Lecture 21: Groundwater

Groundwater Overview

  • Groundwater accounts for 30% of Earth's freshwater supply.

Drought Monitoring

  • Drought impact types:

    • S: Short-Term (e.g. agriculture)

    • L: Long-Term (e.g. hydrology)

  • Intensity Levels:

    • D0: Abnormally Dry

    • D1: Moderate Drought

    • D2: Severe Drought

    • D3: Extreme Drought

    • D4: Exceptional Drought

Infiltration

  • Infiltration: Process of rain soaking into the ground.

  • Infiltration rate depends on soil/rock properties and saturation level.

  • Porosity: Measure of open space in rock/soil.

  • Permeability: Ability of water to pass through a rock; both properties must be present for infiltration.

Soil Types and Permeability

  • Soil types impact permeability:

    • Loose soils are generally more permeable than clay-rich soils.

    • Well-sorted clasts = high porosity; Poorly-sorted clasts = lower porosity.

Rock Types and Groundwater

  • Igneous Rocks: Generally not porous or permeable.

  • Metamorphic Rocks: Typically non-porous due to heat and pressure.

  • Sedimentary Rocks: Can be highly porous and permeable (e.g. sandstone, limestone).

Karst Features

  • Limestone can develop features called karst from acidic groundwater.

  • Examples include caves and sinkholes.

Water Table and Groundwater Movement

  • Water Table: Boundary where soil/rock is saturated with water.

  • Water flows from high to low elevations, influenced by topography.

  • Capillary Action: Water movement against gravity in porous materials.

Aquifers and Aquitards

  • Aquifer: Layer of sediment/rock that transmits water easily.

  • Aquitard: Layer that hinders water flow.

Groundwater Pumping and Depletion

  • Excessive pumping can lower water tables and create a cone of depression.

  • Well types:

    • Regular well: requires pumping to draw water.

    • Artesian well: water flows naturally under pressure.

Groundwater Issues

  • Subsidence: Ground sinking due to water withdrawal (e.g. California).

Mapping Groundwater

  • Water well locations and water table elevations can be mapped using contour lines.

  • Hydraulic Gradient: Change in water table elevation per unit distance indicates flow speed.