Most groundwater originates from rainfall that has entered the earth. The
drawing shows a typical situation with water saturated soils (overburden
aquifer) over a bedrock aquifer. In the overburden aquifer, water
fills the void space between grains of the soil.
Bedrock aquifers underlie the surface soils (overburden) and overburden
aquifers. In the bedrock aquifers, water occurs in fractures and other
voids in the bedrock. Some types of bedrock such as sandstone may also
have additional voids (intergranular voids) that are filled with
As is the case for surface water, groundwater flows from higher elevations
pressures) toward lower elevations (or lower pressures). Groundwater flow
usually toward a groundwater discharge area, as shown in the illustration.
stream in the drawing represents a typical groundwater discharge area.
Groundwater pressure, rather than elevation, controls the rate and
flow in confined (or artesian) aquifers. Those are aquifers that are
isolated under impervious or poorly pervious strata (aquicludes and
The subterranean realm is underground everywhere and most of us are
totally oblivious to its existance. In order to know what exists below the
surface of the earth, subsurface exploration of one type or another
must be done. The illustration shows one type of subsurface exploration,
During drilling, samples of the earth may be brought to the surface
for us to observe. After enough test drilling has been completed, the
the subsurface can be reconstructed. The borings can then be converted
wells by installing pipe with slots or holes, or a screen section to
permit water to enter. Then the depth to water can be measured and
groundwater samples can be collected for analysis to investigate flow
direction, the composition of the water, and if it contains contaminants.;
There are other methods that may be used to explore the subterranean
realm. They include excavation and mining, cavern entry and exploration,
and indirect methods that may involve geophysical techniques.
Groundwater monitoring is performed in a number of situations with varying
objectives. It involves measuring the physical and/or chemical properties
of groundwater on a periodic basis. Concentrations of contaminants are
frequently monitored to determine if they are increasing, decreasing, or
remaining in the same range. Monitoring is also performed at and in the
vicinity of water supply sources to determine the quality of the water and
trends of indicators of water quality. Groundwater monitoring programs
normally involve an array of monitoring or observation wells or
The drawing indicates a shallow overburden aquifer monitoring well
and some of its basic features. A well of this type makes it possible to
measure the groundwater elevation and permits water sampling to test the
composition of the groundwater.
Groundwater Modeling Groundwater Flow Modeling
Groundwater flow modeling is generally used to define the quantity of
groundwater available or direction of dissolved contaminant migration. It
is also used to define the limits of a capture zone for a contamination
recovery well (or well field), or for delineating a water well protection
area (or recharge area) for a water supply. Groundwater scientists
frequently use analytical means to model these situations using the
classical mathematical formulas to estimate the effect on the groundwater
surface. In order to estimate the long-term yield and water-level drawdown
of a recovery or water supply well (or well field) they often use the
classical formulas for making projections.
Modeling by manual calculations (analytical modeling) can be very
time-consuming and, therefore expensive. Therefore,groundwater scientists
have prepared a number of computer groundwater modeling programs that
allow for a more rapid and efficient assessment of groundwater flow under
conditions that may involve the addition of simulated wells and/or
simulated sources of recharge in an existing flow field. The models often
generate contour maps that illustrate relevant data that are related to
groundwater flow. The computer output is usually plotted as groundwater
elevation (or artesian pressure) maps for further analysis.
Solute Transport Modeling
Groundwater quality reflects substances that are dissolved or suspended in
the water. Suspended material is not transported far in most subsurface
materials, but it is usually filtered out.
In general, groundwater flow is very slow and depends on the permeability
(water transmitting ability) of
the subsurface materials, as wells as the hydraulic gradient (slope of the
water-table or pressure gradient for artesian conditions).
The rate of groundwater flow is usually measured in feet (or meters) per
day or feet (or meters) per year. In some situations where flow is slow it
is measured in inches (or centimeters) per year.
Groundwater usually contains higher concentrations of natural dissolved
materials than surface water. The materials dissolved in the water usually
reflect the composition and solubility of the earth materials (soil or
rock) that the
groundwater is in contact with and time that it has been in the
A number of the activities of man pose threats to water quality Some of
these activities include:
Landfill solid waste disposal
Liquid waste disposal basins
Septic waste infiltration systems
Highway deicing with chemicals (eg. salt)
Gasoline service stations
Petroleum bulk storage facilities
Underground storage tanks
Many industrial activities
Livestock feed lots
Urban stormwater infiltration
The illustration presents an example of how a source of groundwater
contamination can pollute millions of gallons of groundwater in an
underlying aquifer. The "industrial area" is presented as a typical source
of contamination. In the illustration, the groundwater contaminants are
volatile organic chemicals (eg. trichloroethene or TCE) that are used as
solvents or degreasers in various industrial processes. The degree of
contamination is indicated by the concentration of total volatile organics
by contours. Degree of contamination is also illustrated by varying colors
in the aquifer. Those closest to the industrial area have the highest
concentrations, while those at a distance or upgradient have lower
concentrations. The downgradient water supply well is being impacted by
the contamination from the industrial area and may have to be shut down
until the aquifer is remediated (cleaned up).
Groundwater scientists have used a number of different computer models to
evaluate the transport of various dissolved organic and inorganic
compounds in groundwater in a number of geologic situations.>
One frequently encountered modeling situation involves the assessment of
the distribution of hydrocarbon concentrations around a contaminant source
to evaluate various remedial scenarios including removal of the source of
contamination, concentration reductions resulting from consumption by
microbes, effect of groundwater recovery wells, and recharge sources.
The illustration on the right shows a simple grid layout for solute
transport modeling. Such models can be used to predict the time required
for aquifer cleanup or for natural concentration reductions by existing
processes in the subsurface.
The illustration shows a schematic drawing for a treatment system
for removing petroleum (gasoline or oil) components from groundwater.
If groundwater contamination is identified on a site, and if contaminant
concentrations are found above regulatory limits, remedial activities or
feasibility studies must be done just to keep a site in
compliance. Such activities vary with the contaminant, medium
that is contaminated, and surrounding environmental factors.
Common remedial methods include the following:
Excavation and offsite removal.
Excavation and onsite treatment
Groundwater "pump and treat"
Soil vapor extraction Sparging
Passive recovery of non-aqueousphase liquids(NAPL)