HARD ROCK HYDROGEOLOGY

The “Hard rock” involves the crystalline volcanic & carbonate rocks along with the Pre-Cambrian rocks, occupying nearly 22, 27, 2000 km2 out of the total landmass of 32, 57,200 sq.km.

Unweathered & Weathered Crystalline Rocks

The storage capacity of weathered or un-weathered hard rocks is restricted to the interconnected system of fractures, joints & fissures. Tensile joints have storage capacity & hydraulic conductivity as compare to tensile fractures. Extensive thickness of saturated weathered zone has more productive aquifer system. Exploration shows that fractured withered zones water yielding within a depth of 100m to 150m.The hydraulic conductivity of the rock mass is a function of fracture spacing / frequency, fracture aperture & fracture conductivity. i.e.

K= ge3/12V.S

Where
e - fracture aperture
v - kinematic porosity, and
S – fracture spacing

The total porosity in fracture medium qt is

q t = qk + q d + q r
Where,

qk - Effective flow porosity
qd - Differentiation porosity
qr - Residual porosity.

PDTA of such rocks show that initially stored water in fracture system released then pressure within the fracture is reduced.

Volcanic Rocks

In India volcanic rocks extrusion is mostly confined as a Deccan traps of central India covering 51,0000 km2 & are hydro geologically unique. The lower massive & upper vesicular horizons make a one trap flow & based on their thickness the Deccan traps are classified as


1. Upper trap 450m
2. Middle trap 1200m
3. Lower trap 150m

The weathered portion as a recharge area, fractured traps, vesicular flows & intertrappeans controls the hydrogeology of Deccan Traps. The ground water occurs under both water table & confined conditions. A hydraulic continuity does exist between the consecutive massive & vesicular units. The weathered and jointed zones in the massive units act as leaking aquitard. Issuance of springs along the slopes of hill & on high grounds due to impervious nature of red boles, flow contacts etc. is a common feature of deccan traps. Secondary porosity because of weathering, brecciation, shrinkage cracks, joints & fracture causes effective permeability with high values of transmissivity. The aquifer mostly an isotropic & heterogeneous and water is mostly bicarbonate-chloride type.

Carbonate Rocks

These are the rocks having 50% of carbonate minerals by weight like limestone, dolomite etc. They have less primary porosity while the secondary porosity due to sink holes, karstification are the major controlling factor over the hydrogeology of carbonate rocks. Chemical composition of solute, temperature variation, climate & solubility strength is the major factors governing the development of secondary porosity.

High Mg calcite ® Aragonite ® Low Mg calcite ® dolomite
¾¾¾¾¾¾¾¾¾¾¾¾ Solubility decreases.¾¾¾¾¾¾¾¾¾¾¾®

Karstic Aquifers

Karstic aquifers are three end members type and they are, a) Porous (granular) b) fissure (fracture) c) conduit (cavernous). Like other aquifers they are further classified as confined, un-confined, leaky & semi-confined.

Summary

Ground water occurrences in hard rock are very much controlled by secondary porosity & permeability, which is ultimately developed by fracture & joints. The concept of fractured aquifers with double porosity system should consider before going for exploration in hard rock terrain.