WASH Cluster – Emergency Training
GWD
Groundwater Development
and Drilling
Session 1
Occurrence of Groundwater
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Session Objectives
Provide an understanding of how
groundwater occurs in nature
Examine how this can influence:
• the location of potential groundwater
sources, and
• the means of developing the source
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What is Groundwater?
After: US Geological Survey website: www.usgs.gov
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Types of rocks
• Three broad groups of rocks are:
– igneous rocks - formed by heat
• crystallized below the surface of the earth (plutonic rocks)
• erupted at the surface through volcanoes (volcanic rocks)
– sedimentary rocks
• deposited in layers in rivers, lakes, the sea or by wind
– metamorphic rocks –
• transformed from sedimentary or igneous rocks under heat
and/or pressure.
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Rocks are aggregates of mineral particles
interlocking crystalline rock (A)
cemented particulate rock (B)
From Longwell, Flint, Sanders 1969, Wiley International
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Typical crystalline rocks
Granitic
rocks
Slate (black) invaded by quartz veins
Gneiss
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Crystalline rocks
Groundwater is stored in fractures in
crystalline rocks - therefore sporadic in
extent and volume.
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Basalt
Variety of different cracks – water is stored in the cracks, and
also flows through the cracks in the subsurface
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Vesicles in basalt – the vesicles are generally not
interconnected. the porosity in basalts is largely through cracks and
fractures
Drill core covering a vertical
profile of basalt
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Sedimentary Deposits
hard, consolidated
OR
unconsolidated
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finely layered cemented fine sandstone
sandstone with sand grains strongly cemented and no porosity
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Coarse river gravel with large open pore
spaces. Thick layers of this material store large
volumes of water
Coarse pebbles cemented together to form conglomerate – these rocks
have low porosity and generally only hold water in fractures
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Consolidated Sedimentary Rocks
Continuous layering (bedding) in flat-lying
sediments, Grand Canyon USA. On RHS
vertical cracks show how these consolidated
rocks fracture
Strongly tilted layers of fine grained siltstone,
with very few visible open cracks
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Unconsolidated sediments
Sand and Gravel forms porous aquifers
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Limestone
• Can be very hard
• OR
• Relatively unconsolidated
• Can have high porosity because of
cavities and caves
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Limestone showing fossil fragments as well as cavities
formed by solution of particles
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Limestone caves and cavities provide huge porosity and
massive flows of groundwater
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Aquifer types and porosity
• Fractured rock aquifers:
– no primary porosity in crystalline rocks and
consolidated sedimentary rocks
– Porosity due to fractures
• Porous media
– unconsolidated granular material with open
pore spaces (unconsolidated sediments)
• “Karst”
– associated with limestone deposits and cave
systems.
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How Aquifers occur
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Why is this useful
• Knowing the rock type helps work out :
– Where to search
– The extent and depth of the aquifer
– How to develop the groundwater source (eg
drilling method)
– The volume available
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CASE STUDY 1:
APPLICATION OF GEOLOGY IN
IDENTIFYING GROUNDWATER
SOURCES IN CHAD
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EXERCISE 1:
WHAT CAN WE FIND FROM A
GEOLOGICAL MAP
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