7. Groundwater
MECAG309
1
7. Groundwater
7.1 Introduction
Groundwater flow through porous material occurs under the influence of energy, flowing
from regions of high energy to regions of low energy. The energy a particle of water
possesses within a flow system is that of elevation, pressure, and velocity. Elevation is with
respect of gravity; water flows down hill. Pressure energy is analogous to that of a
compressed spring, although water may be seen as virtually incompressible if water were so
oceans would be another 50 m higher. In porous systems velocity is low enough that its
contribution to total energy is essentially zero. Thus groundwater flow occurs from positions
of higher energy to that of low. In local flow systems it is essentially follows landscape
contours. In regional flow systems where flow can go very deep, pressures can affect flow
systems.
local flow system
Intermediate flow system
Regional flow system
Fig. 1 Groundwater flow systems.
Groundwater is where the soil or rock is saturated (no air is present) and exists almost
everywhere underground. It is found within pore spaces and cracks of rock and soil. Only in
some types of material, usually limestone, has groundwater dissolved the material to form
large channels and caverns; the majority of groundwater is in pore spaces usually too small to
be seen with the unaided eye.
7.2 Types of water bearing groundwater formations
Within the prairies it is usually found within 3 to 10 m of the surface. The formation of
material containing the groundwater is classified as to its relative permeability and therefore
economic usefulness to humans. An aquifer is an underground formation of permeable rock
or loose material which produces useful quantities of water when tapped by a well. Aquifers
can range in size from only a few hectares in area to thousands of square kilometers and may
range in thickness from as meter to hundreds of meters (Environment Canada, 1990). Useful
aquifers within the prairies are usually from buried gravel beds of old streams and rivers or
coarse sand layers.
309/webnotes/m06groundwater.doc
Dept of Agricultural & Bioresource Engineering, U. of S.
7. Groundwater
MECAG309
2
Saturated zone
Unsatur
ated
zone
Water-table well
Artesian well
potentiometric surface
capillary fringe
Unconfined
Aquifer
water table
Sand
Aquitard
(confining
bed)
Confined
Aquifer
Clay till
Gravel layer
Fig 2. Diagram showing hydrogeological conditions for an unconfined aquifer and a confined aquifer (adapted
from Ward and Elliot, 1995)
Aquitards are water bearing formations that are relatively impermeable. Due to small pore
size or the crystalline nature of rock, water flows through very slowly. Formations of clay,
silt or very compacted sand are aquitards. These layers do not supply water in quantities
sufficent for our needs. Also aquitards form confining layers for aquifers, not allowing them
upward movement.
Aquifers can be that of unconfined if their upward surface is in contact with the unsaturated
pore space of soils, thus they can directly receive drainage waters from infiltration and they
can contribute through capillarity to the root zone or the evaporation front at the soil surface.
In an unconfined aquifer the level of water in a well is referred to as the water table as it
actually represents the saturated depth of water in the soil.
Confined aquifers does not receive direct infiltration of precipitation because it is overlain
by a layer of low permeability. As such it does not have a water table and the water is
confined under a sufficient pressure that when a well is put into this layer the water level will
rise above the base of the overlying confining layer. This type of well is referred to as a
piezometer as it is cased along its entire length except for the bottom part which is screened
so as to allow water in. The level of water within a piezometer is referred to as the
piezometric surface and it represents the pressure of the water in the confined aquifer. A
series of piezometers within the the same confined aquifer will provide an indication of flow
direction.
309/webnotes/m06groundwater.doc
Dept of Agricultural & Bioresource Engineering, U. of S.
7. Groundwater
MECAG309
3
7.3 Flow rates and amounts
How fast does groundwater flow?
The rate at which water flows (q in volume of water per unit cross-sectional flow area per
unit time, eg cm3 cm-2 d-1, m3 m-2 s-1) through the formation is dependent upon the hydraulic
conductivity (K, cm/d) of the material and the pressure gradient (i, cm/cm is equivalent to
slope as expressed in vertical rise over horizontal run).
q=-Ki
Hydraulic conductivity is the permeability of the material to fluid flow, and is an indicator
of the potential for materials to transmit water. The smaller the pores and the more tortuos
the pathway that water must travel between the pores the smaller the hydraulic conductivity;
thus sands having large continuous pores have higher hydraulic conductivities than clays.
Hydraulic gradient (i) is the force driving the water and is analogous to that of a hill slope,
the steeper the slope the greater the force and thus the greater the flow rate, q. If the
piezometric surface were measured in 2 parts of the landscape or at 2 depths within the same
part of the landscape the gradient would then be the difference in piezometric surface
expressed as the height of water (dh) divided by the distance between the piezometers (L);
q = - K dh/L
This relationship between flow rate, hydraulic conductivity, and gradient is known as Darcy's
Law and is commonly used in groundwater studies to determine rates of well recharge, rates
and amounts of contaminant movement, and economic worth of aquifer systems.
Table 1. Hydraulic conductivity values for various types of earthen materials
Material
Shale
Unweathered marine clay
or unweathered glacial till
Weathered glacial till
Silt
Silty sand; mixtures of
sand-silt-clay
Fine sand
Clean sand
Sand-gravel
Clean gravel
Relative
permeability
Hydraulic
conductivity
(cm/d)
Hydraulic
conductivity
(gal/day/ft2)
extremely low
very low
0.0001
0.001
0.00002
0.0002
low
low
low
0.01
0.1
1
0.002
0.02
0.2
low
moderate
high
very high
10
1000
10,000
100,000
2
20
2,000
20,000
Note the hydraulic conductivities presented above are the median value, within each material the
hydraulic conductivity can easily range one or two orders of magnitude on either side of the
above value.
1.0 cm/d = 0.25 gal/day/ft2
309/webnotes/m06groundwater.doc
Dept of Agricultural & Bioresource Engineering, U. of S.
7. Groundwater
MECAG309
4
So to answer the question, how fast does groundwater flow will depend upon the hydraulic
conductivity and the gradient. As an example
How fast does groundwater flow?
Speedy system of a confined aquifer of a sand-gravel system with a hydraulic conductivity of
10,000 cm/d. Two piezometers installed in this aquifer 1 km apart have piezometric surfaces
differing by 50 cm. The porosity of the material is 0.33 cm3/cm3.
Flux = q (cm/s) = K dh/L = 10,000 x 50/100,000 = 5 cm/day
Actual rate of water movment within the aquifer,v = q/n = 5 cm/d / 0.33 = 15.2 cm/d
Distance a particle of water will travel in 1 year = 55 m
Slow system of a aquitard of glacial till with a hydraulic conductivity of 0.01 cm/d. Two
piezometers 1 km apart have piezometric surfaces differing by 50 cm. The porosity of the material
is 0.33%.
Flux = q (cm/s) = K dh/L = 0.01 x 50/100,000 = 5 x 10 -6 cm/d
Actual rate of water movment within the aquifer, v = q/n = 15.2 x 10 -6 cm/d
Time it will take for a water particle to travel 55 m = 55 m/v = about one million years.
How much water can be pumped?
Whether or not a formation can be used as a source of water for human usuage depends upon
how much water can be supplied from its pore space and how fast that water can be pumped
out.
The specific storage (Ss) of a saturated aquifer is defined as the volume of water that a unit
volume of aquifer releases from storage under a unit decline in hydraulic head.
The transmissivity (T) of a confined aquifer of thickness b and hydraulic conductivity, K, is
defined as
T=Kb
and the storativity (S) is defined as
S = Ss b
or as the volume of water that an aquifer releases from storage per unit area of auifer per unit
decline in hydraulic head normal to that surface.
Confined aquifers can be classified according to their ability to transmit groundwater in
sufficient quantities to wells or springs. As water in confined aquifers is under pressure, it
can yield water upon pumping without desaturation of the pore space, however large amounts
of pumping over time can result in surface subsidence if the aquifer is close to the surface.
Wells for rural household water can accept transmissivity values as low as 0.05 m2/d,
however for large volume wells needed for irrigation purposes transmissivity valus should be
greater than 1000 m2/day and ideally as large as 10,000 m2/day.
309/webnotes/m06groundwater.doc
Dept of Agricultural & Bioresource Engineering, U. of S.