Chap+17+Groundwater+F20 (copy)

Chapter 17 - Groundwater

Groundwater Definition

• Hydrogeology: The study of groundwater.

• Groundwater: Water stored beneath Earth’s surface in pores and fractures in rocks.

  • Pores: Spaces between grains in a rock.

  • Fractures: Open cracks in rock.

Importance of Groundwater

• 96% of Freshwater: Groundwater is the largest source of freshwater available to humans.

• Freshwater Share: 96% of all liquid freshwater is groundwater; other sources include:

  • Ice sheets and glaciers: 68.7%

  • Lakes: 1.2%

  • Soil moisture: 0.294%

  • Rivers: 0.004%

  • Water vapor: 0.049%

Fresh Water of the Hydrosphere

• Table of Fresh Water Volume:

  • Ice sheets and glaciers: 24,000,000 km³ (84.945%)

  • Groundwater: 4,000,000 km³ (14.158%)

  • Lakes and reservoirs: 155,000 km³ (0.549%)

  • Soil moisture: 83,000 km³ (0.294%)

  • Water vapor in atmosphere: 14,000 km³ (0.049%)

  • River water: 1,200 km³ (0.004%)

  • Total volume of fresh water: 28,253,200 km³ (100%)

Porosity and Permeability

• Porosity: The amount of void space in rock.

• Permeability: The ability of rock to transmit water, which is:

  • Proportional to flow speed in rocks.

  • Affected by how well the pores are connected.

  • High porosity does not always mean high permeability.

  • Examples:

    • Vesicular basalt: High porosity, very low permeability (pores not interconnected).

    • Clays: Water held by electrostatic charge, low flow.

Examples of Porosity and Permeability

• Uncemented Sandstone: High porosity and permeability.

• Cemented Sandstone: High porosity, low permeability due to cement blocking pathways.

• Vesicular Basalt: High porosity, very low permeability due to unconnected pores.

Aquifers and Aquitards

• Aquifer: Permeable rock strata that transmit groundwater freely (e.g., sand, gravel).

• Aquitard: Impermeable layer preventing water movement (e.g., clay, shale).

Source of Groundwater

• Groundwater Recharge: Occurs when rainwater infiltrates into the ground, enhancing through surface water impoundments (lakes, streams).

• Importance of Wetlands: Enhance groundwater recharge but are decreasing due to development.

Zones of Groundwater

• Soil Moisture: Shallow surface layer wet by rain.

• Zone of Aeration: Space between surface and water table containing both air and water.

• Water Table: Top of saturated zone where groundwater begins.

• Zone of Saturation: Area entirely filled with water.

Water Table Dynamics

• Water Table Features:

  • Shape mimics ground surface.

  • Levels rise and fall with seasons based on rainfall.

• Surface Water Interaction: Levels correlate with water bodies nearby.

Unconfined and Confined Aquifers

• Unconfined Aquifer: Exposed to the surface, water table marks its level.

• Confined Aquifer: Sealed by impermeable layers, pressurized water present.

  • Artesian Pressure: Water rises above the aquifer when punctured by a well.

  • Flowing Artesian Well: Water flows above surface due to pressure.

Darcy's Law and Groundwater Flow

• Darcy’s Law: Describes groundwater discharge (Q) and factors affecting flow:

  • Q = K A [(h1-h2)/d]

    • Q = volume of water flow

    • K = hydraulic conductivity

    • A = cross-sectional area

    • (h1-h2) = hydraulic gradient

    • d = distance

• Higher permeability and steeper slopes increase discharge.

Wells and Cone of Depression

• Well: A hole bored into the saturated zone; must be deeper than fluctuating water table.

• Cone of Depression: Drawdown area around a pumping well, which can affect nearby wells.

Problems with Groundwater Withdrawal

• Nonrenewable Resource: Over-extraction exceeding recharge rates (e.g., Ogallala aquifer).

• Subsidence: Ground sinking due to excessive water removal.

• Saltwater Intrusion: Occurs at coastal wells when freshwater is over-pumped.

Springs and Geysers

• Springs: Natural flows of groundwater where water table meets surface.

• Geysers: Intermittent springs where columns of boiling water erupt.

• Hot Springs: Water 6ºC to 9ºC warmer than ambient temperature.

  • Heated by igneous rock cooling or geothermal gradient.

Groundwater Contamination

• Sources of Contamination: Sewage, industrial waste, agricultural runoff.

  • Cleanup methods include pump-and-treat and biodegradation.

Groundwater Erosion of Limestone

• Limestone Erosion: Acidic rainwater erodes limestone, forming caves and aquifers with rapid groundwater flow.

• Karst Landscapes: Formed by groundwater erosion, characterized by sinkholes and disappearing streams.

Major Features of Karst Topography

• Sinkholes: Collapse of cave roofs creating depressions.

• Sinking Streams: Streams that disappear underground into sinkholes.