BGR Bundesanstalt für Geowissenschaften und Rohstoffe

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TC Indonesia: MANGEONAD Project - Groundwater Quality along the Coast between Sigli and Bireuen in the Province NAD in Northern Sumatra

Report of the project:

Survey areas near Banda AcehSurvey areas near Banda Aceh Source: BGR

Background:
From July to November 2005, the Federal Institute for Geosciences and Natural Resources (BGR) conducted helicopter-borne geophysical investigations and hydrogeological reconnaissance surveys in Banda Aceh and parts of Aceh Besar District (northern coast), between Calang and Meulaboh (western coast), and in Sigli and parts of Pidie District (eastern coast). These activities have been started to contribute to the rehabilitation and reconstruction of the tsunami affected regions in the Province of Nanggroe Aceh Darussalam (NAD), Northern Sumatra (see literature below).

Between January and April 2006, a number of teams engaged by German Agro Action (GAA) / Deutsche Welthungerhilfe cleaned, rehabilitated and sampled 274 dug wells and 16 drilled wells along Aceh’s eastern coast stretching from Sigli to Bireuen. The water samples were analysed in Aceh regarding the number of coliform bacteria and chemical oxygen demand and in the Federal Institute for Geosciences and Natural Resources (BGR) for the inorganic constituents. On request of GAA, the drinking water quality of all wells sampled was assessed according to the Indonesian Drinking Water Standards in the framework of BGR project “Management of Georisks in NAD” (MANGEONAD). In 2007, 19 drilled wells and two dug wells were re-sampled on request of GAA and for the purpose of on-the-job training of DISTAMBEN personnel. The BGR laboratory “Hydrogeochemistry & Environmental Geochemistry” analysed most of the inorganic major constituents and trace elements of these water samples.

Hydrogeology of coastal plain and well locationHydrogeology and well location Source: Directorate Geology of Indonesia

Hydrogeology:
The low lying areas of the coastal plain between Sigli and Bireuen are underlain by unconsolidated to locally semi-consolidated multi-layer aquifers. They consist of an alternating sequence of fine to coarse sand and gravel and clay to sandy clay. The cumulative thickness of clay layers exceeds that of the sandy-gravelly layers. The multi-layer aquifers have a low to moderate transmissivity (T) in the range from 1 to 150 m2/d; the mean T value lies at 20 m2/d. The resulting values of the hydraulic conductivity (K) range from 0.1 to 13 m/d or 8.7x10-7 to 1.5x10-4 m/s. This range of K values is representative for silty sand to clean sand. Thus, the K values found roughly describe the grain size distribution depicted in the well logs; however, they do not cover coarser grain sizes such as gravel or boulder, partially mentioned in the well logs.

Groundwater samplingGroundwater sampling Source: BGR

The multi-layer aquifers are supposed to have a moderate productivity of generally less than 5 l/s, which locally may attain 10 l/s. The deep groundwater is confined; its piezometric head is near or above surface frequently maintaining artesian flow. The mountainous hinterland rising up to 500 m above sea level is built up by fractured hard rocks such as tuffaceous and calcareous sandstones, calcareous mudstones and siltstones as well as limestones.

Groundwater samplingGroundwater sampling Source: BGR

The groundwater recharge is supposed to occur predominantly in the mountainous hinterland as proved by stable isotope analysis. By this, the piezometric head / artesian flow of the coastal multi-layer aquifers can be maintained. The total depths of the drilled wells range from 60 to 157 m; the minimum depth of top of the uppermost screen lies at 43 m below ground level (bgl), while the maximum depth of bottom of the lowermost screen is at 146 m bgl.

The near-surface layers consist of clayey-silty sands and gravels, locally grey, green or yellow clays may prevail. The water table of the shallow groundwater tapped by dug wells is situated between 0.3 and 7 m bgl. Traditionally dug wells are used for domestic water supply in this region. In many cases this groundwater is also used for drinking purposes. Water in open dug wells is highly vulnerable to contamination due to improper sanitation facilities and practises.

Results:
The major concern regarding drinking water quality is related to the parameters manganese and iron as well as sodium and chloride. If these four parameters exceed the guideline values, then the acceptability of the water is challenged either due to a salty taste (Na, Cl) or due to an objectionable taste (Fe, Mn) and objectionable reddish-brown (Fe) or brownish-black (Mn) colour and staining laundry and plumbing fixtures.

Dug well at islamic school in Tenugku DilaweuengDug well in Tenugku Dilaweueng Source: BGR

In 2006, four drilled wells bore arsenic-rich groundwater in the range 12-26 μg/l As; in 2007, the arsenic concentrations ranged from 9 to 26 μg/l As in the same wells. For comparison, the arsenic concentrations in shallow groundwater decreased from 24-92 μg/l As to 14-79 μg/l As from 2006 to 2007, respectively, as monitored in two dug wells. In 2007, 13 wells produced water with elevated iron- and / or manganese concentrations and 12 wells delivered water with elevated sodium (-chloride) concentrations. In total, 18 out of 19 wells sampled were sensu strictu unfit for drinking water purposes. Whereas iron and manganese could be comparatively easily removed by an aeration / filtration process, a reverse osmosis or an ion exchange treatment is needed for the removal of sodium and chloride. However, in 2007 it appeared impossible to raise funds for appropriate raw water treatment facilities in Aceh Province. The re-sampling programme has revealed that the general chemical composition of the deep groundwater occurring in the coastal multi-layer aquifer system stretching between Sigli and Bireuen has not changed significantly from 2006 to 2007.

Spatial distribution of nitrate concentrations in dug wells in Teungku DilaweuengNitrate concentrations in dug wells Source: BGR

Nitrate in shallow groundwater ranges from <0.05 to 243 mg/l NO3, fifteen percent of the water samples exceed the national threshold of 50 mg/l. Almost all of the wells that tap water with elevated nitrate concentrations are located in the villages Cot, Teungku Dilaweueng and Blang Raya in the sub-district Muaratiga, north-west of Sigli. The spatial distribution of nitrate and the results of nitrogen isotope analysis point to the fact, that the elevated concentrations of nitrate originate from septic tank effluents and/or livestock farming inside the villages.

Mixing of fresh groundwater with underlying saline water leads to an increase especially of sodium, chloride, calcium, and magnesium concentrations. This is also reflected by elevated values of specific electrical conductivity (EC), total dissolved solids (TDS), and total hardness. The deep groundwater appears to be predominantly slightly to moderately brackish, hard to very hard, with a pH ranging from mildly acid to mildly basic (pH 6.4-8.0) and with a redox potential reflecting mildly oxidising conditions, where dissolved oxygen and sulfate are frequently depleted. The almost ubiquitous abundance of high iron and manganese concentrations reflects the wide availability of these two elements and the prevailing low oxygen conditions in the coastal aquifers of Aceh Province. Bicarbonate is the predominant anion. The main driving force for the relatively high bicarbonate concentrations is considered the oxidation of organic carbon which in turn accounts for the decomposition of sulfate. Sodium is the predominant cation. There are two sources for elevated sodium concentrations: cation exchange and mixing of fresh groundwater with underlying saline groundwater. These processes result in the Na-HCO3 water type which is abundant in the coastal aquifers of Aceh Province.


Literature:

Contact 1:

    
Dr. Bernhard Siemon
Phone: +49(0)511-643-3488
Fax: +49(0)511-643-2304

Contact 2:

    
Prof. Dr. Thomas Himmelsbach
Phone: +49-(0)511-643-3794
Fax: +49-(0)511-643-2304

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