Chapter 6: Groundwater Flow in Saturated Porous Media

Groundwater

Water located beneath the Earth's surface, filling pores and fractures in soil, sand, and rock, rather than flowing in underground rivers

Saturated Zone

Located below the water table, this area has all voids, cracks, and pores completely filled with water

Unsaturated Zone

Portion of the subsurface above the groundwater table.

The soil and rock in this zone contains air as well as water in its pores, also called the zone of aeration due to the presence of oxygen in the soil

Capillary Fringe

A transition zone directly above the saturated zone where water is drawn upward from the water table by capillary action, saturating or nearly saturating the pore spaces.

Water Table

It represents the level below which the ground is saturated with water, often visualized as the top of the groundwater

Aquifer

Body of porous rock or sediment saturated with groundwater

A geologic formation that stores and transmits groundwater.

Confined aquifers

A type of aquifer which have a layer of impenetrable rock or clay above them

Unconfined aquifers

A type of aquifer that lies below a permeable layer of soil

Porosity

Measure of the void spaces in a material

Maximum volume of water a saturated rock or soil can hold

Permeability

Intrinsic property of the porous medium itself, independent of the fluid passing through it

The ability of a porous material to allow fluids to pass through its interconnected voids.

Darcys

Permeability is often measured in ________

Hydraulic Conductivity

The ease with which a fluid (usually water) can move through pore spaces or fractures.

Expressed as a velocity

Grain Size and Shape

Degree of Sorting

Stratification

Compaction and Cementation

Fluid Viscosity

Factors Affecting Groundwater Movement (5)

Hydraulic Head

Total energy per unit weight of groundwater at a specific point

It is the driving force behind all groundwater movement.

Elevation Head

The vertical distance from a reference datum (usually mean sea level) to the measurement point. It represents potential energy.

Pressure Head

Energy caused by water pressure

Hydraulic gradient

Controls the speed and direction of the flow

Darcy’s Law

Describes the flow of groundwater through porous media (soil/rock)

Laminar Flow

Assumes water moves slowly and smoothly through small pores (Assumptions)

Homogeneity

Assumes the ground is made of the same material (Assumptions)

Saturated Media

Applies specifically to areas where all the gaps in the rock or soil are completely filled with water. (Assumptions)

Laminar Flow

Homogeneity

Saturated Media

Assumptions in Darcy’s Law (3)

Turbulence

In large cracks, where water splashes and swirls. Darcy’s Law isn't designed for this fast movement (Limitations)

Heterogeneity

If the ground is a mix of boulders, clay, and gravel, the law struggles to predict flow because it can’t account for sudden, dramatic changes in the material (Limitations)

Scale

Valid for representative volumes; not meant to track individual water particles through complex, microscopic pathways. (Limitations)

Turbulence

Heterogeneity

Scale

Limitations in Darcy’s Law (3)

Well & Aquifer Management

Contamination Tracking

Engineering & Safety

Applications of Darcy’s Law (3)

Unconfined aquifer

An aquifer whose upper water surface (water table) is at atmospheric pressure.

Confined aquifer

It is a rock or sand that is overlain by a confining layer (e.g. clay aquitard on top) that restricts movement of water into another aquifer

Hydraulic Head

Height to which water will rise in a bore. It is the resting groundwater level.

Hydraulic Gradient

Difference between the hydraulic head measured at two points in an aquifer divided by the distance between them

Recharge

It is the process whereby groundwater is replenished by water entering the groundwater system

Discharge

It is when groundwater leaves the aquifer

Transmissivity

Ability of the aquifer to transmit groundwater throughout its entire saturated thickness

Storage Coefficient/ Storativity

It is the volume of water released from storage with respect to the change in head (water level) and surface area of the aquifer.

S e e p a g e

Slow movement of water through soil or rock

Contaminant transport

The process where groundwater carry pollutants as it flows underground