When rain first falls, it infiltrates into the dry soil. When the soil becomes saturated the excess water must run off of the surface.
When it rains, some of the water infiltrates (sinks) into the ground -
Some of this water get bound up in the top layers of soil in the root zone - called soil moisture belt
Water that is not held in the belt of soil moisture sinks/percolates downward through the zone of aeration or vadose zone.
Zone of Aeration / Vadose Zone
Both air and water occupy the pore spaces.
Contains both the soil moisture belt and the capillary fringe.
Capillary Fringe
Water that is held by surface tension in tiny passages between the grains of soil or sediment.
Zone of Saturation / Phreatic Zone
All of the pore spaces are completely filled with water
Water Table
The zone of aeration and the zone of saturation are separated by the water table.
The water table is not flat but varies with the topography - it stands somewhat higher under hills, and lower under valleys.
Where the water table intersects (or lies above) the ground surface, springs, lakes, swamps, or rivers are present.
Interaction between groundwater and streams
Gaining (or Effluent) Streams
Discharge groundwater into the Stream
Losing (or Influent) Streams
Recharge the groundwater system (i.e. sink into the ground)
Porosity is the amount of pore space in a rock (the spaces between the grains).
Porosity depends on:
Some groundwater is held as a film on the surfaces of grains by surface tension. This water will not flow through the rock and is termed the specific retention (Sr) of the material. The volume of groundwater that can drain from rock or sediment is known as the specific yield (Sy).
Permeability
is the ease with which fluids flow through a rock or sediment. Not all pores in a rock are connected.
A rock is permeable if fluids pass through it, and impermeable if fluid flow through the rock is negligible.
Permeability depends on:
An aquifer is a water-bearing rock. Aquifers have high porosity and high permeability.
Aquitards retard or slow down the transmittion of water. Aquitards have low permeability.
Note that some very impermeable rock types may serve as aquifers if they are highly fractured. Water can travel thorough the cracks in the rock.
Unconfined Aquifer
Confined Aquifer
Movement of Groundwater
Water travels in the pore spaces and fractures in rock. Like surface water, it moves under the influence of gravity. However the its velocity is MUCH lower.
Also important is the pressure of the overlying column of water - water moves to areas of lower pressure.
Darcy's Law
(K) Hydraulic Conductivity - Property of the geologic medium and related to the permeability of the medium.
(h) head - the height of the water above a base level (usually sea level). Wells
Artesian wells