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Chapter 14 Groundwater

Chapter 14 Groundwater

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Chapter 14 Groundwater

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  1. Chapter 14Groundwater

  2. Importance of groundwater • Groundwater is water found in sediment, plus narrow fractures in bedrock • Groundwater is the largest reservoir of fresh water available to humans

  3. Sources of Earth’s water

  4. Distribution of Groundwater

  5. The Water Table • Water table is the upper limit of saturation • Variations in the water table • Depth is highly variable • Varies seasonally and from year to year • Shape is usually similar to surface topography

  6. Factors influencing movement of groundwater - 1 • Porosity – Volume of Pore Space • Permeability – the ability of a material to transmit a fluid through pore space

  7. Factors influencing movement of groundwater - 2 • Aquifer – a permeable rock layer or a sediment that transmits groundwater freely (such as sands and gravels) • Aquitard – an impermeable layer that hinders water movement (such as mudstone - clay)

  8. Springs may result from a perched water table Rollie’s Well

  9. Features associated with Groundwater - 1 • Hot springs • Water is 6-9oC warmer than the mean annual air temperature of the locality • The water for most hot springs is heated by cooling of igneous rock

  10. Features associated with Groundwater -2 • Geysers • Intermittent hot springs • Water erupts with great force • Occur where extensive underground chambers exist near hot igneous rock • Groundwater heats, expands, changes to steam, and erupts

  11. Yellowstone Geyser erupting

  12. Yellowstone Geyser Pool prior to eruption Groundwater flows back in, lowest chamber flashes to steam, erupts again.

  13. Features associated with groundwater - 3 • Wells • To ensure a continuous supply of water, a well must penetrate below the water table

  14. Artesian Well Model Demo: Artesian Discussion: Quicksand

  15. Hydraulic head and urban water supplies Just like an artesian, saves money

  16. Features associated with groundwater - 3 • Wells (con’t) • Pumping of wells can cause • Drawdown (lowering) of the water table • Cone of depression in the water table

  17. Formation of a cone of depression in the water table Discharge exceeds recharge

  18. Other problems associated with groundwater withdrawal • Treating groundwater as a nonrenewable resource • In many places water available to recharge aquifer is less than amount being withdrawn • “Discharge greater than Recharge

  19. Other problems associated with groundwater withdrawal (Continued) • Subsidence ! • Ground sinks when water is pumped from wells faster than natural recharge replaces it

  20. Other problems associated with extreme groundwater withdrawal(Continued) • Dry Wells – cone of depression already seen • Saltwater contamination near coast • Contamination of Wells with sewage • Formation of Collapse Sinkholes

  21. Saltwater contamination due to excessive well pumping WildwoodCrest

  22. Well design requires knowledge of local geology Don’t fire the town engineer!

  23. Geologic work of groundwater • Caverns • Most caverns are created by acidic groundwater dissolving soluble rock at or just below the surface in the zone of saturation • Acid forms as Carbonic Acid (next)

  24. Geologic work of groundwater • Groundwater dissolves rock • Groundwater is often mildly acidic • Contains weak carbonic acid • Forms when rainwater dissolves carbon dioxide from the air and from decaying plants

  25. Geologic work of groundwater • Groundwater dissolves rock • Carbonic acid reacts with calcite in limestone to form calcium bicarbonate, a soluble material • Later cements particles - lithification

  26. Geologic work of groundwater • Caverns • Features found within caverns • Composed of dripstone (travertine) • Calcite deposited as dripping water evaporates

  27. Geologic work of groundwater • Caverns • Features found within caverns • Composed of dripstone (travertine) • Collectively, they are called speleothems • Includes stalactites(hanging from the ceiling) and stalagmites (form on the floor of a cavern)

  28. Geologic work of groundwater • Karst topography • Landscapes that have been shaped by the dissolving power of groundwater on limestone

  29. Geologic work of groundwater • Karst topography • Some common features include • Irregular terrain • Sinkhole or sinks (formed by groundwater slowly dissolving the bedrock often accompanied by collapse) • Disappearing (aka sinking) streams

  30. Solution Stream incision, cave development and karst topography Solution

  31. Sinkholes in Urban Settings

  32. End of Streams and Groundwater