Geology Reference
In-Depth Information
on recharge that is restricted to about 60 m depth. All these studies considered
hard rock aquifer as isotropic and homogeneous, which is not the case as
demonstrated below.
The fresh unaltered massive hard rock is not water bearing, but the
(i) weathered zone and (ii) fissured and jointed zones are productive. In
general, the weathered horizon, when it is water saturated, is poorly
transmissive with high storage while the fissured and jointed zones are
poorly capacitive, but highly transmissive. The problem of flow through
fractures or a fractured environment is primarily a problem of flow through
a dual-porosity media, which includes the porous matrix and the fractured
network. These two components are hydraulically interconnected and cannot
be treated separately. The degree of interconnection between these two
media defines the character of the entire flow domain, and is a function of
the hydraulic properties of each of them. These properties include matrix
hydraulic conductivity and fracture-network distribution, orientation,
apertures, connectivity and, thus, bulk hydraulic conductivity. They will
also determine the heterogeneity and anisotropy of the whole aquifer. The
complexity of flows through fractures makes inadequate the use of classical
techniques such as Theis and Jacob methods for the interpretation of hydraulic
tests. Basically, in hard rock context, assumptions (homogeneity and isotropy)
attached to such methods are not coherent with the reality. Thus, there is a
need for alternative techniques for the evaluation of aquifer parameters able
to involve the specificity of hard rock aquifers.
Four different methods are presented below, for the interpretation of
pumping tests, well adapted to the complexity of groundwater flows in hard
rock aquifers. The method of Neuman (1975) is suited for unconfined
anisotropic aquifers while Gringarten method (Gringarten and Witherspoon,
1972) was developed for the case of a single (horizontal) fracture intersecting
the pumping well. These methods are complementary because they deal
both with anisotropy at the observation and at pumping wells respectively.
The Warren and Root (1963) method takes into consideration the
heterogeneity of the medium, allowing the introduction of a double porosity
aquifer (transmissive fractures and capacitive matrix). Finally, the fourth
method - Barker theory (Barker, 1988)—opens the way for the assessment
of the flow dimension and of the degree of connectivity between the fractures.
All these methods are incorporated and illustrated on observations obtained
from pumping tests carried out in the same study area.
The Maheshwaram watershed located in India (Andhra Pradesh, Ranga
Reddy District) is the main study area of the Indo-French Centre for
Groundwater Research (French Geological Survey/National Geophysical
Research Institute). This watershed, located at 30 kilometres away from
Hyderabad, covers a surface of about 55 km
2
and is mainly constituted by
Archean granites. The weathering profiles are observable through many dug
wells earlier used by the farmers for irrigation. Profiles are generally truncated
