Geoelectrical resistivity survey of the Chennai Sewage Treatment Plants for identifying reclaimed water recharge

Transcription

1 JOURNAL OF COASTAL SCIENCES Journal homepage: ISSN: Volume 2 Issue No Pages Geoelectrical resistivity survey of the Chennai Sewage Treatment Plants for identifying reclaimed water recharge * T. Selvakumar, M. Mathiazhagan, Madhavi Ganesan Centre for Water Resources, Anna University, Chennai , India A B S T R A C T An electrical resistivity survey was carried out in the premises of Chennai Metro water Sewage treatment plants at Koyambedu, Nesapakkam, Kodungaiyur, Perungudi and Anna University to assess the suitability for reclaimed water recharge and also to understand groundwater conditions, such as depth, thickness and location of the aquifer and the type of water. The geo-electrical methods used in the survey are Vertical Electrical Sounding (VES) in and around the Chennai Sewage Treatment Plants. The 5 VES points were conducted using the Schlumberger configuration in the entire study area. The instrument used is ABEM Terrameter SAS The VES data were interpreted using IX1Dv2 software which showed that the area is composed of top soil (clay and Alluvium), clay, sandy clay, weathered gneiss, jointed gneiss and massive charnockite. These layer parameters thus obtained from the analysis were used together with existing boreholes to estimate the depth and thickness of each layer as well as correlating water quality with the aquifer apparent resistivity. The curves are prominently of H-type indicating the presence of three layers followed by combination of curves (HA and HK) indicating the four sub-surface layers. Interpretation reveals the number of subsurface layers, their thickness and their water potential zone within the study area. The resistivity varying from 3.96 Ωm to Ωm and thickness from 0.58 m to m. The EC value for VES 1 (Perungudi) and VES 3 (Koyambedu) are 26 ms/cm and 1.55 ms/cm, and the TDS value for VES 1 and VES 3 are 16,896 mg/l and 993 respectively. A R T I C L E I N F O Received 2 November 2014 Accepted 20 March 2015 Available online 23 March 2015 Keywords Chennai Sewage Treatment Plants Reclaimed water recharge VES Chennai *Corresponding author, address: selva301164@gmail.com Phone: Journal of Coastal Sciences. All rights reserved 1. Introduction When trying to perform an integrated management of water resources, groundwater volumes can be increased by reusing reclaimed water for recharge. Surface spreading is the simplest, oldest and most widely applied method of artificial recharge. The ability of the subsoil and bedrock to treat pollutants adequately depends on the geological and hydrogeological characteristics of the site, particularly the permeability (including the soil one), the thickness of the subsoil and the depth to the water table. As reclaimed water migrates from land surface, it first moves through an unsaturated zone before recharging the saturated groundwater system. When water moves through the unsaturated zone (including soil), physical and chemical processes occur that can affect the water (and potential contaminants) volume and rate of movement. Aquifers associated with thick unsaturated zones (deep water tables) tend to have less recharge than aquifers with thinner unsaturated zones and water tends to move slowly in the first situation. Soil and subsoil provide the most effective protection of groundwater form pollution. Organic and clayey soil, and clayey sands/ gravels and permeable sands (i.e combination of sand, silt and clay) are the most suitable subsoils for effluent disposal and treatment. In contrast, once pollutants enter bedrock the rate of purification declines. Groundwater is most vulnerable and at risk from pollution where bedrock, particularly limestone, is at or close to the surface: where clean, permeable sand/gravel underlies the site; 19 and in sand/gravel where the water table is close to the surface. In areas with these conditions, pollution of wells is common. Therefore, these pedological, geological and hydrogeological characteristics should be examined and mapped in detail, thereby providing a soil and subsoil assessment for any area or site. Modern geophysical methods, such as vertical electrical sounding, can play an important role in these studies (Tapias et al. 2005). The electrical resistivity technique involves the measurement of the apparent resistivity of soils and rock as a function of depth or position. The most common electrical technique needed in hydrogeologic and environmental investigations is vertical electrical soundings (resistivity sounding). During resistivity surveys, current is injected through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The bulk average resistivity of all soils and rock influencing the current. It is calculated by dividing the measured potential difference by the input current and multiplying by a geometric factor specific to the array being used and electrode spacing (Zhody et al. 1974). The use of electrical resistivity survey for assessing the groundwater quality and aquifer potential has increased over recent decades in the world due to rapid advances in geophysical investigations (Lashkaripour and Nakhaei, 2005). The geoelectrical method (VES resistivity survey technique) has been used successfully for investigating the groundwater potential and its quality in different lithological settings because the instrumentation is simple,

2 field operations are easy and the analysis of data is economical and less tedious than other methods. VES survey technique, using resistivity meter, is effective to study groundwater conditions and to assess the subsurface geo electrical layers. This survey technique can also be employed for groundwater investigations. VES data can be analysed using computer software. The outcome of VES data, needs verification especially in case of non-homogenous and an-isotropic aquifers (Alabi et al. 2010; Hibbs, 2001; Urish, 1983; Stewart et al. 1983; Mathiazhagan et al. 2012; Mathiazhagan et al. 2013; Senthilkumar et al. 2012; Sinkandar et al. 2010). For this paper, the methods of investigation include surface geophysical survey, collection and analysis of water quality data from borewells and lithology data/water quality data from Government and private agencies (Sivaraman and Thillaigovindarajan, 2002). The underlying objective of this paper was to test the advantage of using this geophysical method for the continuous characterization of the subsoil structure and suitability of reclaimed water recharge around the Chennai Sewage Treatment Plants. 2. Methodology The resistivity technique examines horizontal and vertical discontinuities in the electrical properties of the ground. It measures earth resistivity by passing an electrical current into the ground and measuring the resulting potentials created. This method involves the supply of direct current or low-frequency alternating current into the ground through a pair of electrodes and the measurement of the resulting potential through another pair of electrodes (potential electrodes) (Zhody et al. 1974). Because the current is known and the potential can be measured an apparent resistivity can be calculated. The apparent resistivity of the subsurface material is a function of the magnitude of the current, the recorded potential difference and the geometry of the electrode array used. The current electrodes spacing (AB) increases after each reading while the potential electrodes spacing (MN) increases only when deemed necessary and controlled by the relation AB/2 5MN/2 as required by the Schlumberger array. For Schlumberger soundings (Fig. 1), the survey was conducted with ABEM SAS 1000 Terrameter. Fig. 1 Schlumberger Sounding or Vertical Electrical Sounding (VES) Then, the sounding curves were interpreted to determine the true resistivities and thicknesses of the subsurface layers. IX1Dv2 software was used for data interpretation. A total number of 5 Vertical Electrical Sounding (VES) were carried out in the Chennai STP s namely Perungudi, Nesapakkam, Koyambedu, Kodungaiyur and Anna University. The maximum outer current electrode spacing was 200 m and inner potential electrode spacing was 10 m. The apparent resistivity data are associated with varying depths relative to the distance between the current and potential electrodes and can be interpreted qualitatively and quantitatively in terms of a lithologic and/or geohydrologic model. In the qualitative interpretation method the shape of the field curve is observed to assess the number of layers and their resistivity. In the quantitative interpretation method, true resistivity ρ and layer thickness h as the fundamental characteristics of a geoelectric layer are obtained. The quantitative interpretation of VES curves in this study was done by the well known method of curve matching. In curve matching technique, the field VES curves are compared with set of theoretical curves to obtain ρ and h. Soil and Water samples were taken for analysis of lithological and hydrogeological information and chemical analysis of the groundwater. These data were used to evaluate the subsurface hydrogeological and lithological conditions and to correlate/calibrate field VES survey curves at these sites. 3. Results and Discussion The results of data analysis are presented as sounding curves, and tables of geoelectric parameters. The predominant field curve in the area is the H type combination curve, a three layer and four layer setup (Figure 2). Table 1 shows that the study area is underlined by three and four geoelectric layers. The first layer with topsoil has a variation. The second layer is shallow having unconfined aquifer with Sandy clay, Sand and Weathered charnockite rock. The third layer is fracture aquifer with Sandy clay, Sand, Shale and Weathered/Fractured charnockite rock. And the fourth layer is deep fractured/massive charnockite rock. Five sets of readings (from VES 1 to VES 5) were taken for Vertical Electrical Sounding (VES) and with these readings, the value of apparent resistivity (ρa) was found out. The ρa value indicates the geological aspects that is type of lithology and type of water present in the aquifer. The EC value for VES 1 and VES 3 are 26.4 ms/cm and 1.55 ms/cm, the TDS values for VES 1 and VES 3 are 16,896 mg/l and mg/l respectively. The Geophysical survey was carried out Chennai STP s. If the apparent resistivity value for water is 10 to 100 ohm-m, the water is of potable use and if the value is above or below this range the water is not fit for potable use. In this case apparent resistivity range from.96 Ωm to Ωm. Almost all the VES locations except VES 5 and VES 3 were totally damaged and it is not fit for potable purpose. Stations ρa 1 ρa 2 ρa 3 ρa 4 h1 h2 h3 Depth Type Curve RMS Error VES H 8.34 VES H 6.81 VES HA VES HA 10.6 VES HK 3.37 Table1 Summary of modeling results for all soundings 20

3 Fig. 2 Vertical Electrical Sounding Interpretation results 21

4 4.1. Correlation of VES results with existing borehole The success of any geophysical survey depends on the calibration of the geophysical data with actual hydrogeological and geological ground truth information. To understand the relationship between geoelectric parameters (formation, thickness and resistivity) and hydrogeological characteristics of formations for the investigated Acknowledgements The authors are thankful to Chennai Metropolitan Water Supply and Sewerage Board and Anna University for granting permission to conduct the geophysical survey at Chennai Metro water Sewage Treatment Plants sites and Anna University STP site. Fig. 3 Geoelectric section and bore-log for the study area area, the results of these soundings were correlated with the results of samples (Soil and Groundwater) collected from existing boreholes at same sites. The results of the Schlumberger electrical soundings interpreted using computer software 1X1Dv2 were found to be in close agreement with the hydrogeological sections (Figure 3). The results from analysis of VES data and existing borehole data corroborates perfectly with the lithology. The top soil with clay/alluvium is the first layer 4. Conclusion This study revealed that Vertical Electrical Sounding measurements are effective to study the suitability of reclaimed water recharge and to understand the present groundwater conditions. Based on interpretation of VES data, following conclusions are drawn. The VES method is a non-destructive and appropriate method to detect depth and thickness of various subsurface geological formations and the groundwater quality within these formations of the layers for the study area. The interpreted VES data using computer software 1X1D v2 was found to be in close agreement with borehole data. The subsurface geoelectrical layers showed different resistivity values at the Chennai Metro water Sewage Treatment Plant sites due to type of formation. The curves are prominently of H type indicating the presence of three layers followed by combination of curves HA and HK indicating the four sub-surface layers. Interpretation reveals the number of subsurface layers, their thickness and their water bearing capacity within the study area. The resistivity varying from 3.96 Ωm to Ωm and thickness from 0.58 m to m. The EC value for VES 1 (Perungudi) and VES 3 (Koyambedu) are 26 ms/cm and 1.55 ms/cm, the TDS value for VES 1 and VES 3 are 16,896 mg/l and 993 respectively. Among the five STPs sites, Anna University and Koyambedu are suitable for reclaimed water recharge. References Alabi, A. A., Bello, R., Ogungbe, A. S., Oyerinde, H. O Determination of Groundwater Potential in Lagos State University, OJO; Using Geoelectrical methods (Vertical Electrical Sounding and Horizontal profiling), Report and Opinion. Hibbs, B. J Geophysical and Hydrochemical analysis of the white river Alluvial Aquifer, Crosby county, Texas, New Mexico Geological Society Guidebook, 52 nd field conference, Geology of the Llano Estacado: Lashkaripour, G. R., Nakhaei, M Geoelectrical investigation for the assessment of groundwater conditions: a case study. Annals of Geophysics, 48, Mathiazhagan, M., Selvakumar, T., Madhavi, G Detection of solid waste dumpsite-induced groundwater contamination leachate using electrical resistivity method, Proceedings of the Sixth IAHR International groundwater Symposium, Kuwait, November, 2012: CRC Press. Mathiazhagan, M., Selvakumar, T., Madhavi, G Geophysical Technique for Sensing of Solid Waste Dump Site-Induced Groundwater Contamination Leachate. Pollution Research, 32(3), Senthilkumar, G., Ramanathan, A. L., Nainwal, H. C., Chidambarm, S Hydrogeophysical investigation of groundwater in Cuddalore Coastal Area, Tamilnadu, India. Geosciences, 2(5), Sinkandar, P., Bakhsh, A., Ali, T., Arshad, M Vertical Electrical Sounding (VES) resistivity survey technique to explore low salinity groundwater for tubewell installation in Chajdoab. Journal of Agricultural Research, 48(4), Sivaraman, K. R., Thillaigovindarajan, S Manual on Rainwater Harvesting Save Water & Save the Nation from the Water Crisis: A Unique Feature of India s Traditional Water Harvesting Methods, Sastha Foundation, Chennai-91 Stewart, M., Layton, M., Lizanec, T Application of resistivity surveys to regional Hydrogeologic reconnaissance. Groundwater, 21(1), Tapias, J. C., Himi, M., Masachs, A., Nieto, C., Brissaud, F., Salgot, M Using electrical imaging for assessing suitability of reclaimed water recharge at 22

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