Piece 1 - The active zone

Where is the aquifer?

How deep does a basement aquifer really extend?

Crystalline-rock aquifers are not uniformly permeable with depth. Field observations show that groundwater circulation is concentrated within a relatively shallow hydrogeologically active zone, where fractures are more abundant and hydraulically conductive.

Investigations at the Maheshwaram and Maddur experimental sites in southern India showed that fracture density and hydraulic productivity decrease markedly with depth. At Maheshwaram, most hydraulically active structures occur within the upper 30 m, while fracture density becomes very low below about 40 m.

Key findings

1. A shallow active zone controls groundwater circulation

Most productive fractures occur within the upper part of the weathered/fractured profile.

2. The upper fractured zone is particularly conductive

At Maheshwaram, the uppermost part of the fractured layer contains a high density of hydraulically conductive fractures.

3. Pumping tests can help determine the aquifer thickness

Multiple pumping tests developed at Maddur provided an estimate of the hydraulic bottom of the aquifer at about 27 m, consistent with geophysical observations.

4. The geological history controls aquifer geometry

The weathering profile can result from several successive phases of weathering and erosion. At Maheshwaram, this produced a stratified profile consisting of regolith, laminated saprolite and a fractured layer.

[Gallery picture]

The transmissive part of a basement aquifer is generally concentrated within a shallow, highly fractured zone rather than extending uniformly to great depths.