Identifying the subsurface profile for riverbank/bed filtration (RBF) using conventional methods such as drilled boreholes is usually labor intensive, costly and acquires only 1-D profiling. Hence, a 2-D ground resistivity method was used as an alternative to obtain a 2-D subsurface profile by extending to 200 m length at Kota Lama Kiri, Kuala Kangsar, Perak site area. Four resistivity survey lines (200 m each) with minimum electrode spacing of 5 m were executed at the site area. The Wenner-Schlumberger array protocol was applied for acquisition data process. The resistivity data were then processed and interpreted using RES2DINV software. The results show the approximate 2-D pseudo-section image of the subsurface resistivity distribution with different soil types. The interpretation of results reveals that the site area consists of two main layers: (a) sand and gravel (300–750 Ωm) from 0 to 10 m depth, and (b) weathered rock (0–350 Ωm) within 10 to 40 m depth. It found that the first layer (0–10 m depth) belongs to the alluvial aquifer type and has a relatively high permeability, thus suitable for RBF. This method is believed to be able to give great assistance for selecting the suitable area for RBF in the future.

INTRODUCTION

Riverbank/bed filtration (RBF) refers to the process of surface water seeping from the bank or bed to the groundwater production wells (Eckert & Irmscher 2006). It offers a low cost and efficient natural alternative technology of water treatment for drinking-water applications. The technology can be applied directly to existing water reservoirs, streams, lakes and rivers (Ray et al. 2003). Most RBF systems are constructed in alluvial aquifers located along riverbanks (Bourg & Bertin 1993). This aquifer can consist of a variety of deposits ranging from sand, to sand and gravel, to cobbles and boulders. Ideal conditions typically include coarse-grained, permeable water-bearing deposits that are hydraulically connected with riverbed materials (Ray et al. 2003). Thus, the collector well should be located adjacent to a surface water body, and over time it should be able to withdraw enough water from the flow system and induce water from the river/lake (Ray 2001).

The soil properties are important factors in selecting the best location for a water abstraction well. Alluvial aquifers are among the keys for water abstraction success, which have the capability of water storage (Hyoung & Jung 2008). Nevertheless, there are some difficulties in acquiring a large area of soil profile. Normally, the site investigation is carried out by using drilled boreholes to gather and collect information and profile depth below the surface, but this is costly and highly labor intensive (Jiarong & Lingjie 2014). Furthermore, such a method merely obtains 1-D soil properties, which often need more than one location of boreholes through heuristic methods. Hence, a geophysical technique can be used to facilitate the task in order to minimize the risk, especially for subsurface profiles.

According to Nordiana et al. (2012), geophysics can provide additional data in order to save cost and time for subsurface characterization. There are a variety of techniques in geophysics, including seismic refraction, seismic reflection, ground penetrating radar, magnetic, gravity and electrical methods including induced polarization, self-potential, and 2-D/3-D imaging resistivity (Anderson & Ismail 2003). 2-D ground resistivity is amongst the choices as far as groundwater exploration is concerned, and has been successful in investigating and mapping water tables, water bearing zones (aquifers), buried channels, unconfined and confined aquifers, sea water intrusion, etc. (Danielsen et al. 2003; Yadav & Shashi 2007; Metwaly et al. 2012). In addition, such methods are non-detrimental to the environment and a rapid observation of the structure (Seger et al. 2014).

Therefore, the aim of the study is to identify the potential soil characteristics for RBF using 2-D ground resistivity for a study area, i.e. at the Lembaga Air Perak (LAP) water intake location at Kota Lama Kiri, Kuala Kangsar, where a RBF well is also located. The 2-D electrical resistivity survey is used as a tool to extend the lithology profile under the site area to create a multi-dimensional map of the subsurface profile.

Brief overview of 2-D ground resistivity methods

Resistivity methods originally started in the 1920s, performed by the Schlumberger brothers, only using four electrodes for a single point of data. 60 years later, Koefoed (1979) improved this method for quantitative interpretation and conventional sounding surveys. Further improvement of this method was made by Griffiths et al. (1990), where a multi-electrode resistivity instrument has been developed, and a new creation of inversion software by Loke (1994) for resistivity survey data interpretation. The main use of resistivity surveys is to determine the subsurface profile where the true resistivity can be estimated by measuring on the ground surface (Loke et al. 2003). An electrical resistivity survey measures the electrical potential differences at specific locations while injecting a controlled electric current at other locations (Loke 2000). Such surveys are usually carried out using 25 to 100 electrodes laid out in a straight line with a constant spacing (Figure 1). A computer-controlled system is then used to automatically select the active electrodes for measurement. The resolution of surface geoelectrical resistivity surveys generally decreases with depth and needs very long layouts to achieve large depth penetrations (Aizebeokhai 2010).
Figure 1

The establishment of electrodes for a 2-D electrical resistivity survey and the sequence of measurements used to build up a pseudo section (Loke 2000).

Figure 1

The establishment of electrodes for a 2-D electrical resistivity survey and the sequence of measurements used to build up a pseudo section (Loke 2000).

Relationship between hydrogeological and resistivity

Electrical resistivity methods measure the resistivity of the ground materials to infer the kind of material beneath the surface. The differences in resistivity of the ground materials are used to identify and infer the different lithology and the hydrogeological condition. Table 1 provides the resistivity values of common rock, soil materials and types of water (Keller & Frischknecht 1966). Granites typically have high resistivity values, since this depends on the degree of fracturing and the percentage of the fractures filled with groundwater. Sedimentary rocks (sandstone, shale and limestone) otherwise, which usually are more porous and have a high water content, normally have lower resistivity values. Fresh groundwater and wet soils have even lower resistivity values. Sandy soil normally has a higher resistivity value than clayey soil. However, the overlapping resistivity values of different classes of rocks and soils need to be taken into account during interpretation. These are due to the resistivity of a particular rock or soil relying on factors such as porosity, the degree of water saturation and the concentration of dissolved salts (Loke 2000).

Table 1

Resistivity of some common rocks, soil and waters in survey area (Source:Keller & Frischknecht 1966)

Type of materialResistivity (Ωm)Type of waterResistivity (Ωm)
Alluvium 10 to 800 Precipitation 30 to 1,000 
Sand 60 to 1,000 Surface water, in areas of igneous rock 30 to 500 
Clay 1 to 100 Surface water, in areas of sedimentary rock 10 to 100 
Groundwater (fresh) 10 to 100 Groundwater, in areas of igneous rock 30 to 150 
Sandstone 8 to 4 × 103 Groundwater, in areas of sedimentary rock > 1 
Shale 20 to 2 × 103 Sea water ≈ 0.2 
Limestone 50 to 4 × 103 Drinking water (max. salt content 0.25%) > 1.8 
Granite 5,000 to 1,000,000 Water for irrigation and stock watering (max. salt content 0.25%) > 0.65 
Type of materialResistivity (Ωm)Type of waterResistivity (Ωm)
Alluvium 10 to 800 Precipitation 30 to 1,000 
Sand 60 to 1,000 Surface water, in areas of igneous rock 30 to 500 
Clay 1 to 100 Surface water, in areas of sedimentary rock 10 to 100 
Groundwater (fresh) 10 to 100 Groundwater, in areas of igneous rock 30 to 150 
Sandstone 8 to 4 × 103 Groundwater, in areas of sedimentary rock > 1 
Shale 20 to 2 × 103 Sea water ≈ 0.2 
Limestone 50 to 4 × 103 Drinking water (max. salt content 0.25%) > 1.8 
Granite 5,000 to 1,000,000 Water for irrigation and stock watering (max. salt content 0.25%) > 0.65 

SITE AREA

The site area is situated in Kota Lama Kiri, Kuala Kangsar, in the state of Perak, Malaysia near to Perak river as depicted in Figure 2, approximately at coordinates: 4°48′3″ to 4°48′13″ North latitude and 100°56′59″ to 100°57′14″ East longitude. The site was chosen due to availability of an RBF well along with a borehole which was owned by LAP located at coordinates of latitude 4°48′7″ North and longitude 100°57′6″ East. The land use and land cover of the surrounding areas can be classified as forest, rangeland, agriculture, waterbodies, built up and barren land areas (Mohd Firdaus et al. 2015). The main river that flows through the site area is the Perak river, measured at 420 km long and having a catchment area of 14,834 km2, which covers about 70% of Perak state. Perak river catchment is the second largest river catchment in Peninsular Malaysia and lies between the mountainous regions of the Perak-Kelantan-Thailand border of the Belum Forest Reserve in the north.
Figure 2

Location of site area in Kota Lama Kiri, Kuala Kangsar, Perak, Malaysia (the image was captured by GeoEye-1 satellite imagery with acquisition date at December 2012).

Figure 2

Location of site area in Kota Lama Kiri, Kuala Kangsar, Perak, Malaysia (the image was captured by GeoEye-1 satellite imagery with acquisition date at December 2012).

Based on geological maps, the site area is generally carboniferous strata. The site area belongs to the Kati formation, typically arena-argillaceous in composition and consisting of interbedded phyllite, metaquartzite, sandstone, shale and siltstone. Besides that, the finer sediments are characteristically laminated and grey in colour. In addition, sedimentary structures observed include rhythmic bedding, rare graded bedding and flute casts (Foo 1983). The hydrological map indicates high aquifer potential type area, which gives the high potential of transmissivity and storability characteristics. The type of soil can be categorized as alluvial soil.

DATA AND METHODOLOGY

Data acquisition

Ground resistivity measurements were performed in the Kota Lama Kiri site area with a multi-electrode resistivity meter system (ABEM Terrameter SAS 4000 instruments) to investigate the hydrogeological conditions. Figure 3 shows the four ground resistivity survey lines of 200 m length each. Each 200 m length survey line is pegged with 41 stainless steel electrodes at 5 m spacing. The connecting cable (jumper) was used to connect the electrode with the dual-purpose cable take-out. To obtain high-quality electrical resistivity measurements, the resistance between the soil and electrode was measured to check if the contacts were reliable and steady. The current was set between 2 and 50 mA. A computer-controlled system was used to automatically select the active electrodes for each measurement. Then, the Wenner-Schlumberger array protocol was applied.
Figure 3

Map shows the ground resistivity survey lines in Kota Lama Kiri, Kuala Kangsar (Line 1 = L1, Line 2 = L2, Line 3 = L3, Line 4 = L4).

Figure 3

Map shows the ground resistivity survey lines in Kota Lama Kiri, Kuala Kangsar (Line 1 = L1, Line 2 = L2, Line 3 = L3, Line 4 = L4).

Data processing and interpretation

The resistivity data processing and interpretation software package ‘RES2DINV’ were used for data processing and interpretation procedure, which is able to produce an inverse model that approximates the actual subsurface structure (Loke et al. 2003; Asry et al. 2012; Ratnakumari et al. 2012; Rai et al. 2015). To process the resistivity data, raw data from the ABEM Terrameter SAS 4000 instrument were converted to DAT format file extension. The least-squares inversion process was then applied to reduce measured resistivity to the apparent resistivity value. The inversion routine utilized by the software was based on the standard constrained technique that attempts to minimize the square of the difference between the observed and calculated apparent resistivity values. In the meantime, root mean square (RMS) error was calculated for accuracy measurement. Next, the resistivity results were interpreted based on the resistivity table scheme by Keller & Frischknecht (1966) as shown in Table 1, along with geological logging of borehole data at the study site area (Table 2). Finally, the approximate pseudo-section image of the subsurface resistivity distribution with different soil type was produced (see Figure 3).

Table 2

Monitoring well borehole data in study area

Soil materialDepth (m)
Silty 0 to 5 
Sand/gravel 5 to 10 
Weathered rock > 10 
Soil materialDepth (m)
Silty 0 to 5 
Sand/gravel 5 to 10 
Weathered rock > 10 

RESULTS AND DISCUSSION

The 2-D profile of L1 to L4 produced with reference to the resistivity table scheme (Table 1) and borehole data logging (Table 2) is shown in Figure 4. The location of each electrode is indicated by small vertical spikes on the horizontal axis representing survey line. Depths of the resistivity variations are marked on the vertical axis and restricted to a depth of 40 m, which is the maximum depth of investigation for a spread length. The first electrode is marked at 0.0 and the 41st at 200 m distance.
Figure 4

Interpreted 2-D ground resistivity for L1, L2, L3 and L4 using Wenner-Schlumberger.

Figure 4

Interpreted 2-D ground resistivity for L1, L2, L3 and L4 using Wenner-Schlumberger.

The interpretation results reveal that the site area consists of two main layers: (a) sand and gravel; and (b) weathered rock. The sand and gravel has a high resistivity value, ranging from 300 to 750 Ωm, which was located at the upper layer with depth of 0–10 m. The high resistivity values obtained from the materials were due to the lower porosity and being acquired during the dry season. During the dry season, the resistivity value of sand and gravel shows higher than in the wet season due to the presence of water. However, as reported by Siti Zahirah et al. (2015), these layers have hydraulic conductivity values in the range of 0.10 to 5.65 cm/s, which shows a relatively high permeability.

The top of the first layer is covered by alluvium soil, but it appears unclear in pseudo section due to overlapping with sand and gravel layers. L1 and L2 (Figure 4) indicate unconfined aquifer at the upper layer because of infiltration process occurring from the river water on the right and left side of L1 and L2, respectively. The profile near the riverbank shows low resistivity due to the presence of water (infiltration proses from river). As aforementioned, these conditions meets the criteria for RBF (Schubert 2002).

The low resistivity values ranging from 0 to 300 Ωm were indicative of a weathered rock layer and observed beneath average depth of 11 to 40 m. The low resistivity at the bottom layer (<300 Ωm) was due to the surface water seepage (high permeability) where the water fills the pores in the bottom layer and moves into deeper layers as a result of the effect of gravity. These are suspected to be a high potential of aquifer and suitable for groundwater exploitation. To confirm the statement, borehole data more than 10 m depth are needed. In addition, according to hydrogeological maps, the site area indicated high potential aquifer for groundwater. The RMS error produced for L1 to L4 is 11.4%, 9.9%, 9.3% and 9.9%, respectively. Therefore, the upper layer consists of sand and gravel type of material, suitable for RBF, as an existing abstraction well (10 m depth) is already constructed at the site area.

CONCLUSION

This study has successfully identified the soil characteristics for potential RBF using 2-D ground resistivity in the Kota Lama Kiri, Kuala Kangsar site area. The 2-D electrical resistivity produced the 2-D pseudo image of the subsurface profile with depth-to-resistivity relationship. Furthermore, the subsurface profile was extended to 200 m length. The interpretation results indicate the site area consists of sand and gravel material (300–750 Ωm) from 0 to 10 m depth and weathered rock (0–350 Ωm) within 10 to 40 m depth. It appears that the upper layer (0–10 m depth) belongs to the alluvial aquifer type and has a relatively high permeability, thus suitable for RBF. This method can assist in the selection of suitable areas for RBF in the future.

ACKNOWLEDGEMENT

The author would like to acknowledge the Ministry of Education Malaysia for providing LRGS Grant on Water Security ‘Protection of Drinking Water: Source Abstraction and Treatment (203/PKT/6720006)’ for sub-project 1.4: ‘Site Characterization of Surface and Subsurface Spatial Data in RBF’.

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