Week 2 beginning Oct. 5th. Section 2 Aquifer properties
Note: Much of the material in this section is based on Brassington (1990:48-64) and
Abbott, B.M. et al. (1973: Unit 5).
Your study of Section 1 has introduced you to geological features that are relevant
to groundwater flow. There are various flow mechanisms that allow measurement of
flow rates through an aquifer and consequently the potential yield of an aquifer,
whether naturally through spring discharge or by borehole extraction. It is also
possible to determine the storage capacity of an aquifer and the residency time of
water in that part of the hydrologic cycle that takes place under ground. Under the
heading Aquifer Properties are included those characteristics that determine
groundwater flow and storage.
1.2.1. Aquifer storage
The amount of water an aquifer can store may be considered in terms of
the porosity and permeability of the lithology and on whether the aquifer is confined or
unconfined.
Hydraulic conductivity is the volume of water that will flow through a unit crosssectional area of an aquifer in unit time. This concept is further explored in section 1.3.
Hydraulic conductivity depends on the properties of aquifer as well as the density and
viscosity of the water. The lithological property controlling hydraulic conductivity is its
intrinsic permeability, which remains constant. Intrinsic permeability can be calculated
when fluid density and viscosity are known. Groundwater flow is usually measured in
terms of the volume of water flowing through an aquifer. Borehole extraction intercepts
the natural flow of the groundwater.
The amount of water a rock can hold depends on its porosity, that is, the proportion of the
volume of the rock made up of empty space or pores that can be filled with water.
Porosity, therefore, is usually measure as a percentage. The principal factors that control
porosity are grain size and shape, the degree of sorting, the extent of chemical
cementation and the amount of fracturing. These properties are illustrated in Figure 1.2.1.
Figure 1.2.2 indicates the relationship between porosity, specific yield, specific retention
and grain size for unconsolidated sediments.
The process of unconsolidated sediments becoming consolidated rocks is principally
brought about by compaction. As sediments increase in thickness, the weight of the
overlying sediments causes closer packing of grains and reduction of pore space, as the
volume is decreased. Any water between the grains is squeezed out as this decrease in
pore space takes place (Figure1.2.3).
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Figure 1.2.3 Compaction of sediments to form sedimentary rock. (a) & (b) rounded grains
in, for example, a sandstone; (c) & (d) platy clay minerals before and after compaction.
Source: Abbott, et al. (1975)
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Dry local clays in an oven and reduce the rock to a fine powder. The particles
may be only a few microns in width. Collect the powder in a container and
introduce water. Describe and explain what happens to the material
What is the relevance of this experiment with respect to the Gaza Coastal Aquifer
and the West Bank Mountain Aquifer?.
Examine figure1.2.4 and answer the following:
 Which of the two sands is better sorted?
 How does porosity vary with sorting of these two sands?
 Select from the following range of porosity values those which you think
are appropriate for each illustration:
<10%; 10-20%; 20-30%; 30-40%; >40%

In Figure 1.2.2 what correlations between permeability and grain size of
sedimentary particles can you recognize?
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Figure 1.2.4 Schematic illustrations of two sand samples.
Grain size, degree of sorting and extent of chemical cementation are the principal factors
controlling porosity and permeability in an aquifer. The yield of an aquifer may be
expressed as:
Y = V x P x Wr
Where
Y = yield
V = volume of aquifer
P = porosity expressed as %
Wr = % of recoverable water
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Figure 1.2.2 summarises the relationship as:
Specific yield = porosity – specific retention

Is there a positive or negative correlation between grain size and porosity in
(i)
unconsolidated sediments
(ii)
consolidated sedimentary rock (except limestone)?
 Does Figure 1.2.2 show a positive correlation between porosity and
specific yield over the whole range of grain sizes in unconsolidated
sediments?
 What is the relationship between specific retention and grain size for
unconsolidated sediments?
 Explain why coarse-grained unconsolidated sediments are less porous
and more permeable than fine-grained sediments, whereas finegrained sedimentary rocks are both less porous and less permeable
than coarse-grained rocks.
Pumping from an aquifer lowered the water table by an average of 5m over an area
of 800 x 103m2
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

If the porosity of the aquifer averages 30% and the specific retention is 10%,
calculate the specific yield.
From this, calculate how much water was actually removed, i.e., the change
in amount of water in storage (this would be the yield, as defined above).
From Figure 1.2.2 suggest what range of grain size material is indicated by
these data.
Porosity is, then a measure of how much water the aquifer rock contains, whereas
permeability determines how fast the water can flow through the aquifer.
Permeability and specific yield therefore are broadly related. In general aquifers with
a high specific yield tend to be more permeable. Less permeable rocks usually have a
lower specific yield.
The values of hydraulic conductivity given in Figure 1.2.5 are on a logarithmic scale.
Thus clean gravel may be expected to have a value of 109 times greater than massive
clay (i.e. 9 orders of magnitude).
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Figure 1.2.5. Hydraulic conductivities in m/day for various rock types.
Source: Brassington (1990).
2.2 Dispersion
Another aspect of groundwater flow related to both the type of porosity and to the
hydraulic conductivity is the way in which a body of water flows through the aquifer.
Figure 1.2.6 As a tracer flows through a rock (a) it splits up each time an alternate
pathway is reached. The result is that the tracer concentration is diluted by
mechanical dispersion. The graph in (b) shows how the concentration of a tracer
varies with time as it flows past a particular point in an aquifer or emerges at a
discharge point.
Source: Brassington (1990).
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If a tracer is injected into an aquifer as a fixed volume at a specific point, it will not
retain its original volume because molecular diffusion and mechanical dispersion will
cause it to be diluted as shown in Figure 1.2.6. Later in the course, the relevance for
tracing pollution through an aquifer will be evident.
2.3 Assessing Hydraulic Properties.
The relationship between hydraulic properties and geology can be used to classify the
different rocks in a study area into potentially good aquifers, poor aquifers and nonaquifers. Likely values of hydraulic conductivity may be estimated as well as specific
yield for each rock formation. It is important to recognise primary and secondary
porosity values as well as a mixture of both. Field work is essential to achieve this
recognition.
In the field grain sizes can be examined in detail as well as the degree of sorting of
sediments and sedimentary rocks. A grain-size chart, as illustrated in Figure 1.2.7
may be used. Each reading must be accurately plotted spatially and vertically. It is a
good idea to bring back to the lab. hand specimens for further study. Your field
notebook should also record joints and fissures. Study bore hole logs that may be
available to you. You should keep in mind that stress release takes place as rock faces
are exposed at the surface and therefore there is far less fissuring at depth than at the
surface. You should record the trend of fissures if fissures are likely to be important
in directing the flow of water through the aquifer. A careful summary of data
collected will allow estimation of the aquifer properties and the value for storage and
extraction. Later in the course you will learn about the construction of groundwater
contour maps and flow nets.
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Figure 1.2.7 Chart showing grain size scales
Source: Brassington (1990)
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