Groundwater Hydrosphere Notes

Groundwater & Hydrosphere

General Overview

  • Hydrosphere: All the water on Earth.

    • Covers 71% of Earth’s surface.

    • Oceans contain the majority of Earth’s water (97.5%).

    • Only 10-15% is readily available freshwater.

Hydrologic Cycle

  • The continuous circulation of Earth’s water among the oceans, atmosphere, and continents.

Hydrologic Processes

  • Evaporation: Water turns from liquid to vapor, moving from the hydrosphere to the atmosphere.

  • Transpiration: Release of water vapor to the atmosphere by plants.

  • Precipitation: Water converts from vapor to liquid in the atmosphere and falls to the surface (single strongest variable).

  • Infiltration: Surface water draining into the ground through cracks in the soil; recharges groundwater.

Groundwater

  • Water stored in the subsurface that flows through connected pores/fractures.

    • Flow is based on pressure and elevation.

    • Moves slower than surface water.

Pores

  • Space between particles, minerals, or surfaces in rocks, sediment, or soil.

Porosity

  • Percentage of rock/sediment that consists of voids.

    • Measurement of rock’s ability to hold water.

    • Depends on the type of rock.

    • Formula: (n) = \frac{volume\,voids}{total\,volume}, (n = \frac{V{voids}}{V{tot}})

    • Types:

      • Primary porosity: in pores.

      • Secondary porosity: in fractures.

Permeability

  • Ability of a material to transmit a fluid.

    • Depends on size, shape, distribution, and packing of grains, and fractures.

    • Factors:

      • Number of available conduits

      • Size of conduits

      • Straightness of conduits

    • Non-proportional to porosity

Hydraulic Conductivity (K)

  • Ease of transmitting groundwater.

    • Aquitard (low K) → Aquifer (high K)

Capillary Fringe

  • Water is pulled above the water table by capillary suction.

Water Table

  • Where fluid pressure is equal to atmospheric pressure.

Phreatic/Saturated Zone

  • Subsurface in which all rock openings are filled with water.

    • Under the water table.

    • Matches topography.

    • Water level at the surface of most lakes/rivers corresponds to the local water table.

    • Surface water is often exposed surface groundwater.

    • Saturated Zone is where all pores are completely filled with water

Aquifers

  • Body of saturated rock or sediment through which water can easily move.

    • Bedrock Aquifer: Forms in fractured rock.

      • Primary rock porosity and permeability are low.

      • Number, size, and connectivity of fractures determine permeability.

    • Unconfined Aquifer: Has a free water table controlled by adjacent elevation and pressure.

    • Confined Aquifer: Bounded above and below by confining beds.

      • Occurs at significant depth below ground surface.

Aquitard

  • Rock/sediment that greatly diminishes groundwater flow due to low porosity/permeability.

Water Flow

  • Downhill water flow:

    • The greater the elevation difference, the more “potential” energy there is as an object falls.

Pressure

  • Force per unit area.

    • Water underground is under pressure from the atmosphere and from the weight of water and materials above it.

Potential Energy

  • The higher the pressure, the more potential energy.

  • Water and material want to move to areas of less pressure.

Hydraulic Head

  • Potential energy available to drive the flow of a given volume of groundwater at a location.

    • Total Hydraulic Head = pressure head + elevation head.

Henry Darcy’s Law

  • Q = -K \cdot I \cdot A

    • K: hydraulic conductivity

    • A: cross-sectional flow area

    • I: head gradient (\frac{h1-h2}{L}, \frac{\Delta h}{\Delta L})

    • H = head, L = length

Recharge & Discharge

  • Recharge Area: Location where water enters the ground and infiltrates down to the water table.

  • Discharge Area: Location where groundwater flows back up to the surface.

Wells

  • Holes in the ground to withdraw water.

Drawdown

  • Lowering of the water near a pumped well.

Cone of Depression

  • Downward-pointing, cone-shaped surface of the water table in a location where the water table is experiencing drawdown due to a pumping well.

Pumping Wells

  • Accelerate flow, reverses flow, causes drawdown, and forms cones of depression.

    • May dry up springs, rivers, wetlands, and other wells.

Groundwater Depletion

  • Extracting water faster than it can be resupplied.

    • Example: Ogallala Aquifer is being depleted at an alarming rate.

Saltwater Intrusion

  • Saline water pulled into formerly freshwater areas.

Groundwater Contamination

  • Addition of chemicals or microbes to the groundwater supply.