HYDROGEOLOGICAL AND GEOELECTRICAL STUDIES OF THE SOUTHEASTERN PARTS OF BANDA DISTRICT UTTAR PRADESH, INDIA

Abstract

An integrated approach of photogeologicali geoelectrical and hydrogeological methods has been employed for studying the southeastern parts of Banda District* India, with an objective to delineate the aquifer horizons and to ascertain the develop ment of groundwater resources. It is a drought prone area and has a general scarcity of water, Physiographically, the area can be divided into two parts,* the northern portion consisting of alluvium with few isolated rock outcrops, and the southern plateau made up of hard rocks• Geologically, the area comprises of Bundelkhand Granite (of Archaean age), which are overlain by sedimentary rocks of Vindhyan Supergroup (1400-900 million years) and the Recent and Sub-Recent Alluvium, The Vindhyan formations consist of Tirohan Dolomite and Breccia (Semri Group)* Kaimur Sandstones (Kaimur Group), and Jhiri Shales, Paisuni Formation and Upper Rewa Sandstones (Rewa group). The Bundelkhand Granite includes granites and gneisses, which are intruded, at places, by metadolerites and quartz reefs. The Tirohan Dolomite, exposed as narrow discontinuous outcrops on the northern face of Vindhyan scarp, is absent north of it, It is evenly fine grained, pelitomorphic in texture. Evidences indicate that the dolomite is cavernous at several places, and continues in subsurface below the Kaimur

Sandstones, both towards south and east of the present expo sures, for considerable distances. The Kaimur Sandstones, mainly exposed in the Vindhyan plateau, is fine to medium grained and quartzitic in texture, The rocks of Rewa group are characterised by the general appearance of Jhiri Shales at their contact with the Kaimurs, except in the southwestern parts of the Manikpur block where it is marked by the presence of Paisuni Formation, The Upper Rewa Sandstones are medium to coarse grained and closely jointed. The Vindhyan formations are generally subhorizontal, with low rolling dips of 3-5° towards southeast. The rock formations exhibit characteristic jointing, the dominant directions being along N135-3150, N120-3000 and N45-22S>° and less Prominently along north-south and east-west directions. The fracture traces and lineaments are mainly observed along N135-3150* N120-3000, NO-1800 and N60-2400 directions, and show a good match with the joints. Intense fracturing, associated with its en echelon pattern and drag effects observed in aerial photographs 4 km south of Manikpur indicate the presence of a ENE-WSW trending shear zone (named as Manikpur Shear Zone), consisting of Displacement ARiedel Shears, conjugate Riedels, and the tension fractures. Analysis of the fracture traces have indicated that the N135-315°» and the NO-180 trending fractures are the later originated joints and may be more important hydrogeologically.

Geoelectrical studies in the present area have been attempted by recording 160 vertical electrical soundings by employing Schlumberger method with maximum current electrode spacing of 1000 m. The geoelectrical data have been discretized from the field apparent resistivity curves and the Kernel (resis tivity transform) function was extracted by using digital linear filters. The theoretical resistivity transform was calculated using Pekeris recurrence relation and the influence matrix was assembled using the partial derivatives of the Kernel with respect to initially assumed layer parameters. For obtaining the solution in terms of actual layer parameters, Marquardt technique of non-linear optimization has been employed in order to obtain stable convergence of the solutions. As a non linear Kernel (resistivity transform) function is linearized, the desired solution is obtained in many iterations and in each iteration, the initially assumed model parameters are automati cally updated till we get final solution. The layer parameters arrived at, with the help of automatic interpretation of resis tivity data, have been correlated with the lithologs of wells, for fixing the ranges of true resistivities for different lithological units. On this basis, the alluvial clays and sands are found to have resistivities less than 19 ohm-m and 19 to 50 ohm-m respectively. The weathered granite and the cavernous limestones have resistivities within the ranges of 57 to 120 and 39 to 150 ohm-m respectively. The weathered sandstone is found to have resistivity between 52 to 80 ohm-m. The differenee in the depth to bed rock interpreted from the sounding data and (viii) from the lithologs of the existing wells ranges between + 2 to 17 per cent. The geological interpretation of resistivity data has also been attempted to delineate aquifer horizons. In the alluvial tract, the depth to bed rock is highly variable, with a maximum of 150-200 m below the ground surface. The trend of contours of maximum depth to bedrock is generally parallel to the fracture traces and lineaments. This could be possibly due to the weathering of bed rock to deeper levels along the fracture traces. In the west, north and northwestern parts of the area, the bed rock appears to be granite, while in the east, southeast, and southern parts, it is sandstone. In the centralsouthern parts of the area, Tirohan Dolomite/Breccia may form the bed rock, which, in turn, is underlain by granite. Below the sandstone, the dolomites and breccia are expected to extend southwards upto the ENE-WSW trending Manikpur Shear Zone. The dolomites are expected to be highly cavernous at some places as indicated by low resistivity (39 to 150 ohm-m) and are expected to occur at depths ranging from 50 to 200 meters below the ground surface and are likely to form good aquifers in the south-central and east-central parts of the area. In unconsolidated alluvium, isolated lenses of fine, silty sand form unconfined aquifers. Besides, 40-80 m thick lensoidal sandy bodies occurring in the lower part of the clayey alluvium, particularly in the eastern portion of the area, have also been delineated. In sandstones and granites,

the main source of groundwater is from weathered and friable zones, while in dolomites, underlying the Kaimur Sandstones, cavernous zones form good aquifers. Fractured sandstones in the vicinity of Manikpur Shear Zone also seem to be important, hydrogeologically. The depth to water table in various formations varies between 1-28 m below the ground surface. In the alluvial tract, the water table slopes towards north and northwest. In the southern upland area, the water table intersects the ground surface at few places, as manifested by the presence of springs. The water table contours for the eastern parts of the alluvial tracts indicate higher hydraulic conductivity of shallow aquifers, while the same for the western portion show lower hydraulic conductivity. The transmissivity of the unconfined aquifers in the alluvium, has been evaluated by ana lysing pumping test data from few large diameter dug wells by using the methods of Papadopulous and Cooper, Papadopulous, and Boulton and Streltsova, The transmissivity varies from 73 to 1036 m /day. The storativity could not be estimated. An analytical relationship between 'modified' trans verse resistance, R1 (product of transverse resistance and ratio of average aquifer water resistivity to water resisti vity at the observation site), and aquifer transmissivity (T) has been developed in the form T - ocR« where a is the product of hydraulic conductivity and modified electrical conductivity at reference points. This relation also takes into considera tion the variations in the quality of groundwater. This relation has been tested successfully for the glacial aquifers of Rhode island, U,S,A. and the alluvial aquifers of the present study area. The field data indicate that the value of a remains constant for an isotropic and homogeneous geological formation. The practical applicability of this relation lies in the fact that if hydraulic conductivity is known for any reference point of a porous and homogeneous aquifer, one can get fairly good idea of the transmissivity of the aquifer at any desired place within the area, from surface resistivity measurements. The above studies have indicated the usefulness of an integrated approach in the aquifer mapping and estimation of their hydraulic characteristics in a geologically compli cated area.