Geo-electrical surveys for groundwater in Kagna river sub-basin, Pargi Mandal, Ranga Reddy District, A.P., India.
Kagna river sub-basin is a part of mighty river Krishna is situated about 60 km west of Hyderabad city. The present investigation is intended to explore the groundwater potentiality of the sub-basin. Generally, a number of geophysical exploration techniques are available which enables an insight to be obtained rapidly in the nature of water bearing layers or aquifers. The choice of a particular method is governed by the nature of the terrain and cost considerations.
The electrical resistivity method will furnish information on the surface geology unobtainable by other methods in groundwater studies. The ability of the electrical method to provide information on depth of the fresh/salt water interface (Pathangey, 1973, and Muralidhar, 1988). The resistivity techniques have been successfully utilized for the assessment of groundwater resource potentials within the basement aquifers.
The area under investigation is located from 17[degrees]07' to 17[degrees]21' North latitudes and 77[degrees]43' to 77[degrees]57' East longitudes (Fig.1), and is forming part of toposheet no.'s 56G/11, 15 and 16 with an areal extent of 335 sq.km. which is constantly drought prone areas in Ranga Reddy District.
RAINFALL AND TEMPERATURE
The area under investigation receives an average annual precipitation of 770mm. The main contributor in this area is the South-West monsoon, i.e., first week of June to the last week of September. Its contribution to the normal annual rainfall is around 80-85%. Rest of the rainfall receives from North-West monsoon, which is active during the period October to December.
The temperature of the area is very hot during summer months especially in April and May. At times the temperature shoots up to 47-48[degrees]C. In the winter seasons the mercury falls down to even 13-15[degrees]C in the months of December and January.
DRAINAGE AND SOILS
The area under investigation is a small sub-basin of the river Kagna. The drainage system of Kagna Sub-basin is sub-parallel to parallel. Parallel drainage is structurally controlled mainly by Joints or faults. Both north and north eastern regions of the basin are structurally controlled and large basaltic flows show this type of parallel drainage. Dendritic pattern is more conspicuous in the southern part reflecting the granitic basement in the area.
The area is essentially covered by lateritic, red sandy and black cotton soils, of which black cotton soil is more predominant and is of insitu type. At few places the red soils are developed on the trap rocks. Thick pile of lateritic soil ranging from 0.5-1.5 mts. observed on massive trap rocks. Black cotton soils are seen to a lesser degree on trap rocks. Black cotton soils are also developed on granites adjacent to the trap rocks and may be of transported type (Jaya Rama Rao, 2002).
GEOLOGY OF THE AREA
The investigated area has granites of Archaean age and is unconformably over lained by the Deccan traps with intertrappean beds. Laterite crust of tertiary period developed over the Deccan traps (Fig.2).
The stratigraphic sequence of the area is as follows:
Recent -- Red Lateritic soil and Black Clayey / Sandy soil Tertiary -- Laterites as caps Upper Cretaceous to Eocene -- Deccan traps with intertrappean beds UNCONFORMITY Archaean -- Granites and Gneisses
The granites of Archean era occupy 30% of the study area and exposed in the western and southern parts. In the northern and eastern parts of the area is mainly occupied by Deccan traps. These Deccan traps cover the hilly terrain in the area and is characterised by the step like topography. These step like features are marked by break in slope which indicates various basaltic flows (six in number) that have been erupted during upper cretaceous to Eocene period (Fig.2). The general slope of the each flow is mostly towards North and Northeast. The younger flows are exposed mainly in North and Northeastern parts of area.
The laterite cappings of younger flows are observed along North-east and South-eastern parts of the area. The end product of latertization is generally pisolitic ferrugenous laterite (Babu and Aseefa 1978).
The contact between granites and Deccan traps are clearly observed at 500 mts North-west of Antaram Village (Jaya Rama Rao 2002). The granites in this area are leucocratic in appearance and vary in color and mineralogical composition.
ELECTRICAL RESISTIVITY METHOD
The electrical resistivity of a rock formation limits the amount of current passing through the formation when an electrical potential is applied. Resistivity of a rock formation varies over a wide range, depending on the rock type, density, porosity pore size and shape, moisture content, quality of ground water and temperature (Kailasam 1963). The resistivity is controlled more by water content and quality within the formation than by the rock resistivity.
The electrical resistivity method reflects the possible lithology occurring in the sub surface between various geological formations, depth and thickness of the weathered zone, quality of ground water and the behaviour of aquifers.
In measuring the electrical resistivity of the formation, the electrode configuration plays a major role. Though there are many electrode configurations, Wenner and Schlumberger configurations are widely used (Todd 1980). Here, the Schlumberger configuration is used because of the field conditions. This configuration is more useful especially in the basaltic and granitic terrain.
RESULTS AND DISCUSSIONS
Electrical resistivity surveys conducted in the study area to understand the electrical properties of the geological formations. In all 36 vertical electrical soundings were carriedout, spreading all over the sub-basin of which 18 soundings each from Deccan trap and Granitic terrain (Fig.3), by using Schlumberger electrode configuration.
The data obtained is interpreted by Schlumberger curve matching method, and the results are presented in the table-1. From the interpretation of vertical electrical sounding data different types of curves were obtained from both the Deccan traps and the Granitic terrains. The interpretation of geo-electrical sections indicates different geological formations (Table-1).
Geo-electrical characteristics of the Deccan traps are rather complex, groundwater bodies are often confined to the near surface weathered and jointed zones. Weathered traps usually show resistivities less than 25 ohm-m. The jointed traps generally show resistivity in the range of 20-100 ohm-m and the compact or hard basalt shows the resistivity above 100 ohm-m, (Balakrishna and Ramanujachary, 1978). The more favourable and weathered zone aquifers are showing the resistivities in the range of 15-25 ohm-m. If the resistivity of a formation is less than 15 ohm-m, it may be due to presence of Clay (Gangadhara Rao, et al, 1983).
More than half of the vertical electrical soundings in the Deccan trap regions are of KH type curves. The resistivity values of second layer range from 35-104 ohm-m, showing the complex nature of trap rocks. At Kondapur village, the resistivity is as low as 14 ohm-m may be due to the presence of clay in the formation.
Generally, laterites show very high resistivity, even upto several hundred ohm-m. This phenomenon can happens under saturated conditions also. The thickness of laterite capping is varying from 18 to 25 mts. (Jaya Ram Rao 2002). In this region of laterites mostly KQ, H.K type curves are obtained (Table-1). These laterite cappings were found as younger flows along north east and south eastern parts of the area. Laterite cappings having more than 350 ohm-m of resistivity are also considered as favourable for tapping the groundwater.
An ideal sketch of litholog with resistivity in the Deccan trap lateritic terrain is as follows: (Not to scale).
In granitic terrain, weathered and fractured zones which form as good aquifer zones. In granitic terrain, resistivity of 20 to 50 ohm-m represent highly weathered zone and 100-250 ohm-m represents a fractured and jointed zones, more than 250 ohm-m is hard and compact granite.
An ideal sketch of litholog with resistivity in the granitic terrain is as follows:(not to scale)
The resistivity of first layer in the granitic terrain is 32-54 ohm-m, which represents soil cover. The second layer resistivity is 4-90 ohm-m, except Dornal represents weathered granite and the third layer resistivity as varying from 22-300 ohm-m (at Darur) representing the fractured and jointed granite.
The resisitivity values obtained in the granitic terrain were also compared with the litholog of the existing bore wells, and at places with well sections. In Kamarpally area 25 m thick fractured zone is observed from vertical electrical soundings. Bore wells drilled into this zone are having good potentials.
Groundwater movement in the Deccan traps is strictly controlled by structural features. It is observed that the movement and the potentiality of the groundwater is influenced by the degree of weathering and fracturing in the massive traps, and the presence of vesicles in vesicular traps and intertrappean red bole horizons are acting as good aquifers in Kagna sub-basin.
Wells located in Antaram, Nagawaram and Kamarpally areas of Granitic terrain supplying copious amounts of water, the resistivity showing deep weathered and fractured conditions, because these areas are found to be on the transition zones. Good to moderate yielding wells of the area also located in Kondapur Khurd, and Gutimukla villages with moderate resistivity values.
The electrical resistivity studies are clearly demarcating the granite to Deccan trap regions. In the transition zone good yielding aquifers are noticed with moderate to low resistivity values. Especially in the lateritic capping areas, very low resistivity is observed due to the presence of clay patches underneath. The resistivity is further increasing towards the elevated areas where granites dominate.
The authors are grateful to the Professor and Head, Geology Department, Osmania University for providing Library and Laboratory facilities. One of the authors, P. Ramakrishna is thankful to the Rajeev Gandhi National Fellowship, New Delhi for Financial Assistance.
1. Babu, S.K. and Aseefa, G. 1978. Mode of lateritization of basaltic rocks. Jour. Ind. Aca. Geosci. Vol.21, p.40.
2. Balakrishna, S. and Ramanujachary, K.R. 1978. Resistivity investigations in the Deccan Trap regions. Geophys. Res. Bull. V.16. No.1, p.31-40.
3. Gangadhara Rao, T. Atvale, R.N. Singh, Muralidharan, D. and Murthy, N.N. 1983. Geophysical exploration for ground water in Deccan Traps of Godavari Purna Basin, Maharastra, Tech. Ref. No.GH 18, GP 10, pp.26-28.
4. Jaya Rama Rao K. 2002: Hydrogeological studies of part of Kagna river basin, Pargi Mandal, R.R. District, A.P. Un published Ph.D. thesis, submitted to Osmania University.
5. Kailasam, L.N. 1963: Geoelectrical investigations of the brain resources of Bharpur, Rajasthan, Jour. Ind. Geo. Sci. V.1, p.22.
6. Muralidhar, M. 1988. Geological and hydrogeological studies in parts of Medak and Ranga Reddy Districts, unpublished Ph.D. thesis: submitted to Osmania University.
7. Patangey, N.S. 1973. Application of Electrical resistivity method for solving certain hydrogeological problems in Minjur-Ponneri area, Hengulpat district, Tamil Nadu, unpublished Ph.D thesis submitted to O.U., Hyderabad.
8. Todd, D. K. 1980: Groundwater Hydrology. John Wiley and Sons, New York.
G. Bala Kotaiah, D. Vdyasagarachary, P. Ramakrishna *, K. Jaya Rama Rao, and K. Malathi
Geology Department, Osmania University, Hyderabad-500 007, A.P., India
* Corresponding author
TABLE: 1 showing the interpreted data of vertical electrical soundings in kagna sub-basin V.E.S No. Location/ Resistivity of different layers Village [[rho] [[rho] [[rho] .sub.1] .sub.2] .sub.3] 1 Mahabub Cottage 46 460 9.2 2 Buragpalli 11 6.0 [infinity] 3 M. chittapalli 52 1040 5.2 4 Sultanpur 24 36 [infinity] 5 Sundapur 24 9.6 120 6 Babapur 10 4.0 50 7 Sultanpur 10 4.0 50 8 Pargi 20 8.0 100 9 Rupkhanpet 89 18 62 10 Naskal 17 340 [infinity] 11 Lkhanapur dam 14 5.6 70 12 Momin kalan 42 105 21 13 Ainapur 103 257.5 5.0 14 Antaram 32 80 160 15 Antaram 6.2 31 124 16 Gutimukla 25 62.5 250 17 Dornal 300 15 300 18 Raghavapur 54 135 2.7 19 Raghavapur 23 58 12.0 20 Sayedmalkapur 14 35 7.0 21 Malkapur 18 7.0 18 22 Pargi 21 31.5 [infinity] 23 P. shakhapur 33 7.0 22 24 Pargi 29 12 140 25 Kamarpalli 36 14 180 26 Dharur 9.0 360 [infinity] 27 Dharur (way to Stn.) 51 10 33 28 Dharur (Rly. Stn.) 5.0 90 [infinity] 29 Ebnur 26 130 [infinity] 30 Karali 21 105 8.4 31 Karali 7.0 70 14 32 Kondapur khurd 7.0 100 14 33 Anantgiri 85 1700 85 34 Keruwalli 140 28 350 35 Anantgiri 90 900 225 36 Anantgiri 18 40 180 Resistivity V.E.S of different No. Location/ layers Thickness of different layers Village [[rho] [h.sub.1] [h.sub.2] [h.sub.3] .sub.4] 1 Mahabub Cottage [infinity] 3.3 3.3 16.5 2 Buragpalli - 5.2 1.0 - 3 M. chittapalli [infinity] 1.9 1.9 9.5 4 Sultanpur - 2.6 13 - 5 Sundapur [infinity] 1.2 3.6 1.2 6 Babapur [infinity] 1.0 3.0 10 7 Sultanpur [infinity] 1.0 3.0 5.0 8 Pargi [infinity] 1.6 4.8 4.8 9 Rupkhanpet 890 4.2 12.6 4.2 10 Naskal - 7.0 70 - 11 Lkhanapur dam [infinity] 1.2 3.6 6.0 12 Momin kalan [infinity] 2.2 11 2.2 13 Ainapur [infinity] 1.2 11 3.6 14 Antaram 320 2.3 6.9 2.3 15 Antaram [infinity] 2.2 6.6 2.2 16 Gutimukla - 1.8 9.8 - 17 Dornal - 1.6 0.8 - 18 Raghavapur [infinity] 3.9 19.5 3.9 19 Raghavapur [infinity] 3.0 15 3.0 20 Sayedmalkapur [infinity] 1.8 1.8 1.8 21 Malkapur 90 1.0 3.0 1.0 22 Pargi - 2.5 7.5 - 23 P. shakhapur 330 1.1 3.3 1.1 24 Pargi [infinity] 1.1 5.5 27.5 25 Kamarpalli [infinity] 1.0 3.0 25 26 Dharur - 2.5 5.0 - 27 Dharur (way to Stn.) 510 1.1 3.3 3.3 28 Dharur (Rly. Stn.) - 2.0 6.0 - 29 Ebnur - 2.0 20 - 30 Karali [infinity] 1.0 1.0 1.0 31 Karali [infinity] 1.8 1.0 1.0 32 Kondapur khurd [infinity] 1.7 1.7 17 33 Anantgiri - 1.1 1.1 - 34 Keruwalli [infinity] 1.7 1.7 1.7 35 Anantgiri [infinity] 1.0 5.0 10 36 Anantgiri [infinity] 1.2 1.2 25
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