soundings have been made. The reflection seismic profiling is based on the echo sounding principle. Shock waves emitted from an impact source, as a hammer or weight dropper, penetrate the subsurface and are reflected back to the surface at the boundaries of geological structures with contrasting rock density values. Since rock types are characterized by different acoustic wave propagation velocities, the shape and the size of individual rock formations can be mapped. The geoelectrical tomography imaging method identifies water bearing geological formations by mapping electrical resistivity contrasts. In this survey, the attention is focused on separating sand and gravel containing clean water, characterized by high resistivity values, from tight shale layers with typical low electrical resistivity.

4.

New definition of hydrogeological units

4.1

Method

The following discussion on the hydrostratigraphy of the investigation area results from a combined interpretation of the geophysical survey and the data from former borehole drilling logs. The Reflection Seismic Sections presented in the CARITAS/CRS report [5] were therefore reinterpreted in collaboration with W. Frei (GeoExpert AG). The reinterpretation of the geophysical results was made with the aim to get a homogeneous model of the hydrostratigraphy in the subdistrict Johan Pahlawan. For this purpose hydrostratigraphic units are defined in contrast to the geological units (as Alluvium, Meulaboh and Tutut Formations). These are time-stratigraphic units. On the other hand aquifers and confining units are hydrogeological units defined by the water transmitting characteristics of the sediments. A hydrostratigraphic unit (including by aquifers and confining units) may be composed of multiple geologic units, parts of two geologic units or a part of only one geologic unit. By defining the hydrostratigraphic units as a succession of underlying confining clay layers and coarse sand/gravel layers above, an attempt was made to combine coarsening upwards clay/sand-cycles, as they are known in progradating delta sediments. The reflectors A, B and C, which separate the units, therefore were laid on the top of a sandy layer. The hydrostratigraphic units defined for the Kecamantan Johan Pahlawan can be applied to a big part of the Meulaboh Embayment.

4.2

Unit I, including the shallow unconfined aquifer

The hydrostratigraphic unit I comprises:  The topmost unconfined shallow aquifer and the underlying confining clay rich sediments.

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The reflector A marks the lower boundary on the seismic profiles. This unit corresponds to the rock formation alluvium and the topmost clays of the Meulaboh Formation. The unconfined aquifer of the unit I is designated in this paper as "shallow aquifer".

4.3

Unit II, including the middle aquifer(s)

The hydrostratigraphic unit II is grouped:  The sandy sediments, appearing directly below the confining clays of unit I.  The underlying clays and lignites of the Meulaboh Formation.  In some parts other sandy layers are intercalated in the clays. The unit II is defined in seismic profiles between the reflectors A and B. The unit corresponds more or less to the Meulaboh Formation without the topmost clays. The semi-confined aquifer of the unit II is designated in this paper as "middle aquifer".

4.4

The units IIIa and IIIb, including the deep aquifers

The hydrostratigraphic units III correspond mainly to:  The Tutut Formation, with its several hundred meters of sediments in the Meulaboh Embayment. The differentiation between IIIa and IIIb is arbitrary. We define IIIa as the part of the Tutut Formation in which the layers are known, on the basis of the old drilling results. The unit IIIa therefore represents the sand and clay intercalations between the bottom of the unit II, marked by the reflector B, and the reflector C in depths between 180 and 200 m. In most places there are no data from the geoelectric survey available at the depth of 100 m. Therefore the upper limit of the unit IIIa can only be defined with security where past drilling results are available. There the reflector C was positioned in the surroundings of the old boreholes on top of a prominent sandy layer. The lower limit of the unit IIIa is positioned at the supposed bottom of the 20 to 30 m thick clay layer in the lowest part of the 1980s boreholes. From these places the reflector C (and consequently the bottom of the unit IIIa) was prolonged to other parts of the seismic profiles. As quoted above, the differentiation between unit IIIa and unit IIIb was not made on the basis of a lithological change, but only because of logistic reasons. The aquifers in a depth between around 180 and 300 m were the main target of the planned drillings. Therefore the sediment body between around 120 and 300 m consisting most probably of the same clay/sand intercalations like in the unit IIIb are painted by a green color in the interpreted reflection seismic profiles (enclosures S1, S2b, S3b, S4). The confined aquifers of the unit III is designated in this paper as "deep aquifers".

5.

A hydrogeological interpretation along the profiles

5.1

The seismic profile S1 (enclosure S1)

The database consists of the seismic profile S1, the geoelectric lines T3 and the nearby CRS borehole. Unit I

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The unit I has a thickness up to 40 m. The shallow aquifer with a thickness of 10 m is present only from S1 0 to 200 m. Closer to the S the ground consists of peat and organic material. Unit II The unit II goes down to depths between 70 and 95 m and has a thickness of 40 to 70 m. Sand/silt of the middle aquifer was found by geoelectrical tomography at the road in a depth of around 50 m. Unit III At the beginning of the profile, three coarsening-upward clay-sand cycles are believed to exist in the unit IIIa in depths between 80 and 170 m. More cycles of the same type seem to be present in unit IIIb. Faults A vertical fault zone is observed between S1 400 to 420 m. The vertical throw at 300 m depth is at least 10 m. The existence of the fault is best evidenced by the inflection of the reflector B at a depth of 70 m below S1 410 m. Surface water The creeks leaving the swampy areas contain dark colored water with a high content of humic acids and are therefore not usable for drinking purposes. Shallow aquifer of unit I The water of the aquifer I is tapped by dug wells. The quality deficiency of the water is known. Middle aquifer of the unit II The first close by the CRS drilled borehole found plenty of gas in a depth of around 50 to 60 m. It is therefore not recommended to use the water of the middle aquifer. The deeper aquifers of the unit III The seismic survey revealed undisturbed intercalations most probably of coarser and finer grained sediments below of a depth of around 80 m in the areas northeast of the road. Here, deep drilling for aquifers between 80 and 300 m can be successful.

5.2

The seismic profiles S2 - S6 - S7 (enclosure S2b)

The database consists of the seismic profiles S2, S6 and S7, the geoelectric lines T1, T3, T4, T6 and the deep boreholes of the 1980s in this region. Unit I The reflector A and therefore the lower limit of unit I lies in a depth of 30 to 50 m between the beginning of S2 until the end of profile S7 with the exception of S6 160 to 260 m (see below). The top aquifer is around 20 m thick and has electrical resistivities between 0 and 40 Ohm*m. Between S6 160 and 260 m an old buried river valley is visible, as they exist numerously in the vicinities of the river Kreung Meureubo (see S3 and chapter 5.4). We assume the existence of a riverbed filled by sand below the swampy ground. Unit II The lower boundary (reflector B) lies between 100 and 120 m. In the most parts of the profile the boundary is in a depth of 100 m. The average thickness is 70 m.

14

The middle aquifer begins in a depth between 15 and 40 m. The geoelectrical tomography revealed at S2 585 m sand/silt-layers at a depth between 25 and 50 m with resistivities between 20 and 160 Ohm*m. The buried river valley is also visible in unit II. It seems that the valley is related to a fault zone (pair of faults). In S7, a salient structural feature of 200 m length straddles the point of intersection with the geoelectrical profile T1 at depths between 30 and 90 m. The shape of this structure is that of a reef. This assumption is corroborated by abnormal high resistivity values found by the geoelectrical sounding VES 7 between 45 and 70 m depth. Further evidence of a buried reef structure is also provided by the high resistivity anomaly on the profile T1. A smaller buried reef like structure is observed at the SW end of the seismic profile S7. Unit III Depositional structures are well imaged down to a depth in excess of 300 m on the NE-half of the profile S2. Correlating to the wells E and C, two major aquifer zones can be interpreted in the unit IIIa, one zone on the top of the unit, and the other zone in the middle. The same feature is applied to the SW-half of the profile S2 and to profile S6. The aquifer sketched in the unit IIIb was only drawn in a hypothetical way. The results of the seismic survey identify long profile segments (S2 0 to 560 m, and 700 to 1000 m, S6 0 to 180 m) on which sedimentary sequences are not affected by compaction faulting or tectonic activities. The data quality at depths in excess of 100 m is very poor in profile S7. The only visible structure is a possible buried reef structure at S7 260 to 380 m. Faults A well defined graben fault structure of about 60 m width is the dominant feature between S6 180 and 240 m. Top aquifer of unit I The water of the aquifer I is tapped by dug wells. The quality deficiency of the water is known. Middle aquifer of the unit II The nearby borehole drilled by the CRS in 2005 found plenty of gas in a depth of around 50 to 60 m. It is therefore not recommended to use the water of the middle aquifer. Deep aquifers of the unit III The results of the seismic survey identify long profile segments (S2 0 to 560 m, and 700 to 1000 m, S6 0 to 180 m) on which sedimentary sequences are not affected by compaction faulting or tectonic activities. Within these subhorizontally layered sequences, prominent reflectors denote sand or gravel formations that may be potential aquifers. The data quality at depths in excess of 100 m is very poor in profile S7. The only visible structure is a possible buried reef structure at S7 260 to 380 m.

5.3

The seismic profiles S3 - S5 0-600 m (enclosure S3b)

The database consists of the seismic profiles S3, S5 (0 to 600 m) and the geoelectrical profiles T4 and T6.

15

Unit I At the beginning of S3 the unit I is only 15 m thick. The shallow unconfined aquifer has a maximum thickness of 8 m (borehole D). Towards the end of S3 the unit increases to 45 m. The thickness of the shallow aquifer varies and can reach 25 m. The sands and gravels show resistivity values of 40 to 160 Ohm*m. The well "Solidarité" at S3 360 m reveals clay and peat between depths of 10 and 30 m [11], which is in perfect agreement with the low resistivity (0 to 10 Ohm*m) zone imaged on the geoelectrical line T6. A seismic low reflectivity zone is observed in this depth range. At the beginning of profile S5 the unit I is 50 m thick. Here typical progradational deposition pattern are visible, i.e. the layers are stronger inclined seawards than the land surface. The base of the progradation is the top of the unit II marked by the reflector A. This means that the top of the unit II represents an ancient land surface. The sediments above this base were deposited seaward step by step. Unit II At the beginning of S3 the unit II is approximately 70 m thick and in correlation to well D, seems to consist mainly of clay rich sediments. A sand layer at the top of the unit II can be interpreted from the beginning to S3 280 m. Between S3 280 and 380 m a buried river bed can be interpreted from the seismic record. On the basis of the drilling results of the well "Solidarité" it can be assumed that a prominent part of the old river valley is filled by sand. To the S the upper sand layer can be followed to S3 680 m. Then, further along the S the data quality degrades. Another buried river bed could be present between S3 680 to 780 m, however, the data quality is too poor to make reliable statements. In profile S5 0 to 600 m progradational sedimentation patterns are visible, similar to unit II. A first sand layer is believed to begin at S5 0 m and end at 200 m. A second sand layer between S5 380 and 620 m is tapped by a private well in a depth of 70 m, where "good" water was found. Unit III The results of the seismic survey identify profile segments (S3 0 to 280 m, and 400 to 600 m, S5 0 to 600 m) on which sedimentary sequences are not affected by compaction faulting or tectonic activities. According to the sediment succession in well D, three sandy layers can be defined at the beginning of profile S3. In the profile segment S to S3 400m, two sandy layers seem to exist. In the profile segment S3 600 to 1140 m there is the possibility of existing aquifers. Between S3 680 and 760 m the existence of a buried riverbed is probable. However, because of the poor data quality no ensured statements can be made. Faults A fault zone between S3 280 m and 400 m seems to be a transcurrent fault intersecting the seismic profile at an oblique angle, i.e. the fault running NNE-SSW.

5.4

The seismic profiles S5 600 m to the end and S4 (enclosures S3b and S4)

The database consists of the seismic profiles S5 600 m to the end and S4, as well as some few existing boreholes. Unit I The thickness of the unit varies between 20 m and 50 m. Due to the lack of borehole data between S5 600 m and 2700 m, the differentiation between the sandy aquifers and the confining clays is made in a very hypothetic manner in this sector of the profile. We believe that between S5 600 m and 2000 m the shallow aquifer consist of alluvial sands. Most 16

probably buried river beds are existing between S5 1180 m and 1360 m as well as between S5 1820 m and 1920 m. And most probably between S5 2100 m and 2380 m there is a filling with reef sands in the top 20 to 30 m. At the end of the profile, according to the strata logs of the wells A and B the unit I can be interpreted to have a thickness of 30 m. The unit consists of coral sand and silt. In the profile 4 the unit I has an average thickness of 30 m and the shallow aquifer is between 5 and 10 m thick. Unit II In profile 5 600 m and 2700 m the lower limit of unit II (marked by reflector B) lies between a depth of 90 and 100 m. The thickness varies between 50 and 80 m. A subdivision of the unit in two parts is evident by the existence of a reflector. In absence of borehole data we don’t interpret the lithology of the strata in this sector of the profile. At the end of the profile, according to the strata logs of the wells A and B the unit II consists mostly of coral and foraminifera sands in the top 40 m and predominantly of clays in the lower part. The correlation with a near-by shallow well (90 m) drilled by the Norwegian NCA gives the possibility to assume that the middle aquifer (in a depth between 20 and 55 m) consist of coral sands and the lower part of the unit II of clays with a thickness of about 50 m. Unit III and faults The unit III in profile S5 600 m to the end seems to consist of a uniform multilayered aquifer system disturbed by only a few faults. Those near S5 700 m, 800 m, 1140 m and 1300 m are affecting the unit III b, whereas the fault near S5 1640 m and especially the fault system between S5 2200 m and 2280 m are crossing reflector C and affecting the units IIIb and IIIa. In profile S4 many faults are present. Between S4 160 and 220 m a pronounced fault cropping near S4 140 m is observed.

6.

A regional geologic interpretation on base of the geophysical survey and aerial photos

6.1

Structural geology and river valleys

6.1.1 Fault zone directions The seismic survey revealed the existence of fault zones in every profile. A correlation between the different profiles shows the faults are running in two main directions at right angles to each other:  NW-SE, parallel to the coastline,  NE-SW, parallel to the river valley axis. The major NW-SE running faults were detected in several profiles:  S1 420m  S6 180 - 240 m (this fault zone is related to a buried river bed)  S3 280 - 400 m, 580 m  S5 820 m, 1290 m, 1650 m, 2200 - 2280m  S4 In S3 there is a faulted zone, which intersects the profile a oblique angle: 17

 S3 260 - 420 m (this fault zone is related to a buried river bed) Even if it has not been proved, we believe that the faulted zone is running NE-SW parallel to the valley axis of the river Kreung Meureubo.

6.1.2 The relation between the fault zones, river valleys and coastlines In the following, we present two hypothetical statements concerning the relation between the fault zones, river valleys and the present and former coastlines: 1. The geographical course of the major river valleys of the Kreung Meureubo (with its mouth in Johan Pahlawan) and the Kreung Bubon (in Samatiga) is controlled by the prominent fault zones running NE-SW, which cut the sedimentary bulk of the Meulaboh Embayment into distances of 10 to 20 km. The seismic survey did not reveal in the Tutut Formation the existence of buried major river valleys in the swampy areas outside of the river plain of the Kreung Meureubo. We suggest that the major rivers ran through the same area during the entire Pliocene. 2. It seems that the old prominent coastlines (see section 6.2.5 below) are accompanied by NW-SE running fault zones (see enclosure A). For example in the fault zone visible in S5 820 m, the fault lies in the prolongation of a presumed former coastline parallel to the old road between Meulaboh and Samatiga. Also, the fault zones in S3 280 – 400 m and S6 180 – 240 m seem to correlate to a former prominent coastline. It is remarkable that in two cases smaller river beds are connected to these fault zones. On the map (enclosure A) it is evident that the creeks Luang Cina and Kreung Leuhan are running in great parts NW-SE, i.e. parallel to the supposed fault zone direction parallel to the shoreline.

6.1.3 The present and older meanders of the Kreung Meureubo Aerial photos (see enclosures E and F) show the actual meanders of the Kreung Meureubo. Former river beds are recognizable particularly to the northwest of the today’s river course. Clearly visible is an old river bed in Lapang running from Mesjid Raya to Lapang (PDAM river water treatment plant) and onward to Gampa 1. Interpreting the aerial photos it seems that the river course oscillated in the past with an amplitude of 2 km.

6.2

Sedimentology and stratigraphy

6.2.1 General As written in section 2.3.2, the Meulaboh Embayment represents a so-called progradating delta sedimentary complex, which is characterized by an oceanward shift of shorelines and a vertical thickening of a sedimentary complex. This leads to a general subsidence of the foredeep and its filling by detritus from the mountains.

6.2.2 Progradational deposition patterns The seismic reflection profiling revealed typical progradational sedimentary patterns, particularly in profile S5 0 - 800 m (see enclosure S3b). Here the profile runs precisely parallel to the valley axis of the Kreung Meureubo. The layers show here an inclination of around 6°. Further to the west (S3 800 – 2740 m) the layers are almost sub-horizontal with inclinations of 1°, the same as in the profiles 2-6-7 (see enclosure S2b). The interpretation of these facts is that typical progradational deposition patterns can be observed, particularly in the river valley perpendicular to the coastline. On the flood plains 18

between the river valleys, fine grained detritic sediments (clay and silt) accumulated when they were flooded by river water. Otherwise the plains were covered by swamps, where peat and lignite sediments were formed. In the coastal region sand was deposited by currents at an oblique angle to the coastline.

6.2.3 Cyclic sedimentation sequences From the literature referring to this topic (not quoted here) it is known that the sedimentary records in deltaic environments are characterized by an increase of the grain sizes from the bottom to the top of certain layer sequences. Such so-called coarsening upwards sequences are recognizable on the logs of the existing boreholes [8]. Major cycles are in the range of 80 to 100 m. They are subdivided into small scale cycles of 20 to 40 m. In the old Dutch borehole B a major cycle exists between 108 and 27 m depth. This cycle is divisible into small cycles at depths of between 108 to 67, 67 to 45, and 45 to 27 m. In borehole A, a major cycle between 183 and 99 m depth and a second between 99 and 14 m can be defined. It can be summarized that the major cycles are 80 to 85 m thick, whereas the sub-cycles have an average thickness between 25 and 30 m. The Tutut Formation has an estimated thickness of 700 m, i.e. the formation is built up of about 9 major cycles or around 26 sub-cycles. This corresponds to the more than 20 cycles of warm and cold periods, which are described in the modern literature to have existed in the Pleistocene. It is assumed that the Tutut Formation represents around 2.6 millions of years. Therefore it can be calculated that one sub-cycle represents approximately 100'000 years.

6.2.4 Old reef structures The existence of a recent active coral reef in Ujong Karong on the promontory of Meulaboh is proved by the observable sediments. Some 14 km to the northwest in Samatiga there is the reef of Lhok Bubon with a similar geographic position. In both cases a river mouth is at a distance of 2 km to the northeast: the mouth of the Kreung Bubon in Samatiga and that of the Kreung Meureubo in Meulaboh. The most evident old reef structure belonging to the unit II was revealed by the reflection seismic in Gampa 2 (see enclosure S2b: profile S7 260-620 m). Another possible reef emerged in profile S4, situated midway between the reefs of Ujong Karong and Gampa 2. Furthermore in Leuhan, the geoelectrical sounding VS3 of the line T3 revealed a structure with high resistivity values (see enclosure T3). This must not be a technical disturbance as mentioned in Caritas/CRS report 2005 [5]. Because of the similarity to the structures in T1 (see enclosure T1: VES 9 and VS7), the existence of an old buried reef can not be excluded.

6.2.5 Old coastlines The horizontal growing of the Meulaboh Embayment delta is recognizable by old coastlines. These are best visible from the air, either from helicopters and airplanes or on aerial photos. The German BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) helicopter pilot 19

Michael Schütt documented this in an inedited paper [12]. He observed between Calang and Meulaboh the existence of dike-likes features, which often run parallel to the actual coastline for several kilometers. The dikes are cut here and there by river mouths. The distance from dike to dike is about 250 m. On a larger scale prominent old coastlines are visible on aerial photos (enclosure A). At a distance of about 2 km from the actual beach there is a clear line evident running parallel to the actual shoreline, both to the northwest and to the southeast of the river Kreung Meureubo. This picture is amplified in the northwest by the presence of the old road joining Meulaboh and Samatiga. Here, 2 km more landward a second linear element parallel to the two mentioned above can be interpreted. A distance of 2 km seems to represent the step amplitude of the lateral increasing of the Meulaboh Embayment delta. As the distance between the coast in the Meulaboh region and the foot of the Berisan Mountains is around 50 km, the number of seaward growth stages can be calculated at around 25, i.e. the same number as the sub-cycles in the vertical sedimentary record (see section 6.2.3).

6.2.5 Progradation rate and accumulation rate The average apparent shoreline shift of the Meulaboh Embayment progradating delta through the past 2.6 million years can be calculated by 2 cm/yr (2 km divided by 100’000 years). Following the numbers presented in section 6.2.3, the average sedimentation rate can be calculated by 25 m (thickness of a sub-cycle) divided by 100’000 years (length of a cycle), i.e. 0.25 mm/yr or 0.25 m/kyr. For comparison, the sedimentation rate of the Bengal delta and thus the rate of delta subsidence - is reported to be 1.2 m/kyr, while that of the Mississippi Delta is >0.01 m/kyr.

6.3

A hydrogeological subdivision of the Johan Pahlawan subdistrict

Summarizing the above explanations (chapters 6.1 and 6.2), the area of the Johan Pahlawan sub-district can be subdivided in three hydrogeological provinces (see enclosure A): A The fluvial plain of the Kreung Meureubo, divided into: a) the river and the river banks b) old river beds c) interlaying flooding areas B The area of the reefs, divided into: a) assured and presumed coral reefs b) interlaying areas with assured or presumed reef sands C The backland a) the border of the swamp area b) the swamp area D The beach area a) present beach ridge b) presumed old major beach ridges

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7

Conclusion regarding the hydrogeology and the water withdrawals

7.1

Water balance, recharge aereas and exploitable water reserves for the Meulaboh County

For Aceh Province only limited water balance data is available. For a water budget we take into account a mean annual precipitation of about 3300 mm. On the Meulaboh Embayment with an area of 3000 km3, per year about 1010 m3/yr (or 330 m3/s) of precipitation is received. Following the IWACO report [9, p.17, Fig. 2.7] we calculate an average annual runoff of 6x109 m3/year (or 200 m3/s). The evapotranspiration has to be high; we assume that 3.5x10 9 m3/yr (or 110 m3/s) are evapotranspirated. In accordance to the estimations of D. Ploethner for the SDC [6], the rest i.e. 0.5x109 m3/yr (or 20 m3/s) would recharge the water-bearing zone. After the BGR report [3, p.12, Fig. 7], the pressure head is built up either in the higher located parts of the embayment or even in the mountainous area further east. In the case of the Meulaboh embayment the beach slope is a major discharge area especially for the groundwater flowing in the unit III. For a model calculation it is estimated that half of the water volume infiltrated to the subsoil recharges the units III and IV; that equals to 2.5x108 m3/yr (or 10 m3/s). This water volume flows mainly in the coarse grained sediments from the eastern parts of the embayment in direction of the Indian Ocean, where it flows out to the sea at the land/seawater interface. Taking into account a water recharging area of 300 km3, a tenth (i.e. 1 m3/s) of the total groundwater recharge to the aquifers of the Tutut Formation in the whole embayment is traversing the sub-district Johan Pahlawan in the subsoil in direction to the ocean.

7.2

Water quality

From a plane or helicopter it can be observed that the coastal plain (2km) of the Meulaboh embayment is covered by water in many places, especially where there are no settlements. During the time when the three formations of interest (Tutut, Meulaboh, Alluvium) were deposited, we estimate the number of major marine transgressions to be in the order of 25, i.e. one transgression phase occurred in average every 100'000 years. At this time at least the costal plain was covered by saltwater. But even in the regression periods saltwater can swash on the costal plain, namely when tsunamis occurred. Contamination caused by saltwater intrusion in the semi-confined aquifers is documented. After the tsunami event of 2004, a local company (Citra) drilled at several sites near the old road Meulaboh-Samatiga 40/50 m, but the saltiness was believed to be from former seawater intrusion. The freshness of the groundwater tends to improve 2 km inland from the beach; however there are many variations in water quality.

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7.4

Summarizing the hydrogeological units and their potential for water withdrawals and additional recommendations when drilling

7.4.1 The hydrogeological unit I General The thickness of the hydrogeological unit I varies between 10 and 70 m. The resistivity values of the sediments are 20 and 380 Ohm*m for the sand layers and 0 and 20 Ohm*m for the clayey layers. Geographic variations of the unit I The thickness depends on the geological/geographical situation of the unit. In the backland (C) and the area of the reefs (B) the unit I is not very prominent and only 10 m to 15 m thick. In the fluvial plain (A) of the Kreung Meureubo the thickness of the unit I increases from 20 m at the border of the plain to 40 m in the vicinities of the river. Here the thickness of the aquifer I reaches 30 m. The seismic survey revealed typical progradational deposition patterns. Where there are buried river beds, the unit I can reach up to 70 m. In the coastal plain towards the beach area (D) the unit reaches a thickness of up to 50 m. Water withdrawals from the shallow aquifer The water potential of the shallow unconfined aquifer I is modest in many parts in the backlands. The aquifer is not protected from above by layers with low permeability. In the absence of other water sources however, the people tap the water in the numerous dug wells. Regarding communal water supply systems we support the conclusion of the IWACO report [9]: Water supply systems based on shallow groundwater are recommended only when specific hydrogeological circumstances occur. Experience so far with systems fed by shallow groundwater elsewhere in Indonesia are not encouraging. Exploitation of the shallow aquifer is often complicated and the water quality is poor compared to deep groundwater. Its vulnerability to pollution and the generally limited extent of the shallow aquifers makes systems fed by shallow groundwater seldom suitable to meet future demands. Main advantages and disadvantages Advantages:  The top aquifer can be tapped everywhere  The drilling or digging costs are relatively low  The local people now how to treat the water. They boil the water prior to drinking or preparing food. Water with elevated concentration of iron ad manganese can be removed by aeration or filtering. Disadvantages: o The quality of the top aquifer water is poor in most places. o The potability of the water of the shallow aquifer differs from place to place on the basis of (1) the extent of saltwater intrusion, (2) the effect of land use, and (3) background concentrations of iron. o When drilling to the top aquifer, there are the following potential hazards according to PT DASA CATEGO BIRKI [10]: - well bore caving/collapse

22

- pipe may become stuck - difficult to install the water well casing

7.4.2 Unit II General The unit II occurs in the depth range between 30 - 50 and 100 -120 m below the land surface. The thickness is 20 m only near the coast and increases landwards up to 80 m. The aquifer II in the upper part of the unit II (in some parts there are two sandy layers) has a thickness between 5 and 30 m and resistivity values between 130 and 250 Ohm*m. Geographic variations of the unit II The thickness depends on the geological/geographical situation of the unit. In the backland (C) the thickness of the unit II varies between 50 and 80 m, the middle aquifer between 10 and 30 m. In the area of the reefs (B) the unit II is up to 70 m thick, and the promising water bearing sand layers at least 15 m, possibly even more in some places. In the fluvial plain (A) of the Kreung Meureubo the thickness of the unit II is between 50 and 70 m. The thickness of the aquifer II varies from place to place, depending if The seismic survey revealed typical progradational deposition patterns. Where there are buried river beds, the unit I can reach up to 70 m. In the coastal plain towards the beach area (D) the unit reaches a thickness of up to 50 m. Water withdrawals from the middle aquifer In the backlands the middle aquifer may be tapped at depths from 30 to 50 m. The water however is of bad quality in many places due to the presence of the gas-producing lignite and peat layers in the same unit. In the fluvial plain (A) old meanders (Ab) of the Kreung Meureubo exist, which were filled up by sediments. These buried river beds are filled by sands, which contain water of an acceptable quality as the well "Solidarité" proved. It can be recommended to tap the sandy river bed fillings by shallow drillings down to 80 m. It is recommended to look for more buried river beds like the one existing between S3 280 and 380 m. One potential site is S3 680 to 780 m. In the area surveyed by profile S5 it may be promising to drill between S5 0 and 600 m to find an aquifer with "good" water. It is advisable to control the quality of the groundwater before the distribution (particularly the one produced in interlaying flooding areas Ac). In the areas where probable structures of buried reefs (Ba) and presumed reef sands (Bb) are present, it is highly recommended to carry out test drillings. There is a considerable chance of finding water of good quality and sufficient quantity in depths less than 100 m. It is highly recommended to check the reef structure and the possible carbonate sands in the surroundings by drillings in order to check their water content. Main advantages and disadvantages Advantages:  The drilling depth is not more than 80 m, which is much easier and less costing zhan to drill to the unit III. Disadvantages: o The quality of the middle aquifer water is poor in many places 23

o o

Aquifer yields appear to be low; a good bore is one with 0.5 L/sec When drilling to the middle aquifer, there are the same potential hazards as drilling to the top aquifer according to PT Dasa Catego Birki [10]

7.4.3 Unit III General characteristics of the unit III The unit III occurs at depths in excess of approximately 100 m. The thickness of the unit III a varies between 80 and 120 m. It is known from the old boreholes that at these depths intercalations of poorly lithified sandstones and mudstones are present (unit IIIa). The seismic survey identified numerous longer profile segments on which sedimentary sequences not affected by compaction faulting or tectonic activities are observed as deep as 500 m (unit IIIb). Geographic variations of the unit III Independent from the geographical position the unit III shows the same characteristics in the most places, namely that of a multilayered aquifer-system. The only exceptions are a) the possible existence of reef structures (C) in Gampa 2 and b) the progradational deposition patterns in the fluvial plain (A, seismic profile S5 0 – 600 m). Water withdrawals from the deeper aquifers In all the areas, within the profile segments, which are not affected by faulting, prominent reflectors indicate sand or gravel formations, which may be potential aquifers. The old PDAM boreholes in the backland (Lapang, Leuhan) show that with a screening length of around 50 m, some 5 L/sec can be produced. When drilling to greater depths even 10 L/s seems to be possible. The volume of drilled freshwater available will depend on well placement, pumpage rate, and aquifer response to withdrawal. In a rough estimate, 300 L/s (30% of the total recharge to the Tutut Formation aquifers, see chapter 7.1) flow on a width of 5 km in the units III between a depth of about 100 and 300 m. The existing old PDAM wells produce around 40 L/sec in total. We argue that the old wells should be closed before drilling new wells. In the present beach area (Da), the topmost sandy layers of the unit IIIa are known very well because it was tapped by many hand drilled boreholes in the past. About 0.5 L/sec water can be produced by one borehole. In the beach area (D) the different freshwater aquifers of the unit III are in close proximity to saltwater. The water-bearing zone is in direct connection with seawater. Future water withdrawals will cause the formation of many potentiometric cones of depression in these aquifers. This condition favors saltwater intrusion by lateral movement and by downward leakage. The southwesternmost location for a major supply well for each aquifer depends on the location of the saltwater front in the aquifer. The existence of deep reef structures (C) in the unit III is hypothetical. Drilling to depths in excess of 100 m to find promising reef sand aquifers bears a certain risk. Main advantages and disadvantages Advantages:  The quality of the water is believed to be better than the one tapped in unit II. The majority of the wells sampled by BGR and UNICEF produce fresh groundwater (see BGR report [3, p. 27].  By handmade drillings the water of the topmost aquifer in unit III can be tapped inexpensively. Disadvantages: 24

o o

o o

o

Rotary drillings to depths in excess of 100 m is expensive. When drilling to the middle aquifer, there are following risks according to PT DASA CATEGO BIRKI [10]: - swelling of grey clay, which can reduce or even close the bore well diameter, deviate the bore or cause the drill pipe to become stuck. With the numbers of deep drilling being contemplated, the deep aquifers can easily be contaminated if measures are not taken whilst drilling and in completion of deep bore holes to high standards. A major concern is related to elevated concentrations of iron, manganese and ammonia in the groundwater (see BGR report [3, p. 27]. On a long term wells have to be equipped with locally manufactured iron removal plants (see BGR report [3, p. 24 Fig. 18-19]. One has to be aware that algae con grow in the boreholes.

7.4.4 Additional recommendations when drilling for groundwater Drilling rigs and drilling staff Due to the nature of the strata, there are often technical difficulties in successfully drilling bores, particularly with deep bore (see section 7.4). As a result:  

suitable drilling rigs and experienced drillers are required employing appropriate drilling techniques; work should be undertaken under the supervision of consultants for groundwater drilling as experienced hydrogeologists and/or groundwater engineers.

Geophysical logging, sampling, reporting The boreholes should be logged, sampled and reported as follows:    

logging with down hole geophysical tools (gamma, possibly fat gamma, SP and calliper), water sampling for complete chemical analysis including specified heavy metals, strata samples retaining and strata logging by hydrogeologists/geologists, writing a report on each borehole.

Pump tests To know better the potential for water withdrawals of a borehole:  

similarly with production boreholes pump tests (min 24 hours preferably min 3 days plus step tests) have to be undertaken, and the hydrogeologists/groundwater engineers should report on the hydraulic characteristics of the aquifer, recommended maximum pumping rates, the efficiency of the borehole and should supervise any bore development required.

Specific interpretation for borehole locations Because of time lack, it was not possible within this report to correlate the geophysical lines in detail, i.e. by constructing NW-SE running hydrogeological profiles. Organizations planning 25

to drill for groundwater may ask a hydrogeologist trying to interpolate the hydrostratigraphy between two seismic and/or geoelectric to get more information about the hydrogeological conditions at the site of their planned boreholes. Old PDAM boreholes There are several old wells in the backland (Lapang and Leuhan), which are not used anymore. Some of them are still discharging artesian water. It is advisable to close or rehabilitate these boreholes, especially when new deep drillings are planned. In the IWACO [9, p.74] study it is recommended: 



Before further action is taken towards expansion of the system, it is proposed to rehabilitate the existing groundwater system by chlorination of the wells to eliminate the mentioned algae problem (….). To carry out test drilling to increase knowledge on the groundwater potential in the areas north of Meulaboh (results of a geo electrical survey are already available).

Groundwater survey The IWACO report [9, p.74] stresses furthermore: Only if groundwater survey confirms groundwater is available in sufficient quantities and of acceptable quality, large scale exploitation of groundwater (….) can be initiated. The aim of the present report is to encourage the organizations which are planning to drill for groundwater to carry out a groundwater survey in the preparation phase. As it is not possible to do that for limited areas, a collaboration with the local authorities and other NGO’s on-site is highly recommended.

8. References 1. CAMERON, N.R. ET AL. (1983): The geology of Takengon quadrangle, Sumatra. Explanatory note and geological map 1:250'000. 2. BARBER, A.J., CROW M.J. AND MILSOM J.S. (2005): Sumatra: Geology, Resources and Tectonic Evolution. Geological Society Memoirs, No. 31, 2005. 3. BGR BUNDESANSTALT FÜR GEOWISSENSCHAFTEN UND ROHSTOFFE (2005): Hydrogeological reconnaissance survey in the province Nanggroe Aceh Darussalam. Northern Sumatra, Indonesia. Survey Area: Calang-Meulaboh. 4. BINNIE & PARTNERS (1986): Aceh Design Unit – Provincial Water Resources Development Plan Inventory of Water Resources schemes. 5. CARITAS SWITZERLAND / CRS (2005): Meulaboh, Aceh Barat; Geophysical Survey for Ground Water Prospection. – Reflection Seismic and Geoelectrical Resistivity Profiling. Field data acquisition: GeoExpert AG & Terratec Geophysical Services. 6. DEZA (2005): Indonesien – Sumatra – Aceh. Bericht zur Situation der Wasserversorgung in Banda Aceh und Umgebung. 27. Dez. 2005. 7. DHV (1983): Eleven Cities Water Supply Project in North Sumatra and Aceh. East Medan Water Supply Project. Meulaboh, Final Design Report. Government of Indonesia (Ministry of Public Works) and Government of the Netherlands (Ministry of Foreign Affairs. May 1983. 26

8. GERMAN HYDROGEOLOGICAL ADVISORY GROUP CTA 40 (1983): Instructions, suggestions, reports elaborated during phase II of the project (1981-1982), Meulaboh area North Sumatra. 9. IWACO CONSULTANTS FOR WATER & ENVIRONMENT (1993): Study of Water Sources Allocation for Water Supply for D.I. Aceh Province. Kabupaten Aceh Barat, Planning for water supply development and raw water sources allocation. Government of Indonesia (Ministry of Public Works). Banda Aceh, September 1993. 10. PT DASA CATEGO BIRKI (2006): Technical and Work Plan Proposal for Water Supply Meulaboh. Proposal No. T-009-01-06-00073. Bandung, February 17th 2006. 11. SOLIDARITÉS (2005): Diagramme forage: Gampa, SOLIND/SB/37. 12. SCHÜTT M. (2005): Tsunami Zeit-, Stärke- und Richtungsskala an der Küste Sumatras? 13. SCHNEIDER M. & SCHOBERT K. (2006): Geological Study in Meulaboh, Aceh Barat, Indonesia. By order of CARITAS Switzerland.

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Appendix Locations “Big Deep Boreholes: A) Mesjid Raya

Dutch

1922

N 04° 07’ 59.2“ E 096° 08’ 48.7”

B) Lido

Dutch

1924

N 04° 08’ 13.3“ E 096° 07’ 47.7”

C) Gampa I

PDAM

1980s

N 04° 10’ 14.1“ E 096° 08’ 36.3”

D) Lapang 1

PDAM

1980s

N 04° 10’ 57.4“ E 096° 08’ 00.3”

E) Leuhan

PDAM

1980s

N 04° 10’ 50.7“ E 096° 08’ 39.4”

F) Old RIver

PDAM

1980s

N 04° 10’ 39.9“ E 096° 09’ 04.2”

G) Lapang 2

PDAM

1980s

N 04° 11’ 03.6“ E 096° 08’ 14.8”

H) Ujoung Blang

PDAM

1980s

N 04° 11’ 08.2“ E 096° 08’ 25.8”

I) Near water intake

PDAM

2004

N 04° 10’ 20.4“ E 096° 09’ 04.2”

J) Water treatment plant

PDAM

2004

N 04° 10’ 22.2“ E 096° 08’ 32.5”

K) Power station

PNL

2004

N 04° 10’ “

E 096° 06’”

28