Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India

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1 J. Ind. Geophys. Union ( April 2007 ) Vol.11, No.2, pp Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India S.N.Das, N.C.Mondal 1 and V.S.Singh 1 Action For Food Production (AFPRO), Hyderabad National Geophysical Research Institute, Hyderabad surendra_n_das@rediffmail.com / ncmngri@yahoo.co.in ABSTRACT Identifying a good site for groundwater exploration in hard rock terrain is a challenging task. In hard rocks, groundwater occurs in secondary porosity developed due to weathering, fracturing, faulting, etc., which is highly variable and varies sharply within very short distances, contributing to near-surface inhomogeneity. In such situations topographic, hydrogeological and geomorphological features provide useful clues for the selection of suitable sites. Initially based on the satellite imageries, topographical, geomorphological and hydrogeological inference promising zones were demarcated in hard rock areas of three Mandals (Seethanagaram, Salur and Pachipenta) of Vizianagaram District, Andhra Pradesh, India, an area of about 750 km 2 to identify good locations for groundwater exploration. Total 100 Vertical Electrical Soundings (VES) were carried out using Wenner electrode configuration and an interactive interpretation of the VES data sharpened the information inferred from geomorphological and hydrogeological reconnaissance. Out of them 42 sites were recommended for drilling. Drilling with Down-The-Hole hammer (DTH) drill method was carried out for drilling in the recommended sites. 42 bore wells are drilled down to 50 to 80 m depths. The interpreted VES results are also correlated with the drilled bore well lithologs, showing good agreement with them. The yields of these bore wells vary from 1500 to 8000 lph, which are obtained from step draw down pumping test. INTRODUCTION There are several difficulties for development of groundwater resources in hard rock areas as wide and erratic variation of vital parameters (i.e. fractures, joints, porosity etc), characterize the groundwater regime. Spatial variation of these characteristic parameters is attributed to, among other causes, to tectonic set-up and degree of weathering of near surface rocks (Barker, Rao & Thangarajan 2001). These processes induce directly or indirectly secondary porosity in the hard rocks to a variable extent. As a result, the groundwater potential also varies significantly from place to place sometimes within few meters and even within the same geological formations. Indian context the situation becomes all the more precarious due to (1) negligible primary porosity and low permeability of host rocks restrict groundwater storage as well as movement, and (2) also low rainfall, high evaporation and surface run-off limit recharge to the groundwater systems (Rangarajan & Athavale 2000). However granite areas, in particular, have shown that a systematic surface exploration succeeds in locating sites where high yielding wells may be drilled. A systematic surface investigation was deployed for groundwater exploration in Seethanagaram, Salur and Pachipenta Mandals, Vizianagaram district, Andhra Pradesh, India. These three Mandals are under Vizianagaram district, Andhra Pradesh, India (as shown in Fig.1). Seethanagaram Mandal is situated to the north part of Vizianagaram district and lies between Latitudes / 24 // / 55 // N and Longitudes / 57 // / 24 // E and extends over an area of about 149 km 2 covering 44 villages. The density of population is 370 per sq.km. Salur mandal is situated to the northwestern part of Vizianagaram district and it lies between north Latitudes / and / and east Longitudes / 23 // and / 30 // and spreads over an area of about 314 km 2 with 90 villages. The density of population is 227 per sq.km. Otherside Pachipenta Mandal is situated to the northwestern part of Vizianagaram district included in Tribal Sub-Plan area and it lies between north Latitudes / and / 26 // and East Longitudes / 43 // and / 29 // and extends over an area of about 287 km 2 covering 54 villages. The population density is 227 per sq.km. Mostly hill ranges and forests occupy the area at 45% to the total geographical area. 79

2 S.N.Das, N.C.Mondal and V.S.Singh Figure 1. Location map of three Mandals, Vizianagaram district, Andhra Pradesh, India. Attempts were made to gather maximum information about the subsurface structures using an appropriate mix of different techniques viz. satellite imageries, hydrogeological reconnaissance and geophysical exploration etc. A conscious integration of the informations, so collected, helped progressively to focus on the targeted zones and locate prospective drilling sites for high yielding wells. Down-The-Hole Hammer (DTH) machine is used for drilling and then drilled bore well data were correlated with the interpreted VES results. The yields of bore wells were also confirmed by step draw down pumping tests. The results of the investigations in the above three mandals are presented in this paper. METHODOLOGY Attempts were made to gather maximum information about the subsurface structures using of different techniques viz., satellite images, hydrogeological reconnaissance and geophysical exploration. An integration of the information, thus collected, helped in locating drilling sites for high yielding wells. The satellite images of these three areas provide useful information about the existence and disposition of large-scale lineaments. Areas of thick vegetation, alluvial patches, and low topography could be demarcated with the help of these imageries as well as toposheets. The surface manifestation of some of these structural features was delineated during hydrogeological reconnaissance. While a few of these structures are exposed on the surface, some others could be seen in well sections and road cuttings. An approximate assessment of the thickness of the weathered zone could be obtained from the existing dug wells at some spots. Some of the well sections provided good indications of the occurrence of alluvial patches and buried river courses/palaeochannels. A careful analysis of the water table contours provided an understanding of the groundwater flow pattern, which in turn, help to identify zones of water accumulation. Some of the hydro-geological features were also 80

3 Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India studied during the field works are described here. (1) Areas of low topography were investigated where chances of occurrence of groundwater is more; (2) The static water level in the existing wells was observed which could give useful information about the depth of the water table and flow direction; (3) The general drainage pattern was studied which controls the recharge potential of the groundwater and suggest existence of fractures and structural anomalies (as shown in Figures 2,3 &4); and (4) Observation of certain type of vegetation was also studied, which are indicative of occurrence of groundwater. Since the structural features like dykes and faults play a significant role in the groundwater movement and accumulation, care was taken to investigate such areas and sites located at these places turned out to be high yielding. HYDROGEOMORPHOLOGY AND HYDROGEOLOGY FEATURES Scheidegger (1973) reported that water is an important agent in the formation of landforms. The satellite images were examined based on visual interpretation and the study areas were divided into several geomorphological units such as flood plains, inselbergs, pediments, and pediplains, residual and structural hills. Flood plains and alluvial plains were found along the Suvarnamukhi river of Seethanagaram Mandal (Fig.2). The alluvium consists of sand, silt and clay with the thickness varies from 5.00 to more than m. The thickness of alluvium varies from 1.50 to m. The residual hills are located in the northern site near Guchchimi, Ippalavalasa and Nidagallu villages. The structural hills identified and found in and around Repativalasa and western side of Jaggunayudupeta village. These hills are gently sloping surfaces towards eastern side. The trend of the structural hills ranges from NNE to SSW. The runoff is high with negligible infiltration, making them poor aquifers (Raju, Rao & Kumar 1989). Major parts of this mandal are underlain by Archaean group of rocks, while few patches are occupied by recent alluvium along the Suvarnamukhi River and other hill streams. The Archaean group of rocks comprising khondalites, prophysoblastic gneissic granite, gneisses Figure 2. Drainage pattern of Seethanagaram Mandal, Andhra Pradesh, India. 81

4 S.N.Das, N.C.Mondal and V.S.Singh Figure 3. Drainage pattern of Salur Mandal, Andhra Pradesh, India. 82 Figure 4. Drainage pattern of Pachipenta Mandal, Andhra Pradesh, India.

5 Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India and intrusive quartz veins. The total depth of weathered zone varies from 7.00 to m (SGWD 2005). The ground water occurs in water table conditions in weathered and fracture zones in this mandal. The depth of dug well varies from 8.00 to m. The depth of water level in these dug wells ranges from 2.00 to m (bgl). The yield of the dug well ranges from 40,000 to 70,000 lpd depends upon the nature of geological formation. In Salur Mandal, there are few patches which are occupied by recent alluvium along the Vegavathi, Suvarnamukhi and Gomukhi rivers and of the valley fill areas (Fig.3). The alluvial and residual material resulted by the weathering of parent rock material have been transported and deposited in the valley fill areas which mainly comprises of fine sands, silts and talus material. The valley fills have found to be very productive aquifers. The saturated thickness of alluvium ranges from to m. The residual hills are located in the eastern site near Gudivalasa, Mamidipalli, Redda Chipuruvalasa and Sonadavalasa villages of Salur Mandal. The pediplains are also found in eastern site of this mandal. Salur Mandal is also underlain by Archaean group of rocks, while few patches are occupied by recent alluvium along the Vegavathi, Suvarnamukhi and Gomukhi rivers and of the valley fill areas. The Archaean group of rocks consisting Khondalites, Granites, Gneisses and charnockites occupy a major part of this Mandal. The depth of weathered zone varies from 8.00 to m. The ground water meets under unconfined conditions in shallow depth and unconfined to semi-confined conditions in weathered and fractured zones in deeper depth. The depth of dug wells varies from 8.00 to m. The depth to water level ranges from 5.40 m to m (bgl). The yield of the dug wells ranges from 30, 000 to 60, 000 lpd. Recent alluvium occupies along the Pedda Gedda and Chitte rivers and of the valley fill areas in Pachipenta Mandal (Fig.4). Its general trend is NE-SW direction. The extent and thickness of alluvium along the stream is ranges from to m. The residual hill is located in and around Chindivalasa and Gongadivalasa villages. The flood plain lies in the central part of the area. Major parts of Pachipenta Mandal are underlain by Archaean group of rocks, while few patches are occupied by recent alluvium along the rivers and of the valley fill areas. The Archaean group of rocks consisting Khondalites, Granites, porphyroblastic gensises, charnockites and general trend is northeast-southwest direction. The thickness of weathered zone varies from 8.00 to m. The ground water is only available in weathered and fractured zones under water table conditions. The depth of dug wells varies from 8.00 to m. The depth to water level ranges from 2.00 to 8.50 m (bgl). The yield of dug wells ranges from 30, 000 to 65, 000 lpd. GEOPHYSICAL INVESTIGATIONS Based on the above geomorphological and hydrogeological inference, promising zones were demarcated for carrying out detailed vertical electrical soundings and obtaining specific and reliable information about the exiting and geometry of subsurface structures particularly those, which do not have any surface signatures. Although indirect, the geophysical methods provide information on aquifer geometry that is useful in conceptualization of the aquifer system. A vertical electrical sounding (VES) method was used and a good resistivity contrast was measured between weathered rock containing water and the hard rock without water. Thus, 100 vertical electrical soundings (including at least 2-3 VESs in and around each village) were carried out covering the entire three mandals, using the Wenner electrode configuration. Considering the variation in the apparent resistivity for each electrode separation, keeping the place of observation constant made quantitative interpretation of the electrical resistivity data. Sanker Narayan & Ramanuja Chary (1967) showed that it is possible to solve the field equation directly to get resistivities and thicknesses of the subsurface layers from the field data by inverse slope method. It can be seen that the field data falls on discrete straightline segments with sharp changes in slope in all the cases, representing the subsurface resistivity variations. 42 sites out of 100 VES locations are given good indication of water bearing zones. The results of VES at one site of each Mandal are presented in Table 1. The depths to the basement derived from the sounding interpretations were in good agreement with the actual depth obtained from the lithologic data collected during the drilling bore wells. This interpretation has helped us to understand the subsurface layering, degree of saturation and depth to basement of these aquifers. STEP DRAW DOWN PUMPING TEST A step draw down test helps in computation of well efficiency, well productivity and also determining the safe discharge rate for any well for selection of a suitable pump set (Jacob 1947; Rorabaugh 1953). In this test the time draw down and time recovery data 83

6 S.N.Das, N.C.Mondal and V.S.Singh Table 1. Results of geoelctrical layers at the selective sites of Seethanagaram, Salur and Pachipenta Mandals Sl No Village Mandal Resistivity ( r ) Thickness Depth (d) Inferred lithology in Ohm-m (h) in m in m 1 Tamarakandi Colony Seethanagaram Topsoil, clay, kankar and sand Highly weathered and fractured Khondalites, gneissic granite & gneisses. High Hard rocks (gneissic granite & gneisses) with minor fractures. 2 Velagavalasa Salur Topsoil, clay and fine sand Highly weathered Khondalites Weathered and fractured Khondalites, gneissic granite & gneisses. High Hard rocks (gneissic granite & gneisses). 3 Suragaddavalasa Pachipenta Topsoil, clay and fine sand Clay and coarse sand Highly weathered and fractured Khondalites, gneissic granite & gneisses. High Hard rocks (gneissic granite & gneisses). Table 2. Results of step draw down pumping test data at selective sites of Seethanagaram, Salur and Pachipenta Mandals Discharge Max Draw Sp.Draw Sp.Capacity Well Aq.loss Well.Loss Optimum Rec. (Q) Sl No Village Mandal Steps (Q) in lpm down (Sw) down (Sw/Q) (Q/Sw) in efficiency Co.eff (B) Co.eff (C) yield (Qp) Discharge in m in m/lpm lpm/m (Ew) in % in lpm in lph TamarakandiColony Seethanagaram Vellagavalasa Salur Suragaddavalasa Pachipanta

7 Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India for different steps were plotted on semi-log graph. The time required for achieving complete recovery (SWL) is estimated by proper extrapolation of the recovery curve. The recovery period thus estimated is subtracted from 24 hours, which gives the optimum time period for pumping the well. The time draw down curves is extrapolated to this time and the draw down noted for each step. The ratio of draw down (S 1 ) and the corresponding discharge rate (Q 1 ) are calculated. Values of (S 1 /Q 1 ) are plotted against Q 1, and the least square straight line is fitted through the points. The intercept and slope give the coefficient of the aquifer losses and well losses (B & C) by trial and error. Specific capacity and well efficiency: Specific capacity (Q/S w ) of a well is the discharge per unit draw down in the well and is usually expressed in lpm/m. This is a measure of well efficiency. The specific capacity of well is not constant but decreases with the increase in pumping rate (Q) and prolonged pumping (t). If the well is small in diameter and is pumped at higher rate, the frictional losses will increase. The maximum safe yield of a well is the capacity of the aquifer to supply water without causing a continuous lowering of the water table and is therefore, limited by the rate at which the ground water is replenished by the recharge. Excessive lowering of the ground water table by pumping may affect the surrounding water sources. The efficiency of the well for a particular discharge is calculated by BQ/S w. It is shown in Table 2 for one site of each Mandal. RESULTS AND DISCUSSION The investigations carried out and conclusions arrived is presented here to illustrate the process of identification of potential aquifer zones in hard rock terrains. In most of the cases any single technique alone would not have been so reliable or useful. Figure 1 shows the locations and areas of investigation of 42 favorable sites. Three case studies were discussed here. Seethanagaram Mandal: Tamarakandi colony of Seethanagaram Mandal is one of 11 sites chosen for its location close to a major lineament and on the bank of the Suvarnamukhi river (Fig.2). The VES carried out at the village Tamarakandi colony. Figure 5 reflects a 20 m top layer of clay kankar and sand zone with resistivity Ohm-m. The water saturated was first at 23.0 m depth in the highly weathered and fractured Khondalites, gneissic granite and gneisses with an initial yield of 1500 lph. The main aquifer occurs at a depth of 40 m, which extends down to 50 m depth producing a yield of 5500 to 9000 lph. The interpreted geoelectrical section matches well with the borehole litholog. It shows weathered and highly fractured zones having resistivity of 58.31Ohm-m and that of massive gneissic granite and gneisses with minor Figure 5. Typical VES plot at Tamarakandi colony of Seethanagaram Mandal. 85

8 S.N.Das, N.C.Mondal and V.S.Singh Figure 6. Comparison of geoelctrical layers and lithologs. Figure 7. Cross plot of (S w /Q) vs Q for determination of B & C at Tamarakandi Colony (Seethanagaram Mandal). fractures with high resistivity (as shown in Fig.6). The above yield of this bore well was also confirmed by conducting the step draw down pumping test which showed recommended yield of 9,000 lph and optimum yield of 15, 480 lph. The cross plot S w /Q vs. Q is shown in Fig.7. Similar matching was obtained at other 10 sites of this Mandal. Salur Mandal: Velegavalasa of Salur Mandal is one of 22 sites. In this village the site was chosen for its close location to the drainage, and belongs to the Pediplain (Fig.3). The VES was carried out at this village and typical curve of it is shown in Fig. 8. It shows that the top layer is completely dry being resistivity Ohm-m. The water struck at 30 m in the first fractured zone yielding 800 lph. The main aquifer was met at 40 m depth extending up to 50 m producing the yield of lph. The interpreted geoelectrical section matches well with the observed lithologs (as shown in Fig.9) indicating the weathered zone resistivity as Ohm-m and that of fractured zone as Ohm-m and the massive gneissic granite and gneisses with high resistivity. The above yield of this bore well was also confirmed by conducting the step draw down test which showed recommended yield of 3600 lph and optimum yield of 3720 lph. The cross plot S w /Q vs. Q is shown in Fig.10. Similar agreements were also achieved in the other 21 sites in this Mandal. Pachipenta Mandal: Suragaddavalasa of Pachipenta Mandal is one of 9 sites. The sounding was carried out at this village falls on a lineament as inferred from the hydrogeomorphological map and being located in the structural hill (Fig.4). Here, the result of VES has shown four geoelctrical layers (as shown in Fig. 11). The drilling results show that water struck at 6.0m depth with an initial yield of 800 lph. The second aquifer was found at 17.5 m depth, which increases the yield to 3000 lph. The main aquifer meets at 32.0 m depth and extending upto depth of 50 m producing the yield of 5000 to 8000 lph. The interpreted geoelectrical section well correlates with the borehole litholog (as shown in Fig. 12) showing the highly weathered Khondalites zones resistivity as Ohmm and, weathered and fractured Khondalites, gneissic granite & gneisses showing resistivities Ohmm and that of massive gneissic granite and gneisses with high resistivity. The above yield of this bore well was also confirmed by conducting the step draw down test. This test showed recommended yield of 7,500 lph and optimum yield of 17, 400 lph. The cross plot S w /Q vs. Q is shown in Fig.13. Similar agreements were also found in the other 8 sites of this study mandal. 86

9 Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India Figure 8. Typical VES plot at Velegavalasa, Salur Mandal. Figure 9. Comparison of geoelctrical layers and litho-logs. 87

10 S.N.Das, N.C.Mondal and V.S.Singh Figure 10. Cross plot of (S w /Q) vs Q for determination of B & C at Velegavalasa, Salur Mandal. Figure 11. Typical VES plot at Suragaddvalasa, Pachipenta Mandal. 88

11 Groundwater Exploration in Hard Rock Areas of Vizianagaram District, Andhra Pradesh, India CONCLUSIONS In general, the hard rock formations do not have good groundwater potential, still integrated studies help to ascertain presence of hidden water bearing formations. Resistivity sounding proves to be an effective tool to locate the productive zones, when interpreted in conjunction with hydrogeological and hydrogeomorphic data. Of course, all the relevant and situation-specific features would have to be taken into consideration while deciding the overall approach for groundwater exploration strategy. ACKNOWLEDGEMENTS Authors wish to acknowledge Mr. D.K. Manavalan, Executive Director, AFPRO, New Delhi, and Mr. K. Siva Prasad, Principal Specialist cum coordinator, AFPRO field Unit VI, Hyderabad for their kind permission to publish this paper. The authors are grateful to VISHWAS for allowing utilization the data. REFERENCES Figure 12. Comparison of geoelctrical layers and lithologs. Figure 13. Cross plot of (S w /Q) vs Q for determination of B & C (Suragaddvalasa, Pachipenta Mandal). Barker, R., Rao, T.V. & Thangarajan, M., Delineation of contaminant zone through electrical imaging technique, Curr. Sci., 81, 3, Jacob, C.E., Draw down test to determine effective radius of artesian well, Trans. Amer. Soc. of Civil Engrs., 112, 2321, Raju, K.C.B., Rao, G.V.K. & Kumar, B.J., Analytical aspects of remote sensing techniques for groundwater prospecting in hard rocks, In: Proc. of the 6 th Asian Conference on Remote Sensing, Rangarajan, R. & Athavale, R. N., Annual replenishable groundwater potential of India an estimate based on injected tritium studies, J. Hydrol., 234, Rorabaugh, M.J., Graphical and theoretical analysis of step-draw down test of artesian well, In: Proc. Amer. Soc. Civil Engrs., 79, 362, Sanker Narayan & Ramanuja Chary, An inverse slope method determining absolute resistivity, J. Geophys., Scheidegger, A.E., Hydrogeomorphology, J. Hydrol. 20, 2, State Ground Water Department (SGWD), District report, Vizianagaram (Accepted 2007 May 15. Received 2007 April 14; in original form 2006 December 26) 89

12 S.N.Das, N.C.Mondal and V.S.Singh Mr. S.N. Das (Hydrogeologist, AFPRO) obtained his M.Sc. in Geology from Banaras Hindu University, Varanasi and M.Phil. in Geology from Utkal University, Bhubaneswar in 2000 and 2001 respectively. He has been working from last five years from NGO sectors for ground water development in rural sectors. He has extensive experiences in the field of groundwater exploration in hard rock terrains and specialist is RS & GIS. He has published over 5 research papers in National and International Scientific Journals. He is members of IAH & IGU. Dr. N.C. Mondal (Scientist, NGRI) obtained his M.Sc.Tech. in Exploration Geophysics from BHU, Varanasi and Ph.D. in Geophysics from Osmania University, Hyderabad in 2000 and 2005 respectively. He was pursued his research in the field of Groundwater Modeling since 2000 at NGRI. He was awarded of Senior Research Fellow from CSIR, New Delhi during Developed the efficient and versatile technique for estimation of groundwater potential zones in hard rock terrain. Published over 50 research papers in National and International Scientific Journals. He is members of IAH, IHA & SPG and visited in two times to Singapore for presenting his research findings. Dr. V.S. Singh (Deputy Director, NGRI) obtained his M.Sc. and Ph.D. in Geophysics from BHU, Varanasi, India in 1972 and 1987 respectively. He was pursued his research in the field of Groundwater Hydrology since 1972 at NGRI. He was awarded German Fellowship during at NLfB, Hannover, Germany and also British Fellowship at University of Birmingham, U.K. during 1984 and Developed efficient and versatile techniques for estimation of aquifer parameters considering realistic field conditions, particularly, prevailing in Asian countries. Published over 140 research papers in National and International Scientific Journals and edited two books. Supervised one Ph.D. student. Dr. Singh has been conferred with NATIONAL HYDROLOGY AWARD. 90