Groundwater Flashcards

Groundwater Overview

• Groundwater is located underground in pore spaces of sediments and rocks, or within fractures and cavities in rocks.
• Constitutes approximately 51% of drinking water for the entire U.S. population and 99% for the rural population.
• Represents a small portion of the Earth's hydrosphere.

Hydrologic Cycle

• Describes the continuous circulation of water from the Earth's surface to the atmosphere and back.
• Involves processes: evaporation, transpiration, precipitation, infiltration, and runoff.
• Solar energy drives the cycle, facilitating water vapor's journey through these processes.

Earth's Water Budget

• Balance of water can be expressed as:
  • Precipitation = Runoff + Evapo-transpiration + Infiltration.

Factors Influencing Runoff vs. Infiltration

• Intensity and duration of rainfall affect runoff; lower intensity favors infiltration.
• Soil texture governs porosity and permeability, influencing infiltration rates.
• Topography: flat and gently sloping surfaces enhance infiltration.
• Land use changes: vegetation (grass, forest) promotes infiltration; bare or urban land increases runoff.
• Climate conditions: arid climates favor runoff due to higher evaporation rates and less vegetation.

Storage and Movement of Groundwater

• Porosity is the percentage of void space in sediment or rock, indicating groundwater storage capacity.
  • Primary porosity develops when rock forms.
  • Secondary porosity results from fractures and cavities formed after rock formation.

Permeability

• Defined as the capacity of materials to transmit fluids, relevant for groundwater movement.
• Expressed in rates, e.g., inches/day or centimeters/day.

Structure of Water Table

• Comprises three zones: Unsaturated Zone, Saturated Zone, and the Water Table.

Unsaturated Zone

• Located just below the land surface, where pores contain both water and air.
• Water in this zone is referred to as soil moisture, not groundwater.

Saturated Zone

• Area beneath the surface where all pores are filled with water.

Water Table

• The upper surface of the saturated zone, fluctuates with water recharge and discharge.

Factors Affecting Groundwater Velocity

• Velocity depends on hydraulic conductivity (measure of permeability) and hydraulic gradient (slope of the water table).
• Darcy's Law illustrates the relationship:
  • V = K (h/L)/n, where V is velocity, K is hydraulic conductivity, n is porosity, h is head difference, and L is horizontal distance.

Aquifer

• Composed of porous and permeable materials, capable of storing and transmitting water.
• Varies greatly in size and thickness, ranging from small areas to extensive regions below the surface.

Aquiclude

• A formation that restricts water movement, typically made up of less permeable materials compared to an aquifer.