Aquifer Characteristics groundwater2.pdf

1. Porosity of Sediments and Rocks

* Depending on grain size, particle shape and arrangement, diagenetic
features, actual values of porosity can range from zero or near zero to
more than 60%.

+ In general, for sedimentary rocks, the smaller the particle size, the
higher the porosity.

+ Totalporosity amount of pore space

nr= V,/V,= (V,-V,)/V,

V, : void volume,

V, : total volume

Vs:solid volume

* Primary porosity: interstitial porosity (original in the rock)

* Secondary porosity: fracture or solution porosity

* Effective porosity: percentage of interconnected pore space available
for groundwater flow.

2. n = 47.65 % n= 25.95%

(a5 Cubic packing of spheres
(b) Rhombohedral packing

3. PERMEABILITY is the capability of a rock to allow the passage of
fluids. Permeability is dependent on the size of pore spaces and to
what degree the pore spaces are connected. Grain shape, grain
packing, and cementation affect permeability .

Low porosity High porosity High porosity
High permeability Low permeability

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Permeability

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Permeability Pervious Semi-Pervious Impervious
Aquifer potential Good Por None uN
A , \ SEEN
Clean CleanSandor Sand & Very Fine Sand, Silt, Loess,
Unconsolidated Gravel Gavel Loam
sediments / Soils

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Pumping Well Terminology

Cone of depression: A depression in the
groundwater table or potentiometric
surface that has the shape of an
inverted cone and develops around a
well from which water is being
withdrawn. The slopes of the cone
become increasingly steep the closer
they are to the well.

Static Water Level [SWL] (h,) is the
equilibrium water level before
pumping commences

Pumping Water Level [PWL] (h) is the
water level during pumping
Drawdown (s = h, - h) is the
difference between SWL and PWL
Well Yield (Q) is the volume of water
pumped per unit time

Specific Capacity (Q/s) is the yield per

6. Cone of Depression

. High K, aquifer

lo
Low K;, aquifer

Radius of Influence (R) for a well is the maximum horizontal extent of
the cone of depression when the well is in equilibrium with inflows.

A zone of low pressure is created centered on the pumping well.
Drawdown is a maximum at the well and reduces radially.

Head gradient decreases away from the well and the pattern
resembles an inverted cone called the cone of depression.

The cone expands over time until the inflows (from various
boundaries) match the well extraction.
The shape of the equilibrium cone is controlled by hydraulic

conductivity. =

7. Cone of Depression
Unconfined aquifer

= Cone of depression expands very slowly (drainage through gravity)
= Increased drawdown in wells and in aquifer (dewatering of aquifer)
= Wellis pumped at a constant rate and equilibrium has reached.

Land surfoce __

Limits of cone Land surface
of depression

| Water table

2 Cone of
Drowdown N depression

Confining bed

Y,
Unconfined

Confining bed Confining bed

Confined aquifer
= Cone of depression expands very rapidly and no dewatering takes
place

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Superposition of Multiple Wells

E E a a: 2. Ground surface
Original piezometric Û 2 a
race

CRD EEE TER:

Composite drawdown
curve for oil three
Sells pumping

for Qs only

Impermeable

Confined aquifer

ZT TT O
Impermeobie
— - = - = + am

However linearity of the governing equation allows for superposition of single well
solutions.

For the case of multiple wells in a confined aquifer, the actual drawdown
resulting from all wells can be obtained simply by summing up the drawdown from
each single well. Note: Summation of drawdowns not strictly applicable for
unconfined aquifers since equations not linear in À (and therefore not linear in

drawdown),

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SPECIFIC YIELD (Sy) is the ratio of the volume of water drained
from a rock (due to gravity) to the total rock volume. Grain size
has a definite effect on specific yield. Smaller grains have larger
surface area/volume ratio, which means more surface tension.
Fine-grained sediment will have a lower S, than coarse-grained
sediment. y v,

a
IS

SPECIFIC RETENTION (S,) of the aquifer is the amount of
water retained as a film on the surface of grains or held in small
openings by molecular attraction is the ratio of the volume of
water a rock can retain (in spite of gravity) to the total volume of
rock. a 2

r

Fr

10. TABLE 4.2 Selected values of porosity, specific yield, and specific retention
(values in percent by volume)

Material Porosity Specific Yield Specific Retention
Soil 55 40 15

Clay 50 2 48
Sand 25 22 3
Gravel 20 19 1
Limestone 20 18 2
Sandstone (semiconsolidated) 11 6 5)
Granite 0.1 0.09 0.01
Basalt (young) 11 8 3

Source: From Heath (1989).

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° Transmisivity is the rate of flow through a vertical strip of

aquifer (thickness b) of unit width under a unit hydraulic
gradient.

T=K*b
T= Transmisivity [L?/T] e.g., m?/d
K= Hydraulic conductivity; b: aquifer thickness

B

Storage Coefficient is storage change per unit volume of
aquifer per unit change in head.

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Storativity (coefficient of storage)

Storativity (S): The volume of water that a
permeable unit will absorb or expel from storage

per unit surface area per unit change in hydraulic
head

R

Storativity is a dimensionless property

S = volume of water/(unit area) (unit head change)
=L3/(L2* L) = m?/m?

13

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Storativity (Coefficient of Storage)

Water removed froma unconfined aquifer:
= Water is drainage of water from pores

= Early stage: water comes from expansion of water and
compression of matrix

= Later stage: water comes from gravity drainage

Water removed from a confined aquifer:
= Hydraulic head decreases - water level in wells falls
= Fluid pressure decreases in the aquifer.

= Porosity decreases as the granular skeleton contracts
(aquifer collapses slightly) and increase in pressure
from overburden

14

14. Water Storage in Aquifers

Unconfined aquifers:
> Storage changes correspond directly to change in water table
level (water table increases = water going into storage and vice
versa).

Storage parameter: “specific yield” or storage coefficent (S,) =
volume of GW released per unit decline in water table (per unit
area).

Confined aquifers:

> Storage changes correspond mainly to compression of aquifer as
weight of overlying material is transferred from liquid to solid
grains (change in porosity) when water is removed (or vice versa)
storage parameter: “specific storage” (S,)

15. Confined aquifer

Unit cross-sectional area

Unit decline of
ic surface

Unconfined aquifer

Unit cross-sectional area

Impermeable Impermeable
(a) @)

Illustrative sketches for defining storage coefficient of (a) confined and (b)
980))

MBAS

16. Meander bolt built
up abowo floodplain — Verticál accrotion
deposits
(a)

Figure 4.6 Contrasting geometry of (a) meandering and (6) braided rivers (from Walker and Cant,

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Groundwater potential in hard-rock terrain

Depth (m)

18. Groundwater Overdraft

Overpumping will have two effects:

1. Changes the groundwater flow
direction.

2. Lowers the water table, making it
| necessary to dig a deeper well.

« This is a leading cause for
desertification in some areas.

* Original land users and land owners
often spend lots of money to drill new,
| deeper wells.

Streams become permanently dry.

19. Threats to groundwater

20. Book: Groundwater Hydrology
by

David Keith Todd