• Just as the catchment is the basic unit of surface hydrology, the aquifer is the fundamental unit of groundwater hydrology.
• Aquifers are defined by their hydraulic properties rather than strictly by geology.
• Understanding aquifer types and properties is crucial for:
  • Groundwater management
  • Well design and sustainability
  • Resource assessment

• An aquifer can be:
  • Coextensive with geologic formations
  • A group of formations
  • Part of a formation
  • Independent of geologic units (cutting across formations)

• Units of low permeability that bound an aquifer are called confining beds.
• The designation is context-dependent:
  • In areas with prolific aquifers → low permeability unit = confining bed
  • In groundwater-poor regions → same deposit might be considered an aquifer

Important note

In practice, this ambiguity is resolved by explicitly defining hydraulic conductivity or porosity values that characterise the unit.

• Also called water table aquifers
• The water table forms the upper boundary
• No confining layer above
• Wells or piezometers installed in unconfined aquifers approximately define the water table position
• Water is under atmospheric pressure at the water table

• Also called artesian aquifers
• Bounded by confining beds on both top and bottom
• Water is under pressure greater than atmospheric
• The water level in a well occurs above the upper boundary of the aquifer
• Occasionally, water level occurs above ground surface (flowing artesian well)

• An unconfined aquifer that develops above the regional water table
• Forms on top of a low hydraulic conductivity layer
• An unsaturated zone exists below the perched aquifer
• Often temporary or seasonal
• Important for local water supply in some areas

Key feature: Unsaturated zone both above and below the perched zone

Types of Confining Beds

Three terms are occasionally used to describe different types of confining beds:

Aquitards play important dual roles:

• Influence rates of recharge between aquifer layers
• Can protect underlying aquifers from contaminants migrating downward from the ground surface
• Act as a barrier to vertical contaminant transport
• Provide slow release of water to adjacent aquifers during pumping

• Detailed characterisation in the midcontinent region of North America shows that simple concepts don’t capture reality.
• Runkel et al. (2018) use the term “aquitardifers” to describe confining beds that behave both as an aquitard and aquifer at the same place.

Extreme anisotropy in hydraulic conductivity:

• Horizontal direction:

  • Permeable partings parallel to bedding
  • Can transmit water horizontally like an aquifer

• Vertical direction:

  • Layering of low permeability units
  • Sufficiently low K_v to inhibit vertical transport of contaminants

In some shallow settings, aquitardifers develop vertical fractures:

• Create fast-flow pathways
• Can extend over relatively large areas
• Provide direct connection between surface and deeper units
• Completely change the protective function of the confining bed
• Challenge simple predictions of contaminant transport

Key Aquifer Properties

When a pump is turned on in a well:

1. Water level in the well casing (and hydraulic head) is reduced
2. This causes groundwater to flow from the aquifer into the well
3. Much of the pumped water initially comes from “storage” in the aquifer

Therefore, aquifers have at least two important characteristics:

• Ability to store groundwater
• Ability to transmit this water to a nearby well

• Similar concept to hydraulic conductivity
• Main difference: applies across the vertical thickness of an aquifer
• Integrates conductivity over aquifer thickness

If the thickness of the aquifer is b, the transmissivity T is:

Where:

• T = transmissivity [L^2/T]
• K = hydraulic conductivity [L/T]
• b = aquifer thickness [L]

Units: m²/day, ft²/day, or gpd/ft (gallons per day per foot)

For a homogeneous confined aquifer, Darcy’s equation can be written in terms of transmissivity:

Apply Darcy’s law across the aquifer:

Where:

• Q = discharge [ L^3/T ]
• T = transmissivity [ L^2/T ]
• W = width perpendicular to flow [ L ]
• i = hydraulic gradient [dimensionless]

Aquifers can store water. How this storage is accomplished differs depending upon whether the aquifer is confined or unconfined.

Confined aquifer — Two mechanisms supply water:

1. Water expansion: Lower pressure → water molecules expand slightly
2. Aquifer compression: Lower water pressure → less support for skeleton → grains pushed closer together → squeezes water out

Before pumping:

• Water pressure holds grains apart
• Aquifer skeleton is supported
• Pore space is at maximum

During pumping (head declines):

• Water pressure decreases
• Weight of overlying rocks pushes down
• Grains move closer together
• Pore space reduces → water squeezed out

Result: Only small amounts of water released (S = 10^{-5} to 10^{-3})

When pumping is excessive or prolonged:

• Initial compression is elastic (recoverable)
• Continued compression becomes plastic (permanent)
• Aquifer structure permanently altered
• Land surface subsides

Why clay layers are critical:

• Clay is ~1000× more compressible than rock
• Major contributor to subsidence
• Once compressed, cannot recover

Unconfined aquifer:

• Main source of water is drainage of water from pores as the water table declines
• Much more water released per unit decline in head
• For a comparable unit decline in hydraulic head, an unconfined aquifer releases much more water from storage than a confined aquifer

• Dimensionless quantity
• For confined aquifers: S ranges between 10^{-3} to 10^{-5}
• For unconfined aquifers: S (or specific yield) ranges between 0.01 to 0.30

Units: [1/L] (e.g., 1/m or 1/ft)

Relationship between S and S_s:

where b is the aquifer thickness.

Mathematical Formulation of Storage

For a confined aquifer, specific storage reflects storage from:

• Compression of the granular matrix
• Expansion of water

The mathematical definition (Domenico and Schwartz, 1998):

Where:

• \rho_w = density of water [ML^{-3}]
• g = gravitational constant (9.81 m/sec²) [LT^{-2}]
• n = porosity of the aquifer
• \beta_p = vertical compressibility of rock matrix
• \beta_w = compressibility of water

Compressibility (\beta) quantifies how much a material compresses under pressure.

Units: inverse of pressure [1/Pa] or [m^2/N]

Key insight:

• Higher compressibility = more compressible
• Lower compressibility = more rigid/incompressible

Water compressibility (\beta_w):

• 4.4 \times 10^{-10} m²/N at 25°C

The equation S_s = \rho_w g (\beta_p + n\beta_w) can be used to estimate specific storage and storativity.

Example calculation:

• Clay layer with n = 0.3
• \beta_p = 1 \times 10^{-7} m²/N
• \beta_w = 4.4 \times 10^{-10} m²/N
• \rho_w g \approx 10^4 N/m³

S_s \approx 1 \times 10^{-3} m⁻¹

In an unconfined aquifer, the groundwater response to pumping is different from a confined aquifer:

Early time (no significant water level change):

• Water comes from expansion of the water
• Water comes from compression of the grains
• Similar to confined aquifer behaviors

Later on (water table falls):

• Water mainly comes from gravity drainage of pores
• This is the dominant storage mechanism

The storativity of an unconfined aquifer is expressed as:

Where:

• S_y = specific yield of the aquifer
• b = aquifer thickness
• S_s = specific storage

In some cases, an aquifer may be:

• Confined at early stage of pumping
• Becomes unconfined at late time

Process:

1. Initial pumping creates cone of depression
2. Water levels decline
3. Head falls below the top of the aquifer
4. Aquifer dewaters from the top down
5. Storage mechanism changes from elastic to gravity drainage

As the aquifer changes from confined to unconfined, storativity values change accordingly.

Expressed as the ratio of:

Where:

• V_d = volume of water that drains by gravity
• V_T = total volume of soil or rock (after being saturated)

Not all water initially present is released from storage.

Where:

• V_r = volume of water retained against gravity
• V_T = total volume of soil or rock

Specific retention increases with:

• Decrease in grain size (more surface area)
• Decrease in pore size (stronger capillary forces)

Typical values:

• Coarse gravel: S_r \approx 0.01 to 0.05
• Fine sand: S_r \approx 0.05 to 0.15
• Silt: S_r \approx 0.15 to 0.30
• Clay: S_r \approx 0.30 to 0.50

Examples of Real-World Aquifer Systems

Principal Aquifers in the United States organized by aquifer type and showing pumping rates for different uses

• This collection accounts for about 90% of daily groundwater production in the US.

Characteristics:

• Intergranular porosity
• Primarily unconfined conditions
• High permeability (varies with clay content and sorting)
• Most extensively pumped aquifer type in US

Four main categories:

1. Basin/valley-fill: Central Valley (California), Arizona basins
2. Blanket sand and gravel: High Plains aquifer (most pumped)
3. Glacial deposits: North of glaciation line (outwash, terraces)
4. Stream-valley aquifers: Local, small extent (not mapped)

Key characteristics:

• Consolidated sand → reduced porosity
• Joints and fractures transmit most groundwater
• Low to moderate hydraulic conductivity
• Extend over large areas → provide large water quantities

Composition:

• Calcite (CaCO₃) → limestone
• Dolomite (CaMg(CO₃)₂) → dolostone
• Marine origin (shells, corals, algae, precipitates)

Key processes:

• Post-depositional: compaction, cementation, dolomitisation
• Solution enhancement: dissolution by acidic groundwater
• Creates tubes, widened joints, and caverns

Solution openings range from:

• Small tubes and widened joints
• To caverns tens of meters wide
• Extending hundreds to thousands of meters in length

Hydraulic properties:

• Well-connected networks → large yields to wells
• Undissolved rock between openings → nearly impermeable
• Extreme heterogeneity

Crystalline-rock aquifers:

• Fractured granite, basalt, metamorphic rocks
• Very low primary porosity
• Flow occurs primarily in fractures
• Secondary porosity controls productivity
• Important in New England, Canadian Shield
• Well yields highly variable

Volcanic-rock aquifers:

• Basalt flows with high permeability
• Flow in cooling fractures and lava tubes
• Vesicular zones store water
• Examples: Hawaiian volcanic, Columbia Plateau
• Can be extremely productive locally
• Complex interbedded structure

Summary of Aquifer Systems

Basin-Fill Aquifers:

• Structural basins in mountain regions
• Hundreds to thousands of meters thick
• Examples: Basin and Range, Central Valley
• Long, narrow bodies with interbedded materials
• Major water sources in western US

Fluvial Aquifers:

• Deposited by river systems
• Two types: meandering vs braided
• Braided → more continuous and productive
• Long, narrow, thin geometry
• Mississippi River Valley, High Plains

Key characteristics:

• High permeability (varies with sorting)
• Unconfined conditions
• High specific yield (0.1-0.3)
• Susceptible to contamination
• Most extensively pumped in US

Categories:

• Basin/valley-fill
• Blanket sand and gravel
• Glacial deposits
• Stream-valley aquifers

Semiconsolidated Sediments:

• Gulf Coast aquifer systems
• Wedge-shaped geometry (thicken toward coast)
• Confined updip, unconfined downdip
• Risk: saltwater intrusion, land subsidence
• Example: Houston subsided >9 ft (>2.75 m)

Sandstone Aquifers:

• Cemented sand with reduced porosity
• Flow in fractures and bedding planes
• Low to moderate hydraulic conductivity
• Large areal extent
• Example: Dakota Sandstone (~46% recharge through confining beds)

Carbonate-Rock Aquifers:

• Limestone and dolostone
• Solution-enhanced permeability
• 10 orders of magnitude range in K
• Large yields where well-connected
• Example: Floridan aquifer system

Other Types:

• Crystalline-rock: fractured granite/basalt, flow in fractures
• Volcanic-rock: basalt with cooling fractures and lava tubes

Formation requirements:

1. Chemically aggressive groundwater
2. Fractures present for transmission
3. Groundwater can drain out

Solution features:

• Start as small conduits (~1 cm)
• Grow to cave size (kilometers long)
• Highest yields at fracture intersections
• Extremely heterogeneous permeability

Karst landforms:

• Sinkholes (dissolution, cover subsidence, cover-collapse)
• Stream sinks (disappearing streams)
• Dry valleys
• Large spring systems
• Underground cave networks

Management challenges:

• Rapid contaminant transport
• Unpredictable flow paths
• Surface-subsurface connectivity