Home
* Contact * INDEX *
Current Issues * Priority Issues *
Reference Index * Selwyn's
Profile * Your
Comments
Ground
Water Recharge
Groundwater
recharge is the replenishment of an aquifer with water from the land surface. It
is usually expressed as an average rate of millimetres of water per year,
similar to precipitation. Thus, the volume of recharge is the rate times the
land area under consideration, and is typically expressed in millions of cubic
meters per year.
The quantity
of recharge to an aquifer has been considered equivalent to the "safe
yield" or quantity of water that could be removed from an aquifer on a
sustainable basis. It is believed that the "sustainable yield" of an
aquifer is almost always appreciably less than recharge. This is because
sustainable yield must also allow for adequate provision of water to sustain
groundwater-dependent ecosystems.
Nevertheless,
a sustainable yield figure is derived from a recharge determination, and any
sustainable yield study will usually involve the determination of recharge as a
necessary first step. However, recharge is not bore understood, so it is
difficult to estimate aquifer sustainability unless the recharge-related
processes are carefully studied.
What is an
Aquifer?
An aquifer is
a body of saturated rock through which water can easily move. Aquifers must be
both permeable and porous and include such rock types as sandstone,
conglomerate, fractured limestone and unconsolidated sand and gravel.
Fractured
volcanic rocks such as columnar basalts also make good aquifers. The rubble
zones between volcanic flows are generally both porous and permeable and make
excellent aquifers. In order for a bore to be productive, it must be drilled
into an aquifer. Rocks such as granite and schist are generally poor aquifers
because they have a very low porosity. However, if these rocks are highly
fractured, they make good aquifers.
A bore is a
hole drilled into the ground to penetrate an aquifer. Normally such water must
be pumped to the surface. If water is pumped from a bore faster than it is
replenished, the water table is lowered and the bore may go dry. When water is
pumped from a bore, the water table is generally lowered into a cone of
depression at the bore. Groundwater normally flows down the slope of the water
table towards the bore.
Is an Aquifer
an Underground River?
No. Almost all
aquifers are not rivers. Since water moves slowly through pore spaces in an
aquifer's rock or sediment, the only life-forms that could enjoy floating such a
'river' would be bacteria or viruses which are small enough to fit through the
pore spaces. True underground rivers are found only in cavernous rock formations
where the rock surrounding cracks or fractures has been dissolved away to leave
open channels through which water can move very rapidly, like a river.
Ground water
has to squeeze through pore spaces of rock and sediment to move through an
aquifer (the porosity of such aquifers makes them good filters for natural
purification). Because it takes effort to force water through tiny pores, ground
water loses energy as it flows, leading to a decrease in hydraulic head in the
direction of flow. Larger pore spaces usually have higher permeability, produce
less energy loss, and therefore allow water to move more rapidly. For this
reason, ground water can move rapidly over large distances in aquifers whose
pore spaces are large or where porosity arises from interconnected fractures.
Ground water moves very rapidly in fractured rock aquifers with the spread of
contaminants being difficult or impossible to prevent.
What does an
aquifer look like?
Every aquifer
is unique, although some are more generic than others. The boundaries of an
aquifer are usually gradational into other aquifers, so that an aquifer can be
part of an aquifer system. The top of an unconfined aquifer is the water table.
A confined aquifer has at least one aquitard at its top and, if it is stacked
with others, an aquitard at its base.
Figure 1 shows
an example of an aquifer system
How Does an
Aquifer Work?
An aquifer is
filled with moving water and the amount of water in storage in the aquifer can
vary from season to season and year to year. Ground water may flow through an
aquifer at a rate of 50 feet per year or 50 inches per century, depending on the
permeability. But no matter how fast or slow, water will eventually discharge or
leave an aquifer and must be replaced by new water to replenish or recharge the
aquifer. Thus, every aquifer has a recharge zone or zones and a discharge zone
or zones.
Figure 2 is a
simple cartoon showing three different types of aquifers: confined, unconfined,
and perched.
Recharge zones
are typically at higher altitudes but can occur wherever water enters an
aquifer, such as from rain, snowmelt, river and reservoir leakage, or from
irrigation. Discharge zones can occur anywhere; in the diagram, discharge occurs
not only in springs near the stream and in wetlands at low altitude, and also
from bores and high-altitude springs.
The amount of
water in storage in an aquifer is reflected in the elevation of its water table.
If the rate of recharge is less than the natural discharge rate plus bore
production, the water table will decline and the aquifer's storage will
decrease. A perched aquifer's water table is usually highly sensitive to the
amount of seasonal recharge so a perched aquifer typically can go dry in summers
or during drought years.
Why is
Groundwater So Clean?
Aquifers are
natural filters that trap sediment and other particles (like bacteria) and
provide natural purification of the ground water flowing through them.
Like a coffee
filter, the pore spaces in an aquifer's rock or sediment purify ground water of
particulate matter (the 'coffee grounds') but not of dissolved substances (the
'coffee'). Also, like any filter, if the pore sizes are too large, particles
like bacteria can get through. This can be a problem in aquifers in fractured
rock.
Clay particles
and other mineral surfaces in an aquifer also can trap dissolved substances or
at least slow them down so they don't move as fast as water percolating through
the aquifer.
Natural
filtration in soils is very important in recharge areas and in irrigated areas
above unconfined aquifers, where water applied at the surface can percolate
through the soil to the water table. For example, in Figure 1, a protective
layer of silt in the southern valley provides natural protection to the aquifer
from septic systems, pesticide application, and accidental chemical spills.
Despite
natural purification, concentrations of some elements in ground water can be
high in instances where the rocks and minerals of an aquifer contribute high
concentrations of certain elements. In some cases, such as iron staining, health
impacts due to high concentrations of dissolved iron are not a problem as much
as the aesthetic quality of the drinking water supply. In other cases, where
elements such as fluoride, uranium, or arsenic occur naturally in high
concentrations, human health may be affected.
How is an
Aquifer Contaminated?
As shown in
Figure 3, an aquifer can be contaminated by many things we do at and near the
surface of the earth. Contaminants reach the water table by any natural or man-made
pathway along which water can flow from the surface to the aquifer.
In general,
any activity, which creates a pathway that speeds the rate at which water can
move from the surface to the water table, has an impact.
In Figure 3,
wastewater leaking down the casing of a poorly constructed bore bypasses the
natural purification afforded by soil. Excessive addition of fertilizer, and
agrichemicals, coupled with the enhanced recharge from crops, golf courses and
other irrigated land and along road ditches, are common reasons for
contamination arising from non-point sources. Removal of soil in excavations and
mining reduces the purification potential and also enhances recharge; in some
cases, the water table is exposed and becomes directly vulnerable to the entry
of contaminants.
RETURN TO: INDEX
