Aquifer Characteristics in groundwater 1.pdf

Aquifer

1. Groundwater

Infiltration
Surface materials
Topography
Vegetation
Precipitation
Groundwater Distribution
Zone of Aeration (unsaturated zone)
Canillary Fringe
Zone of saturation
Water Table
Availability of Groundwater
Porosity
Permeability
Groundwater Flow
Hydraulic conductivity
Hydraulic gradient

2. Aquifers
An aquifer is a formation that allows water to be accessible at a usable rate
(storirig/transmitting significant amounts of water). Aquifers are permeable
layers such as sand, gravel, and fractured rock. Flow still governed by Darcy
Law (P > 0)
* Confined aquifers have non-permeable layers, above and below the
aquifer zone, referred to as aquitards or aquicludes. These layers restrict
water movement. Clay soils, shales, and non-fractured, weakly porous
igneous and metamorphic rocks are examples of aquitards.

+ Sometimes a lens of non-permeable material will be found within more
permeable material. Water percolating through the unsaturated zone will be
intercepted by this layer and will accumulate on top of the lens. This water is a
perched aquifer.

+ An unconfined aquifer has no confining layers that retard vertical water
movement.

+ Artesian aquifers are confined under hydraulic pressure, resulting in free-
flowing water, either from a spring or from a well.

3. Confining beds

| | water table well ring extesien

A Pr potentiometric surface of
confined aquifer

water table

unconfined or water
table aquifer

confining unit

inapenmesble bediock.

A well whose source of water is a confined (artesian) aquifer. The water level
in artesian wells stands at some height above the water table because of
the pressure (artesian pressure) of the aquifer. The level at which water
stands is the potenti tric (or pressure) surface or piezometric surface

4. Water Table

Groundwater flows in ES ge
direction of decreasing | Unconfined
hydraulic head

Hydraulic gradient = Ah/A
Capillary <—_—_—_
Fringe $ Vadose or
unsaturated zone
Water Table

Saturated zone

5. Groundwater
à

> Unconfined aquifers:
> Pores saturate by “pooling up” on top of an impervious or low
conductivity layer
> Aquifer upper boundary is the water table (w.t.) [p=0 at w.t.]
> Aquifer supplied by recharge from above
> Elevation of w.t. changes as storage in aquifer changes
> Piezometric surface corresponds to w.t.

> Confined aquifers:
> Saturated soil that is bounded above and below by low conductivity

layers

> Boundary of aquifer does not change in time

> Major recharge occurs upstream or via leaky confining layers

> Water generally under high pressure; piezometric surface is above
top of aquifer

6. Components of groundwater

infiltration N Soil Moisture

Unsaturated (vadose) Zone Fou fringe

WATER TABLE
Saturated Zone

The rate of infiltration is a function of soil type, rock type and time. The
vadose zone includes all the material between the Earth's surface and the
zone of saturation. The upper boundary of the zone of saturation is called the
water table. The capillary fringe is a layer of variable thickness that directly
overlies the water table. Water is drawn up into this layer by capillary action.

7. Aquifer: A layer of soil or rock that has relatively higher porosity
and permeability than the surrounding layers, enabling
useable quantities of water to be extracted (sand or
gravely sand).

Aquitard: A layer of soil or rock that has relatively lower porosity
and/or permeability than the surrounding layers, limiting
the movement of groundwater through it and the
capacity to extract useable quantities of water. (sandy

diay)

Aquiclude: A saturated geologic unit which does not transmit a
significant quantity of groundwater under ordinary
gradients (clay)

8. Soil Profile and Groundwater Zones

Unsaturated Zone:
Water in vadose zone

Capillary Fringe:
Water is pulled above the
water table by capillary
suction

Water Table: where

fluid pressure is equal to
atmospheric pressure __,

Saturated Zone:
Where all pores are
completely filled with water.

_ Phreatic Zone: Saturated zone below the water table

9. Perched aquifer: Unconfined aquifer developed above
regional water table (lens) caused by a rm
layer

Unconfined aquifer

Spring: A place where ground
water naturally comes to
the surface at the

intersection of the water

table and land surface.

10. Unconfined vs. Confined Aquifers

Well i 5
! iw ee well

Water piezometric Flowing well in Ground
5 surface confined aquifer surface

Regional E lo
water table . A -

Confining layers Unconfined
oye “J aquifer

11. Recharge

Natural Artificial
* Precipitation + Recharge wells
+ Melting snow + Water spread over land
* Infiltration by streams in pits, furrows, ditches
and lakes « Small dams in stream

channels to detain and
deflect water

12. Perennial Stream (effluent)

+ Humid climate

« Flows all year - fed by groundwater
base flow (1)

« Discharges groundwater

Ephemeral Stream (influent)

+ Semiarid or arid climate

* Flows only during wet periods (flashy runoff)
« Recharges groundwater

13. Groundwater Movement

HYDRAULIC HEAD/ FLUID POTENTIAL = h (length units)

+ Measure of energy potential (essentially is a measure of
elevational/gravitational potential energy)

+ Hydraulic head is used to determine the hydraulic gradient

Hydraulic head = the driving force that moves groundwater. The
hydraulic head combines fluid pressure and gradient, and can be
though of as the standing elevation that water will rise to in a well
allowed to come to equilibrium with the subsurface, Groundwater
always moves from an area of higher hydraulic head to an area of
lower hydraulic head. Therefore, groundwater not only flows
downward, it can also flow laterally or upward.

14. Groundwater Movement

General Concepts 1= HYDRAULIC GRADIENT
= dh/dL

ri
+ Hydraulic gradient for an | pssiore un = Slope
unconfined aquifer = (dh) p opa
approximately the slope of run

the water table.
* Porosity = fraction (or %) of void space in rock or soil.

* Permeability = Similar to hydraulic conductivity; a measure of
an earth material to transmit fluid, but only in terms of material
properties, not fluid properties.

« Hydraulic conductivity = ability of material to allow water to
move through it, expressed in terms of m/day (distance/time). It
is a function of the size and shape of particles, and the size,
snape, and connectivity of pore spaces.

15. Groundwater Movement

Movement of groundwater depends on rock and sediment
properties and the groundwater’s flow potential. Porosity,
permeability, specific yield and specific retention are important
components of hydraulic conductivity.

HYDRAULIC CONDUCTIVITY = K

units = length/time (m/day)

Ability of a particular material to allow water to pass
through it
The definition of hydraulic conductivity (denoted "K" in hydrology
formulas) is the rate at which water moves through material.
Internal friction and the various paths water takes are factors
affecting hydraulic conductivity. Hydraulic conductivity is
generally expressed in meters per day.

PA we

16. Groundwater Movement -- Darcy’s Law

The velocity of groundwater is based on hydraulic
conductivity (K), as well as the hydraulic head (I).
The equation to describe the relations between subsurface
materials and the movement of water through them is
Q=KIA
D

Q = Discharge = volumetric flow rate, volume of water
flowing through an aquifer per unit time (m?/day)

A = Area through which the groundwater is flowing, cross-
sectional area of flow (aquifer width x thickness, in m2)

e e e

17. Henry Darcy’s Experiment Da rcy’s Law
k

Darcy investigated ground water flow under controlled conditions

O: Volumetric flow rate [L®/T]

u A: Cross Sectional Area (Perp. to flow)

K: The proportionality constant is added
to form the following equation:

Ä rae = : Hydraulic Gradient

x

K units [LT]

18. Turbulence and Reynolds Number

+ The path a water molecule takes is called a streamline. In
laminar flow, streamlines do not cross, and the viscous
forces due to hydrogen bonds are important.

* In turbulent flow acceleration and large scale motion
away from a smooth path is important (this is the familiar
inertial force ) and streamlines cross.

+ We could take the ratio of inertial to viscous forces. When
this number is “large,” inertial forces are more important,
and flows are turbulent.

* This ratio is known as the Reynolds number Re:

Turbulent

R
_ Inertial Forces

Mes = R
Laminar e

Viscous Forces

19. Limitations of Darcy’s Equation

Rangeof x
Applicability |

—> y Turbulent Flow
| Re > 100

Darcy's Law

Specific
Discharge,
q

| Re<10

|
|
| |
|
| ¡tan a= 1/K
|

1.0 10
Darcy’s Law works for 1.0
<Re<10

Hydraulic Gradient, dh/9l 2=-ka(dh/aL

20. A Linear Non-linear , Turbulent
fair] mir 1 — =

Darcy's law valid

Specific discharge

Re < (1.10)!
———=4 grad H

Darcy's law was established in certain circumstances: laminar flow in
saturated granular media, under steady-state flow conditions,
considering the fluid homogenous, isotherm and incompressible.
Darcy' law is valid for Reynolds number less than 1, but the upper limit
can be extended up to 10. Reynolds number is grater than 100 in

turbulent flow.

The turbulent flow can be located at Reynolds numbers greater than
60...100. Between the laminar and the turbulent flow there is a

21. WELL SORTED POORLYSORTED WELL SORTED
Coarse (sand-gravel) Coarse - Fine Fine (silt-clay)

Permeability and Hydraulic Conductivity

Low

High

Sorting of material affects groundwater movement. Poorly sorted
(well graded) material is less porous than well-sorted material.

22. Groundwater Movement
Porosity and hydraulic conductivity of selected earth materials

R

Hydraulic
Porosity Conductivity

Material (%) (m/day)
Unconsolidated

Clay 45 0.041

Sand 39 32.8

Gravel 25 205.0

Gravel and sand 20 82.0
Rock

Sandstone 15 28.7

Limestone or shale 5 0.041

Granite 1 0.0041

23. Unconsolidated

®K depends on:
— grain size,
— mineral composition,
— Sorting
K (clay) <3 x 104 m/d
K (coarse gravel) = 100 m/d
In common aquifers

24. Groundwater Movement

The tortuous path of groundwater molecules through an
aquifer affects the hydraulic conductivity. How do the following
properties contribute to the rate of water movement?

* Clay content and
adsorptive properties

« Packing density

« Friction

* Surface tension

+ Preferred orientation
of grains

* Shape (angularity or
roundness) of grains

* Grain size

+ Hydraulic gradient

25. Groundwater Flow Nets
Water table contour lines are similar to topographic lines on a map.
They essentially represent "elevations" in the subsurface. These
elevations are the hydraulic head of a region.

Water table contour lines can be used to determine the direction
groundwater will flow in a given region. Many wells are drilled and
hydraulic head is measured in each one. Water table contours (called
equipotential lines) are constructed to join areas of equal head.
Groundwater flow lines (Stream lines), which represent the paths of
groundwater ‘downslope, are drawn perpendicular to the
equipotential lines.

A map of groundwater contour lines (equipotential lines) with
groundwater flow lines (Stream lines) is called a flow net.

Remember: groundwater always moves from an area of higher
hydraulic head to an area of lower hydraulic head, and perpendicular

26. Hydraulic gradient

° Equipotential lines =
Lines of equal h = 100m
hydraulic head h=80

* Gradient = contour Direction of
interval/horizontal groundwater AIDE =
: flow
distance

27. Flow Net Theory

1. Streamlines Y and Equip. lines d are L.
2. Streamlines Y are parallel to no flow boundaries.

E ES B
3. Grids are curvilinear squares, where diagonals
cross at right angles.

4. Each stream tube carries the same flow.

28. Portion of a flow net is shown below

Equipotential

channels

29. EXPLANATION
-----Water table Piezometer
-20—Line of equal hydraulic head | Beso level
~4¢— Direction of ground-water flay > Head level

Elevation scal