All monitoring wells should be developed to create an effective filter pack around the
well screen, to rectify damage to the formation caused by drilling, to optimize
hydraulic communication between the formation and the well screen, and to assist in
restoring the natural water quality of the aquifer near the well. Development stresses
the formation around the screen, as well as the filter pack, so that mobile fines, silts,
Monitoring Well Design and Construction
32
and clays are pulled into the well and removed. The process of developing a well
creates a graded filter pack around the well screen. Development is also used to
remove any foreign materials (drilling water, muds, etc.) that may have been
introduced into the well borehole during drilling and well installation, and to aid in the
equilibration that will occur between the filter pack, well casing, and the formation
water.
The development of a well is extremely important to ensuring the collection of
representative ground-water samples. If the well has been properly completed, then
adequate development should remove fines that may enter the well either from the
filter pack or the formation. This improves the yield, but more importantly it creates a
monitoring well capable of producing samples of acceptably low turbidity. Turbid
samples from an improperly constructed and developed well may interfere with
subsequent analyses.
When development is initiated, a wide range of grain sizes of the natural material is
drawn into the well, and the well typically produces very turbid water. However, as
pumping continues and the natural materials are drawn into the filter pack, an
effective filter will form through a sorting process. Inducing movement of ground
water into the well (i.e., in one direction) generally results in bridging of the particles.
A means of inducing flow reversal is necessary to break down bridges and produce a
stable filter pack.
The common methods for developing wells are described by Aller et al. (1989) and
Driscoll (1986) and include:
• Pumping and overpumping,
• Backwashing,
• Surging with a surge block,
• Bailing,
• Jetting,
• Airlift pumping, and
• Air Surging.
Aller et al. (1989) provide a detailed overview of well development and should be
consulted when evaluating well development methods. Overall, the most effective
and efficient method available for inducing flow reversal during well development is
the careful use of a properly-constructed surge block. To be effective, the surge
block may need to be lifted and lowered throughout the well screened interval for
several hours, with periodic pumping or bailing of the fines. However, use of a surge
block can result in potential damage to the well screen and filter pack. In lowpermeability
zones, excessive fines may penetrate the filter material. Depending on
the depth of the water, the hydraulic conductivity of the aquifer, and the diameter of
the well, pumping may effectively achieve well development.
Monitoring Well Design and Construction
33
The following is a general procedure for developing a well by surging and pumping of
fines:
1. Record the static water level and total well depth.
2. Set the pump and record the pumping rate. Pump until turbidity reaches the
desired level as measured using a turbidity meter.
3. Discontinue pumping and begin surging using a properly designed surge
block and proper surging technique.
4. Measure and record well depth to determine the amount of fines, and repeat
Step 2. If the well has been properly designed, the amount of pumping
required to achieve the desired turbidity level will be substantially less than
the amount of pumping required during the first pumping cycle.
5. Repeat surging and pumping until the well yields water of acceptable turbidity
at the beginning of a pumping cycle. A good way to ensure that development
is complete is to shut the pump off during the last anticipated pumping cycle,
leaving the pump in place, and re-start it at a later time. The turbidity of the
discharge water should remain low.
Effective and efficient well development is possible only with adequate flow rate
during water withdrawal. Additionally, any fines that have been drawn into the
well should be removed to the greatest degree possible. Therefore, Cal EPA
recommends that one of the following pumping methods, listed in the order of
preference, be used in conjunction with a properly designed surge block:
1. Centrifugal pump capable of removing fines if the water level is within
suction-lift distance.
2. Electric submersible pump capable of pumping fines.
3. Properly designed and operated air-lift system (with Cal EPA approval).
Well development methods and equipment that alter the chemical composition of the
ground water should not be used. Development methods that involve adding water
(including water pumped from the well) or other fluids to the well or borehole, or that
use air to accomplish well development, are discouraged. Consequently, methods
that are unsuitable in most cases for monitoring well development include
backwashing, jetting, airlift pumping, and air surging. Approval should be obtained
from the lead regulatory agency prior to introducing air, water, or other fluids into any
well for the purpose of well development. Any water introduced into the well during
well development should be chemically analyzed to determine its potential impact on
water quality. The well development methods that are generally accepted by Cal
EPA are bailing, surging with a surge block, pumping (and overpumping), or
combinations of these methods. Airlift pumping may be acceptable if the RP can
demonstrate that appropriate measures will be taken for preventing air contact with
the formation, and from preventing the entry of compressor oils into the well.
Monitoring Well Design and Construction
34
Monitoring wells should not be developed before well sealant materials have set or
cured (generally a minimum of two days after its emplacement).
Ground water should be collected and measured for turbidity periodically during well
development and at the completion of well development. The final turbidity
measurement should be recorded on the well construction log. A well that cannot be
developed to the point of producing low turbidity water (<5 NTUs) may be considered
by Cal EPA to have been improperly completed (e.g., mismatched filter pack and
formation materials or filter pack and screen slot size). If a well is not producing low
turbidity ground-water samples (<5 NTUs), the RP should demonstrate to the
satisfaction of the appropriate regulatory agency that proper well completion and
development measures have been employed. Failure to make such a demonstration
could result in a determination by Cal EPA that the well should be decommissioned
and replaced.
Cal EPA emphasizes that proper well construction and development procedures, as
well as proper sampling procedures (e.g., selection of appropriate well purging and
sampling rates), are necessary to yield ground-water samples that are
representative of ambient water quality. Cal EPA recognizes that ground water in
some wells (both high and low yield) in fractured rock or karst aquifers may become
muddy after periods of rainfall, even though during fair weather the water is free of
turbidity. Wells completed in very silty geologic units also may produce consistently
turbid samples. Wells of this type will normally be considered to have been properly
installed and developed, and turbid water samples will be considered representative
of mobile constituents in the aquifer. Information obtained from any aquifer tests
conducted on the well should be used to establish the initial yield of the well, and
these data can be used for periodic redevelopment and maintenance assessments.
If well drilling, installation, or completion have altered ground-water quality chemically
in the vicinity of the well, well development should aid in restoring ground-water
quality within the well to natural ground-water quality. The ability of a well
development method to remove clays from the sides of the borehole should be
considered, because clays retained in the borehole may alter the chemical
composition of ground water in the well. Periodic monitoring of ground water during
well development, for water quality parameters such as specific conductance,
temperature and pH, should be performed. The reproducibility of these field
parameters indicates that ground-water chemistry in the well has been restored to
natural quality. The volume of water withdrawn from a well during development
should be recorded.
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