Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 6.2
MONITORING WELL INSTALLATION AND ABANDONMENT

6.2 MONITORING WELL INSTALLATION AND ABANDONMENT

Monitoring wells at investigation sites can serve several purposes including the determination of the potential presence, extent, and movement of contaminant plumes in groundwater, as well as the assessment of aquifer characteristics (for use in groundwater models). Monitoring wells must therefore facilitate hydrologic testing and facilitate the collection of representative groundwater samples. There is no "typical" monitoring well for achieving this objective.

Monitoring well construction and materials are a function of the anticipated nature of the contaminants, groundwater quality, desired sampling depth(s), the aquifer's lithology and its overburden, the borehole diameter, and the drilling procedure. Chemical incompatibility could preclude the use of some materials. Soil grain size may dictate the filter material grain size. A deeper or larger diameter well will need a stronger casing than a shallow or smaller diameter well.

To ensure that samples are representative of groundwater conditions, monitoring well design should reflect anticipated contaminant properties. For example, if the contaminant is a light non-aqueous phase liquid (LNAPL), it will float on top of the water table; in this case, the well screen must be installed across the water table. If the contaminant is a dense non-aqueous phase liquid (DNAPL), it will sink to the bottom of the topmost aquifer where it may pool on a confining layer; in this case, the well screen must provide access to the bottom of the aquifer. If the contaminant is soluble in water, it may impact the entire saturated zone; in this case, multiple screen intervals may be required to assess the vertical extent of the dissolved contaminant plume.

In many cases a mixture of different chemicals will be present, including soluble as well as insoluble chemicals. As a practical matter, therefore, monitoring wells may be of various designs on the same site.

Well design also must be protective of the aquifer that it accesses. The well must not provide a pathway for surface contaminants into the aquifer or for contaminant transport between hydraulically separated water bearing units.

6.2.1 Permanent Monitoring Wells

All permanent groundwater monitoring wells have certain design components in common (USEPA, 1991a). A schematic of a standard groundwater monitoring well is presented in Figure 6-1. The monitoring well design components are:

  • Casing consisting of solid riser, screen intervals, and a top and bottom cap
  • Filter pack(s)
  • Annular seal(s)
  • Well head protection
Figure 6-1

Figure 6-1. Diagram of a Typical Groundwater Monitoring Well
This illustration shows the screen interval across the water table, which is used for the assessment of LNAPLs.
[Source: US Navy, 2007.]

6.2.1.1 Well Casing

In general, the casing consists of a solid well riser and a well screen, which keeps the borehole open and provides access to groundwater for the collection of a water sample. The casing should always include a bottom cap to exclude material from entering the bottom of the well and a top cap to exclude surface material or water from entering the well from the surface.

Well casing materials include steel, polyvinyl chloride (PVC), fiberglass, ABS plastic and polytetrafluoroethylene (PTFE) commonly referred to as Teflon®.

Well diameters range from 1 inch to greater than 12 inches and depend upon the proposed well use. Typical well diameters for environmental investigations are 2 to 4 inches, depending on the objective of the well installation (e.g., site investigation or groundwater remediation) and the proposed monitoring and test equipment. When practical, the casing diameter should not exceed 4 inches to avoid generating large volumes of potentially contaminated soil and groundwater requiring management and disposal during well installation, development and purging activities. Larger diameter wells may be considered for use in free product recovery activities.

Consider the following in the selection of the casing material (USEPA, 1991a; CalEPA, 1995b).

  1. The casing material must not alter the groundwater chemistry by leaching, sorbing or desorbing
  2. It must be strong enough to withstand the forces acting upon it, such as the hydrostatic pressure
  3. It must be resistant to chemical or physical deterioration for the life of the well
Figure 6-2

Figure 6-2. Portion of PVC Well Screen and Flush Threaded Bottom Cap
The filter pack, typically consisting of clean silica sand is installed around the well screen and allows groundwater to flow into the monitoring well for the collection of groundwater samples.


Figure 6-3

Figure 6-3. Hollow Stem Auger with PVC Well Casing.
Hollow stem auger with 2 inch diameter PVC well casing. To form the filter pack surrounding the well screen, clean silica sand is poured down the center of the auger as the auger is slowly withdrawn from the subsurface.

For additional guidance on casing material selection, see the following:

  • Standard Practice for Design and Installation of Ground Water Monitoring Wells, (ASTM, 2004a).
  • Handbook of Suggested Practices for the Design and Installation of Groundwater-Monitoring Wells (USEPA, 1991a).
  • Test Methods for Evaluating Solid Waste, Volume II, Chapter 11. SW-846 (USEPA, 1991c).
  • Monitoring Well Design and Construction for Hydrogeologic Characterization, Guidance Manual for Ground Water Investigations (CalEPA, 1995b).
  • The Chemical Composition of Leachate from a Two-Week Dwell-Time Study of PVC Well Casing and Three-Week Dwell-Time Study of Fiberglass Reinforced Epoxy Well Casing (Cowgill, 1988).
  • Sorption of Aromatic Hydrocarbons by Materials Used in Construction of Ground-Water Sampling Wells (Gillham et. al., 1990).
  • Adsorption of Selected Organic Contaminants onto Possible Well Casing Materials (Jones et. al., 1988).
  • Evaluation of Four Well Casing Materials for Monitoring Selected Trace Level Organics in Ground Water (Parker et. al., 1989).

In general, the well riser and well screen should be of the same material. Various mechanisms are used to join the casing sections. The recommended method uses flush threaded joints, which ease the installation process and the later use of downhole equipment by eliminating ridges on the outside and inside of the casing. An example of a well screen with a flush-threaded bottom cap is illustrated in Figure 6-2.

Joining methods that use solvent welding are not acceptable (USEPA, 1991a), since the practice may introduce solvents into the well. The United States Environmental Protection Agency (USEPA) also excludes the use of welding for stainless steel casings (USEPA, 1991a).

Select the screen for a large percentage of open area, non-clogging slots, resistance to corrosion, and sufficient structural strength (USEPA, 1995c). Select the screen slot size such that it will retain 90 to 100 percent of the filter pack material (USEPA, 1991a). For typical investigations of shallow groundwater in Hawai`i, a 2-inch diameter PVC casing with a screen slot size of 0.020 inches combined with a filter pack constructed of clean silica sand is typically employed.

The maximum recommended screen length is typically 10 feet. The screen length should be kept as short as possible to accomplish the required task. For initial investigations or sites where LNAPLs are present, typical shallow monitoring wells are constructed with 7 feet of screen interval below the water table (i.e., the saturated interval) and 3 feet of screen interval above the water table. This allows for the assessment of groundwater level fluctuations as well as the accumulation of free product on the water table surface.

The casing at the bottom of the well screen must be closed against soil infiltration from the bottom by an attached cap. The bottom of the casing, i.e. the cap, must not be more than 0.5 foot below the bottom of the lowest screen. A solid riser below the screen may lead to stagnant environments, which may alter the groundwater chemistry (USACE, 1998).

The total borehole depth should not be more than 1 meter below the planned bottom of the monitoring well in order to avoid preferential vertical groundwater flow originating from below the well screen. If a borehole extends greater than 1 meter below the planned bottom of the monitoring well, backfill the lower portion of the borehole with a chemically inert, low permeability material such as grout. In highly permeable aquifers, where vertical flow is not a concern, the borehole may be backfilled with chemically inert sand up to the design depth.

The well casing must be free of foreign matter (labels, soil, grease, etc.). Utilize a new and factory cleaned well casing when possible, or wash the casing prior to installation. Washing is not necessary if the casing is used directly out of its intact manufacturer's packaging.

Whenever possible, install the well screen and casing within the center of the drill casing (in most cases, the hollow stem auger). Following placement of the well screen and casing, install the filter pack and annular seal while slowly removing the drill casing from the subsurface. Exercise caution to prevent borehole collapse during installation of the filter pack and annular seal. Figure 6-3 illustrates a 2 inch diameter PVC well casing in place in the center of a hollow stem auger drill rod and the placement of a filter sand pack.

6.2.1.2 Filter Pack

The purpose of the filter pack is to prevent particle infiltration from the surrounding formation into the well. The filter pack is installed in the annular space surrounding the well casing. In most geologic settings it is necessary to install an artificial filter pack. The use of natural formation material as the filter pack only is not acceptable.

The filter pack must be a chemically inert granular material with a grain-size larger than the screen slots but smaller than the particles in the surrounding formation. Due to its chemical inertness and its abundant natural occurrence, the use of well-rounded, well-sorted silica sand is encouraged. For permanent wells, there should be at least 2 inches clearance around the screen (but no more than 8 inches) to allow for addition of the filter pack and to help prevent bridging during filter pack installation. For example, for 2 inch and 4 inch diameter monitoring well screens/casings, the borehole or auger used should have an inside diameter of at least 6 and 8 inches, respectively (USEPA, 1991a). Placing the filter pack slowly down the borehole will minimize bridging and surging of the well.

The use of well centralizers may be considered for wells deeper than 20 feet. When used, they should be of PVC, PTFE, or stainless steel and attached to the casing at regular intervals by means of stainless steel fasteners or strapping. Centralizers should not be attached to any portion of the well screen.

The filter pack should extend from below the bottom of the well screen to 2 to 5 feet above the well screen, if feasible. In areas of shallow groundwater, a site-specific plan is recommended before installation of the wells begins. The filter pack above the screen interval buffers any settling during well development and maintains the separation of the annular seal from the screen. In deep wells the filter material may not compress when initially installed and subsequent settling may be significant. Therefore, the deeper the well the higher the filter pack must be installed above the top of the screen. In wells more than 200 feet deep, the filter pack must be installed to 5 feet above the screen. Care must be taken to ensure that filter material does not extend across a confining layer into an overlying aquifer.

To prevent intrusion of annular sealant material into the top of the filter pack, consider installing a layer of fine sand between the primary filter pack and the annular seal (CalEPA, 1995b). The secondary filter pack should be at least 6 inches thick, but should not exceed 2 feet.

Calculate and record the volume of filter pack material expected based on the construction design. Also record the actual volume used during well construction. Explain any discrepancy between the calculated and actual volume.

Well development can be performed in two stages: predevelopment and development (see Subsection 6.2.1.7). Predevelopment takes place upon installation of the well casing and filter pack, but prior to placement of the annular seal. Predevelopment may be necessary in aquifers of low permeability. Clean water may be circulated down the well through the screen into the filter pack and out of the top of the borehole to aid in removal of the fines (USACE, 1998). All water introduced into the borehole must be removed. It is therefore important to record the water volume injected and the water volume removed from the well. Do not use this technique if the borehole bridges water bearing units that are not hydraulically connected, since it may induce cross contamination.

6.2.1.3 Annular Seal

The annular seal extends from the top of the filter pack to the top of the well and generally consists of bentonite (pellets, chips, or powder) or cement, or bentonite/cement mixtures (USEPA, 1991a). Bentonite is a naturally occurring clay mineral that expands upon hydration. Neat cement is a mixture of Portland cement (Type I for general use) and water, at a ratio of 5 to 6 gallons of water per 94-pound bag of cement. The grout must have a minimum design strength of 2,500 pounds per square inch.

The annular seal must prevent vertical migration of liquids into the borehole annular space. The annular seal should:

  1. Provide a lasting seal between the borehole and casing that is less permeable than the surrounding formation. The sealant permeability is typically at least one to two orders of magnitude lower than the formation permeability.
  2. Be chemically inert to resist chemical deterioration.
  3. Resist physical deterioration.

As shown in Figure 6-1, the annular seal consists of a bentonite seal immediately above the filter pack. Above the bentonite seal, a bentonite/cement grout is used as the annular seal to the approximate ground surface.

The annular seal material and placement is dependent upon whether the screen interval is below the water table (fully saturated screen interval) or extends across the water table.

For a fully saturated screen interval, a 3 to 5 foot thick bentonite seal, if feasible, is installed above the filter pack. In areas of shallow groundwater, a site-specific plan is recommended before installation of the wells begins. Pelletized bentonite is preferred for this application. The thickness of the bentonite is measured prior to hydration. The pelletized bentonite must be allowed to completely hydrate in conformance with the manufacturer's instructions prior to filling the remainder of the annular space with a bentonite/cement grout, as described below.

For a fully saturated screen interval, a bentonite seal can be installed through the use of a slurry mixed from powdered bentonite and clean water. It must contain a minimum of 20 percent solids by weight and have a density of 9.4 pounds per gallon or greater. The slurry should have a batter-like, high viscosity consistency. The project plan should provide and discuss the design and installation details for slurry seals since the bentonite/cement grout installed above the slurry seal is usually denser and may intrude into the slurry.

For screen intervals extending across the water table, a 3 to 5 foot thick bentonite seal is also installed above the filter pack. Bentonite pellets and tablets are preferred since bentonite chips have higher moisture content and are slower to swell (USACE, 1998). The bentonite should be installed in 6 to 12 inch lifts and should be allowed to hydrate for approximately 30 minutes prior to installation of the next lift. The bentonite must be allowed to completely hydrate in conformance with the manufacturer's instructions prior to filling the remainder of the annular space with a bentonite/cement grout, as described in Subsection 6.2.5.3. The use of slurry seals when the filter pack is above the saturated zone is not recommended due to the potential for infiltration and clogging of the filter pack.

Following installation of the bentonite seal, the remainder of the annular space is filled with a bentonite/cement grout. The grout must be placed from the bottom up using a tremie pipe equipped with a side discharge.

When installing the bentonite seal and the bentonite/cement grout, calculate and record the volume of bentonite and bentonite/cement grout expected based on the drilling log. Also record the actual volume used during well construction and explain any discrepancy between the calculated and actual volume.

Figure 6-4

Figure 6-4. Installation of Well Head Protection
Well head consists of flush-mounted well vault sealed with cement above the annular seal.


Figure 6-5

Figure 6-5. Traffic Rated Flush Mounted Well Head Box


Figure 6-6

Figure 6-6. Above Ground Well Head Box with Steel Bollard Guards

6.2.1.4 Well Head Protection

Protect the top of the permanent monitoring well casing with a well head box. The purpose of the well head box is to prevent contaminant infiltration into the subsurface and vandalism or damage of the monitoring well. The well head box must allow access to the well with all anticipated measuring devices.

When setting well head boxes, they must be anchored in the surface seal placed above the bentonite/cement grout. A flush mounted box must be set slightly above the surrounding elevation to prevent pooling of surface water on the well. Provide protection from infiltration into the annular space by installing a surface seal made of a water-tight material such as neat cement or concrete. Install the surface seal on top of the annular grout seal so that it extends down to at least one foot below ground surface. The surface seal must form an apron at ground surface that is at least 2 feet wide and 4 inches thick. The concrete apron must slope away from the well (a minimum of 1 percent) to prevent surface water leakage into the well head.

A flush mounted well head box during installation is illustrated in Figure 6-4. A traffic rated flush-mounted well head box installed in a parking lot is illustrated in Figure 6-5. The top of the well casing should be closed with a water tight cap, such as a compression cap.

Protect the well head from unauthorized access by using well head boxes and well caps equipped with locking or tamper proof mechanisms. As needed, install concrete or steel bollards to prevent accidental damage to the well head projecting from the ground surface. Install bollards outside the surface seal apron. An installation featuring guards is illustrated in Figure 6-6. Bumper guards must not be installed in the protective apron to avoid cracking of the surface seal, should a vehicle accidentally hit the guard. In instances where wells are installed in areas where heavy machinery is operated, wellhead protection may require enhanced construction such as well boxes specifically designed to withstand heavy vehicle traffic and reinforcing the concrete.

6.2.1.5 Well Installation Log

Throughout the drilling and well installation process, record detailed observations. It is essential that all relevant data be recorded during the field activities, so that detailed well logs may be generated for inclusion in the final report. Prepare a well installation log for each monitoring well. An example well installation log is illustrated in Figure 6-7.

At a minimum, the log will include:

  1. Project name and location
  2. Well designation and location relative to contaminant source
  3. Date and time of well installation start and completion
  4. Environmental consulting company and on-site consultant
  5. Drilling company
  6. Drilling method
  7. Volume of drill fluid and/or pre-development fluid lost into well during installation
  8. A graphical depiction of the well
  9. Casing material type, diameter, joint type, and screen slot size
  10. Filter pack material, calculated and actual volume
  11. Annular seal material, calculated and actual volume
  12. Annular grout seal material, calculated and actual volume
  13. Placement method for filter pack, seal and grout (tremie, pumped, gravity)
  14. Borehole diameter
  15. Depth to bottom of borehole
  16. Depth to bottom of casing
  17. Depth to bottom and top of screen interval
  18. Depth to bottom and top of solid riser
  19. Depth to bottom and top of filter pack
  20. Depth to bottom and top of annular bentonite seal
  21. Depth to bottom and top of annular grout
  22. Depth to bottom of surface seal
  23. Depth to the water table
  24. Surface seal and well apron design
  25. Protective box/casing and cap designs
  26. Ground surface elevation
  27. Top of casing elevation
Figure 6-7

Figure 6-7. Example Monitoring Well Installation Log

6.2.1.6 Well Survey

All wells being used to assess the hydraulic gradient and the groundwater flow direction must be surveyed by a licensed professional surveyor. Record the well locations in the Hawai`i State Plane Coordinate System. Determine the top of casing elevations within ±0.01 foot. Reference the elevations to an established National Geodetic Vertical Datum. Resurvey the top of well casing if the protective cover and or well casing sustain damage.

The surveyor must mark the surveyed reference point on the top of the casing. Reference all downhole measurements to the surveyed reference point. Using a reference point on other parts of the well head is not acceptable, since other parts of the well head are exposed and therefore are more readily disturbed.

6.2.1.7 Well Development

The purpose of well development is to restore the aquifer's hydraulic conductivity following drilling and monitoring well installation. Well development removes drill cuttings, mud, and mobile particulates and gases from the well, the filter pack and the adjacent formation. Development must extract a sufficient water volume from the well casing, filter pack, and adjacent formation such that the resulting inflow is representative of the groundwater flow in the surrounding aquifer.

Final well development proceeds after well installation is completed and the grout has had time to cure. Initiate well development no sooner than 48 hours and no later than 7 days after well completion.

Acceptable well development methods are mechanical surging, pumping, backwashing, bailing, and high velocity hydraulic jetting. The use of air for well development is not acceptable (CalEPA, 1995b; USACE, 1998).

During well development by pumping, water is extracted from the well at high rates, dislodging and removing loose material in the process.

During well development by surging, a surge block is moved up and down in the well similar to a piston in a cylinder. The surge block is usually attached to a drill rod or stem and operated by a drill rig. The movement of the surge block pushes and pulls water through the well screen, dislodging fine particulates from the screen, filter pack and adjacent formation. Surging is alternated with groundwater pumping to remove groundwater and sediment accumulated in the well during surging. Use of a vented, rather than un-vented, surge block is recommended to minimize the volume of water in the well that is forced into the formation on the down stroke of the surge block.

During well development by backwashing, water is pumped through the well into the filter pack and formation. The backwashing dislodges particulates stuck in the well screen and filter pack. Backwashing is alternated with groundwater pumping to remove groundwater and the sediment suspended in the groundwater due to well development. A combination of surging, backwashing, and pumping may be used to develop a well in the presence of fine particulate materials.

During well development by bailing, a bailer is used in a similar manner as a surge block to agitate the well water and dislodge particulates in the filter pack and well screen. After surging, water is removed from the well by withdrawing the full bailer to the surface. When surging with a bailer, a surging period of 10 to 20 minutes is recommended prior to removing water from the well.

During well development by high velocity hydraulic jetting, water is jetted through the well screen from several horizontal jets. The water jet dislodges particulates from the well screen, filter pack and adjacent formation. Hydraulic jetting is alternated with groundwater pumping to remove groundwater and sediment accumulated in the well by jetting.

When using water during drilling, pre-development or hydraulic jetting, at least three times the volume of water added should be removed during well development.

The following criteria are typically achieved during well development (USEPA, 2002b):

  1. Removal of at least three times the calculated volume of standing water in the well. The calculated volume of standing water should include the saturated filter pack (assuming 30 percent porosity of the filter pack).
  2. The well water pH stabilizes to within plus or minus (±) 0.1 pH units for three successive readings. Readings are separated by the removal of one well volume of water.
  3. Well water temperature stabilizes to within ±1 degree Celsius.
  4. Well water conductivity stabilizes to within ±3 percent.
  5. Well water oxidation-reduction potential stabilizes to within ±10 millivolts.
  6. Well water dissolved oxygen concentration stabilizes to within ±0.3 milligrams per liter.
  7. The well water is clear to the unaided eye, in areas where the local groundwater is known to be clear and the turbidity readings are below 10 nephelometric turbidity units (NTUs).
  8. Turbidity stabilizes to within ±10 percent at concentrations larger than 10 NTU. In areas of known turbid groundwater, the final well water may be turbid to the eye.
  9. The sediment thickness in the well is less than 1 percent of the well screen length or less than 0.1 foot for wells with screens less than 10 feet long.

Use the methods listed in Table 6-1 to determine the water quality parameters. For the specific methods refer to the USEPA document entitled "Methods for Chemical Analysis of Water and Wastes" (USEPA, 1983) and the ASTM standards identified in Table 6-1.

Table 6-1
Test Methods for Water Quality Parameters
Water Quality Parameter EPA Method ASTM Standard
pH 0150.1 D1293, D5464
Temperature 0170.1  
Conductivity 0120.1 D1125
Turbidity 0180.1 D1889
Oxidation Reduction Potential   D1498
Dissolved Oxygen   D888, D5462

If the well recharge rate is so slow that (1) the required water volume cannot be removed within 48 hours of development, (2) excessive sediment remains in the well after development, or (3) high turbidity persists, then the Hawai`i Department of Health (HDOH) Hazard Evaluation and Emergency Response Office (HEER Office) should be contacted for consultation.

During all purging and sampling activities, prevent potentially contaminated water from spilling onto the ground surface surrounding the well. All water and sediment extracted during well development must be placed in containers conforming to requirements of the United States Department of Transportation. The containers must be properly labeled and stored on site pending disposal. The label should include at a minimum: project name, project location, date, container contents, emergency contact name and phone number.

6.2.1.8 Well Development Log

Present a well development log for each monitoring well. ASTM Standard D5521 (ASTM, 2005a) presents additional guidance on groundwater well development. An example well development log is illustrated on Figure 6-8.

At a minimum, the log should include the following information:

  1. Project name and location
  2. Well designation and location
  3. Well construction including total depth of well and screen length
  4. Date and time of well completion
  5. Date and time of well development
  6. Well development technique
  7. Average pumping or water extraction rate
  8. Estimated recharge rate
  9. Static water level from top of casing before development
  10. Sediment level from top of casing prior to development
  11. Static water level from top of casing- 24 hours after development
  12. Sediment level from top of casing- 24 hours after development
  13. Volume of liquids lost into well during drilling, predevelopment and development
  14. Volume of water standing in the well casing and saturated filter pack, assuming a 30 percent filter pack porosity
  15. A running log of:
    1. Time
    2. Water volume removed, both incremental and total
    3. Field measurement of pH, temperature, conductivity, oxidation- reduction potential, and turbidity
    4. Field measurements of dissolved oxygen
    5. Visual and olfactory observations such as color, clarity, passive odor observations, particulates, etc.
  16. Total volume of water removed
  17. Total time needed for development
  18. Investigation derived waste (IDW) inventory including type and number of IDW containers, location of IDW storage

6.2.1.9 Timeline for Drilling, Well Installation, Development, and Sampling

Commence monitoring well installation within 12 hours of borehole completion for holes that are uncased or only partially cased with temporary drill casing. For holes that are fully cased with temporary drill casings, commence installation within 48 hours of borehole completion. Once well installation is begun, proceed without interruption until the monitoring well casing is installed and grouted and the drill casing removed (USACE, 1998).

Commence well development no sooner than 24 hours and no later than 7 days after well completion.

The HEER Office recommends that groundwater gauging, purging and sampling be conducted no sooner than 14 days after well development. The 14-day hiatus is a "rule-of-thumb" (USACE, 1998). Guidance from other agencies for the time interval between monitoring well completion and groundwater sample collection ranges from 24 hours (US Navy, 2007) to 48 hours (SC DHEC, 2005) to several weeks (Puls and Barcelona, 1989). Generally, high permeability formations require less time (e.g. 24 to 48 hours) to equilibrate than low permeability formations (e.g. days to weeks).

If a different time frame from 14 days (e.g. 24 to 48 hours) is proposed based upon known site conditions, overall project considerations, or other pertinent information, the interval, rationale, and evidence supporting the proposal must be noted in detail in the SAP or by consultation with the HEER Office.

Figure 6-8

Figure 6-8. Example Well Development Log

Figure 6-9

Figure 6-9. Diagram of Pre-Pack Monitoring Well Assembly
Assembly during installation (left) and following retraction of drive casing (right).
[Source: GeoInsight Online, 2008.]

6.2.2 Temporary Monitoring Wells

Temporary groundwater monitoring wells or sampling points are generally installed in boreholes driven by direct push technology (DPT). After completion of the borehole, a well casing consisting of a solid riser, well screen, and bottom cap is inserted into the borehole. A filter pack may or may not be installed. Hydraulic connectivity to the surrounding formation is then established by purging the monitoring well according to the procedures outlined in Subsection 6.4.

The use of temporary groundwater monitoring wells is not as rigorous as the construction and development of permanent groundwater monitoring wells, and the samples collected may not be representative of the aquifer. However, the information collected from temporary monitoring wells may be useful in screening for potential contaminants of concern or for use in guiding the final placement of permanent monitoring wells during future investigation phases. In general, the use of temporary monitoring wells may present a lower cost alternative to the initial characterization or delineation of potential groundwater impacts.

While the use of temporary monitoring wells may be advantageous in some instances, they are not suitable for long-term monitoring of groundwater or for final decision making purposes. The intended use of data collected from temporary monitoring wells should be described in the SAP and discussed with the HEER Office.

6.2.3 Other Wells

Other wells used for groundwater sampling include micro wells, existing production wells and potable wells. The following sections provide guidance for the use and sampling of these wells.

6.2.3.1 Micro Wells

Microwells are also referred to as small diameter monitoring wells. Microwells are generally installed in boreholes driven by DPT and are typically less than 2-inches in diameter. Microwells installed by the direct push method must be constructed using a pre-packed well screen. An example of a pre-packed well assembly designed to be installed via direct push equipment is illustrated in Figure 6-9. A sand grout barrier, installed directly above the pre-packed well screen, prevents grout from entering the screens.

For a microwell, the sand grout barrier shall:

  1. Be placed directly above the pre-packed well screen in the annulus between the well casing (riser pipe) and the borehole wall as the probe rods are retracted.
  2. Extend not more than 2 feet above the top of the pre-packed well screen.

An annular seal extending from the filter pack to the top of the well should be installed in general conformance with Subsection 6.2.1.3. The annular seal must prevent vertical migration of liquids into the borehole annular space. Installation requirements presented in Subsections 6.2.1.4 through 6.2.1.9 apply to the construction of microwells.

6.2.3.2 Production/ Potable Well Sampling

Collection of water samples for analysis from wells other than groundwater monitoring wells should be approved by the HEER Office. The request should be accompanied by the following information:

  1. Well installation date
  2. Well construction logs including casing material, well depth, depth of bottom and top of well screen, filter pack material and depth, seal material and depth
  3. Elevation of screen interval
  4. Depth to groundwater table and bottom of unconfined aquifer
  5. Depth to top and bottom of confined aquifer(s)
  6. Contaminants of concern
  7. Characteristics of contaminants of concern (LNAPL, DNAPL, solubility etc.)
  8. The depth at which contaminants are expected to occur. The depth estimate should be supported by existing site-specific data
  9. A description of how the sampling data will be used

Collect raw water samples from a supply well as close to the well head as possible (before any treatment). Purge the well long enough to obtain a representative sample of groundwater with a minimal residence time in the collection/distribution system. The purge volume may be substantial for large diameter wells with long screen intervals.

6.2.4 Special Considerations for Monitoring Wells

Consider special designs for:

  1. Monitoring wells that are intended to permanently hold downhole equipment.
  2. Monitoring wells where free-phase LNAPL or DNAPL plumes are anticipated.
  3. Monitoring wells that are installed in fractured bedrock.
  4. Monitoring wells that are installed in semi-confined caprock.

6.2.4.1 Monitoring Wells with Permanent Downhole Equipment

Dedicated sampling equipment that will reside within a groundwater monitoring well must not alter the chemistry of the groundwater and must be resistant to chemical or physical deterioration. Inspect the equipment periodically for damage, deterioration, and proper operation. The equipment must not interfere with aquifer tests, well maintenance, and water level measurements (CalEPA, 1995b).

6.2.4.2 Monitoring Wells at Sites with LNAPL Plumes

If free phase LNAPL plumes are present or suspected to be present, design and install wells to provide access to the part of the aquifer where the free-phase plumes reside. For LNAPL plumes, provide sampling access across the water table and capillary fringe of the uppermost water bearing unit. If the aquifer in question is tidally influenced, free phase liquid may be trapped in pore spaces below the capillary fringe when the water table is at its highest and above the capillary fringe when the water table is at its lowest.

In addition, provide access to the part of the aquifer where dissolved phase contaminants are expected. To delineate the dissolved contaminant plume vertically, screen wells below the depth of the LNAPL plume. In a tidally influenced aquifer, the uppermost sampling interval for dissolved contaminants must be below the free phase plume smear zone. In addition, install wells cross, up and down gradient to delineate the horizontal extent of the free phase and dissolved contaminant plumes.

6.2.4.3 Monitoring Wells at Sites with DNAPL Plumes

Most DNAPLs that are commonly found in soil and groundwater contamination fall into four groups (USEPA, 2004c):

  1. Chlorinated solvents used in metal finishing, semiconductor manufacturing, dry cleaning, chemical manufacturing, and equipment maintenance
  2. Creosote mixtures used in treating wood products
  3. Polychlorinated biphenyls (PCBs) used primarily in electrical transformers and condensers
  4. Byproducts (e.g., coal tars and oils) from manufactured gas plants (MGP)

The tendency of DNAPLs to move independently of groundwater flow makes it difficult to delineate and remediate free phase DNAPL plumes. Most DNAPLs are slightly soluble or contain components that are soluble. Some of these soluble ingredients are deemed to be a great health risk and environmental risk, and clean up goals are set at low dissolved concentrations. If a free phase DNAPL plume resides in the saturated zone, it becomes a constant source for dissolved contaminants. Therefore, it is important to locate and remediate the free phase plume.

Most DNAPLs are relatively immiscible in water and tend to remain in a separate non-aqueous phase due to their low solubility. If their density is sufficiently high compared to water density and they are present in a great enough free phase volume, they will sink through the saturated zone and pool on the uppermost confining unit. DNAPLs with a high enough density, therefore, migrate vertically rather than following groundwater movement. In addition, they may migrate according to the slope of the uppermost confining unit, which may differ from the regional groundwater flow direction. The migration of DNAPLs with a density (specific gravity) closer to 1 will be influenced by groundwater movement to a greater degree.

The USEPA has published a guidance document that helps in the selection of delineation techniques at DNAPL contaminated sites. The document is entitled: "Site Characterization Technologies for DNAPL Investigations." Techniques entail geophysical as well as non-geophysical methods for characterization. The following DNAPL properties help in selecting the appropriate delineation techniques (USEPA, 2004c):

  • As a chemical class, DNAPLs are electrically resistive (non-conductive)
  • Chlorinated solvents are generally volatile and may be found in soil gas plumes
  • The dissolved phase of chlorinated solvents is relatively mobile and sufficiently soluble to be readily detectable
  • Most PCBs are not volatile and are not sufficiently soluble to be readily detectable in groundwater. The lighter end PCBs do have some solubility [3 milligram per liter (mg/L) range] and will volatilize to some extent
  • Coal tar byproducts are a mixture of phenols and cresols; the aromatics benzene, toluene, ethylbenzene, and xylenes (BTEX); naphthalenes and light oils; and tars and heavy oils rich in polynuclear aromatic hydrocarbons (PAH). The aromatics and smaller polynuclear aromatics are volatile and sufficiently soluble to be detected as a groundwater plume
  • Coal tar creosote mixtures are very diverse and may or may not be associated with groundwater plumes. They may contain several chemicals that fluoresce

The HEER Office requires that both the free phase and dissolved plumes are delineated at DNAPL sites. Borings and groundwater monitoring wells should be designed to accommodate the selected investigation techniques.

6.2.4.4 Monitoring Wells in Fractured Bedrock

In fractured rocks, the fractures form a net of connected discontinuities. These discontinuities form the main passages for fluid flow, and the rock matrix between the fractures are considered impermeable. Whether a continuum approach or discontinuum approach is applied to the site depends on the connectivity and density of the fracture net compared to the scale of the project site. Choose a continuum approach if the fracture net across the site is dense enough that the fractured formation is hydraulically equivalent to a porous medium (Domenico et. al., 1990).

In most cases, on the scale of a typical response site, the condition of hydraulic equivalence to a porous medium is not satisfied. The flow must be described in relation to individual fractures and fracture sets. This has many consequences. For example, in a hydraulically isotropic, porous formation, the groundwater flow direction is perpendicular to the equipotential lines of the water table or potentiometric map. This assumption cannot be made in a fractured formation, since the flow direction will follow the fracture orientation. The following parameters need to be known to describe flow in fractured rock: orientation, fracture density, degree of connectivity, aperture opening, and smoothness of fractures (Domenico et. al., 1990).

The environmental consultant must carefully plan well placement and design, in fractured bedrock, and discuss the basis and assumptions in the SAP.

Guidance on hydrology in fractured rock is available at the United States Geological Survey (USGS) website (USGS, 2008). The USGS has been researching the hydrology of fractured rocks under its National Research Program and has compiled a reference list of its research spanning from 1998 to the present. This reference list is available on the USGS web site.

6.2.4.5 Monitoring Wells in Confined Aquifers

Confined aquifers are saturated zones that occur below a confining geological layer or unit. The confining layer or unit, referred to as an aquitard, prevents water contained in the aquifer from rising to its elevation of hydraulic equilibrium. The elevation of hydraulic equilibrium for a confined aquifer is the potentiometric surface. The potentiometric surface for a fully saturated confined aquifer lies above the bottom of the confining unit.

The aquitard prevents hydraulic communication between a confined aquifer and an overlying or underlying aquifer. The degree of hydraulic separation of the aquifers adjoining the same aquitard depends on the difference in permeability between the aquitard and aquifers. Hydraulic separation also prevents cross contamination between the aquifers.

Any boring and well installation that penetrates a confining unit becomes a potential pathway between the water bearing units and can result in cross contamination.

In many cases, the overlying aquitard protects the underlying confined aquifer from contamination. To determine if the confining layer will prevent cross contamination, collect soil samples from the aquitard for permeability testing. In addition, the HEER Office may require pumping tests to prove hydraulic separation. Soil sampling of the aquitard and pumping tests must be performed outside the contaminant plume(s) to avoid cross contamination of the confined aquifer.

Further investigations of the confined aquifer will be based on the outcome of the permeability test and pumping test. Design borings and wells that penetrate into the confined aquifer such that they do not open vertical water and contaminant pathways. Keep in mind that a well installed into a confined aquifer may be artesian and plan accordingly.

6.2.5 Monitoring Well Abandonment

The purpose of well abandonment is to prevent surface water from infiltrating into the subsurface and to prevent vertical groundwater movement within the aquifer (HDLNR, 2004). By eliminating water movement vertically within the former borehole, the borehole will cease to be a potential conduit for contaminant dispersion (USEPA, 1991a).

A groundwater monitoring well that is no longer needed, sustains damage serious enough to potentially affect the well's structural integrity, or is determined to be improperly installed must be decommissioned. The HEER Office will typically only issue "No Further Action" letters after submittal of sufficient documentation to verify that monitoring wells and soil borings are properly abandoned in accordance with this guidance. Any exceptions are judged on a site by site basis and approved by the HEER Office.

The well abandonment guidance presented in this document does not address any other county, state, or federal agency requirements; nor does it serve to absolve the owner of any responsibilities associated with past use of the well or any event that may occur after the well is decommissioned to the satisfaction of the HEER Office.

6.2.5.1 Well Abandonment Planning

All soil borings and groundwater monitoring wells will eventually require decommissioning and closure in accordance with the guidance provided in this document. Soil borings cannot remain open under any circumstances. Soil borings must be pressure grouted as detailed below. For wells with a diameter exceeding 12 inches, revised procedures should be submitted to the HEER Office for approval.

If soil borings and monitoring wells are constructed without HEER Office oversight, provisions must still be made for the decommissioning and closing of the borings and wells in accordance with the guidance provided in this document.

Notify the HEER Office at least one week prior to any well abandonment operations. A representative from the HEER Office may elect to be on site to witness the well or boring decommissioning. If so, the potentially responsible party (PRP) will be notified.

If borings/wells have been decommissioned without proper notification of the HEER Office, the PRP may be required to re-excavate the borings/wells and close them under proper HEER Office supervision.

A geologist or soil engineer should plan, supervise, and document well abandonment.

6.2.5.2 Well Abandonment Procedures

The HEER Office recognizes three abandonment procedure options. Note that of the three options presented in this guidance, express advance approval must be sought from the HEER Office for the third option (Remove Well Materials and Backfill with Clean Soil). Descriptions of the options follow:

Option 1: Remove Well Material and Grout

  1. Retain a geotechnical or environmental consultant to plan, supervise and document the abandonment procedures.
  2. Notify the HEER Office at least one week in advance of the fieldwork.
  3. Pull or overdrill and remove the entire casing from the borehole together with the annular fill.
  4. Remove any sediment or sludge to the original well/borehole depth.
  5. Pressure grout the borehole to within 5 feet of ground surface or to above the groundwater table, whichever is shallower (CalEPA, 1995b). Pressure grouting is described in more detail in Subsections 6.2.5.3 and 6.2.5.4.
  6. Calculate the required volume of grout based on the depth and diameter of the borehole and record. Also record the volume actually used. Explain any discrepancy between the calculated and actual volume.
  7. Remove the surface seal and protective surface box.
  8. Backfill the top of the borehole with clean soil and match the surrounding surface completion (asphalt, concrete, soil, other).
  9. Documentation must, at a minimum, include the information required for the completion of the "Abandonment of Monitoring Well Summary Report" (included in Section 18).

Option 2: Perforate and Grout Well Casing

  1. Retain a geotechnical or environmental consultant to plan, supervise and document the abandonment procedures.
  2. Notify the HEER Office at least one week in advance of the fieldwork.
  3. Remove any sludge or sediment from the well.
  4. Rip or perforate the entire well casing.
  5. Pressure grout the well to within 5 feet of ground surface or to above the groundwater table whichever is shallower (CalEPA, 1995b). Pressure grouting is described in more detail in Subsections 6.2.5.3 and 6.2.5.4.
  6. Calculate and record the required volume of grout based on the depth and diameter of the borehole. Record the volume actually used. Explain any discrepancy between the calculated and actual volume.
  7. Remove the well head including the surface seal and the uppermost 3 to 5 feet of casing.
  8. Backfill the top of the borehole with clean soil and match the surrounding surface completion (asphalt, concrete, soil, other).
  9. Documentation must at a minimum include the information required for the completion of the "Abandonment of Monitoring Well Summary Report" (included in Section 18).

Option 3: Remove Well Materials and Backfill with Clean Soil

This option may be used only in areas where groundwater is not a current or potential source of drinking water or where contamination has not been left on-site. Express advance approval must be obtained from the HEER Office for this option.

  1. Retain a geotechnical or environmental consultant to plan, supervise and document the abandonment procedures
  2. Notify the HEER Office at least one week in advance of the fieldwork
  3. Pull or overdrill and remove the entire casing from the borehole together with the annular fill
  4. Remove any sediment or sludge to the original well/borehole depth
  5. Backfill the borehole to within 5 feet of ground surface or to above the groundwater table whichever is shallower
  6. The backfill must be composed of silty clay or clayey silt, free of deleterious materials. Any rock aggregates within the soil must not exceed 1 to 2 inches in diameter and constitute less than 50 weight percent of the backfill. Use on-site soil where possible. Documentation (generally by laboratory analysis) that the soil is not impacted by contaminants should be presented to the HEER Office in advance for approval when using this option
  7. Keep the moisture content of the soil at an optimum for compaction. Add clean water as needed
  8. Place the soil into the borehole in thin lifts and compact using mechanical means. Use this backfilling method to a depth of 5 feet below surface grade
  9. In the upper 5 feet, compact the soil with appropriate compaction equipment to match or exceed the surrounding soil density. Field soil density tests are not a requirement for the summary report. However, provisions must be made to allow for additional compaction if satisfactory compaction cannot be achieved by the prescribed method as determined by the HEER Office
  10. The HEER Office considers the backfilling complete when the final lift for the well and any ancillary excavations is compacted, and its finished grade matches the surrounding grade. Surface completion must match the surroundings (asphalt, concrete, soil, other)
  11. Calculate and record the required volume of backfill based on the depth and diameter of the well. Record the volume actually used. Explain any discrepancy between the calculated and actual volume
  12. Documentation must at a minimum include the information required for the completion of the "Abandonment of Monitoring Well Summary Report" (included in Section 18)

If unanticipated conditions arise during well closure that prevent the execution of the prescribed procedures, halt the field work until the HEER Office concurs with the procedural changes needed to complete well abandonment.

Properly manage all waste generated during well abandonment, label any containers used and store on site until disposal. Record the waste management method used as well as numbers and contents of any containers in the field log. Track the waste until final disposal. Complete proper disposal manifests. Any manifests should be documented in the project report files provided to the HEER Office.

6.2.5.3 Grouting

Any grouting must proceed from the bottom of the well up to the top using a tremie pipe. Feed the grout into the borehole/well under pressure to ensure adequate penetration of the annular space and surrounding formation. Submerge the tremie pipe opening in the grout by two feet or more to prevent bridging. Free-fall placement of grout into the borehole is not acceptable (USEPA, 1991a; CalEPA, 1995b; USACE, 1998).

When the grout has reached to 5 feet below surface, leave it to settle and cure. After 24 hours, check the grout for settling and add additional grout as needed. Repeat checking and adding of grout as needed (USACE, 1998). The HEER Office will consider the grouting complete when the neat cement has hardened and no settlement has occurred.

If it is anticipated that the well location will have to be located in the future, a piece of metal such as rebar may be embedded into the grout near the surface to allow locating of the well location via toning.

Calculate and record the volume of grout required to complete the abandonment. Also record the actual volume used, and explain any discrepancy between the two.

6.2.5.4 Grout Composition

Grouting materials acceptable for use to permanently abandon wells and borings are neat cement, concrete, sand-cement slurry, or cement-bentonite mixture (HDLNR, 2004). Cement used for these mixtures shall conform to the requirements of ASTM C150 for Portland cement, Type I (hereafter referred to as "Portland cement"). The different grout types and their mixtures are described as follows:

  1. Neat Cement. Neat cement shall be mixed at a ratio of one 94-pound sack of Portland cement to not more than six gallons of potable water.
  2. Concrete. Concrete shall contain 5.3 sacks of Portland cement per cubic yard of concrete and a maximum of 7 gallons of water per 94-pound sack of cement. The aggregate shall consist of 47 percent sand and 53 percent coarse aggregate conforming to ASTM C33.
  3. Sand-Cement Slurry. Sand-cement for grouting shall be mixed at a ratio of not more than one part sand to one part Portland cement, by weight, and not more than six gallons of water per sack of Portland cement. Clean well-sorted sand shall be used.
  4. Cement-Bentonite. A slurry of Portland cement, bentonite and water. The amount of bentonite added shall not exceed 8% bentonite per dry weight of cement (7.5 pounds of bentonite per 94 pound sack of cement). The volume of water used in preparing these slurries is limited to three quarters (0.75) of a gallon per 94 pound sack of cement for each 1% of bentonite added. The actual percentage of bentonite for a specific location should be based on the degree of volume expansion needed.

Different types of grout mixtures can be used for well abandonment. The materials selected depend on field conditions and long-term needs for each location (HDLNR, 2004). The HEER Office recommends use of cement-bentonite grout for the saturated zone for most monitoring well abandonments. It is not recommended to use neat cement in the saturated zone, particularly if the groundwater is acidic. Water with an acidic pH may corrode the cement (USEPA, 1991a). The rationale for the choice of grout mixture should be documented in the project-specific workplan and include consideration of factors such as depth and width of the soil boring or well borehole, and water composition in the saturated zone. After grouting, all wells must be sounded to determine if the grout has settled.

Grout should have a minimum design strength of 2,500 pounds per square inch. Strength capabilities may be demonstrated by documenting proper mix proportions and procedures based on the specific type of grout used. If cement/grout is ordered as a ready mix from a vendor, a verification of mixture (e.g., purchase order or receipt with order specification) should be submitted as part of the documentation of soil boring or well abandonment in the investigation report. The receipt must document that the grout was mixed as specified in this document and that it had the specified design strength. In instances where the depth of the well to be abandoned is greater than 100 feet, or where there is a specific need to withstand high load stress (e.g., high traffic areas or locations where heavy equipment is used), a materials testing laboratory should be used to test the final cement for 7-day and 28-day compressive strength.

One mix of cement-bentonite may be used for both the water-bearing and vadose zone when abandoning wells in locations where load-bearing is not a concern and where groundwater is encountered at a depth of less than 20 feet below ground surface.

6.2.5.5 Well Abandonment Reporting

Report well abandonment to the HEER Office within 30 days of fieldwork using the "Abandonment of Monitoring Well Summary Report" presented in Section 18. Alternatively, if this form is included in a site closure, monitoring or investigation report, submit the report within 60 days of well abandonment. Submit the following information together with the form (i.e., Abandonment of Monitoring Well Summary Report):

  1. A copy of the original boring log
  2. The well construction log
  3. An abandonment log
  4. A site map showing the location of the abandoned monitoring well
  5. Disposal documentation for wastes generated during the abandonment process