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Design and Installation of Monitoring Wells. HEDRICK. What are Monitoring Wells and why are they important?. 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

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Presentation Transcript
slide3

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
slide4

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.
correctly designed monitoring wells can allow the collection of
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
slide7

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.
slide8

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.

monitoring well components
Monitoring Well Components
  • Bore Hole
  • Well Casing
  • Well Screen
  • Filter Pack
  • Annular Seal
  • Surface Protection
slide11

Surface Protection

Protective Outer Cap

Annular Seal

WellCasing

Bore Hole

Filter Pack

Water Table ↓

Well Screen

slide12
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
slide16
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
slide18

(4)

External protective cap

(5)

slide20
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.
slide21
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
slide23

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
slide25
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.
slide27
4. Improper well screen length and placement2
  • the retrieval of water quality data from discrete zones is impossible.
slide29
5. Improper selection of filter pack materials2
  • Causes well sedimentation, well screen plugging, chemical alteration of ground-water samples, or well failure.
slide31
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
slide33
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.
slide35
Now That we Know Some Potential Pitfalls, What is the Role of Site Characterization in Monitoring Well Design?
tools and methods for site characterization include
Tools and Methods for Site Characterization Include:
  • Soil and rock sampling
  • Field analytical methods
  • Remote Sensing and geophysical methods
slide37
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
site specific design variables include
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
i know about the site i know where my wells need to go now what
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
monitoring well completion types
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
single casing single screen wells
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
multiple casing single screened wells
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
bedrock completions
Bedrock Completions
  • Cased with Conductor or surface casing
  • Cased
  • Open-bedrock bore holes
slide49
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)

slide50
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)

slide51
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)

monitoring multiple cased vertically separated zones well clusters
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
single casing multiple screen wells
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
slide56

(2)

(2)

nested wells
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
single casing long screen wells
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
multilevel monitoring system
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
slide63

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 depthCan 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)

slide64

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)

monitoring well casing and screen materials
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
slide68
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
requirements of casing and screen materials physical strength
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 tensile strength of the casing itself
slide72
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
slide74
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
what are the main types of well casing and screen materials
What are the main Types of Well Casing and Screen Materials?
  • Thermoplastic materials
      • PVC and ABS
  • Fluoropolymer materials
      • PTFE, TFE, FEP, PFA, and PVDF
  • Metallic materials
      • Carbon steel, low carbon steel, galvanized steel, and stainless steel
  • Fiberglass reinforced materials
      • FRE and FRP
pvc physical characteristics
PVCPhysical 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
pvc chemical characteristics
PVCChemical 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
metallic materials
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
types of corrosion
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
geochemical indicators of corrosion conditions
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
stainless steel
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
string jointing
String Jointing
  • Threaded joints preferable
  • Glued joints can lead to solvation and sample contamination problems
factors affecting casing diameter
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 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
  • Anticipated Well Depth and Casing strength 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
factors affecting casing diameter86
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
primary filter pack
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
slide90

Secondary Filter Pack

Primary Filter Pack

(5)

secondary filter pack
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
how are filter packs installed
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
what is the purpose of well screens
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
basic well screen types
Basic Well Screen Types
  • Slotted Casing
  • Continuous-slot wire-wound
  • Louvered (shutter-type)
  • Bridge-Slot
  • Prepacked
slide102

Internal Screen

External Screen

Prepacked filter pack material in between

(2)

well screen slot size
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
screen slot size for naturally developed wells
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
step one
Step One
  • The Formation sample is dried and massed

(2)

step two
Step Two
  • Sieve the sample of the natural formation of the zone of interest surrounding the well screen

(2)

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

Step Three

(2)

step four
Step Four
  • Plot the data asgrain size vs. cumulative percent of the sample retained
  • The result is a grain size distribution curve

(2)

step 5
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)

step 6
Step 6
  • Determine uniformity coefficient
  • Uniformity coefficient is the ratio of D60/D90

(2)

step 7
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
slide116
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)

slide117
Slot size for sieve analysis rarely matches that of commercially available slot size, so the nearest smaller commercially available slot size is used
step 1
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
step 2
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
step 2a
Step 2a
  • Calculate D30, D60, and D10 of the filter pack
  • For this, you employ the use of multipliers
step 2b
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
step 2c
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
step 3
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)

finis step 4 determine screen slot size
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)

caveat soil piping
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
slide128
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
slide129
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
slide130
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
slide131
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
slide133
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.
slide135
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
slide137
[Information from ASTM (1992), Aller and others (1989), Hardy and others (1989), Driscoll (1986), Gillham and others (1983), and Claassen (1982)]
  • BENTONITE
      • (A hydrous aluminum silicate composed primarily of montmorillonite)2
  • Advantages: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
slide138
NEAT CEMENT (Uses Portland Cement)
    • (Composed of calcium carbonate, alumina, silica, magnesia, ferric oxide, and sulfur trioxide with pH ranges from 10 to 12)
  • Advantages:
    • 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.
methods of annular seal installation
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
methods of annular seal installation143
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
surface seal
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.
slide150
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.
slide152
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.
documentation report
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
end note literature cited
End Note Literature Cited
  • Fetter, C.W. Applied Hydrogeology. Prentice Hall, Fourth Edition, 2001.
  • Nielsen, David M., Editor. Environmental Site Characterization and Ground-Water Monitoring. CRC Press, Taylor

and Francis Group, Second Edition, 2006.

  • 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.

  • 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.