Geology Reference
In-Depth Information
17
Simulation of Flow in
Weathered-Fractured Aquifer
in a Semi-Arid and
Over-Exploited Region
Shakeel Ahmed and P.D. Sreedevi
Indo-French Centre for Groundwater Research
National Geophysical Research Institute, Hyderabad 500 007, India
INTRODUCTION
The granites in and around Hyderabad, India, form part of the largest of all
granite bodies recorded in Peninsular India. Alcaline intrusions, aplite,
pegmatite, epidote, quartz veins and dolerite dykes traverse the granite. There
are three types of fracture patterns (Fig. 1) in the area, viz. (i) mineralised
or weathering fractures, (ii) fractures traversed by dykes, and (iii) late-stage
fractures represented by joints. The vertical fracture pattern is partly
responsible for the development of the weathered zone and the horizontal
fractures are the result of the weathering. Hydrogeologically, the aquifer
occurs both in the weathered zone and in the underlying weathered-fractured
zone. The Maheshwaram watershed of about 53 km² in the Ranga Reddy
district (Fig. 2) of Andhra Pradesh, India, is underlain by granitic rocks. This
watershed is a representative Southern India catchment in terms of over-
exploitation of its weathered hard rock aquifer, its cropping pattern, rural
socio-economy, agricultural practices and semi-arid climate. The objective
of this study is to develop and test well-suited modelling approaches to
simulate the flow in the existing aquifer system that consist of two layers
with any separating strata.
However, due to deep drilling and heavy groundwater withdrawal, the
weathered zone has now become dry. About 150 dug-wells were examined
and the nature of the weathering was studied. The weathered-zone profiles
range in thickness from 1 to 5 m below ground level (bgl). They are followed
by semi-weathered and fractured zones that reach down to 20 m bgl. Joints
