Session GWD 1: Occurrence of
Groundwater
What is groundwater?
Groundwater is water stored below the ground surface (the subsurface) in openings in rocks and
soils. The soil profile contains varying amounts of water, increasing from the ground surface
downwards.
The water table is where the subsurface materials become saturated with groundwater. Above the
water table is an interval known as the unsaturated zone which contains some water but is not fully
saturated.
As shown in the figure, the unsaturated zone also contains air in the pore spaces.
After: US Geological Survey website; www.usgs.gov
Aquifers
A water bearing layer is known as an aquifer. This is generally referred to as the material below the
water table,
Water is held in the pore spaces in the soil and rocks. The greater the porosity the larger the
amount of water stored in the aquifer. It will also govern the rate of movement of groundwater
through the aquifer – This is discussed later.
The characteristics of the rocks (ie how the porosity occurs in rocks in an area) need to be known to
understand the characteristics of groundwater in an area.
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Rocks and Porosity
Rocks are aggregates of mineral particles. The particles are held together to varying degrees by
interlocking of crystals that have crystallised together, or particles that have been deposited
together and subsequently bound by a cement of some type. The way in which the particles are
held together strongly influences the porosity and the occurrence of groundwater.
comparison of interlocking crystalline rock (A) with cemented particulate rock (B)- from
Longwell, Flint, Sanders 1969, WileyInternational
The porosity of a rock or soil is dependent on the types of spaces in which water can be stored,
the arrangement of the material in a soil or rock mass and how this affects the interconnection
of the pores. The figure below shows a range of ways in which porosity occurs in rocks.
A: well sorted granular material with open pores and high porosity
B: poorly sorted granular material, pores filled with smaller particles, low porosity
C: well sorted granular material, open pores between particles that are porous themselves, very
high porosity
D: well sorted granular material, interstices filled by cementing mineral matter, low porosity
E: consolidated rock, solution cracks and cavities providing the porosity
F: consolidate rock, interlocking fractures providing the porosity
from Bear and Verruijt 1987, after Meinzter, 1942
Types of Rocks
The discussion below presents information on different rocks and some of the features to be
considered when trying to identify potential groundwater resources. Different rock types comprise
aquifers and they have different characteristics. This section describes some geological features that
can assist in giving an indication of the presence of groundwater.
The three broad groups of rocks are:
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igneous rocks - formed by heat
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o crystallized below the surface of the earth (plutonic rocks, such as granite);
o erupted at the surface through volcanoes (volcanic rocks such as basalt)
sedimentary rocks deposited in layers in rivers, lakes, the sea or by wind
o layers of sediment derived from erosion of already formed rocks
o layers of the shells of marine animals such corals
o layers of peat forming coal
metamorphic rocks – rocks transformed from sedimentary or igneous rocks under heat
and/or pressure.
For this discussion of aquifers and groundwater occurrence, the rocks are grouped into the following
categories which reflect the arrangement of particles comprising the rocks and the type of porosity:
TABLE OF ROCK TYPES AND POROSITY
Aquifer
category
Crystalline
Basement
Volcanic rocks
Consolidated
sedimentary
rocks
Unconsolidated
sediments
Example Rock
types
Granite,
granodiorite,
gabbro
Gneiss, schist,
hornfels
Basalts
Scoria
Means of Formation
Arrangement of
minerals
Interlocking crystals
Fractures
Metamorphic (heat
and pressure)
Igneous (volcanic)
Interlocking crystals
Fractures
Interlocking crystals
Sometimes granular
Rhyolite,
rhyodacite,
pumice
Igneous (volcanic)
Particles often
welded together,
Sometimes granular
Fractures, layers
between different
lava flows
Fractures, layers
within different lava
flows sometimes
granular
Conglomerate,
sandstone,
siltstone, shale
Limestone
Deposition of particles
by rivers, sea,wind
Particles compacted
and cemented
together
Particles cemented
together
Coal
Deposited mainly in
swamps
Particles bound
together
Gravel, sand, silt,
clay
Deposition of particles
by rivers, sea,wind
Limestone
Deposition in the sea
Particles generally
uncompacted very
little cementation
Particles weakly
cemented together
Igneous (plutonic)
Deposition in the sea
Type of porosity
Fractures, bedding
planes (depostional
layers)
Fractures, bedding
planes.
cavities in caves and
solution cracks
Fractures, bedding
planes
Between the
particles
Between particles,
Cavities in caves and
solution cracks
Aquifers are often referred to by the characteristics of the porosity. Thus:

Fractured rock aquifers comprise: crystalline basement rocks , volcanic rocks and
consolidated sedimentary rocks
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Porous media – essentially unconsolidated sedimentary aquifers such as gravel and sand as
well as some non consolidated volcanic rocks such as scoria or pumice
A more complex yet widespread aquifer type associated with limestone deposits and cave systems is
known as “Karst”.
Crystalline Basement rocks
The “crystalline basement” rocks are massive features that generally contain no layering. They are
typically granite and metamorphic rocks that contain limited numbers of cracks. They often form
high hills.
The massive nature of these types of rocks is shown in FigXX from Sierra Leone where there are
massive granite outcrops, Elsewhere they are weathered at the surface and form extensive plains
with occasional rock masses outcropping (eg the plains in eastern Chad with granitic rock outcrops).
Granite outcrop, Kailahun district, eastern Sierra Leone
Granite outcrops in eastern Chad. Note that the plains
surrounding the outcrops are weathered crystalline rocks crossed by some drainage lines (right foreground)
Typical rocks types in “Crystalline Basement” include
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igneous rocks such as granite, granodiorite, diorite and gabbro
metamorphic rocks, such as gneiss, schist and slate
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Granitic rocks
Gneiss
Slate (black) invaded by quartz veins
Groundwater is stored in fractures in crystalline rocks and therefore is sporadic in extent and
volume. Often it occurs at the interface between hard unweathered rock and soils formed on the
crystalline rock.
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Sedimentary rocks
Sedimentary rocks are commonly deposited in large sedimentary basins and have wide extent. The
sedimentary layers exposed in the Grand Canyon show how extensive the layering can be. It also
indicates that the deposits in a sedimentary basin can be very thick.
Sedimentary rocks have varying structures and degree of consolidation (and therefore strength)
depending on their age and the geological history of the area in which they occur.
Continuous layering (bedding) in flat-lying sediments,
Grand Canyon USA. On RHS vertical cracks show how these consolidated rocks fracture
Some sedimentary deposits are strongly consolidated into rock while others, generally younger
sediments, have not been consolidated. Rather than being rock they are essentially soft sediments
such as gravel, sand, silt,clay and porous limestone.
Consolidated sedimentary rocks
Consolidated sedimentary rocks are conglomerate, sandstone, siltstone, shale and limestone.
The sedimentary rock layers generally have a finite thickness and may rest on crystalline basement
or on older sedimentary layers.
Depending on the location, the consolidated rocks are sometimes strongly tilted so that the layers
have vertical or semi-vertical orientation.
They also have varying degrees of fractures. The abundance, orientation of the layers and fractures
and how often the fractures intersect, controls the amount of storage and the rate of flow of
groundwater.
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Horizontal layer in consolidated siltstone and sandstone, with
vertical cracks.
Strongly tilted layers of fine grained siltsone, with very few
visible cracks
The consolidated sedimentary rocks are much harder and the particles which form them are strongly
bound together.
sandstone with sand grains strongly
cemented and no porosity
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finely layered cemented fine sandstone
Coarse pebbles and sand cemented together to form
conglomerate – these rocks have low porosity and generally only hold water in fractures
Hard limestone with fossil fragments
including large coral remains. solution of the calcium carbonate can produce cavities and porosity in limestones
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Unconsolidated Sediments
The sedimentary materials that form very good aquifers are layers of gravel and sand that are not
strongly consolidated or cemented. These sediments are also layered as shown in the photo below
and can be interspersed with silt and clay. In contrast to the strongly cemented sandstones and
conglomerates such as those described above, they have sufficient open pore spaces to allow water
to pass through.
Layered fine gravel and sand in the vertical section. In
the foreground are sands in the bed of a wadi. The wadi sands comprise important aquifers
Weakly cemented sandstone (part of an outcrop of
aquifer material)
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Coarse river gravel with large pore spaces. Thick
layers of this material store large volumes of water
Soft limestone with skeletal remains of
animals. These are weakly cemented and there is porosity between the particles. In addition there can be solution of calcium carbonate to
form cavities.
Volcanic Rocks
Magma that originated below the earth erupts through volcanoes. There are various types of
volcanic rocks which have resulted from different types of original magma and the type of eruption.
Basalts
In many parts of the world there are extensive aquifers composed of basaltic rocks. These are
volcanic rocks erupted as either lava flows or as scoria.
Basalts are extensive across large areas eg Ethiopia, the Deccan Traps in India and can be many
hundreds of metres or more thick. The porosity of basalt is due to the interconnection of cracks and
fractures. In some areas the basalt flows produced very blocky rock outcrops which provide
abundant pathways for water to enter and pass through. Basaltic rocks also have cavities (known as
vesicles) that form from gases in the lava, although these are generally not connected cavities and
provide little porosity to the rock mass.
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Fractured basalt – fractures are the main method of water
storage and permeability in basalts
”Stony rise” country comprising blocky basaltic rock on ground
surface, Bahir Dar, Ethiopia
Vesicles in basalt – the vesicles are generally not
interconnected. the porosity in basalts is largely through cracks and fractures
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Drill core covering a vertical profile of basalt –
even within a few metres, there is great variation in rock type.
In some areas in which there has been recent volcanism, scoria cones are preserved and these are
made up of highly porous lumps of very vesicular rock. Scoria cones often contain high volumes of
very fresh groundwater.
Other volcanic rocks
The volcanic equivalents to granite and granodiorite are rhyolite and rhyodacite. These may be
flows of very stick lava although they often erupt in very explosive eruptions. The erupted particles
(mineral crystals, fragments of rock and volcanic ash) often weld together in hte intense heat to
form hard crystalline rock. In some instances the ash is not welded and remains porous (pumice)
Rhyodacite showing crystals and fragements of rock
(black) bound by hard grey glassy rock.
Further descriptions of a wide range of different rock types that may be encountered on a geological
map are shown on http://geology.about.com/library/bl/images/blrockindex.htm . There are photos
of hand specimens of igneous, metamorphic and sedimentary rocks for viewing by individuals.
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Spatial distribution of rock types
The relationship of the different rocks and groups of rocks in an area influences how the aquifers are
positioned and how groundwater occurs. From a water resource perspective it is why it is important
to appreciate the differences in rock type and occurrence.
The following figure is a diagrammatic cross section of how different rocks may be found in nature.
Eg it shows that:
A. some consolidated sedimentary rocks are strongly tilted into folds
B. consolidated sedimentary rocks that are not so strongly tilted (have relatively flat layering)
can rest on older rocks and their thickness is often known .
C. flat unconsolidated deposits generally rest on top of consolidated sedimentary rocks – they
are generally younger and their distribution and thickness is often known
D. volcanic rocks originate at depth and erupt via volcanoes at the ground surface, and
sometimes are interlayered with sedimentary rocks
E. granites shave been intruded into rocks such as consolidated sediments and can occur at
depth or are exposed at the ground surface when erosion occurs
F. Faults (large fractures in the earth’s crust resulting from large earth movements) can push
different rock types against each other
Geological and Hydrogeological maps
The distribution of different rock types are shown at various scales on geological maps. This
information forms the basis of understanding types of aquifers and how groundwater might flow
through the aquifers. Even more useful are hydrogeological maps that consolidate much of the
geology into aquifer maps.
Subsurface Distribution of Aquifers
In some areas there are multiple aquifers present and these can occur at different depths below the
ground. These are typically regions where sedimentary rocks comprise the aquifers although they
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may also include layers of volcanic rocks such as basalts. Such layered sequences generally rest on
basement rocks (such as crystalline rocks).
The thickness and the geographic extent of different layered aquifers (including basalts) layers are
studied in the subsurface by making geological cross sections. This information is obtained by
records of bore drilling. The mapped aquifers, differentiated on similarities in rock type or groupings
of rock types, can be identified from the cross sections as targets for groundwater investigations.
Groundwater distribution in different rocks
In order to recover water from the subsurface (for example in a well), the stored water in the pore
spaces must be able to be released. This is an important concept in understanding where there may
be suitable resources to meet emergency needs. Of particular note:
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clays have high porosity, and contain significant amounts of water, but because the pore
spaces are so very small, the water does not easily pass through the clay and is bound by the
molecular forces onto the fine clay particles. Hence a well dug into clay will not yield a great
deal of water even though it may be saturated.
crystalline rocks rocks such as granite, and many volcanic rocks, and consolidated
sedimentary rocks such as sandstone and mudstone, have very low porosity and water is
only held in the rock mass in cracks. These rock masses comprise Fractured Rock aquifers
sand and gravel which is the most common aquifer type have relatively high porosity and
water is relatively easily stored and extracted from these materials
water commonly rests at the interface of low porosity bedrock and an overlying saturate
layer
This is shown in the figure below which gives an indication of the moisture content in a
diagrammatic geological rock profile (not to scale). Note that in the unsaturated zone the water
content increases towards the water table, below which all of the pores are full of water.
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After: Don Armstrong, Australian Groundwater School, AMF course notes 1987 (unpublished)
The above profile shows that a more readily accessible target for groundwater is gravel rather than
the more hit and miss occurrence of water in fractures in bedrock. The following case study (CASE
STUDY 1) is a good example of the direct application of this.
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CASE STUDY 1: GEOLOGY AND
GROUNDWATER FOR REFUGEES AND
IDPs IN EASTERN CHAD
Summary
The relationship between geological features and groundwater sources is well observed in the
supply of groundwater at a number of locations in eastern Chad, where refugees from neighbouring
Darfur have been living since around 2003.
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The geology was assessed to see if possible aquifers were present.
The two key aquifers identified from geological maps, air photos, topographic maps and field
observation are wadi sands and fractured bedrock.
The location and extent of wadis was mapped to indicate a significant groundwater resource
that was readily developed. T.ese deposits are relatively continuous and homogeneous
In areas where there were no significant wadis, groundwater supplies were found in
fractures in crystalline rocks – the groundwater zones were investigated by mapping of rock
geological structures, topography and geophysics. The sources found were isolated and
localised. Not all areas investigated in the bedrock yielded successful wells due to the
irregular distribution of fractures
Humanitarian Situation:
In 2005 there were three refugee camps in the Eastern border of Chad where refugees from Darfur
were located. Up to 20,000 people were residing in each of the three camps at Treguine, Breidjing
and Farchana, around 40-50 km from the border with Sudan. As the crisis in Sudan escalated,
refugees continued to enter the country. This led to stress on resources in the existing camps with
the potential for another refugee camp to be established at Gaga further inside Chad.
Further south around Goz Beida, there were additional refugees in the Djabal refugee camp, and by
2007 large numbers of Internally Displaced People (IDPs) were also in this area requiring aid.
The needs of large numbers of people as well as possible impacts of climate requiring water at much
large volumes than existing sources were able to provide pointed to a potential water crisis.
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Indicative refugee
situation in Eastern Chad, 2004 – from UNHCR records
Water supply situation
Rainfall records for Abeche, a city around 100km from the border indicates a average maximum of
nearly 400 mm in August from September to March with some falls in April and May. Water supply
in this dry environment is thus largely provided by groundwater.
Prior to the influx of refugees and IDPs many local communities sourced their water supply from
shallow wells often in the wadis or wadi margins. Water was also recovered from wadi sands, that
are often at risk of contamination from surface pollution, including the impact of animal waste.
Water levels in the larger wadis are commonly shallow (around 1-3m depth to water) although this
will vary seasonally and with use.
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Camel herds on Wadi – water in the wadi sediments potentially
damaged by animal waste.
Further south around Goz Beida, a borefield was operating supplying groundwater from bedrock
fractured rock aquifers to the town of Goz Beida. As the Djabal refugee camp was established
nearby, this source was used to provide water to the refugees. Stress on the fractured rock aquifer
increased.
The need for sustainable sources of groundwater to meet the demands of the rapidly increasing
population was evident. The aquifer systems were not well documented and knowledge of existing
wells limited.
Adopted options to meet emergency needs
A range of water supply options focussed on groundwater have been developed for these areas for
water supply.
1. WadiSands: It was recognised that the wadi sands are potentially large sources of
groundwater that has the potential to be recharged annually during teh wet season.
Estimates of thickness in some of the larger wadis (eg teh Wadi Hamra at Treguine and
Breidjing) are in excess of 40m (Salas, 2004). Assuming a wadi width of around 200m,
saturated thickness of say 20m and porosity of 30% for the coarse sand, - along a 100m
length of wadi there may be sufficient water stored in the wadi sands to supply 20,000
people for a year (assuming 15L/p/day, the Sphere Standard). However it must be noted
that this volume would need to be replenished by recharge from flooding if the entire
volume were extracted
Wadi sands have been developed at Treguine and Breidjing using shallow wells with suction
pumps attached. Lytton et al (2007) report pumping volumes of 6L/sec.
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Wadi within fractured crystalline
rocks can be a major source of groundwater
Rocky outcrop of crystalline metamorphic
basement rock containing quartz veins forming a hill. A wadi containing sandy sediment is visible in the left background.
MDPE Discharge line from 6m deep tube well in the sands in the bed of the
Wadi Hamra. Water pumped by petrol powered suction pump to 70,000L water tanks. Water was treated with Chlorine before
disctribution through the network to tapstand tapstands.
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2. In the Farchana camp to the north east, the Wadi Hamra is thinner and narrower than at
Breidjing and does not present the same potential for high yields, so bedrock was the
available target aquifer . In this location, two wells up to around 40m deep are located
around 3km from the camp and the water pumped by submersible pump to the camp.
These are drilled into fractured bedrock and have yielded 2.3 and 3 L/sec although there has
been significant drawdown of water levels. There was a need to provide additional yield to
this camp and further sources were investigated.
The diagrammatic section across the wadi shows how mapping the extent of different
aquifer types can help in the search for a groundwater source. The extent of the Wadi
sediments was obtained by mapping the width of the wadis and drilling of shallow bores.
Shallow drilling of wells in the wadi was undertaken for short term assistance, while in the
meantime investigation using geophysics of potential sites for drilling in the bedrock was
undertaken. A well adjacent to the wadi but not in the wadi bed was drilled with a yield of
5L/sec.
Diagrammatic cross section across a Wadi in Chad.
3. At Goz Beida in Southeast Chad, up to 5 existing Government wells more than 50m deep
were supplying the community at GozBeida. The wells equipped with submersible pumps
were drilled into fractured bedrock. As the increased demand was placed on these wells
from the refugee camp, careful monitoring and pump scheduling was required to maintain
supply to users.
4. The bedrock aquifers were investigated in the area around Goz Beida to provide water for
IDPs. There was a serious water shortage, geophysical surveys (resistivity) coupled with
examination of geological maps and air photos targeted drilling sites. Bores up to 60m deep
were drilled and installed as production wells. Not every interpreted site provided a
successful bore.
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Developing a successful bore at Gouroukun drilled
in bedrock with PAT Drill (mud rotary) to approximately 60m
References:
Lytton, L., Arafan, S. and Hazell, R. 2007. A low-tech drilling method for refugee water supplies.
WaterLines Vol 26, No 1, pp 5 – 7
Salas, G. 2004. Groundwater Resources for Refugees in Eastern Chad. Reprt to UNHCR and RedR
Australia.
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Similar approaches to that in the case study could apply to a wide range of situations where there
may be alluvial deposits along streams or rivers bound within outcropping older rocks.
This is common as shown in the plot of the ground surface in the area of an alluvial aquifer system in
Northern Australia. The flat land along the valley of the Mulgrave River is underlain by alluvial sand,
silt and clay deposits with the elevated country made up of crystalline rocks. The black dots
represent groundwater bore locations in the alluvial unconsolidated sedimentary aquifers.
Prepared by Chris Nicol, GHD
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Relevance to Emergency situations:
The learning from this is that when entering an emergency situation, a rapid assessment of
the type of the geology in the area may provide guidance about whether there may be a
readily accessible source of groundwater to consider as part of the water supply design.
Different rock and soil types have different capacity to provide water in sufficient quantities
to meet the emergency needs-
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Key resources
Key texts for this are:
CW Fetter: Applied Hydrogeology,4th Ed, Prentice Hall, 2001
Weight, W. D.,and Sonderegger, J.L. Manual of Applied Field Hydrogeology, McGraw-Hill, 2000
Freeze, R.A. and Cherry, J.A., Groundwater, Prentice Hall, 1979
Key web-based links are:
US Geological survey: http://pubs.usgs.gov/of/1993/ofr93-643/
UK Groundwater Forum: www.groundwateruk.org
Cornell University: http://www.mqtinfo.org/planningeduc0019.asp
From BRS website Science for Decision Makers
http://daff.gov.au/brs/publications/series#science
Understanding Groundwater
http://adl.brs.gov.au/brsShop/data/sfdm_groundwater_lores.pdf
Specifically targeted to humanitarian programs:
WEDC (the Water, Engineering and Development Centre):
To gain access to WEDC learning materials, login to the website www.wedc.lboro.ac.uk/
Oxfam:
www.oxfam.org.uk/resources/learning/humanitarian/watsan.html
http://www.oxfam.org.uk/resources/learning/humanitarian/tbn_list.html
Davis, J and Lambert, R., – Engineering in Emergencies, A practical Guide for Relief Workers, (2nd
Edition), ITDG Publishing in association with RedR UK.
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