Untitled Flashcards Set

SUBSURFACE FLOW - GROUP 4

SUBSURFACE ZONES

Groundwater is a plentiful, renewable resource that is naturally clean and secure, easily developed for quick irrigation and consistent water supply. Its main challenges are locating its underground zones and recharge areas.

Groundwater is a vital and replenishable resource beneath the Earth's surface, supporting ecosystems, drinking water, irrigation, and industry. The subsurface is divided into distinct zones:

1. Soil Water Zone (Vadose Zone):

   • Uppermost layer, closest to the surface.

   • Contains both water and air in pore spaces.

   • Water is held by surface tension and available for plant roots.

   • Moisture content varies based on rainfall, evaporation, and plant activity.

   • Critical for infiltration, the process where water moves from the surface into the ground.

2. Capillary Zone:

   • Located beneath the soil water zone.

   • Water is held by capillary action rising from the saturated zone.

   • Fills smaller pore spaces.

   • The height of the capillary fringe depends on the pore size distribution of the soil.

   • Represents a transition between the unsaturated vadose zone and the fully saturated zone.

3. Saturated Zone (Phreatic Zone):

   • Deepest zone where all pore spaces are completely filled with water.

   • The top of this zone is called the water table.

   • Groundwater in this zone is under hydrostatic pressure.

   • Primary source for wells and springs.

AQUIFER TYPES

Aquifers are geological formations that can store and transmit significant quantities of groundwater. They are classified based on their geological structure and hydraulic properties:

1. Unconfined Aquifers (Water Table Aquifers):

   • Upper boundary is the water table, which is free to fluctuate.

   • Typically recharged directly by rainfall infiltrating from the surface.

   • More susceptible to contamination than confined aquifers.

2. Confined Aquifers (Artesian Aquifers):

   • Occur when a porous formation holding groundwater is between two impermeable layers (aquicludes).

   • Under pressure, recharged by a natural source at a higher elevation (e.g., rainwater at an outcrop or a river).

   • Piezometric surface represents the initial water level at the recharge source.

     • If the piezometric surface is above ground level, it forms a flowing artesian well where water naturally flows to the surface.

     • If the piezometric surface is below ground level, it forms a non-flowing artesian well, requiring pumping.

   • Wells can be drilled through multiple artesian aquifers to access various pressurized water sources.

3. Leaky Aquifers (Semi-Confined Aquifers):

   • Intermediate between unconfined and confined aquifers.

   • Overlain by aquitards (semi-permeable layers).

   • Allows limited vertical water movement at a slower rate.

   • Can receive recharge from direct rainfall (like unconfined aquifers) and lateral flow through the aquitard (like confined aquifers).

AQUIFER PROPERTIES

Aquifers are geological formations capable of storing and transmitting significant quantities of groundwater. Important properties include:

1. Porosity (n):

   • Percentage of void space in a rock or sediment.

   • Determines the amount of water an aquifer can store.

2. Permeability (k):

   • Measure of the ability of a porous material to transmit fluids.

   • Depends on the size and interconnectedness of pore spaces.

   • Higher permeability allows easier water flow.

3. Storage Coefficient (S):

   • Describes the volume of water an aquifer releases or absorbs per unit area when the water level changes by a unit depth.

   • In unconfined aquifers, it is equivalent to specific yield (water that drains from pores).

   • In confined aquifers, it reflects:

     • Compressibility of the aquifer matrix.

     • Expansibility of the water itself.

4. Transmissibility (T):

   • Represents an aquifer's capacity to transmit water per unit width under a unit hydraulic gradient.

   • Mathematically: T=KimesbT = K imes b, where:

     • KK = permeability (hydraulic conductivity).

     • bb (or HH) = saturated thickness.

   • Confined Aquifers: Water flow capacity remains constant regardless of pressure changes.

   • Unconfined Aquifers: Water flow capacity changes with water levels; lower levels decrease ease of flow.

FLOW MECHANISMS

Groundwater flow is governed by Darcy's Law, a fundamental principle in hydrogeology:

• Darcy's Law:

  • States that groundwater flow rate through a porous medium is proportional to the hydraulic gradient and the medium's permeability.

  Mathematically:

  Q=−kA(dhdl)Q = -kA \left( \frac{dh}{dl} \right)

  Where:

  • QQ = Volumetric flow rate.

  • kk = Hydraulic conductivity (related to permeability).

  • AA = Cross-sectional area of flow.

  • dhdl\frac{dh}{dl} = Hydraulic gradient (change in head over distance).

  • Essential for understanding and modeling groundwater flow in various applications:

    • Well pumping.

    • Aquifer recharge.

    • Contaminant transport.