I frequently get questions about
groundwater and why water is in the ground - AND - more specifically, how to
find water in the ground.
Groundwater comes from
rain that gets into the soil and then flows downhill underground.
When we drill a well, it is this groundwater that we are tapping into.
I can not explain it nearly as well as the Water Encyclopedia does.
information is excerpted from an article at the
Water Encyclopedia site:
Groundwater is water that occurs below
the Earth's surface at depths where all the pore (open) spaces in
the soil, sediment , or rock are completely filled with water
(see Figure 1).
Figure 1. In the unsaturated zone, pore spaces contain air;
hence, no groundwater can be pumped from this zone. Usable
groundwater occurs in the saturated zone, where pore spaces
are completely filled with water.
FROM THE WATER ENCYCLOPEDIA
A shallow wellor a deep well , originates and
is replenished (recharged) by precipitation.
Groundwater is part of the
hydrologic cycle, originating when part of the precipitation that
falls on the Earth's surface sinks (infiltrates) through the soil
and percolates (seeps) downward to become groundwater. Groundwater
will eventually come back to the surface, discharging to streams,
springs, lakes, or the oceans, to complete the hydrologic cycle.
A well that is drilled
will first pass through a zone called the unsaturated (vadose) zone
where the openings in the soil, sediment, or rock are primarily
filled with air (Figure 1). Water exists here only in transit
downward. The thickness of this zone depends on such factors as
climate, elevation, season of the year, and area-wide groundwater
withdrawals through pumping. In the rainy season of humid areas, the
unsaturated zone may be a fairly thin layer, extending from the land
surface to only a few meters (10 feet or so) below the surface. But
in drier months of the year, the unsaturated zone may extend deeper
as recharge to the aquifer declines and withdrawals increase. In
arid regions, the unsaturated zone may be a thick layer, extending
from the land surface to 300 meters (1,000 feet) or more below it.
Further drilling will
reach a zone called the saturated zone where all of the openings are
filled with water, and where the water is known as groundwater. If
the saturated zone is permeable enough to supply a well with water
under normal hydraulic gradients, this saturated zone is called an
aquifer. Importantly, an aquifer is not an underground river, lake,
or pool. Rather it consists of geologic materials whose open spaces
(pore spaces) are filled with water that moves down a pressure
gradient, and which can be tapped productively by wells. The top of
the saturated zone is called either the water table (if the aquifer
is unconfined) or the potentiometric surface (if the aquifer is
confined): see Figure 2.
To visualize the
zones, imagine a bucket filled with gravel. Ample pore space exists
between the individual pieces of gravel. If water is poured on top
of the gravel, the water will percolate down through the pore spaces
and begin to fill these spaces from the bottom up. The water in pore
spaces at the bottom of the bucket represents groundwater; that is,
all the pore spaces are filled with water. If holes were punched in
the bottom of the bucket, water would flow out. Using this analogy,
the bucket of gravel is like an aquifer: water is stored within in
it and will move through it toward a discharge point—in this case,
the hole in the bucket.
determined by the size of pores and the degree to which they are
interconnected, and hence, the ease by which water can flow through
the material. Highly permeable aquifers, such as those comprised
primarily of coarse sand and gravel, can supply more water than less
permeable aquifers, such as those comprised of silts or clays. In
this example, the pores in sand and gravel are larger than those in
silt and clay, so water moves through sand and gravel more quickly.
In some aquifers, especially in sedimentary bedrock, water occurs in
fractures (cracks) instead of pore spaces in sediments. The yield
from a fractured rock aquifer can vary from less than 1 liter per
minute, or about 0.3 gallons per minute (if the well encounters few
fractures) to large quantities of groundwater—for example, more than
300 liters per minute, or about 100 gallons per minute (if the
fractures are numerous and large).
FROM THE WATER ENCYCLOPEDIA
The water table defines the top
of an unconfined aquifer. Water in a well penetrating an
unconfined aquifer will remain at the elevation of the water
table. Some streams and lakes intercept the water table,
allowing direct groundwater–surface water exchange. The
potentiometric surface reflects the water pressure of a
confined aquifer, and is the level to which water in a well
will naturally rise (i.e., to an elevation above the
confined aquifer it penetrates). WELLS. All wells in
confined aquifers are considered artesian wells. If the
elevation of the potentiometric surface is above than the
elevation of the land surface, groundwater will flow
naturally (without pumping) from the well, known as a
flowing artesian well. RECHARGE. Recharge to
unconfined aquifers occurs over a wide area of the
unsaturated zone, directly above the aquifer. Recharge to
confined aquifers occurs only where there is a pathway
(e.g., a fracture) through the confining layers, or where
the confined aquifer is exposed at the surface and becomes
unconfined. Generally, the recharge area for a confined
aquifer is at a higher elevation than the aquifer itself,
and may be many kilometers from the well.
Aquifer and Well Types.
Aquifers are divided into two types: unconfined and confined. An
unconfined aquifer is often shallow, and the vadose zone above it
primarily contains permeable material. The top of the aquifer is the
water table. The water table moves up and down on a seasonal basis.
It is highest during the wet season owing to higher recharge and
lower pumping rates (e.g., no irrigation), and lowest during the dry
season because of limited recharge and higher use (e.g., a high rate
Confined aquifers may be shallow or deep, and are characterized by
being separated from the surface by low-permeability strata (e.g.,
geologic layers) that confines the groundwater below it. In a
confined aquifer, groundwater is generally under pressure. This
water pressure may vary seasonally, similar to the water table in an
Because groundwater in a confined aquifer is under pressure, it will
rise in a well bore above the level of the aquifer penetrated by
that well. One way to visualize this is to squeeze a milk or juice
pouch that is punctured at the top by a straw. If the straw fits
firmly into the squeezed pouch, the liquid will rise up into the
straw, above its level inside the pouch.
Artesian and flowing artesian wells are typical of wells drilled
into confined aquifers. An artesian well is one in which the
groundwater rises above the level of the penetrated aquifer. The
water in an artesian well will rise to an elevation at which the
pressure of the water in the aquifer is matched by the pressure
reflected by the elevation of the water in the well; this level is
known as the hydrostatic level. If groundwater reaches all the way
to the surface under its own pressure, the well is called a flowing