Transcript Document

Design and Installation of
Monitoring Wells
HEDRICK
What are
Monitoring
Wells and why
are they
important?
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• A Monitoring Well is a well designed to detect, and monitor
through time, trace levels of both inorganic and organic
contaminants in groundwater systems.2
• Monitoring wells are installed to determine the ground-water
quality at localities such as landfills, industrial facilities, service
stations, Superfund sites, waste-water treatment facilities,
mines, petrochemical plants, and areas of suspected or known
ground-water contamination.1,2
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•
Contamination from anthropogenic activities is a common
problem for groundwater. For example, "BTEX" (benzene,
toluene, ethylbenzene and xylene), which comes from
gasoline refining, and MTBE - which is a fuel additive, and
many industrial solvents are common groundwater
contaminants, often the result of leaking subterranean storage
tanks and dumping. 2
•
Cleanup of contaminated groundwater tends to be very costly.
Effective remediation depends on the detection and tracking
of contaminants in the groundwater system.
•
As part of a comprehensive monitoring program, strategically
placed Monitoring wells can help.
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Correctly designed monitoring wells can allow
the collection of:
•
Representative samples from a target monitoring zone to allow detection and
monitoring of contaminant plumes 1,2
• Use of materials that don’t react with target analytes provide representative samples
•
Accurate hydraulic parameter data 2
• Hydraulic conductivity
• Definition of preferential flow pathways
• Calculation of ground-water flow velocity
•
Accurate ground-water level data at a specific location in the ground-water flow
system 2
• Allows for the construction of potentiometric surface contour maps
• Allow for definition of ground-water flow direction in the horizontal plane
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How important is well
design and installation?
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• Thousands of Monitoring wells are installed each year.
• Many of these are designed and installed by contractors not
aware of proper monitoring well design and construction
practices. 2
• As a result, many monitoring wells have design flaws and
were installed using materials and methods that may adversely
affect the integrity and quality of samples retrieved from those
wells.
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• The goal of the water monitoring system is to obtain
groundwater samples that are representative of the
groundwater system, retaining the physical and chemical
properties of the groundwater, and that are minimally affected
by the sample collection process. 2
• Proper ground-water monitoring well design and installation
techniques are necessary to minimize the chemical alteration of
samples.
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What are the Primary
Components of Monitoring Wells?
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Monitoring Well Components
• Bore Hole
• Well Casing
• Well Screen
• Filter Pack
• Annular Seal
• Surface Protection
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Protective Outer Cap
Surface Protection
Annular Seal
Well Casing
Bore Hole
Water Table ↓
Filter Pack
Well Screen
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•
The well casing is a length of solid pipe and can be made of a variety of
materials from PVC pipe to stainless steel, and isolates the well from the
surrounding rock or soil, and is necessarily smaller in diameter than the
borehole 1,2
•
The well screen is used to allow water into the well and filter out the soil and
sediment. It is often a piece of pipe with holes, slots, gauze, or a continuous
wire wrapped around it. The top is usually installed above the water table. It
is preferable that the well screen be professionally manufactured and not
conjured in the field with a knife and makeshift materials. The screen is
attached to the end of the casing by threaded joints. 2
•
The Casings and screens are made from various materials that must be
chosen carefully on the basis of cost, durability, and potential reactivity with
the ground water. Teflon is the most costly, least durable, and most inert.
Stainless steel is the most durable, is moderately costly, and is also
essentially inert. PVC pipe is often used because of low cost. 1
•
Only PVC pipe that is threaded should be used, as PVC pipe joined with
solvents may add organic contaminants to the water. 2
•
Due to high cost and structural weakness, Teflon is inferior to stainless steel
as well casing material. Stainless steel may react negatively with acidic or
saline ground waters. Under such conditions, PVC with threaded joints would
be better. 2
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(2)
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(2)
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(2)
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• The filter pack is often sand, and the grains must be
necessarily larger than the filter screen slots. The filter pack
surrounds the casing inside the borehole, and fills the annular
space up past the well screen in the bore hole. The filter pack
is often capped by a layer of fine sand. 2
• The annular seal is often comprised of a layer of expandable
material such as bentonite pellets, capped by a layer of fine
sand capped by a layer of grout which is pressurized in place. 1
• Capping the annular seal is the surface protection: a layer of
concrete surrounding the casing, which can protrude above
ground level, and a protective cover, which often includes a
lockable cap. The layer of concrete effectively seals the area
between the well and the borehole (the annular space) from
movement of contaminants and pollutants 2
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What does a finished
Monitoring Well look like?
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(4)
External protective cap
(5)
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What Are the Most
Common Errors in Well
Design?
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• A basic requirement for proper and effective
ground-water monitoring well design and
installation is the working use of flexible
guidelines that are adaptable to a range of
chemical and geological environments.
• To develop said guidelines, it is necessary to
identify common problems in well design and
construction.
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1. Use of a well casings or screens that
are not compatible with the
hydrogeological environment or the
known or anticipated contaminants 2
•
Results in chemical alteration of samples or
well failure
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(2)
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2. Use of a well screen that is not
commercially produced. 2
• Well sedimentation or turbidity in collected
samples during the life of the monitoring
program
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(2)
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3. Use of a single well screen and-filter pack
combination for all of the wells at a
particular site, regardless of the geology or
grain size distribution. 2
• Causes sample turbidity, invasion of overlying well
construction materials, and lower than expected well
yields.
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(2)
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4. Improper well screen length and
placement 2
• the retrieval of water quality data from discrete zones is
impossible.
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(2)
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5. Improper selection of filter pack
materials 2
• Causes well sedimentation, well screen plugging,
chemical alteration of ground-water samples, or well
failure.
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(2)
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6. Improper selection and placement of
annular seal materials 2
• Results in alteration of sample chemical
quality, plugging of the filter pack and well
screen, or cross-contamination from geologic
units improperly sealed off
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(2)
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7. Inadequate surface protection
measures 2
• Results in surface water entering the well,
alteration of sample chemical quality, and damage
to or destruction of the well.
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(2)
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Now That we Know Some
Potential Pitfalls, What is the Role
of Site Characterization in
Monitoring Well Design?
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Tools and Methods for Site
Characterization Include:
• Soil and rock sampling
• Field analytical methods
• Remote Sensing and geophysical
methods
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• A thorough site characterization can provide
a detailed knowledge of site specific
geologic, hydrologic, geochemical, and
microbiological conditions to aid in site
specific well design. 2
• Aids in Monitoring well placement, well type, and
determining desired monitoring well depth 2
• Aids in determining drilling method 2
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Site specific design variables
include:
• Objective of the ground-water monitoring project
• Water quality monitoring versus water level monitoring 2
• Surficial conditions such as drainage, topography, seasonal
climate changes, and site access 2
• Geologic setting, flow pathways, degree of heterogeneity,
porosity type, recharge or discharge conditions 2
• Ground-water / surface water interrelationships 2
• Ground water chemistry and microbiology 2
• Site specific contaminants and their chemistry, density,
viscosity, reactivity and concentration 2
• Anthropogenic influences
• Human induced changes in hydraulic conditions 2
• Regulatory requirements 2
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(2)
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I know about the site, I know
Where My Wells Need To Go,
now what?
• Design the type of monitoring well
needed based on the characteristics of
the site and project objectives
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What Are the Types of Monitoring
Well Completions?
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Monitoring Well Completion Types
•
Single-Casing, Single-screen Wells
•
Multiple-casing, Single Screen Wells
•
Bedrock Completions
• Cased
• Cased with Conductor
• Uncased
•
Monitoring Multiple vertically Seperated Zones
• Well Clusters
•
Single-Casing, Multiple Screened Wells
• Nested Wells
•
Single-casing, Long Screen Wells
•
Multilevel Monitoring Systems
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Single-Casing, Single-Screen
Wells
• Simplest, most common type
• This type of well is most useful for
monitoring water table fluctuation or a
single discreet interval, such as a thin sand
seam within a matrix of silt and clay 2
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(2)
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(2)
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Multiple-Casing, Single-Screened
Wells
•
Referred to as Telescoping casing wells
•
Often used where it is necessary to drill through a one or more
contaminated zones to complete a well in a formation below
•
A larger diameter bore hole is drilled to just below the contaminated
zone, terminating at the top of a confining layer where a large
diameter surface or conductor casing is installed and pressure
grouted in place. 2
•
A smaller diameter bore hole is then drilled from the bottom of the
pilot zone down to the zone of interest 2
•
The monitoring well is then completed with the surface casing in tact
to prevent hydraulic communication between the upper zone and
zone of interest 2
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Sand Filter Pack
(2)
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Bedrock Completions
• Cased with Conductor or surface casing
• Cased
• Open-bedrock bore holes
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• Cased with Conductor or surface casing
2
•
Drill a hole through bedrock and complete as in the multiple –casing, single-screened
method
•
Insures the zone of interest is isolated from the penetrated by the borehole, minimizing
interzonal flow
•
Good for monitoring discrete zones beneath confining beds or beneath known contaminated
zones
•
Good for monitoring zones directly beneath overburden
(2)
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• Cased
2
• Drill a hole through bedrock to the zone of interest and complete in the
same manner as a single-casing, single-screened well
• Good for monitoring discrete zones
(2)
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•
Open-bedrock bore holes (Uncased) 2
•
•
•
•
•
•
•
Drill and case a larger borehole down through overburden to competent bedrock
From there drill a smaller borehole down to the zone of interest
Bottom bore hole is not cased, there is no well screen, no filter pack
Impossible to monitor a specific zone
The entire open bore hole interval contributes to the well
Used as a screening tool to monitor thick sequences where only horizontal flow occurs
Not recommended in areas of vertical gradients or where data from a discreet zone is
desired
(2)
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Monitoring Multiple Cased
Vertically Separated Zones
Well Clusters
• Used where the objective is to monitor
several different vertical intervals in the
same location or within the same
formation or in different formations, or
where the goal is to study vertical
differences in hydraulic head or vertical
differences in water quality 2
• May be costly 2
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(2)
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(2)
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Single-casing, Multiple-screen Wells
•
An alternative to well clusters; the objective is to monitor several different vertical
intervals in the same location or within the same formation or in different formations, or
where the goal is to study vertical differences in hydraulic head or vertical differences in
water quality 2
•
Consists of alternating sections of well screen adjacent to zones of interest and well
casing between the zones of interest all within a single bore hole
•
Filter pack sand is installed all around and just above and below each screened interval
and annular seal is installed between the screened and filter packed zones to inhibit
hydraulic movement between zones of interest
•
Mechanical, inflatable packers must be installed in the cased portions of the well to inhibit
movement of water between screened zones
•
To allow discrete hydraulic head measurements and ground-water samples from the
screened zones, the installation of pressure transducers and sampling pumps is required
in these zones. 2
•
It is possibly less costly than well clusters, but the savings are often offset by all the
required in well equipment 2
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(2)
(2)
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Nested Wells
•
A third type of well completion designed to monitor several different vertical
intervals
•
Several, small diameter, single-casing, single-screen wells are installed in a
very large diameter bore hole
•
The screened intervals are filter-packed
•
Areas between screened intervals are separated by annular seal material
•
With this completion, it is often difficult to insure proper construction and
function, but this can be tested by introducing tracers into the wells. 2
•
The costs of this type of completion, including additional costs for drilling the
larger bore hole, are similar to costs for clustered wells, and there are no real
advantages for using this type over the others. 2
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(2)
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(2)
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Single-Casing, Long-Screen Wells
•
Another commonly used alternative to monitoring sveral vertical zones
•
Uses a long screen which spans all of the vertical zones of interest, or the
entire saturated thickness of a formation
•
Designer expectations are ususally that flow through the well will be horizontal
•
And by placing a pump next to a specific zone of the screen will allow
sampling for that part of the screen 2
•
Problems usually arise, which include movement of water within the screen
from zones of higher hydraulic head to zones of lower hydraulic head, and
therefore the screen serves as a vector for groundwater and possible
contaminants 2
•
Thus the water samples collected are not representative of any one zone, but
represent a composite of the water conditions across the entire screen 2
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(2)
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Multilevel Monitoring System
• Installed in a single bore hole to monitor
several vertical zones of interest
• Can be used to better characterize
contaminant plumes 2
• Two methods:
• A) One for sandy soils and sediments
• B) One for Bedrock or cohesive, glacial till type
sediments
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Method A includes a casing device of rigid PVC pipe inside of
which are multiple tubes, each of which ends at a sampling port
of a different depth
Can be used to get a detailed picture of the vertical distribution of
contaminants, but water levels usually can’t be monitored with
this method 2
(1)
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•Method B is similar to Method A except inflatable packers are installed above and
below each sampling port. Each zone to be sampled is isolated by inflating the packers
above and below the sampling ports
2
(2)
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(2)
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Design Components of
Monitoring Wells
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Monitoring Well Casing and
Screen materials
• The purpose of casings is to provide access from
the surface to some zone of interest in subsurface
geologic material 2
• Well casings prevent the collapse of geological
material into the borehole 2
• Well casings help prevent hydraulic communication
between several water-bearing zones penetrated
by the bore hole 2
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• Historically, the selection of casing material for wells
was based on the material’s structural strength 2
• Because it is now possible to analyze ground water
chmeicals in ppb, a premium is now placed on
materials that don’t react with chemicals in the water
and alter the chemical integrity of the collected
sample
• Therefore, the selection of casing materials needs to
be based on: 2
• Physical strength
• Chemical resistance
• Chemical interference potential
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Why Is the Physical Strength of
the Casing Material Important?
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Requirements of Casing and
Screen materials:
Physical Strength
•
Monitoring Well casing and screen materials must be able to withstand all the
forces exerted on them by the surrounding geological materials and the forces
exerted on them during well construction and installation, and maintain
structural integrity for the expected operating life of the well 2
•
The three components of structural integrity are:
•
•
•
•
Tensile strength
Compressive strength
Collapse strength
The tensile strength is the most important factor 2
•
The material must have enough tensile strength to support its own weight while suspended,
from the surface, in an air-filled bore hole.
•
The maximum installation depth of a material can be calculated
by dividing the tensile strength of a material by its linear weight.
•
The joints are the weakest point in a casing string, therefore the
tensile strength of the casing joints is more important than the
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tensile strength of the casing itself
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(2)
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• Compressive strength is the load required to deform the
casing while compressing it longitudinally 2
• Compressional strength critical at higher casing weights 2
• Important factors in determining compressional strength are:
• Yield strength
• Stiffness
• And to a lesser degree the dimensional parameters
• Casing length
• Casing wall thickness
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(2)
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• The collapse strength refers to the ability of the material to
maintain cross sectional integrity, and prevent the casing
walls from caving in, a critical factor, especially at depth 2
• Collapse strength is determined mainly by dimensional
parameters 2
• Collapse strength of a material is proportional to the cube
of its wall thickness 2
• Therefore, even small increases in wall thickness provides
significant increases in collapse strength
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(2)
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What are the main Types of Well
Casing and Screen Materials?
•
Thermoplastic materials
•
•
Fluoropolymer materials
•
•
PTFE, TFE, FEP, PFA, and PVDF
Metallic materials
•
•
PVC and ABS
Carbon steel, low carbon steel, galvanized steel, and stainless steel
Fiberglass reinforced materials
•
FRE and FRP
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PVC
Physical characteristics
•
Good strength, rigidity, and temperature resistance allows PVC to handle loads and
stresses of handling and installation
•
Has complete resistance to electrochemical corrosion, high resistance to abrasion, high
strength to weight ratio,durability, flexibility, workability, low maintenance, and low cost
•
Good chemical resistance except to low molecular weight ketones, aldehydes and
chlorinated solvents
•
In comparison to metallic materials, the tensile, compressive, and collapse strenght of
PVC is relatively low.
•
The specific gravity of PVC is 1.4, not much higher than water.
•
Therefore, the buoyant force for PVC is very high, increasing the maximum string length
for that portion immersed in water by 40% 2
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PVC
Chemical Characteristics
•
PVC is superior in some respects of chemical resistance because it does not
conduct electricity, and is therefore is not affected by electorchemical or
galvanic corrosion 2
•
Is resistant to biological attack, chemical attack by soil,water and other
naturally existing substances in the subsurface 2
•
PVC is susceptible to solvation by certain organic solvents such as THF,
MEK, MIBK, CH, DMF, and acetone 2
•
Chemical interference is low, sorbs and leeches negligibly, RVCM levels
improved markedly since 1976 2
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Metallic Materials
• Stronger, more rigid, and less temperature sensitive
than PVC
• Expensive
• Strength and rigidity characteristics to meet virtually
any subsurface stress, force, or condition
• All steels are subject to corrosion
• Corrosion weakens material over time, and
introduces sampling bias 2
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Types of Corrosion
•
Oxidation “rusting”
•
Selective corrosion or loss of one element of an alloy, creating a
structurally weaker material
•
Bi-metallic corrosion – creation of a galvanic cell where two metals
are in close proximity
•
Pitting corrosion – highly localized loss of metal by pitting or
perforation
•
Stress corrosion – Corrosion in areas of metal under high stress
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Geochemical Indicators of
Corrosion Conditions
•
Low ph (<7), water is acidic, conducive to corrosive conditions
•
Dissolved Oxygen If DO is >2ppm, corrosive conditions exist
•
Presence of Hydrogen Sulfide (as little as 2ppm), can cause corrosion
•
TDS, if greater than 1000ppm, Ec of water can cause electrolytic corrosion
•
If Carbon Dioxide exceeds 50ppm, corrosion can occur
•
If Chloride ion exceeds 500ppm, corrosion can occur
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Stainless Steel
•
Performs well in most corrosive environments, namely under oxidizing
conditions
•
Stainless steel requires exposure to oxygen to develop its highest corrosion
resistance
•
Types 304 and 316, types widely used as casing material, sorb arsenic,
chromium, and lead, while leeching cadmium, so chemical interference in the
collected samples is a possibility 2
•
Resitance to corrosion of both types can be improved with Nitric acid and
Potassium dichromate 2
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String Jointing
• Threaded joints preferable
• Glued joints can lead to solvation and
sample contamination problems
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What Decides Well and Casing
Diameter?
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Factors affecting Casing Diameter
•
The Nominal diameter of most monitoring well bore holes is either 2 or 4
inches. 2
•
Large diameter bore holes are suitable to determine large scale aquifer
characteristics such as transmissivity and storativity 2
•
In situations where high yield measurements are not the objective, small
diameter bore holes work better
•
Small-diameter bore holes are less expensive
•
•
•
Smaller materials are installed
Costs per foot are lower because less costly methods can be used
Quantities of potential contamination are smaller
•
Larger bore holes have more capacity for down-hole tools and equipment and
can handle larger pumps for pumping tests 2
•
A wider variety of well installation methods is available for small bore holes
•
•
•
Anticipated Well Depth and Casing strength
•
•
2
For example, wells of 6 in nominal diameter or less can be installed using a hollow-stem
auger
Only wells of 2in. Diameter or less can be installed using the direct-push methods
2
For shallow wells, the strength characteristics of all diameters of casing materials are
adequate
Deeper wells require larger-diameter, thicker-walled casings to prevent bends, casing
failure, and difficulties during construction procedures
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Factors Affecting Casing Diameter
• Ease of well development 2
• Smaller diameter wells take less time to develop
• However, development in smaller wells may not be as effective or as effiecient
as in larger wells
•
Purge Volume 2
• Volume of the purge increases exponentially
• The volume of water in a 4 in. diameter well is four times that of a 2 in. well
• Increase in volume can increase sample collection time
•
Rate of recovery of the well
• It takes less time for a small-diameter well to recover than a large-diameter one
•
Unit Cost of materials and drilling
• Unit costs of both materials and drilling go up with an increase in diameter
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Centering Casings in the Borehole
Most
Common
Type
(2)
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What Are Filter Packs and What
Are They Made Of?
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Primary Filter Pack
•
Primary Filter Packs are sand or gravel packs packed in the annular
space around and just above the well screen in which the sediment
of the surrounding natural formation is replaced with coarser sand
grains introduced from the surface 2
•
The filter pack stabilizes the area surrounding the casing and well
screen to prevent bore hole and well collapse
•
Filter-pack grain size is designed to permit only the finest grains and
sediments to enter the well screen during well development, resulting
in mostly sediment-free ground water for sampling after well
development.
•
The primary filter pack should consist of as chemically inert material
as possible, like quartz, should not consist of limestone or other
carbonate materials such as shell fragments, and contain no organic
material such as wood or coal. However, filter-pack material of known
chemistry such as glass beads, can be used. 2
•
Should extend from the bottom of the well screen to about 3 ft. above
the top of the well screen in case filter pack settling occurs
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Secondary Filter Pack
Primary Filter Pack
(5)
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Secondary Filter Pack
• The secondary filter pack is a finer grained material than
the primary filter pack 2
• It is placed in the annular space between the primary filter
pack and the annular seal above
• The purpose of the secondary filter pack is to prevent
material used for the annular seal from infiltrating and
clogging the filter pack and affecting water chemistry. 2
• The secondary filter pack should consist of inert material,
similar to that of the primary filter pack. A length of
secondary filter pack of about 1 to 2 ft is recommended 2
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How Are Filter Packs Installed?
• Filter Packs are installed by: 2
• Gravity placement (free fall) in only very shallow wells
• Placement by Tremie Pipe – introduced through a partially
flexible pipe or tube via gravity – the most recommended
method
• Reverse circulation – water and sand mixture are poured into
the annulus. The water passes through the screen filter and
into the well where it is pumped out
• Backwashing – Sand is allowed to free fall down the annulus
while water is poured into the well casing, through the well
screen, and back up the annulus
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(2)
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Well Screens
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What is the purpose of Well
Screens?
• To provide access to a specific portion of the
subsurface materials for sample collection
• To provide designed openings for ground water
to flow through the well
• Provides structural support for the filter pack
• Prevents filter pack material from entering the
well
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Basic Well Screen Types
• Slotted Casing
• Continuous-slot wire-wound
• Louvered (shutter-type)
• Bridge-Slot
• Prepacked
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Slotted – limited open area
(2)
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Continuous-Slot Wire-Wound
More Open Area
(2)
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Louvered (shutter-type)
Open Area limited
(2)
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Bridge-Slot
(2)
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Prepacked
•Very good for fine
grained formations
(2)
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•Internal
Screen
•External
Screen
•Prepacked
filter pack
material in
between
(2)
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Well Screen Slot Size
•
Well Screen slot size and filter pack grain size are normally determined at the
same time
•
Filter pack grain size must be larger than that of the surrounding natural
formation 2
•
•
This allows for higher hydraulic conductivity through the well while minimizing the
entrance of fine grained materials into the well
•
The well can recharge between samplings without clogging with sediment
•
Samples will be low in suspended sediment and low in turbidity
•
Sediment free samples decrease sampling time and minimize the need for sample
filtration
Screen slot size must be smaller than the grain size diameter of the filter pack
material to prevent filter pack material infiltration of the well 2
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How Do I Determine Filter Pack
Grain Size and Well Screen Slot
Size?
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Screen slot size for naturally
developed wells
• Naturally developed wells allow the natural formation to
collapse around the well screen instead of using an artificial
filter pack from the surface
• These wells are useful in areas of coarse materials 2
• Allow for good efficiency for developing the formation and and
removing drilling detritus from the well 2
• Downside is the required time for well development and
removal of fine-grain natural formation sediment 2
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First, Should I use a Naturally
Developed Well or a Filter-Packed Well?
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Determine natural or Filter packed
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Step One
•
The Formation sample is dried and
massed
(2)
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Step Two
• Sieve the sample of the natural formation of the
zone of interest surrounding the well screen
(2)
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Step Three
• Mass the amount of sample retained by each of the sieves
•Beginning with the largest sieve, calculate the cumulative
percent retained for each successive sieve
(2)
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grain size vs.
cumulative percent of the
sample retained
•
Plot the data as
•
The result is a grain size distribution curve
Step Four
(2)
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Step 5
•
Plot the data on specialized graph paper with
U.S. standard sieve numbers
•
Determine the effective size
•
Effective size is the equal to the sieve size that
retains 90% of the formation material
•
This is termed D10
(2)
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Step 6
•
Determine uniformity coefficient
•
Uniformity coefficient is the ratio of
D60/D90
(2)
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Step 7
•
If the effective grain size is grain size is greater than .010 inches
And
•
The uniformity coefficient is greater than 3
A natural well can be developed 2
•
If a natural well can be developed, determine well screen slot size
•
If a natural well can’t be developed, an artificial filter pack must be
installed
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Determine screen slot size for
your naturally developed well
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•
If uniformity coefficient is greater than 6
• Slot size should be that which retains atleast 50% of the formation material (D50)
•
If the uniformity coefficient is greater than 3 but less than 6
• Slot size should be that which retains no less than 60 % of the materials (D40)
•
Where the uniformity coefficient is less than 3
• Slot size should be at 70%, or D30
(2)
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• Slot size for sieve analysis rarely matches
that of commercially available slot size, so the
nearest smaller commercially available slot
size is used
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Filter pack and Well screen slot
size for Filter Pack (artificial) wells
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Step 1
• Perform sieve analysis, using steps one
through 3, as displayed previously, from
the procedure for determining screen slot
size for naturally developed wells
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Step 2
• Plot the data on the specialized graph
paper as shown previously, BUT
• This time you will not be plotting a grain
size distribution curve for the natural
formation, but a filter pack grain size
distribution curve
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Step 2a
• Calculate D30, D60, and D10 of the filter pack
• For this, you employ the use of multipliers
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Step 2b
• To calculate D30, multipliers are used
• If the formation is relatively fine-grained and
well sorted, use a factor of 3
• If the formation is relatively coarse-grained or
poorly sorted, use a factor of 6
• Therefore, thus D30 of the filter pack will
have a grain size 3 to 6 times larger than
D30 of the formation
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Step 2c
• Calculate D60 and D10
• Using an ideal uniformity coefficient of 2.5 (a ratio: D60 /D10),
calculate D60 and D10
• These are calculated by trial and error by using grain sizes
that are close to D30, remember the ratio of the two is 2.5
•
Plot all three data points on the specialized graph paper and connect with a
smooth curve
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Step 3
•
Define the permissible range in grain sizes for the filter pack,
• The permissible range is 8% on either side of the grain size curve
• Draw the boundaries of the filter pack envelope on either side of the filter
pack grain distribution curve
(2)
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Finis! Step 4: Determine screen slot size
•
The screen slot size should be
designed to retain 90-99% of the
materials, equivalent to D10 and D1
respectively
(2)
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Caveat! Soil Piping
• In formations with grains that are finer than fine
to very fine sands, soil piping can occur,
bringing formation soil into the well2
• For these conditions, due to manufacturing
difficulties of screens, installation of Pre
packed screens is recommended2
• Pre packed screens are capable of much smaller
screen slot sizes
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Impact of Depth/Length of Well
Screen
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•
No matter length of the screen, the data collected from a well will generally
represent the average of the conditions that exist over the length of the
screen2
•
Before deciding on the length of the screen, define the objectives the wells
must satisfy
•
Long screened wells can be used to detect the presence of contaminants2
•
Short screened wells can be used to measure absolute concentrations of
contaminants that may be present in a specific zone2
•
The differences in the above two methods could provide profoundly different
data, and prompt very different decisions
•
Short screens are required to accurately measure flow direction or
contaminant distribution2
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•
Workers have found that concentrations of contaminants can vary one to
three orders of magnitude over a vertical distance of a few inches to a few
feet2
•
Contaminant plumes can be forced beneath the water table by hydraulic head
differentials in areas of aquifer recharge2
•
Contaminant plumes can be forced to the surface due to hydraulic head in
areas of aquifer discharge2
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• Wells with long screens cannot provide data of sufficient
quality to define the three dimensional distribution of hydraulic
head or ground water chemistry because of the averaging
effect that occurs in such well screens2
• Because concentrations of contaminants are highly variable at
such small scales, to truly detect the 3 dimensional distribution
and movement of contaminant plumes, multiple wells with
short screens at close intervals, or multilevel monitoring, is
needed. 2
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•
Wells installed to specifically monitor the presence of LNAPLs, well screen
length must be determined by the extent of water table fluctuation. 2
•
The screen needs to be long enough to remain within the water table during
both highs and lows, meaning the well designer will have to take into account
historical water table values in the study area
•
Wells which are used to detect LNAPLs , and in which LNAPLs are found,
should not be used for detection of dissolved-phase concentrations, the
boundary between the dissolved phase and colloidal state is transitional and
the particulates can’t be excluded from the sample2
•
Multilevel monitoring wells with short screens are more suited to detect
DNALPs
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What Are Annular Seals and What
Are They Made Of?
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•
Annular seal consists of a sealing layer of either bentonite or
Neat Cement, overlain by a layer of fine sand, overlain by a layer
of grout
•
Annular seals are installed from above the secondary filter pack
to near land surface
•
This seals the annular space between the casing and borehole
wall.
•
The annular seals prohibit vertical flow of water between aquifers
and prevent cross-contamination of aquifers by contaminants.
•
They also protect against infiltration of water and contaminants
from the surface.
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(2)
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•
A three to five ft plug should be placed above secondary filter pack.
•
The annular seal plug is formed from a hydrated material such as bentonite or
cement that acts as a sealant.
•
The choice of a sealant material must minimize possible effects on the constituents
to be analyzed from the well
•
Penetration of the sealant into the underlying filter pack is necessarily limited to
less than a few inches
•
A layer of fine sand overlies the initial Annular seal layer, sealing the grout from
trickling below
•
The remaining upper part of the annular seal is grouted up to just below the frost
line.
•
•
The grout prevents movement of ground water and surface water within the annular space
between the well casing and borehole wall. It also maintains the structural integrity and
alignment of the well casing.
Grout can be bentonite ore neat cement
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(5)
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•
[Information from ASTM (1992), Aller and others (1989), Hardy and others (1989),
Driscoll (1986), Gillham and others (1983), and Claassen (1982)]
• BENTONITE
•
•
Advantages:2
–
–
–
–
–
•
(A hydrous aluminum silicate composed primarily of montmorillonite) 2
Readily available and inexpensive.
Pellets and granules are easy to use.
Remains plastic and will not crack if it remains saturated.
Expands from 10 to 15 times dry volume when hydrated.
Low hydraulic conductivity (about 1 x 10-7 to 1 x 10-9 centimeters per second)
Disadvantages:2
–
–
–
–
–
–
–
–
Effectiveness of seal difficult to assess.
Complete bond to casing not assured.
Because of rapid hydration, bentonite can stick to walls of annulus and bridge annulus.
May not be an effective seal in unsaturated zone because of desiccation.
Can affect the chemistry of the surrounding ground water by cation exchange of Na, Al, K,
Mg, Ca, Fe, and Mn from the bentonite with other cations in the ground water.
Sets up with a pH between 8.5 and 10.5, which can affect the chemistry of the
surrounding ground water.
Most bentonites contain about 4-6 percent organic matter, which might affect the
concentration of some organic constituents in ground water.
Not suitable for use in arid climates
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• NEAT CEMENT (Uses Portland Cement)
–
•
Advantages:
–
–
•
(Composed of calcium carbonate, alumina, silica, magnesia, ferric oxide, and sulfur trioxide with pH ranges
from 10 to 12)
Readily available and inexpensive.
Can assess continuity of placement using temperature or acoustic-bond logs.
Disadvantages:
–
–
–
–
–
–
–
–
–
Requires mixer, pump, and tremie pipe for placement.
Generally more cleanup required than with bentonite.
Contamination can be introduced to borehole by the pump.
Failure of the grout to form a seal can occur because of premature and/or partial setting of the cement,
insufficient grout column length, voids and/or gaps in the grout column, or excessive shrinkage of the
cement.
Pure cement will shrink during the curing process, resulting in a poor seal between the cement and both
the casing and the borehole wall.
Additives to the cement to compensate for natural shrinkage can cause an increase in pH, dissolved solids,
and temperature of the ground water during the curing process. The increased pH causes precipitation of
calcium and bicarbonate ions from the ground water.
Soluble salts in the cement can be leached by the ground water, thereby increasing the concentrations of
calcium and bicarbonate in the ground water.
Cement may cause unusually high values of pH in ground-water-quality samples.
Heat of hydration during curing can deform or melt thermoplastic casing such as PVC.
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(2)
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(2)
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Methods of Annular Seal
Installation
•
Bentonite
• Before putting Bentonite into the bore hole, the amount of material
needed to fill the space must be calculated
• Can be placed in a bore hole as a dry solid material or as a grout
• Pellets, chips, or granules can be placed dry
• Granular or powdered Bentonite can be mixed with water and pumped
into the annulus
• In shallow wells, chips may be delivered via gravity fall method
• In deep wells, bridging may occur, and bentonite must be tamped to
ascertain that no gaps are present
• To avoid bridging, chips or pellets must be poured at a rate not to exceed
2 or 3 minutes per 50lb. Bag
• It requires 1 to 2 hours for bentonite to hydrate enough to hold back a
column of grout
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(2)
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Methods of Annular Seal
Installation
• Neat Cement
• Must be properly mixed, pumped, and placed
in the bore hole correctly
• Neat cement must not be poured, but
pumped under pressure through a tremie pipe
using a Positive Displacement pump
• Curing time is 48 to 72 hours
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What is Surface Completion?
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Surface Seal
•
The surface seal consists of two parts
•
•
Concrete seal around the base of the well casing
An outer protective cap
•
The surface seal prevents surface runoff from flowing down into the annulus of the
well
•
In situations in which a protective casing around the well is needed, it holds the
protective casing in place.
•
The depth of installation of a surface seal can range from several feet to several
tens of feet below land surface.
•
Local regulatory agencies might specify a minimum depth of installation.
•
A cement surface seal is recommended. Bentonite Desiccates easily
•
Should not extend more than 2 ft. away from casing in colder climates due to frost
heave. Frost heave can crack the surface seal and allow water to enter the bore
hole.
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(5)
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(2)
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(2)
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(2)
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•
An external protective cover should be installed around the well
to prevent unauthorized access to the well and damage. The
protective cover is installed simultaneously as the surface seal
and should extend to just below the frost line
•
One design for protective casing is a steel casing with vented
locking protective cover and vent hole, which permits
condensation to drain out of the annular space between the
protective casing and well casings
•
Coarse sand or pea gravel or both are to be placed in the annular
space between the protective casing and the well to prevent
entry of insects. 2
•
A second design, flush to grade, is a steel casing with a locked
manhole cover enclosing a well that is flush with the land surface.
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(2)
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Well Development
• Well development removes the fine-grained material to improve
the hydraulic efficiency of the well.
• Hydrologic efficiency is achieved when a large fraction of the fine
materials from both the filter pack and aquifer material adjacent
to the borehole no longer clog the pump or well screen.
• There are several different methods of well development which
include
– mechanical surging with bailing or pumping,
– over pumping,
– air lift pumping, and
– jetting.
•
The well development procedure should be slow and is site specific.
Once the pH is stabilized the well can be used for monitoring the site.
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•Installation Summary
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(3)
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Documentation Report
•
Those installing wells should document the details of the well construction
•
A complete report will allow future workers to judge the utility of the well,
•
And to allow workers trying to determine if anomalies in data collected from
the well can be tied to well construction
•
Most states and some local governments require the installation of wells be
thoroughly documented
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(2)
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Finis!
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End Note Literature Cited
1.
Fetter, C.W. Applied Hydrogeology. Prentice Hall, Fourth Edition, 2001.
2.
Nielsen, David M., Editor. Environmental Site Characterization and Ground-Water Monitoring. CRC Press, Taylor
and Francis Group, Second Edition, 2006.
3.
http://ewr.cee.vt.edu/environmental/teach/gwprimer/montwell/montwell.html
Boyd, Timothy S. and Jolly, Robert S. Monitoring Well Construction. Groundwater Pollution Primer, 1996.
4.
http://www.dec.state.ny.us/website/environmentdec/2005a/enforcementinit030405.html
DEC Releases Results of Enforcement Initiative. DEC Newsletter, New York State Department of Environmental
Conservation, April 2005.
5.
http://www.dec.state.ny.us/website/dshm/sldwaste/wellpag2.htm
Groundwater Monitoring Well Design. Solid Waste Management Facility, New York State Department of Environmental
Conservation, 2006.
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