WASH Cluster – Emergency Training GWD GWD1 1 Groundwater Development and Drilling Session 1 Occurrence of Groundwater

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WASH Cluster – Emergency Training GWD

1

Groundwater Development and Drilling

Session 1

Occurrence of Groundwater

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Session Objectives

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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?

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After: US Geological Survey website: www.usgs.gov

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• 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.

Types of rocks

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Rocks are aggregates of mineral particles

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From Longwell, Flint, Sanders 1969, Wiley International

interlocking crystalline rock (A) cemented particulate rock (B)

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Typical crystalline rocks

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Slate (black) invaded by quartz veins

Gneiss

Granitic rocks

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Crystalline rocks

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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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hard, consolidated

OR

unconsolidated

Sedimentary Deposits

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sandstone with sand grains strongly cemented and no porosity

finely layered cemented fine sandstone

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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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WASH Cluster – Emergency Training GWDConsolidated Sedimentary Rocks

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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

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Sand and Gravel forms porous aquifers

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• Can be very hard • OR • Relatively unconsolidated

• Can have high porosity because of cavities and caves

Limestone

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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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• 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.

Aquifer types and porosity

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How Aquifers occur

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• 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

Why is this useful

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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

Documents

WASH Cluster – Emergency Training GWD GWD1 1 Groundwater Development and Drilling Session 1 Occurrence of Groundwater