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Groundwater Development and Drilling

Groundwater Development and Drilling. Session 1 Occurrence of Groundwater. 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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Groundwater Development and Drilling

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  1. Groundwater Development and Drilling Session 1 Occurrence of Groundwater GWD1

  2. 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 GWD1

  3. GWD1

  4. What is Groundwater? After: US Geological Survey website: www.usgs.gov GWD1

  5. 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. GWD1

  6. Rocks are aggregates of mineral particles interlocking crystalline rock (A) cemented particulate rock (B) From Longwell, Flint, Sanders 1969, Wiley International GWD1

  7. Typical crystalline rocks Granitic rocks Slate (black) invaded by quartz veins Gneiss GWD1 Slate (black) invaded by quartz veins

  8. Crystalline rocks Groundwater is stored in fractures in crystalline rocks - therefore sporadic in extent and volume. GWD1

  9. Basalt Variety of different cracks – water is stored in the cracks, and also flows through the cracks in the subsurface GWD1

  10. 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 GWD1

  11. Sedimentary Deposits hard, consolidated OR unconsolidated GWD1

  12. finely layered cemented fine sandstone sandstone with sand grains strongly cemented and no porosity GWD1

  13. 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 GWD1

  14. 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 GWD1

  15. Unconsolidated sediments Sand and Gravel forms porous aquifers GWD1

  16. Limestone • Can be very hard • OR • Relatively unconsolidated • Can have high porosity because of cavities and caves GWD1

  17. Limestone showing fossil fragments as well as cavities formed by solution of particles GWD1

  18. Limestone caves and cavities provide huge porosity and massive flows of groundwater GWD1

  19. 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. GWD1

  20. How Aquifers occur GWD1

  21. 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 GWD1

  22. CASE STUDY 1: APPLICATION OF GEOLOGY IN IDENTIFYING GROUNDWATER SOURCES IN CHAD GWD1

  23. EXERCISE 1: WHAT CAN WE FIND FROM A GEOLOGICAL MAP GWD1

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