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M. Thangarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500007, IN

Abstract

The groundwater flow models so far used represent porous media having continuous interconnected pore space. The flow problem in the fractured rocks has always been a chalIenging one. Evolving conceptual model of a fractured system requires either a gross simplification or a detailed description of the aquifer properties controlling the groundwater flow. At present, there is only a basic conceptual understanding of flow in the vicinity of weathered and fractured hard rock aquifers. NormalIy this conceptual understanding is not translated into the quantitative interpretation procedures. Often, simple continuum models are applied to analyse pumping test data, and the results then used to produce quantitative catculations on a regional scale. Even if the regional system can be represented using the continuum equivalent approach, it is unlikely that the results of applying continuum models at the local scale have any general validity, and the aquifer parameters so derived may be different from the aquifer parameters appropriate for describing regional flow in quantitative terms. Hence, there is a need to develop appropriate methods for analysis of pumping test data and appropriate simulation techniques to improve the success rate and yield of welts in fractured rocks. The analysis should provide cost-benefit analysis for locating new high yield wells. To do this, it is necessary to investigate in detail the flow in the vicinity of a pumping borehofe, and to apply appropriate non-continuum models. This paper reviews the following models viz. (i) equivalent porous medium (EPM), (ii) dual porosity medium, (iii) discrete fracture network (DFN), (iv) stochastic continuum (SC), and (vi) channel network (CN), which are used to conceptualize fractured systems. The utility of integrated DFN and SC modelling approach to simulate fractured aquifer systems is brought out through a case study in Ireland.

Keywords

Hard Rock Aquifer, Non-Continuum Models, Discrete Fracture Network Model, Stochastic Continuum Model.

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M. Thangarajan ¹, M. Masie ², T. Rana ², Vincent Uhl ², T. B. Bakaya ², G. G. Gabaake ³

Affiliations
1 National Geophysical Research Institute, Hyderabad-500 007, IN
2 Eastend Investments (Pty.) Ltd., Gaborone, BW
3 Department of Water Affairs, Govt. of Botswana, Gaborone, BW

Abstract

A multi-leaky aquifer system in Shashe river valley (Okavango delta), Botswana was simulated to study the aquifer response and thereby evolve optimal pumping schemes. The river valley falls in an arid region and has three aquifers separated by two aquitards. The upper alluvial aquifer below the narrow riverbed is being exploited to augment drinking water supply to the nearby wildlife tourist center, Maun. The bottom aquifer is saline. Continuous increase of abstraction from the upper unconfined and middle semi-confined aquifers has been causing progressive decline of water levels in these aquifers, as well as migration of saline water to the middle aquifer. In order to quantify the upward flow from the saline unit, a multi-layer model was constructed and calibrated for both steady state and transient condition. The calibration was reasonably good even with the given limited water level history for the middle (semi-confined) aquifer system. The calibration has shown that vertical conductivity (K¹) of the confining layers has a considerable influence on the hydrodynamics of multi-aquifer system. As a natural corollary, groundwater movement to the middle (semi-confined) aquifer is found to be dependent on the vertical downward leakage from the upper phreatic aquifer and upward leakage from the lowermost saline unit. As the river-derived recharge to the phreatic aquifer has been negligible due to no-flow condition existing in the riverbed for nearly a decade, even a moderate pumping rate of 3150m³/day may cause severe damage to the middle aquifer. The upper reaches of the middle semi-confined aquifer are likely to become unconfined and the upward leakage from the underlying saline aquifer will increase the concentration of total dissolved solids (TDS). The situation may be improved by reconfiguration of well fields. The longevity of wellfields will increase if the pumping rate from the lower reach is reduced and that from middle and upper reaches is increased.

Keywords

Groundwater, Multilayer Model, Okavango Delta, Shashe Well Field, Water Management.

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M. Thangarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007 (A.P.), IN

Abstract

Numerical simulation of groundwater flow regime in weathered hard rock region is a valuable tool to assess the groundwater resources and to understand its hydrodynamics of a basin. One such study was canied out in Bukdaleru River basin, a hard rock region in Nalgonda district, Andhra Pradesh, India. Recurring droughts and increased exploitation of groundwater to meet the growing needs of drinking water and irrigation had resulted in the decline of regional groundwater levels, causing deterioration of groundwater quality in the basin. In order to evolve future pumping schemes, a preliminary mathematical model of phreatic aquifer was conceptualised, constructed and calibrated in two stages viz., steady and transient conditions. The calibrated model was then used to predict the aquifer behaviour for the next five years. The model study indicates that the phreatic aquifer can sustain the present withdrawal of 28 x 10⁶ m³/year without furlher decline of water level. Any additional increase of withdrawal in the shallow aquifer will result in progressive decline of water level, as the average annual dynamic reserve is only about 28 x 10⁶ m³. If the present model is refined and validated through additional field data, then optimal utilisation schemes can be evolved.

Keywords

Groundwater, Hard Rock Aquifer, Numerical Simulation, Model Calibration of Aquifer, Andhra Pradesh.

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M. Thangarajan ¹, V. S. Singh ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

With the growing needs of water, particularly in developing countries, groundwater resources are being exploited indiscriminately. Evolution of pumping schemes for the development of groundwater resources has become an imperative need to minimise the environmental hazards. This, in turn, requires a reliable estimation of hydrogeological parameters and quantitative appraisal of interference between wells. Pumping tests using a distributed battery of wells may give a representative estimate of aquifer parameters valid over a large area.

One such study was carried out in the Neyveli basin, Tamil Nadu (India), using pumping test data on wells distributed over a total distance of about 2 km. The spread of observation wells was about 4 km. The pumping was continued till a steady state condition was achieved which was monitored through continuous recording of water levels in the observation wells. The drawdowns at any observation point due to pumping of a number of wells is calculated using the principle of superposition. Forward modelling has been employed for the estimation of aquifer parameters through comparison of computed and observed drawdowns. This approach, besides enabling estimation of hydrogeological parameters, can also provide a first-order basis to design an optimal spatial pattern of wells and pumping rate to achieve a specified drawdown in a certain area.

Keywords

Hydrogeology, Aquifer Parameters, Neyveli Basin, Tamil Nadu.

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M. Thangarajan ¹, V. S. Singh ¹, M. Sriman Narayan ²

Affiliations
1 National Geophysical Research Institute, Hyderabad-500 007, IN
2 Public Works Department, Government of Tamil Nadu, Chennai - 600 005, IN

Abstract

Hydrodynamics of groundwater flow regime in a hard rock region and its quantification are not yet fully understood. This is mainly due to several uncertainties involved in quantifying the hydrogeological parameters, Numerical techniques are found to be potential tools in interpreting hydrogeological parameters in hard rock regions as well as studying the aquifer response for given input/output stresses.

One such study was carried out in Palar river basin, Tamil Nadu, where pumping tests carried out on large diameter wells were interpreted using a numerical technique. The aquifer parameters (transmissivity - T and storage coefficient - S) evolved through pumping tests were then used to characterize the groundwater flow regime.

Keywords

Groundwater, Hard Rock Terrain, Hydrodynamics, Palar Basin, Tamil Nadu.

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M. Thangarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The primary concern in selecting waste site is the need for isolating the waste site from the fresh water zones, e.g., water table. It implies that there should be an impermeable zone for pollutant migration between any waste site and groundwater regime. The unsaturated zone, otherwise called the vadose zone, that lies between the topsoil and water table forms the most important natural attenuating zone for the migration of leachate. Thus, there is a need to select a site, which has very low permeable and absorbing vadose zone. Selection of an ideal site needs rigorous field investigations, followed by mass transport modelling. In case of scarce resources of money and time, one needs a few faster methods based on empirical approach to identify the disposal site. One such empirical method is to identify a site, which has good attenuation potential for the leachate produced by the waste pile. This is termed as waste aquifer isolation principle (WAIP). This will restrict the contaminant migration to reach the water table within short span of time. This principle makes use of the hydrogeologic criteria using the data related to type of waste and area to be disposed, quantum of pollutant, effective porosity, thickness of unsaturated zone, and vertical permeability of the medium. A computer code WASP, based on the above principle, was used to compute an index to decide upon the suitability of the site.

The above approach was tested in an existing site near Ranipet (125 km southwest of Chennai) in Vellore district. Tamil Nadu. Tamil Nadu Chromates and Chemicals Ltd., (TCC) at Ranipet Industrial Estate produces sodium dichromate and disposes of its solid wastes containing soluble hexavalent chromium (Cr⁶⁺), which is toxic, for the last 20 years in an area of 35000 m² and has contaminated the groundwater regime with high level chromium concentration. It was found through this study that the site is unsuitable for disposing toxic wastes. This approach may be followed as a first step, wherever quick solution is needed with minimum resources, but this cannot and does not replace the need for quantification of pollutant migration through mass transport modelling. This quick method is based on hydrogeological criteria with minimum computational effort, as such it can be employed easily by any field hydrologist.

Keywords

Environmental Geology, Solidwaste, Disposal Site, Hexavalent Chromium, Ranipet, Tamil Nadu.

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V. S. Singh ¹, M. Thangarajan ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

Kodaganar river basin is a hard rock terrain where the upper shallow weathered part followed by deep fractured rocks form the aquifer system The area is drought prone and indiscriminate exploitation of groundwater over decades has resulted in depletion of water level Therefore, assessment of groundwater potentia1 is warranted which in turn requires the knowledge of aquifer parameters The common practice of determining the aquifer parameters is by carrying out pumping test on wells and further by interpreting the time drawdown data In the present study the aquifer parameters are determined through pumping tests on dug wells duly taking into account the actual field conditions while interpreting the data Numerical method has been adopted to carry out forward modelling of pumping test data to estimate aquifer parameters.

Keywords

Hard Rock Terrain, Aquifer Characteristics, Numerical Method, Kodagana River Basin, Tamil Nadu.

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Ron Barker ¹, T. Venkateswara Rao ², M. Thangarajan ²

Affiliations
1 University of Birmingham, Birmingham B15 2TI, GB
2 National Geophysical Research Institute, Hyderabad.- 500 007, IN

Abstract

Much of the seventy five percent of India that is underlain by shallow granitic and gneissic basement suffers from a shortage of water, but it is precisely these areas where siting successful boreholes for water supply is a difficult problem. Geophysical methods, especially resistivity sounding, are commonly employed to improve the drilling success rate, but the techniques are time consuming, subject to considerable ambiguity and still have scope for considerable improvement. In the Dindigul area of Tamil Nadu, we have tested new electrical tomography (imaging) techniques to provide detailed 'pictures' of the sub-surface that show variations in bedrock topography and the nature and thickness of the overburden. The resulting images can be used to plan borehole investigations more cost effectively.

Keywords

Electrical Imaging, Potential Boreholes, Hard Rock Region, Kodaganar River Basin, Dindigul District, Tamil Nadu.

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V. S. Singh ¹, M. Thangarajan ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad, 500 007, IN

Abstract

Aquifer parameters viz. transmissivity (T) and storage coefficient (S) are a pre-requisite for the assessment of groundwater resources in an area. A large number of dugwells tapping the top phreatic aquifer (comprising mostly weathered material) are being utilized for irrigation and/or domestic purposes. Use of such wells makes the estimation of aquifer parameters easy and cost effective. A case study in Vaippar river basin, a hard rock region in Tamil Nadu (India) is presented.

Keywords

Hydrogeology, Aquifer Parameters, Pumping Test, Large Diameter Wells, Recovery Data, Tamil Nadu.

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V. S. Singh ¹, M. Thangarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

In many situations pumping tests are carried out without observation well and drawdown/recovery, data recorded in the pumped well itself. Although, the time drawdown/recovery data are, being precisely recorded in the pumping well, the interpretation of such data often gives ambiguous results due to non-accountabiiity of well loss and well storage in many situations. This is particularly true in the case of low permeable aquifer. In order to overcome such difficulties, finite difference, method is used to estimate aquifer parameters. Field examples are presented to illustrate the above method.

Keywords

Hydrogeology, Aquifer Parameters, Pumping Test, Well Storage, Well Loss.

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