CONFINED AND UNCONFINED AQUIFERS

7.2 CONFINED AND UNCONFINED AQUIFERS

If there is homogeneous porous formation extending from the ground surface up to an imper- vious bed underneath (Fig. 7.1), rainwater percolating down in the soil saturates the forma- tion and builds up the ground water table (GWT). This aquifer under water table conditions is called an unconfined aquifer (water-table aquifer) and well drilled into this aquifer is called a

water table well .

1 Zone of aeration

Precipitation

Recharge area at

2 Zone of saturation

outcropping of formation

Flowing artesian

Water table

Non-flowing

Piezometric

artesian well

W.T. W.T.

water table

G.S

1 1 Imp Imp

layer layer

GWT GWT

Cap. fringe Cap. fringe

2 2 W.T. Aquifer W.T. Aquifer

Confining Confining

layer layer

Water Water table table

aquifer aquifer Confined or artesian Confined or artesian

Aquiclude Aquiclude

aquifer aquifer

(Impervious) (Impervious)

Bed rock Bed rock Fig. 7.1 Types of aquifers and location of wells

On the other hand, if a porous formation underneath is sandwiched between two imper- vious strata (aquicludes) and is recharged by a natural source (by rain water when the forma- tion outcrops at the ground surface—recharge area, or outcrops into a river-bed or bank) at a higher elevation so that the water is under pressure in the aquifer (like pipe flow), i.e., artesian condition. Such an aquifer is called an artesian aquifer or confined aquifer. If a well is drilled into an artesian aquifer, the water level rises in the well to its initial level at the recharge source called the piezometric surface. If the piezometric surface is above the ground level at the location of the well, the well is called ‘flowing artesian well’ since the water flows out of the well like a spring, and if the piezometric surface is below the ground level at the well location, the well is called a non-flowing artesian well. In practice, a well can be drilled through 2-3 artesian aquifers (if multiple artesian aquifers exist at different depths below ground level).

HYDROLOGY

Sometimes a small band of impervious strata lying above the main ground water table (GWT) holds part of the water percolating from above. Such small water bodies of local nature can be exhausted quickly and are deceptive. The water level in them is called ‘perched water table’.

Storage coefficient . The volume of water given out by a unit prism of aquifer (i.e., a column of aquifer standing on a unit horizontal area) when the piezometric surface (confined aquifers) or the water table (unconfined aquifers) drops by unit depth is called the storage coefficient of the aquifer (S) and is dimensionless (fraction). It is the same as the volume of water taken into storage by a unit prism of the aquifer when the piezometric surface or water table rises by unit depth. In the case of water table (unconfined) aquifer, the storage coefficient is the same of specific yield (S y ).

Since the water is under pressure in an artesian aquifer, the storage coefficient of an artesian aquifer is attributable to the compressibility of the aquifer skeleton and expansibility of the pore water (as it comes out of the aquifer to atmospheric pressure when the well is pumped) and is given by the relation.

HG ...(7.2)

S =γ w nb +

nE s KJ

where S = storage coefficient (decimal) γ w = specific weight of water n = porosity of soil (decimal)

b = thickness of the confined aquifer K w = bulk modulus of elasticity of water

E s = modulus of compressibility (elasticity) of the soil grains of the aquifer. Since water is practically incompressible, expansibility of water as it comes out of the

pores has a very little contribution to the value of the storage coefficient. The storage coefficient of an artesian aquifer ranges from 0.00005 to 0.005, while for a

water table aquifer S = S y = 0.05–0.30. The specific yield (unconfined aquifers) and storage coefficient (confined aquifers), values have to be determined for the aquifers in order to make estimates of the changes in the ground water storage due to fluctuation in the GWT or piezometric surface (ps) from the relation.

∆GWS =A aq × ∆GWT or ps × S or S y ...(7.3) where

∆GWS = change in ground water storage

A aq = involved area of the aquifer ∆GWT or ps = fluctuation in GWT or ps S or S y = storage coefficient (confined aquifer) or specific yield (unconfined aquifer). Example 7.1 In a certain alluvial basin of 100 km 2 , 90 Mm 3 of ground water was pumped in a year and the ground water table dropped by about 5 m during the year. Assuming no replenish-

ment, estimate the specific yield of the aquifer. If the specific retention is 12%, what is the porosity of the soil?

Solution (i) Change in ground water storage

∆GWS =A aq × ∆GWT × S y

GROUND WATER

90 × 10 6 = (100 × 10 6 )×5×S y

S y = 0.18

(ii) Porosity n = S y +S r = 0.18 + 0.12 = 0.30. or 30% Example 7.2 An artesian aquifer, 30 m thick has a porosity of 25% and bulk modulus of

compression 2000 kg /cm 2 . Estimate the storage coefficient of the aquifer. What fraction of this is attributable to the expansibility of water?

Bulk modulus of elasticity of water = 2.4 × 10 4 kg /cm 2 .

HG +

Solution S =γ w nb +

nK s KJ HG . 2 14 × 10 8 . 0 25 × 2 × 10 7 KJ

Storage coefficient due to the expansibility of water as a percentage of S above 7500 × . 0 467 × 10 − 8

× 100 = 2.28%, which is negligibe Note In less compressible formations like limestones for which E ≈ 2 × 10 5 s kg/cm 2 , S=5

× 10 –5 and the fractions of this attributable to water and aquifer skeleton are 70% and 30%, respectively.