Ground Water Hydraulics

Pore space can vary tremendously depending on the rock or sediments that make up the subsurface. The size, shape, regularity, and continuity of the pore space will determine how much water can be extracted or stored in the subsurface, how fast water can move through the pore space, and how contaminants are distributed within the subsurface.

Porosity
The porosity of a sediment or rock formation is defined as the fraction of the material’s volume that is not occupied by solids:
porosity [in %] = (volume of pores/total volume) x 100 = (1 – volume of solids/total volume) x 100
Groundwater fills the entire pore space, but not all of that groundwater will be available for pumping. Some pores may be entirely isolated from the rest of the pores (Figure 12). Other pores may be so fine that water molecules are held tightly to the soil, particle, or mineral surfaces by adhesion (magnet-like forces on the surface of water molecules that attract them to the surfaces of mineral grains, especially clay). Adhesion immobilizes water molecules. Even if pores are not fine enough for adhesion, capillary forces can hold water back against gravity drainage.
The total porosity (total pore space) of a rock or sediment formation is therefore divided into effective porosity, consisting of the fraction of pore space that is interconnected, and isolated pore space. Effective porosity is further subdivided into specific retention and specific yield. Specific retention is that volume fraction of water that is held back by adhesion and capillary forces when an aquifer is drained. Specific yield is the percentage of water that is actually available for pumping when sediments or rocks are drained out due to the lowering of the water table near a well.
Peat has by far the highest effective porosity: 90% of its total volume consists of pores, half of which is drainable
(specific yield: ~45%). Clay, silt and fine sand also have relatively high effective porosities. But their specific yields vary widely: The specific yield of clay is only 2% or less, while that of fine sand is over 20%. The highest specific yield is obtained in coarse sands and gravels (20% - 30%), where little water is held back by retention (large pores have only small capillary forces). This makes sand and gravel aquifers good storage reservoirs for groundwater.
In an unconfined aquifer (sometimes also called a water table aquifer), the water pumped from a well is obtained by
lowering the water table and draining the volume of rock or sediment just above the water table. Pumping water from the well creates a lower-than-normal water level surface in the vicinity of the well; this is known as the cone of depression.
The volume of water pumped from the well is equal to the specific yield of the sediments (or rocks) times the volume of the cone of depression (the total volume of sediments that have been drained). As explained above, most of the water contained in an aquifer becomes part of the the specific yield, when the water level is lowered and sediments (or rock fractures) are drained.
In a confined aquifer, water and the sediments or rocks containing the water are under pressure and therefore
slightly compressed. When pumping from a well in a confined aquifer, the water table inside the well is lowered, resulting in lower water pressure in the deep part of the well.
The water available for pumping as a result of lowering the pressure head and thus decompressing water and
sediment structure is, of course, only a tiny portion of the total water contained in the confined aquifer. In
fact, only about one-millionth of the amount of water contained in a cubic foot of aquifer is released from the
aquifer per 1-foot water pressure drop. That tiny amount of water that is packed into the confined aquifer
by pressure is called the specific storage (or storativity) of the aquifer. It is measured in cubic feet of water
released per cubic foot of aquifer for each foot of pressure drop. It typically ranges from 10-6 to 10-5 (1/
ft).
In a confined aquifer, the pressure drop induced by a pumping well occurs simultaneously over the entire
thickness of the aquifer. To compute the amount of water available for pumping from a confined aquifer,
hydrogeologists multiply the specific storage coefficient by the thickness of the aquifer. This number is called
the storage coefficient of the aquifer.
storage coefficient = specific storage x aquifer thickness
Like the specific yield, storage coefficient usually is reported in percent. Typically, it ranges from 0.00005
(0.005%) to 0.005 (0.5%).
To sustain a given pumping rate (e.g., 500 gpm), the water pressure in the confined aquifer has to drop much faster and over a much larger area than the water table in the unconfined aquifer, where water is actually draining from pores. for the same pumping rate, the water level in a well penetrating a confined aquifer will drop much faster and by a much larger amount than the water level in a well penetrating an unconfined aquifer. For the same reason, the water level (pressure head) in a confined aquifer well is much more sensitive to nearby groundwater extraction than a well in an unconfined aquifer.
As a result of continuous pumping, the pressure head in the confined aquifer will drop until the confined aquifer either becomes partially unconfined (at which point actual drainage of the aquifer begins), or until the lowering of the pressure head has created a steep pressure gradient between the aquifer being pumped and an overlying or underlying
source aquifer. The steep pressure gradient across the aquitard that separates the overlying (or underlying) source aquifer from the pumped aquifer accelerates the rate of water flow through the aquitard.
The pressure gradient increases until water flow through the aquitard to the pumping well is sufficient to match the depletion created by the quantity of water removed from the well.