Aquifers are underground layers that store and transmit groundwater. They come in two main types: confined, bounded by impermeable layers, and unconfined, with the water table as their upper limit. These differences affect how water moves and is extracted.

Aquifer properties like porosity, specific yield, and hydraulic conductivity determine how much water they can hold and how easily it flows. Geology plays a big role too, with unconsolidated sediments, consolidated rocks, and fractured crystalline formations creating diverse aquifer types.

Aquifer Types and Characteristics

Confined vs unconfined aquifers

• Confined aquifers bounded by impermeable layers (aquitards or aquicludes) above and below
  • Groundwater under pressure causing water levels in wells to rise above the top of the aquifer (artesian wells)
  • Pressure surface known as the potentiometric surface represents the level to which water would rise in a well
• Unconfined aquifers bounded by an impermeable layer at the bottom and the water table at the top
  • Water table is the upper surface of the saturated zone exposed to atmospheric pressure
  • Water levels in wells coincide with the water table fluctuating with recharge and discharge (shallow wells)

Porosity and retention in aquifers

• Porosity ($n$) is the ratio of the volume of voids (pores) to the total volume of the aquifer material
  • Expressed as a decimal or percentage indicating the amount of water an aquifer can store
  • Varies with grain size, shape, and packing arrangement (well-sorted sand vs poorly-sorted gravel)
• Specific yield ($S_y$) is the ratio of the volume of water that drains from the aquifer by gravity to the total volume of the aquifer material
  • Represents the amount of water that can be extracted from an unconfined aquifer
  • Depends on grain size and interconnectedness of pores (fine sand vs coarse sand)
• Specific retention ($S_r$) is the ratio of the volume of water retained in the aquifer material after gravity drainage to the total volume of the aquifer material
  • Represents the amount of water that remains in the pores due to capillary forces and cannot be extracted
  • Influenced by grain size and surface tension (clay vs sand)

Hydraulic properties of aquifers

• Hydraulic conductivity ($K$) measures an aquifer's ability to transmit water
  • Depends on the properties of both the aquifer material (permeability) and the fluid (density and viscosity)
  • Expressed in units of length per time (m/s or ft/day)
  • Varies with grain size, sorting, and cementation (clean sand vs silty sand)
• Transmissivity ($T$) is the product of the hydraulic conductivity and the aquifer thickness ($b$)
  • $T = Kb$
  • Represents the rate at which water is transmitted through a unit width of the aquifer under a unit hydraulic gradient
  • Expressed in units of length squared per time (m²/s or ft²/day)
  • Important for estimating well yields and groundwater flow rates (high $T$ = productive aquifer)

Types of aquifers by geology

• Unconsolidated sedimentary aquifers composed of loose materials such as sand, gravel, and silt
  • High porosity and permeability due to large pore spaces between grains
  • Examples: alluvial aquifers (river deposits), glacial outwash aquifers (meltwater deposits)
• Consolidated sedimentary aquifers composed of rock formations such as sandstone, limestone, and dolomite
  • Porosity and permeability depend on the presence of fractures, joints, and solution cavities
  • Examples: sandstone aquifers (porous rock), karst aquifers (dissolved limestone)
• Igneous and metamorphic rock aquifers composed of crystalline rocks such as granite, basalt, and gneiss
  • Primary porosity is typically low, but secondary porosity can develop through fracturing and weathering
  • Examples: fractured bedrock aquifers (cracked granite), volcanic rock aquifers (vesicular basalt)