a. Groundwater System Geometry b. Groundwater Storage Groundwater management has been increasingly delegated to local authorities in recent decades. Local agencies are allowed to create groundwater management plans and raise revenue to pay for management. defining the geometric elements of an aquifer or a groundwater system is the first and most important step in the majority of hydrogeologic studies. It is finding the answers to the following questions regarding the groundwater : “where is it coming from?” (contributing area), “where is it entering the system?” (recharge area), “where is it flowing?” (throughout the aquifer extent), and “where is it discharging from the system?” (discharge area).

1. “Groundwater
Management”
Antonius L. Antjak
176060400111010
Chapter I
a. Groundwater System Geometry
b. Groundwater Storage

2. A. Groundwater System Geometry
FIGURE 1.1 Key spatial elements of a groundwater system.A, extent;
B, recharge area; C, contributing (drainage) area; D, discharge area
Recharge area is the actual land surface
through which the system receives water
via percolation of precipitation and surface
runoff, or directly from surface water
bodies such as streams and lakes. When
part of a groundwater system, an aquifer
may receive water from adjacent aquifers,
including through aquitards, but such
contact between adjacent aquifers is
usually not referred to as the recharge (or
discharge) area.

3. In an unconfined aquifer with a shallow water table, water is also lost
via direct evaporation and plant root uptake, which may be significant
if riparian vegetation is abundant
An area that gathers surface water runoff, which eventually ends up
recharging the system, is called drainage or contributing area
Discharge area is where the system loses water to the land surface,
such as via direct discharge to surface water bodies (streams, lakes,
wetlands, oceans) or discharge via springs.

4. In summary, defining the geometric elements of an aquifer or a groundwater system is the first and most
important step in the majority of hydrogeologic studies. It is finding the answers to the following
questions regarding the groundwater :
“where is it coming from?” (contributing area),
“where is it entering the system?” (recharge area),
“where is it flowing?” (throughout the aquifer extent), and
“where is it discharging from the system?” (discharge area).

5. B. Groundwater Storage
1. POROSITY AND EFFECTIVE POROSITY
The nature of the porosity of porous media (sediments and all rocks in general) is the
single most important factor in determining the storage and movement of groundwater in
the subsurface.
Many quantitative parameters describing “life cycle” of water and contaminants (when
present) within a groundwater system directly or indirectly depend on porosity
Here are just a few: infiltration of precipitation into the subsurface, rock (sediment)
permeability, groundwater velocity, volume of water that can be extracted from the
groundwater system, and diffusion of contaminants into the porous media solids

6. Porosity (n) is defined as the percentage of voids (empty space occupied by water or air)
in the total volume of rock, which includes both solids and voids :
n =
Vv
V
× 100% (1.1)
Assuming the specific gravity of water equals unity, total porosity, as a percentage, can be
expressed in four different ways (Lohman, 1972):
where :
Vv = volume of all rock voids and
V = total volume of rock
n =
Vi
V
=
Vw
V
=
V −V 𝑚
V
= 1 -
Vm
V
× 100% (1.2)

7. where
n = porosity, in percent per volume
V = total volume
Vi = volume of all interstices (voids)
Vm = aggregate volume of mineral (solid) particles
Vw = volume of water in a saturated sample
Porosity can also be expressed as:
n =
𝜌𝑚 −𝜌𝑑
𝜌𝑚
= 1 -
𝜌𝑑
𝜌𝑚
× 100% (1.3)
where
ρm = average density of mineral particles (grain density) and
ρd = density of dry sample (bulk density).

8. Primary porosity is the porosity formed during the formation of rock
itself, such as voids between the grains of sand, voids between
(consolidated) rocks, or bedding planes of sedimentary rocks.
Secondary porosity is created after the rock formation mainly due to
tectonic forces (faulting and folding), which create micro- and
macrofissures, fractures, faults, and fault zones in solid rocks.
Both the primary and secondary porosities can be successively altered
multiple times, thus completely changing the original nature of the
rock porosity.

9. (1) The shape and arrangement of its constituent particles,
(2) The degree of assortment of its particles,
(3) The cementation and compacting to which it has been subjected since its
deposition,
(4) The removal of mineral matter through solution by percolating waters,
and
(5) The fracturing of the rock, resulting in joints and other openings
The porosity of a sedimentary deposit depends chiefly on :

10. FIGURE 1.2 Diagram showing several types of rock interstices and the relation of rock texture to porosity
(a) Well-sorted sedimentary deposit having high porosity
(b) Poorly sorted sedimentary deposit having low porosity
(c) Well-sorted sedimentary deposit consisting of pebbles that are themselves porous and thus the deposit as
a whole has a very high porosity
(d) Well-sorted sedimentary deposit whose porosity has been diminished by the deposition of mineral matter
in the interstice’s
(e) Rock rendered porous by solution
(f) Rock rendered porous by fracturing (Meinzer, 1923).

11. The porosity of unconsolidated sediments (gravel, sand, silt, and clay) is often called intergranular porosity
because the solids are loose detritic grains. When such rocks become consolidated, the former intergranular
porosity is called matrix porosity.
Good examples are fractured clays and glacial till sediments, or residuum deposits, which have preserved
the fabric of the original bedrock in the form of fractures and bedding planes. Sometimes, microscopic
fissures in rocks are also considered part of the matrix porosity as opposed to larger fissures and fractures
called macro porosity.
In general, rocks that have both the matrix and the fracture porosity are referred to as dual-porosity media.
Plots of average total porosity and porosity ranges for various rock types are shown in Figs. 1.3 and 1.4

12. FIGURE 1.3 Porosity range (horizontal bars) and average
porosities (circles) of unconsolidated and consolidated
sedimentary rocks
FIGURE 1.4 Porosity range (horizontal bars) and average
porosities (circles) of magmatic and metamorphic rocks.

13. 2. SPECIFIC YIELD AND COEFFICIENT OF STORAG
The specific yield of the porous material is defined as the volume of water in the pore space that can
be freely drained by gravity due to lowering of the water table. The volumeof water retained by the
porous media, which cannot be easily drained by gravity, is called specific retention. Together, the
specific yield and the specific retention are equal to the total porosity of the porous medium (rock).
This is schematically shown in Fig. 1.5, in the case of groundwater withdrawal from an unconfined
aquifer.

14. FIGURE 1.5 During pumping of an unconfined
aquifer, water is released due to gravity drainage.
Within the cone of depression (volume of aquifer
affected by drawdown) not all water is released
rapidly because of delayed gravity rainage, and
some may be retained permanently. (Modified
from Alley et al., 1999.)

15. Figure 1.6 shows the forces interacting in a confined aquifer: total load exerted on a unit area of the aquifer (σT), part of
the total load borne by the confined water (ρ), and part borne by the structural skeleton (solids) of the aquifer (σe).
FIGURE 1.6 Left: In a confined aquifer system, the total weight of
the overlying rock and water (σT) is balanced by the pore-fluid
pressure (ρ) and the intergranular or effective stress (σe). Right :
Groundwater withdrawal reduces fluid pressures (ρ). As the total
stress (σT) remains nearly constant, a portion of the load is shifted
from the confined fluid to the skeleton of the aquifer system,
increasing the effective stress (σe) and causing some compression
(reduction in porosity). Extended periods of lowered hydraulic
head may result in irreversible compaction of the skeleton and
land subsidence. where ρw = density of water
g = acceleration of gravity
α = compressibility of the aquifer skeleton
n = total porosity
β = compressibility of water

16. FIGURE 1.7 As the hydraulic head decreases during pumping of a
confined aquifer, the fluid pressure of the stored water decreases
as well and water is squeezed out of the pore space by the
encroachment of solid grains (reduction of porosity). The aquifer
remains fully saturated, while its skeleton (solid grains) undergoes
slight compression since it has to bear a larger portion of the
overburden load.
Assuming that the total load exerted on the aquifer is constant, and if σ is reduced because of pumping, the load borne by
the skeleton of the aquifer will increase. This will result in a slight compaction (distortion) of the grains of material, which
means that they will encroach somewhat on pore space formerly occupied by water and water will be squeezed out (Fig.
1.7).
At the same time, the water will expand to the extent
permitted by its elasticity. Conversely, if ρ increases, as in
response to cessation of pumping, the hydraulic (piezometric)
head builds up again, gradually approaching its original value,
and the water itself undergoes slight contraction. With an
increase in ρ there is an accompanying decrease in σe and the
grains of material in the aquifer skeleton return to their former
shape.

17. 3. GROUNDWATER STORAGE AND LAND SUBSIDENCE
Groundwater is always withdrawn from storage in the porous media, regardless of the
conditions of a groundwater system recharge. In other words, prior to its extraction from the
subsurface, water had to be stored in the porous media voids, i.e., in the storage.
with groundwater pumpage during long periods without significant aquifer recharge, such as
multiyear droughts

18. FIGURE 1.8 Schematic presentation of different groundwater
storage components in an unconfined aquifer. Note that
groundwater is flowing in both the dynamic and the static
parts of the storage.
Figure 1.8 shows some key concepts of natural groundwater
storage. The portion of the saturated zone that changes its
thickness in response to natural recharge patterns represents
dynamic storage.
This storage volume can vary widely in time depending on
seasonal and longterm fluctuations of precipitation and other
sources of recharge
The portion of the saturated zone below the multiyear low
water table has constant volume of stored groundwater and is
therefore referred to as long-term or static storage, even
though groundwater in it is constantly flowing
In the presence of artificial groundwater withdrawals, the
long-term static storage can decrease if the extracted volume
of water exceeds the dynamic storage.

19. if the extracted volume of water exceeds the dynamic storage, This is called aquifer mining and is evidenced by the
continuing excessive decline of the hydraulic heads or decrease of spring flows (Fig. 1.9)
FIGURE 1.9 Examples of aquifer mining. Top: Water levels for monitoring well SM Df1 screened in the Aquia aquifer at
Naval Air Station Patuxent River, MD, 1943–2006, showing response to groundwater withdrawals in excess of 1.0 million
gal/d from at least 1946 through 1974, about 1.0 Mgal/d from 1975 through 1991, about 0.8 Mgal/d from 1992 through
1999, and about 0.7 Mgal/d from 2000 through 2005. (Modified from Klohe and Kay, 2007.)

20. In this book explains that for, the renewable dynamic storage can also increase in cases of induced
natural recharge caused by groundwater pumpage near surface water bodies for example. pumping may
reverse the hydraulic gradients and result in inflow of surface water into the groundwater system as
shown in Fig. 1.10.
At the same time, the system does not discharge into the surface stream, which also increases the
dynamic storage.
FIGURE 1.10 Induced aquifer recharge due to groundwater withdrawal near a surface water body.
(Modified from Alley et al., 1999.)

21. THANKYOU
FOR YOUR ATTENTION