Groundwater quality assessment of a freshwater wetland in the Selangor (Malaysia) using electrical resistivity and chemical analysis

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1 255 IWA Publishing 2014 Water Science & Technology: Water Supply Groundwater quality assessment of a freshwater wetland in the Selangor (Malaysia) using electrical resistivity and chemical analysis Mahmoud Khaki, Ismail Yusoff and Nur Islami ABSTRACT Groundwater quality of the Paya Indah Wetland (PIW) was targeted for the present study using integrated two-dimensional electrical resistivity imaging (ERI) and hydrochemical surveys. Electrical resistivity and the influence of variations in the lake level were investigated using daily water balance measurements, water sampling and geochemical analysis, and well-logging. The geoelectrical resistivity surveys comprised 14 resistivity traverses in the PIW. The resistivity surveys predicted the high-permeability areas separated in order to provide pathways for lake drainage. The resistivity inverse model showed that the freshwater zone is extremely clear. Chemical analyses of the groundwater samples obtained from seven boreholes from 2010 to 2013 were studied. The total dissolved solids and electrical conductivity were represented as being less than 500 mg/l and 600 μmhos/cm, respectively, and freshwater was therefore indicated. The lake water level was proportionate to rainfall fluctuation with the level measured in The soil and groundwater confirmed there to be fresh and brackish water zones in the study area, this result being obtained from ERI and the hydrochemistry of the groundwater samples. Key words groundwater quality, hydrochemical, Paya Indah, resistivity Mahmoud Khaki (corresponding author) Ismail Yusoff Nur Islami Department of Geology, University of Malaya, Kuala Lumpur, Malaysia mahmoud.khaki@gmail.com INTRODUCTION Groundwater is one of the most important natural global resources in the preparation of drinking water for urban and rural communities and the provision of water required for irrigation. Groundwater sustains the flow of streams and rivers, and provides the required water sources for the maintenance of riparian and wetland ecosystems. The availability of rainfall and recharge is an important and critical issue in the hydrological cycle. The Paya Indah Wetland (PIW) is located in the Kuala Langat District in the State of Selangor. A total of 14 lakes are found in the PIW, of which the three major lakes are Lake Lily, Telipok Lake and Lake Kemuning. It is well-known that shallow lakes and wetland systems are almost invariably connected hydraulically to the surrounding unconfined aquifer system. The electrical resistivity imaging (ERI) method gives reliable results in hydrogeological investigation, where the situation of the aquifer can be suitably delineated. The electrical resistivity method has been extensively used in near-surface geophysics for mapping groundwater contamination and bedrock topography. ERI has also been successfully used to image clay sand stratigraphy in unconsolidated sediments. The advantage of a multielectrode tomography survey over the conventional resistivity method is that it has benefits such as the interpretation of subsurface geophysical anomalies quantitatively from 2D resistivity models of subsurface geological formations. Besides, it is capable of extracting the range of true resistivity from the inverted resistivity models, it covers the large amount of density data with better resolution and the data acquisition time is considered to be less. The geoelectrical imaging method, which is used to determine the resistivity distribution of structures, has been applied in both environmental and geotechnical studies for improved mapping of complex geological structures (Sultan & Santos 2008). There are many doi: /ws

2 256 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply examples of studies in which the ERI method has been successfully used for groundwater investigations (Lagabrielle & Teilhaud 1981; Bradbury & Taylor 1984; Lenkey et al. 2005), groundwater contamination studies (Kundu et al. 2002; Islami et al. 2011, 2012), and saltwater intrusion problems (Wilson et al. 2006; Koukadaki et al. 2007). An application of the resistivity method and water quality analysis to an investigation of the groundwater in the PIW area is attempted in the present study (Figure 1(a)). The efficiency of the geoelectrical imaging method used to investigate the subsurface profile is considered in the current research. The reason for using the 2D interpretation was its ability to provide information on both vertical and horizontal extensions in the subsurface. Providing more precise information to characterize the shallow aquifer, delineating the depth of bedrock and determination of the boundary between the fresh and brackish groundwater by using 2D ERI are the objectives of applying a resistivity survey and water quality analysis in the PIW. GEOLOGY OF THE STUDY AREA The PIW is situated in the Kuala Langat District in Selangor state, and extends to an area of approximately 3,100 hectares (Figure 1(a)). The geological setting of the area was established through seven drilled boreholes and from a detailed surface geological map of the area. According to the hydrogeological investigation of the present study, the available geological data was used to divide the PIW model into three layers with distinct hydraulic properties. The geological circumstances of the groundwater were established on Quaternary sediments, including unconsolidated gravel, sand, silt and clay of the Simpang Formation in the Pleistocene, and the Gula and Beruas Formations in the Holocene. Quaternary sediments consist of four basic layers: the first layer combines peat and peaty clay with a thickness of approximately 5 8m; the second layer has a thickness of around 20 m and comprises a silty clay/sandy clay aquitard; the third layer comprises a silt sand/silty gravel aquifer, which is approximately m thick; and the last layer is the bedrock. The geological profile of the PIW shown in Figure 2 is based on a field reconnaissance (Minerals and Geosciences Department of Malaysia 2002), which conforms entirely to the drilling well. Sand and silt that have been determined by previous mining activities to be the secondary deposit cover the majority of the Paya Indah lakes. Information obtained from the sediments of Drought Lake, which has an approximately similar status to the Paya Indah lakes, presents the possibility of an impermeable layer, such as silt and clay, at the bottom of the lake, the existence of which is assumed from observations of the different water levels in each lake. Figure 1 (a) Location and geological map of the study area (modified after GSD (1985)); (b) geographical location of the site showing the location of the ERI profiles, boreholes and lakes. South of Kuala Lumpur, Malaysia.

3 257 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply Figure 2 Schematic geological profiles in the PIW (JICA 2002). CLIMATE AND HYDROGEOLOGY The main aquifer, which is topped by peat materials and clay, is realized as a confined aquifer. There are five different ways of recharging the groundwater: the flow of rainfall through the aquitard; water penetration around the edges of the bedrock where there is either a thin or no aquitard; the flow of rainfall from more permeable bedrock; penetration from the riverbed in stretches where the river bottom is connected with more penetrable sandy horizons; and penetration from ponds and wetland areas into the upper aquifer or where the aquitard has been removed and replaced by more permeable materials. Two inter-monsoon periods were reported by JICA

4 258 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply (2002) from the area in the Langat River Basin that is affected annually by the northeast and southwest monsoons. These inter-monsoons, which are characterized by variable winds and thunderstorms in the afternoon, occur in April and October between the two monsoon periods: the northeast monsoon occurs from November to March and the southwest monsoon from May to September. Rainfall intensity is strongly dependent on rain falling in April and November, when mean rainfall is approximately 280 mm. The lowest rainfall is reported (mean rainfall of 115 mm) in June. The wet seasons take place in the monsoon transitional periods from March to April and October to November. According to JICA (2002) the monthly rainfall average is 180 mm, whereas annual precipitation is approximately 2,400 mm. It is possible to determine a wet season from July to December and a dry season from January to June (Figure 3(a)). These two seasons are determined from an analysis of local precipitation events and the average annual rainfall data from 2000 to 2012, whereas the mean rainfall is reported to be and mm for the wet and dry seasons respectively. The temperature throughout the year remains fairly constantly within the range of 24 to 32 W C and at a mean of 27 W C. Figure 3 (a) Average monthly rainfall from 2000 to 2012; (b) mean water level of lakes and rainfall in the PIW for 2008.

5 259 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply LAKE WATER LEVEL Fluctuations in lake levels in PIW were analyzed based on water level data obtained in Variations of lake water level in eight lake stations over time are summarized in Figure 3(b), clearly identifying recharge periods during the season from September to December as a result of high rainfall during this time. Rainfall data in the PIW was also measured for the same period (Figure 3(b)). There are two seasons in the study area, as shown in Figure 3(a): a dry season from February to June and a wet season from September to December, which includes rainfall. Water levels in Visitor Lake and Main Lake are dependent on rainfall, and their variations predict there to be an adequate relationship between rainfall and water levels (Figure 3(b)). In the period after heavy rainfall, rises in water level in the upper lakes, such as Visitor Lake and Main Lake, occur more quickly than the rise of water level in the lower lakes, such as Chalet Lake and Lotus Lake. This suggests that the upper lakes control the water in the lower lakes. This property can be interpreted as the water recharging to the Paya Indah lakes having originated from a relatively small inflow of groundwater (or the inflow from a large catchment area). GROUNDWATER MONITORING Geochemical analysis was carried out to assess the properties of the groundwater samples collected from monitoring wells in the study area. Groundwater level measurements and groundwater quality monitoring were undertaken using observations of the wells in both March and October from 2010 to A handy analyzer, simple chemical testing on site and laboratory testing were used for analysis of water quality. The defined parameters in the field included ph, conductivity, well depth, total dissolved solids (TDS), salinity and temperature. Groundwater tables also were recorded during the groundwater sampling. The hydrogeochemical parameters measured were the major cations and anions. Two containers were used to divide and maintain the groundwater samples, which were retained with nitric acid (HNO 3 ) for cation analysis. The groundwater samples were filtered for anion analysis and the temperature maintained at 4 W C. The concentration of TDS varied by many orders of magnitude. Table 1 describes the scheme used to classify the groundwater, based on TDS (Freeze & Cherry 1979). RESISTIVITY SURVEYS ERI studies were applied to optional profiles of the study area. Significant penetration of groundwater has affected the lake levels; a possible groundwater flow zone to and from the lake was located using an electrical resistivity technique, and ground surface measurements defined the distribution of subsurface resistivity. Consequently, the true resistivity of the substrate could be estimated. The mineral and fluid content, porosity and degree of water saturation in the rock are effective parameters of ground resistivity. The ERI method uses direct electrical currents to image the electrical resistivity of the subsurface, which, in hydrology, may be associated with the degree of fluid saturation (Parasnis 1997). The technique has been used to locate the water table, water- or air-filled fracture zones, faults and karst conduits (Seaton & Burbey 2000). A Wenner array was used in the present research to obtain the subsurface electrical resistivity profiles surrounding the lake (Figure 1(b)). An ERI survey was performed at each of the selected sites using an ABEM Terrameter SAS 1000 resistivity meter with a multi-electrode switch system with 64 channels; in order to select automatically the active electrodes for each electrode set-up a computer-controlled system which included an ABEM SAS1000 instrument was employed in the study. The Wenner arrays were used along 14 traverse lines each of length 400 m and with an electrode space of 5 m. Resistivity model profiles were created by inverting the measured resistivities using the software Res2DInv and a non-linear least-squares Table 1 Relationship between lithology and TDS of groundwater Class of water TDS (mg/l) Freshwater 0 1,000 Brackish water 1,000 10,000 Saline water 10, ,000 Brine water > 100,000

6 260 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply optimization technique (Loke 2007). In this scheme, the true resistivity distribution of the subsurface is obtained by a linearized least-squares inversion of apparent resistivity pseudosections acquired along profiles. RESULTS AND DISCUSSION Electrical resistivity A maximum depth of 65 m was considered for the resistivity surveys. The ERI surveys included measurements made along several profiles inside and outside the area of the PIW (Figure 1(b)). The ERI surveys detected considerable variations in subsurface resistivity within the shallow alluvial sediments. However, there are some serious limitations in such investigations because current technology is not capable of distinguishing the difference between formations of similar resistivity, such as saline clay, saline sand and cases with low resistivity as a result of the water quality. Therefore, the resistivity data should be interpreted with adequate control on surface and subsurface geology, that it can be accessible from boreholes. The resistivity survey profiles (profiles 1 to 5) were undertaken near the lake in the center of the PIW. The data for the 2D geoelectrical images of profiles 1 to 4 were acquired in an area close to the lake. The survey line for profile 1 had an almost east west orientation. The brackish groundwater boundary in the Wenner inverse model for profile 1 (Figure 4(a)) was clearly shown as a steeply dipping curve. The Wenner inverse model of profile 1 shows an almost wavy interface. Figure 4 Selected 2D resistivity inverse models of the apparent resistivity data measured in the PIW. Elevation in meters is relative.

7 261 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply The differences in the value of resistivity are because of various lithologic types and variations in water saturation. Classification of fresh to saline water based on resistivity of layered regolith (Singhal & Gupta 2010) is shown in Table 2, where the boundary between the fresh and saline groundwater zone is measured at 20 Ω m and resistivity values of less and more than 20 Ω m are estimated to be saline and freshwater zones respectively. The source of the freshwater is assumed to be the groundwater recharge process, which is driven directly by rainfall. Consequently, the region of low resistivity values (less than 20 Ω m) in the bottom section (below 30 m) of the Wenner inverse is related to brackish water. The concentration of brackish water in this position of the section could be due to the concentration of marine deposits in that area. The freshwater layer floats on top of brackish water, since freshwater has a lower density than brackish water. Relatively high and low resistivity values occur in the top, which is related to more compacted material alternating with softer material. The traverse of profile 3 (Figure 4(b)) is located parallel to profile 1 at a distance of approximately 1 km. The depth for the same resistivity zone exactly matches both lines at approximately 30 m depth. The region of low resistivity values in the bottom section (below 35 m) in the Wenner inverse model (Figure 4(b)) is related to brackish water. The other ERI profiles for the PIW are almost identical. Profile 5, which is the same as profile 1 and 3, near to borehole 2, is shown in Figure 4(c). Using data obtained from borehole 2, as a reference, we identify the boundary between the bedrock, sand with gravel (aquifer) and clay layers. According to the data the thickness of the clay layer is around 10 meters, which is shown clearly in the resistivity pseudo-section in Figure 4(d). The aquifer (sand and gravel) is identified in the region 40 Ω m to 80 Ω m. Table 2 Aquifer prospect as related to resistivity (Ω m) of layered regolith (Singhal & Gupta 2010) 0 20 Clays with limited prospect (or saline water) Optimum weathering and groundwater prospect Medium conditions and prospect Little weathering and poor prospect > 200 Negligible prospect Figure 4(d) gives a detailed view of the upper section of the same low-resistivity area, which is related to profile 6. The minimum true subsurface resistivity for freshwater for the interval depth of 2 10 m was observed to be 30 Ω m, whichmayberelatedtothetopaquifer. GEOCHEMISTRY The groundwater samples were collected once per year from 2010 to 2013 from seven existing wells in order to cover all the different lithological units. Figure 2 shows the locations of the wells in the study area. Concentrations of elements based on the chemical analysis of the borehole water samples (in mg/l) are listed in Table 3. Observations at borehole 5 indicated that the electrical conductivity (EC) value was higher than for the other boreholes; high EC values may be due to the composition of sand in the mine, which contains various minerals and has dominated the water quality of the area. According to Table 3, the TDS value in the PIW is less than 548 mg/l, which corresponds to freshwater. A piper diagram (Figure 5) shows that the water chemistry from 2010 to 2013 in the study area ranged from calcium bicarbonate water in the wells to water with a more neutral ionic distribution, found in the streams in siliciclastic rocks. Annual water chemistry analysis of the PIW catchment is plotted on the piper diagram. Based on the results for the concentrations of cations, the water type was shown to be NaþK (with one outlier in 2010) in terms of calcium type. An abundance of sodium in the area can be interpreted as the intrusion of seawater into ground- and/or surface waters. Therefore, salinity and the presence of organic matter play a significant role in controlling exchangeable cations in the study area. However, the results for anions cannot be interpreted in the same way. Anion concentrations are not limited to a specific type of water. The chemistry of water samples is more controlled, with a higher concentration of HCO 3 and Cl (analysis of samples yielded a straight line in anion triangles close to the HCO 3 -Cl side) due to the type of bedrock (i.e. limestone) and salinity in the area. Groundwater facies (central quadrilateral of the piper diagram) vary between marine and deep ancient groundwater (on the right side) and deeper groundwater influenced by ion exchange (at the bottom)

8 Table 3 Concentration of elements based on chemical analysis of borehole water samples from 2010 to 2013 Parameters (mg/l) BH Depth(m) Year Ca Mg Na K Cl SO 4 HCO 3 NO 3 Fe Mn Cu Pb Ni Ba Zn Al PH Cond a NTU TDS BH < 5 45 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 5 54 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 0.1 < 0.01 < < 0.1 < BH < < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < < 1 < 5 7 < 0.5 < 0.1 < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 1 < < 0.1 < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < < 5 4 < 0.5 < 0.1 < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 1 12 BH < 5 90 < < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 5 34 < < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 1 < 5 27 < < 0.1 < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < BH < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 5 84 < < < < 0.1 < BH < < 0.1 < 0.01 < 0.1 < 0.1 < < < < < 0.1 < < < < 0.1 < 0.01 < < 0.1 < < < < 0.1 < 0.01 < < 0.1 < BH < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 5 25 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 1 < 5 18 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 5 19 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < BH < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 1 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < 1 < 5 < 1 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < < < 1 < 5 5 < < 0.1 < 0.01 < 0.1 < 0.1 < 0.1 < a Cond is in (μmhos/cm). 262 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply

9 263 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply Figure 5 Graphical presentations (piper diagram) of the hydrochemical analysis data of the water samples collected from wells in the PIW, 2010 to over the three-year analysis of water chemistry obtained from the catchment. It can be concluded that, although water type varies due to rate of rainfall and, consequently, variation in ion concentration based on seasonality, the presence of seawater, the clay mineralogy and organic matter are important factors influencing the characteristics of the surface water in the study area. The integrated resistivity surveys compared with the chemical analysis results gave a clear picture about the subsurface section. Low TDS (<500 mg/l) and chloride content (<50 mg/l) of groundwater were found by our chemical analysis of groundwater samples from the wells. Resistivity profiling (profile 5) was carried out near well 2 (Figure 4(c)) to determine the subsurface structure and type of water. The comparison between the results of the resistivity model and the geochemical analysis indicate that the resistivity ranged from 40 to 80 Ω m for zones of fresh groundwater containing chloride less than 200 mg/l and TDS value less than 500 mg/l. In contrast, a resistivity of less than 20 Ω m corresponded to areas affected by brackish water. CONCLUSION ERI is an effective, quick and inexpensive method that can provide continuous two-dimensional images of the subsurface. ERI and periodic hydrochemical analysis of groundwater samples were found to be a highly effective method for determination of the boundary between fresh and brackish groundwater and characterization of groundwater quality in the PIW. The integrated methods applied in the

10 264 M. Khaki et al. Groundwater quality assessment using resistivity and chemical analysis Water Science & Technology: Water Supply present study consisted of geoelectrical resistivity and geochemical methods. A combination of interpretations of the resistivity inverse model with the results from boreholes would be beneficial for mapping the thickness of the aquifer and for defining the subsurface layer as well as the depth of the bedrock. The bedrock and aquiferous sand and gravel zones, which are located below the clay layer, were mapped during the research. Interpretation of the resistivity model clearly shows the thickness of the aquifer ranging between 10 to 20 m. The depth to bedrock estimated from the resistivity inverse model generally varies from 30 to 40 m. The accuracy of interpretation in comparison with reference well logs is defined via the obtained result from resistivity imaging. The efficiency and advantages of geoelectrical imaging for groundwater investigations were proven using an inverse model section in the PIW; this is known to be a useful tool for defining the boundary between freshwater and brackish water due to its inherent ability to detect variations in pore water EC. The brackish water zone, with a resistivity value of less than 20 Ω m and position at around 25 m depth, was observed clearly using the resistivity pseudosection model. According to the results obtained from chemical analysis, water quality in the area is freshwater with a low TDS value (<500 mg/l) and EC content (<600 μmhos/cm). ACKNOWLEDGMENTS The authors would like to thank the Department of Mineral and Geosciences of Malaysia for their cooperation. This work was supported by the University of Malaya under research grant PV A. REFERENCES Bradbury, K. R. & Taylor, R. W Determination of the hydrogeologic properties of lakebeds using offshore geophysical surveys. Ground Water 22 (6), Freeze, R. A. & Cherry, J. A Groundwater. Prentice-Hall, Englewood Cliffs, NJ. Geological Survey Department of Malaysia (GSD) 1985 Geological Map of Peninsular Malaysia. 8th edn. 1:500,000. Islami, N., Taib, S., Yusoff, I. & Ghani, A. A Time lapse chemical fertilizer monitoring in agriculture sandy soil. International Journal of Environmental Science and Technology 8 (4), Islami, N., Taib, S., Yusoff, I. & Ghani, A. A Integrated geoelectrical resistivity, hydrochemical and soil property analysis methods to study shallow groundwater in the agriculture area, Machang, Malaysia. Environmental Earth Sciences 65 (3), JICA and MDGM 2002 The Study on the Sustainable Groundwater Resources and Environmental Management for the Langat Basin in Malaysia, Japan International Cooperation Agency (JICA) and Mineral and Geoscience Department Malaysia (MDGM) Report, Vol. 3. Koukadaki, M. A., Karatzas, G. P., Papadopoulou, M. P. & Vafidis, A Identification of the saline zone in a coastal aquifer using electrical tomography data and simulation. Water Resources Management 21, Kundu, N., Panigrahi, M., Sharma, S. & Tripathy, S Delineation of fluoride contaminated groundwater around a hot spring in Nayagarh, Orissa, India using geochemical and resistivity studies. Environmental Geology 43, Lagabrielle, R. & Teilhaud, S Prospection de gisements alluvionnaires en site aquatique par profils continus de resistivite au fond de l eau. Bull. Liaison Lab. Ponts Chauss. 114, Lenkey, L., Hámori, Z. & Mihálffy, P Investigating the hydrogeology of a water-supply area using direct-current vertical electrical soundings. Geophysics 70, Loke, M. H Rapid 2-D Resistivity & IP Inversion Using the Least-squares Method. Geotomo Software, Malaysia. Parasnis, D. S Principles of Applied Geophysics. Chapman and Hall, London. Seaton, W. J. & Burbey, T. J Aquifer characterization in the Blue Ridge physiographic province using resistivity profiling and borehole geophysics: geologic analysis. Journal of Environmental and Engineering Geophysics 5 (3), Singhal, B. & Gupta, R. P Applied Hydrogeology of Fractured Rocks, 2nd edn. Springer, Heidelberg. Sultan, S. A. & FAM Santos 2008 Evaluating subsurface structures and stratigraphic units using 2D electrical and magnetic data at the area north Greater Cairo, Egypt. International Journal of Applied Earth Observation and Geoinformation 10 (1), Wilson, S., Ingham, M. & McConchie, J The applicability of earth resistivity methods for saline interface definition. Journal of Hydrology 316, First received 9 April 2013; accepted in revised form 3 September Available online 18 September 2013