Ground Water and Aquifers Overview

Ground Water

• Ground water is a replenishable resource unlike other Earth resources.
• Modern trends focus on recharging ground water from natural sources (rain, percolation dams).
• Ground water is free from pollution and atomic attacks.
• It's economical and efficient for irrigation, especially in times of low surface water supply.

Types of Aquifers and Formations

• An aquifer is a water-bearing geologic formation that transmits water at a rate suitable for extraction.
• Good aquifers include:
  • Unconsolidated gravels, sands, and alluvium
  • Lake sediments and glacial deposits
  • Sandstones
  • Limestones with cavities (caverns)
  • Granites and marble with fissures and cracks
  • Weathered gneisses and schists
  • Heavily sheeted quartzites
  • Vescicular basalts
  • Slates
• An aquiclude absorbs water but doesn't transmit it significantly (clays, shales).
• An aquifuge neither absorbs nor transmits water (basalts, granites).
• An aquitard transmits water at a slow rate (clay lenses interbedded with sand).

Confined and Unconfined Aquifers

• Unconfined Aquifer (Water-table aquifer): A porous formation extends from the ground surface to an impermeable layer below. Groundwater table (GWT) develops. Wells drilled into these aquifers are called water table wells.
• Confined Aquifer (Artesian aquifer): A porous formation is sandwiched between two impermeable layers (aquicludes). Water pressure within is high, and wells may be flowing artesian wells (water level above ground surface) or non-flowing artesian wells (water level below ground surface).

Specific Yield and Specific Retention

• Specific yield (S): The percentage of total volume of saturated aquifer that drains by gravity.
• Specific retention (Sr): The percentage of the volume of water held in the void spaces by molecular and surface tension forces.
• Porosity equals specific yield plus specific retention.

Storage Coefficient

• The volume of water discharged from a unit prism of aquifer when the water table drops by one unit. It's dimensionless.
• For artesian aquifers, the storage coefficient accounts for the compressibility of the aquifer and the expansion of pore water.

Darcy's Law

• Darcy's Law (1856) states the relationship between the velocity of water flow in a porous medium and hydraulic gradient (slope).
• Velocity of flow is directly proportional to the hydraulic gradient
• Q = KiA where Q is the volume flow rate, K is the hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional area.

Transmissibility

• Transmissibility (T) is a measure of an aquifer's capacity to transmit water.
• It is the flow capacity per unit width under a unit hydraulic gradient.
• For confined aquifers, T = Kb; for water table aquifers, T = KH.

Well Hydraulics

• Steady radial flow into a well.
• Water table conditions (unconfined aquifer) The rate of flow is calculated using equations specific to the conditions.
• Artesian conditions (confined aquifer).

Well Hydraulics

• Steady radial flow into a well.
• Water table conditions (unconfined aquifers) and artesian conditions (confined aquifers)
• Calculations for yield and drawdown are presented using formulas involving transmissibility (T), hydraulic conductivity (K), and geometric factors related to well and aquifer dimensions.

Specific Capacity

• The discharge per unit drawdown in a well (usually expressed as liters per minute per meter of drawdown).

Cavity Wells

• A type of well especially used in alluvial strata that has a stiff clay layer underlying it.
• High yields are possible because flow into the hollow cavity can be relatively easy.
• The design and characteristics of these wells are different from more conventional well designs.

Well Spacing

• Wells should be sufficiently far apart so that their cones of depression do not overlap.
• Spacing is calculated according to the size of the aquifer and the anticipated pumping rate. A wider spacing is recommended in rockier areas.

Open Well Construction

• In alluvial areas with vertical impervious steining, the well size and depth directly influence the water yield.
• In rocky areas, wider well diameters generally aren't required but deeper wells may be beneficial instead. Alternate bands of masonry are sometimes laid dry (without cement mortar) to improve well efficiency.

Well Tests (Pumping and Recuperation Tests)

• Used to estimate a well's yield.
• Pumping tests involve pumping at a constant rate and measuring the drawdown (drop) in water levels over a period.
• Recuperation tests involve stopping pumping and measuring the time it takes for water levels to return to their previous state.