Water Lecture 3

GEOL 142: Water

Lecture 3: Groundwater

Groundwater and Aquifers

• Groundwater: Water located underground, filling pore spaces in rock, sediment, or soil. Moves slowly downhill where permeability allows.

• Aquifer: A body of permeable rock or sediment (e.g., sand, gravel, sandstone, limestone) saturated with water that allows groundwater movement.

Groundwater Terminology

• Porosity: The percentage of total volume in bedrock, sediment, or soil occupied by pore spaces.

  • Depends on particle size, shape, compaction, fractures, and cementation.

  • Example: Sandstone (~20% porosity) vs. Igneous/metamorphic rock (~1% porosity, mostly from fractures).

• Permeability: The ability of a material to transmit fluids, depending on pore size and connectivity.

  • High permeability: Sand, gravel (large openings).

  • Low permeability: Clay, shale (small openings, surface tension effects).

Groundwater Zones

• Unsaturated Zone: Pore spaces contain mostly air.

• Saturated Zone: All openings are filled with water.

• Water Table (Groundwater Table): The upper surface of the saturated zone, following surface topography and sloping toward streams or lakes.

Groundwater Movement

• Percolation: Rainwater seeps downward through the unsaturated zone until reaching the water table.

• Movement: Below the water table, groundwater flows slowly along curved paths from high to low water table areas.

• Recharge Area: Where water enters the saturated zone.

• Discharge Area: Where groundwater exits into streams, lakes, or surface water bodies.

Groundwater Flow & Velocity

• Groundwater flow rate is proportional to:

  • Cross-sectional area (A) – Larger area = more water flow.

  • Permeability (K) – Higher permeability = faster flow.

  • Hydraulic gradient (h/l) – Steeper slope = faster flow.

• Darcy’s Law: Describes groundwater discharge Q  = (A) (K) (h/l), Where:

  • Q = Discharge (flow rate)

  • A = Cross-sectional area

  • K = Permeability coefficient

  • h/l = Hydraulic gradient (slope of the water table)

• Water flows from high-pressure to low-pressure areas, with velocity dependent on permeability and pressure difference.