Land And	EPA 510-B-17-003
Emergency Management	October 2017
United States	5401R	www.epa.gov/ust
Environmental Protection
Agency
&EPA
How To Evaluate Alternative
Cleanup Technologies
For Underground
Storage Tank Sites
A Guide For Corrective
Action Plan Reviewers

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Appendix A
Horizontal Remediation Wells

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Contents
Introduction	1
Advantages Of Horizontal Remediation Wells	2
Limitations Of Horizontal Remediation Wells	4
Uses Of Horizontal Remediation Wells	6
Air Sparging And Soil Vapor Extraction	6
Hot Air Or Steam Injection	9
Bioremediation	9
Chemical Injection	9
Removal Of Light Non-Aqueous Phase Liquid (LNAPL)	9
Plume Containment	10
Injection Of Treated Water	10
Sampling	10
Drilling A Horizontal Remediation Well	11
Planning Activities	11
Precautions To Take When Planning A Horizontal Remediation Well	13
HDD Equipment	14
Drilling Tools	18
Two Ways Of Installing Horizontal Wells: Surface-To-Surface And Blind Drilling	21
Surface-To-Surface/Continuous Wells	21
Blind Wells	25
Locating Technologies	27
Walkover Locators	27
Remote Magnetic Systems	28
Gyroscopic Steering Tools	29
Drilling Fluids	30
Containing Contamination	31
Casing And Well Screen Selection	32
Sealing The Well	34
Well Development	35
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Operation And Maintenance	36
Conclusion	37
References	38
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List Of Exhibits
Exhibit A-l. Schematic Of Typical Horizontal Remediation Well	1
Exhibit A-2. Comparison Of Zone Of Influence Of Horizontal Well Versus Vertical Well	3
Exhibit A-3. Air Sparging And Soil Vapor Extraction System	7
Exhibit A-4. How Horizontal Air Sparge Wells Can Fail To Deliver Air Uniformly	8
Exhibit A-5. Segmented Horizontal Remediation Wells	11
Exhibit A-6. Borehole Radius Of Curvature	13
Exhibit A-7. Smaller HDD Drill Rigs	15
Exhibit A-8. Larger HDD Drill Rigs	16
Exhibit A-9. Typical Drillhead (With Cavity Open)	17
Exhibit A-10. Typical Drillhead	17
Exhibit A-ll. Drill Bit Types	18
Exhibit A-12. Toothed Drill Bit	19
Exhibit A-13. Steering The Borehole	19
Exhibit A-14. Use Of Jetting Tool To Drill A Curve	20
Exhibit A-15. Mud Motor And Hydraulic Jetting Tools	21
Exhibit A-16. Drillhead Emerging From Ground	22
Exhibit A-17. Process Of Drilling Surface-To-Surface Well	23
Exhibit A-18. Back Reamer (Right) Attached To Well Casing (Left) Prior To Pullback	24
Exhibit A-19. Back Reamer (Right) Attached To Well Casing (Left) After Pullback	24
Exhibit A-20. Process Of Drilling Blind Well (Open-Hole Completion)	26
Exhibit A-21. Walkover Locator Receiver In Use	27
Exhibit A-22. Walkover Locator (Cross-Section)	28
Exhibit A-23. Gyroscopic Steering Tool	29
Exhibit A-24. Inadvertent Returns During HDD	32
Exhibit A-25. Well Screen In HDPE Pipe	34
Exhibit A-26. Schematic Of Horizontal Well Construction	35
Exhibit A-27. Hydraulic Jet Used For Horizontal Well Development	36
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Appendix A
Horizontal Remediation Wells
Introduction
This appendix describes the use of horizontal remediation wells for cleaning up underground storage
tank sites. While the chapters in this guide How to Evaluate Alternative Cleanup Technologies for
Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers describe remediation
techniques, this appendix describes one well installation technology that can be used to facilitate
various remediation techniques.
Horizontal directional drilling (HDD) is a technology widely used to install underground utilities and to
extract oil and natural gas. HDD equipment and methods are also used to install wells at environmental
cleanup sites; these are known as horizontal remediation wells (HRWs) or horizontal environmental
wells. HDD uses a specialized drill rig and drill-head locating equipment to create a curved borehole
along a pre-determined borepath, as shown in Exhibit A-l. The drill rig either pulls or pushes the well
casing into the curved borehole. A HRW can be installed either using both an entry hole and an exit
hole, known as a surface-to-surface, continuous, or double-ended well; or using only an entry hole,
known as a blind well. HRWs are able to access locations beneath surface obstructions, and are able to
place long well screens in contact with the contaminated area. HRWs wells can be thousands of feet
long, with hundreds of feet of well screen.
Exhibit A-l, Schematic Of Typical Horizontal Remediation Well
Target
(Reprinted from Horizontal Directional Drilling Good Practices Guidelines (2008) by the HDD Consortium with
permission of the North American Society for Trench less Technology (NASTT). Copyright 2008 by NASTT. All rights
reserved.)
The oil drilling industry developed HDD in the 1930s and continues to update the technology (Bardsley
2014a; Van Heest 2013). HDD was adapted for installing underground utilities in the 1970s (Bardsley
2014a). This use of HDD, known as trenchless technology, is now used to install various underground
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utilities such as water, sewer, gas, electric, telephone, and fiber optic cables (Directed Technologies
Drilling (DTD) 2004; Van Heest 2013).
Starting in the late 1980s, the U.S. Department of Energy sponsored a series of demonstration projects
applying HDD technology for the installation of HRWs (U.S. Department of Energy 1998). These
demonstration projects helped improve HRW installation technology and techniques (Denham and
Lombard 1994). Following the Department of Energy demonstration projects, other federal agencies
began to consider the use of horizontal wells for remediation projects (U.S. Army Corps of Engineers
1996; U.S. Environmental Protection Agency 1997). HDD contractors have been designing and installing
HRWs for more than 20 years (Van Heest et al. 2013). Thousands of HRWs have been successfully
installed in the U.S. (Bardsley 2014a).
Advantages Of Horizontal Remediation Wells
HRWs have two main advantages:
	Ability to access places beneath surface obstructions
	Greater contact with contamination
HDD drill rigs can install wells in locations where conventional vertical drilling or trenching would be
undesirable or impractical; for example, they can be used beneath some types of active facilities,
buildings, neighborhoods, roads, railyards, airport runways, tank farms, rivers and lakes, and sensitive
ecological areas (Bardsley 2014a; Doesburg 2013; Doherty et al. 2000; DTD 2004; Griffin 2004; Lubrecht
2011; Lubrecht 2012; Sequino 2014; U.S. Environmental Protection Agency 2007; Van Heest 2013;
Walters 1999/2000).
Most geologic deposits or sediments near the surface have horizontal layers with different vertical
strata. This causes fluids in the subsurface to preferentially flow in a horizontal direction. Therefore,
contaminant plumes are longer and wider than they are thick. A horizontal well may be installed within
a specific stratigraphic layer to maximize contact with the contamination (Bardsley 2014a; DTD 2004;
Griffin 2004; U.S. Environmental Protection Agency 2012; Van Heest 2013). Plume geometry is often a
main factor when deciding between using HRWs, conventional vertical wells, or other delivery
mechanisms; plume geometry determines the number and length of wells, driving the cost comparison.
Large, elongated plumes often favor an HRW application.
HRW well screens can be hundreds of feet long (Mott-Smith et al. 2011; Lubrecht 2012). Multiple
vertical wells with overlapping zones of influence (ZOI) might be needed to achieve what a single
horizontal well can achieve with a single zone of influence, as shown in Exhibit A-2. A typical ZOI for a
HRW is about 80 feet or a 40-foot radius around the well. The ZOI can be measured using vertically
installed piezometers or wells, as is done for conventional vertical remediation wells. Lundegard et al.
(2001) found during a pilot study that at least 30 vertical air sparge wells would be needed to achieve
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the benefit of one horizontal well with 90 meters of well screen. Installing many vertical wells would
have a much greater adverse impact on sensitive surface habitats.
Exhibit A-2. Comparison Of Zone Of Influence Of Horizontal Well Versus Vertical Well
Map View
Vertical
Well\,	Horizontal
Cigar-shaped outline of early ZOI	: \
Screen	ftliB% M
Elliptical outline of late ZOI
(Reprinted from Directional Drilled Horizontal Wells and Engineered Horizontal Remediation Wells Screens
Accelerate Site Closure (2013) by Garry Van Heest, Michael Sequino, and George Losonsky with permission of
Directional Technologies, Inc.)
Having a longer well screen in communication with the subsurface contamination, may potentially result
in a faster cleanup (Sequino 2014). In addition, HRWs may be less prone to short-circuiting than vertical
wells because HRWs are screened within a layer of uniform permeability (Van Heest 2013).
Because the HDD crew steers the drill in three dimensions during the drilling process, the wellbore can
follow a curved path to follow plume geometry, target a Non-Aqueous Phase Liquid NAPL layer, target a
specific geologic layer, or avoid underground utilities as needed (Bardsley 2014a; DTD 2004; Parmentier
and Klemovich 1996).
Depending on site-specific circumstances, additional potential advantages of HRWs include:
	Intersecting multiple fractures, if vertically fractured bedrock is present, (Carlisle et al. 2002;
Kiriner et al. 2005)
	Reducing disruption in the vicinity of the site by (Bardsley 2014a)
o Reducing number of wellheads
o Locating well heads away from existing surface infrastructure
o Reducing construction time on site
	Reducing time needed for injections, because of the long well screens (Van Heest 2013)
	Resulting in less business disruption because of the smaller footprint (Van Heest 2013)
	Requiring fewer pumps and manifolds than vertical well networks (DTD 2004)
	Helping achieve green and sustainable remediation goals through reduced energy use and less
disruption of surface ecology (Lubrecht 2012)
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 Recently developed proprietary technology that allows the placement of multiple multi-purpose
wells within a single horizontal borehole (EN Rx 2015)
Although installing a HRW is more expensive per foot than installing a vertical well, incorporating
horizontal wells may make the overall cleanup project less expensive by reducing the number of wells
needed, shortening the time to closure, and reducing disruption to businesses or other activities (DTD
2004; Fournier 2002; Parmentier and Klemovich 1996; Sequino et al. 2012; Van Heest et al. 2013;
Walters 1999/2000).
Limitations Of Horizontal Remedi	ells
	Higher installation costs for HRWs than vertical wells
	Design, drilling, and installation of HRWs more challenging than vertical wells
Upfront HRW installation costs are greater per foot than vertical wells (Griffin 2000; Parmentier and
Klemovich 1996). Typical HRW costs include: mobilization, $10,000 - $20,000; boring, $90 - $120 per
foot; well materials, $10 - $20 per foot; and wire-line tracking, if needed, $30 - $40 per foot. Other
potential costs include handling, characterizing, and disposing of the large volume of investigation-
derived waste often produced during HRW installation, typically 0.08 cubic yards per foot of drilling,
depending on the borehole diameter. Larger projects typically require roll-off containers and frac tanks
to containerize drilling waste. If off-site disposal is required, costs range from tens to hundreds of
thousands of dollars for moderate to large projects.
Although HRWs have been used for decades, the environmental remediation industry and
environmental regulators are more familiar with vertical wells. This can pose a hindrance to the
widespread adoption of HRWs (Griffin 2000). While many professionals agree that this technology is
sound, there is a lack of published, peer-reviewed literature assessing the performance of HRWs.
A HRW engineer pointed out that "proper development of a horizontal well, drilled with drilling mud,
generally takes a higher level of care than developing wells that were constructed using auger drilling
technology" (Lubrecht 2012). See additional explanation in Drilling A Horizontal Remediation Well
section. Installing HRWs puts great strain on the well materials, and this increases the potential for
system failure due to well screen deformation.
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HRWs can have limitations based on site features such as, landscape footprint, geometry, and depth.
Planning the bore path for a HRW is more complicated than for a conventional vertical well; the
horizontal well driller must consider the three dimensional subsurface geology, the location of
underground utilities, and where to locate the entry and exit bores (Lubrecht 2011). HRWs typically
descend to the desired depth at a shallow angle, so the entry and exit bores may need to be located a
large horizontal distance away from the treatment area as shown in Exhibit A-l. At some sites, the
required setback distance may place the entry and exit bores off of the contaminated property on
parcels owned by other parties. In these cases, one must obtain permission to conduct drilling activities
on those properties.
A fluctuating water table can also pose a problem for HRWs screened near the water table as shown in
Exhibit A-3. For example, a soil vapor extraction well installed just above the water table could flood if
the water table rises. Alternatively, a well installed to extract LNAPL at the water table's surface could
fail to extract LNAPL if the water table rises or falls (DTD 2004; see Removal Of LNAPL section).
HRWs are installed by specialty HDD contractors, some of whom are more familiar with utility
installation than with the requirements for environmental wells and the procedures for working on
cleanup sites (Bardsley 2014b; Griffin 2004). Griffin notes that "Horizontal wells require proper design,
construction, and development procedures that are not normally understood by directional drillers"
(Griffin 2000). Some researchers have cautioned that horizontal wells could "provide a conduit for
contaminated groundwater to be drawn into the well, conveyed a large distance, and injected into an
uncontaminated region of an aquifer" (Steward and Jin 2001). For this reason, exercise extra care to
prevent cross-contamination when installing HRWs with long screened intervals, as discussed in
Planning Activities section.
HRW installers may also face regulatory hurdles. Some states' well construction regulations apply to
conventional vertical wells; special permission may be needed to use horizontal wells (Bardsley 2014b).
For instance, California has requirements for companies using HDD near a California roadway (Caltrans
2015). Drilling underneath rail lines may also require special permits and fees.
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Uses Of Horizontal Remediation Wells
HDD has been used to install extraction, injection, and monitoring wells to address contaminated soil
and groundwater (Bardslev 2014b). Less commonly, HDD has been used to collect soil samples. HRWs
have been used to address petroleum contamination and chlorinated solvents (U.S. Army Corps of
Engineers 2010). HRWs have been used for the following remediation technologies (DTD 2004; Bardslev
2014a; Bardslev 2014c):
	Groundwater extraction
	Soil vapor extraction1
	Dual phase extraction2
	Free phase contaminant removal
	Air sparging3
	Hot air/steam injection
	Bioremediation4
	Nutrient injection
	In-situ chemical oxidation5
	Electrical resistance heating
	Radio frequency heating
	Permeable reactive barriers
	Infiltration or injection of treated groundwater
A hybrid approach, using both HRWs and vertical wells, may be suitable at some sites depending on
plume geometry and project goals.
The following subsections describe the more common uses of HRWs.
Air Spa	ci Soil Vapor Extraction
HRWs have been widely used for air sparging (AS). By bubbling air through contaminated groundwater,
AS systems transfer volatile contaminants from the groundwater to the overlying vadose zone. AS
systems are often used together with soil vapor extraction (SVE) systems, which induce a vacuum within
the overlying unsaturated soil or vadose zone to extract contaminated vapors from the subsurface soil
gas and then collect them for treatment at the surface, as shown in Exhibit A-3.
For more information, see the following chapters in How to Evaluate Alternative Cleanup Technologies for
Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers
1	Soil Vapor Extraction, Chapter II
2	Dual Phase Extraction, Chapter XI
3	Air Sparging, Chapter VII
4	Biosparging, Chapter VIII
5	Chemical Oxidation, Chapter XIII
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Exhibit A-3. Air Sparging Arid Soil Vapor Extraction System
Air
Stripper
Blower/
Vacuum
Pump
Soil Vapor Extraction
Water
Table
O  O g oO o
Air Sparging
HRWs have been used to mitigate the potential for vapor intrusion into buildings (U.S. Environmental
Protection Agency 2012; Van Heest et al. 2013). The system works by extracting air from beneath the
building's foundation slab, preventing vapor phase contaminants from transferring from the vadose
zone into the building. Horizontal drilling can install such a sub-slab depressurization system from
outside the building, preventing disruption to building activities (Van Heest et al. 2013).
Horizontal well screens can disperse air broadly across a contaminated area; however, the well must be
designed and installed properly to ensure that air will be dispersed as uniformly as possible across the
entire screened interval, as shown in Exhibit A-4 (Doesburg 2013; DTD 2004; Lundegard et al. 2001). For
both AS wells and SVE wells, well screens with more or larger openings at the end of the screen that are
farther from the pump, called zone well screen designs, can help make the flow uniform by equalizing
the pressure along the length of the well screen (Doherty et al. 2000). Lundegard et al. (2001)
determined that "if the well airflow rates can be estimated, a custom perforation pattern can be
designed that will minimize pressure drop within the well and result in effective delivery of air to the
formation."
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Exhibit A-4. How Horizontal Air Sparge Wells Can Fail To Deliver Air Uniformly
^ t i ,
l I I I I I I I
I I I I I I I


~
Low Permeability


. I
\ t


1 1 1
^ | | |
i ii ii
i i i i


High Permeability


~


~

i
I .
~


1 l'l
/ i .

Top: Excessive pressure drop caused by poor screen design
Middle: Excessive pressure drop caused by natural variation in permeability
Bottom: Excessive pressure drop caused by downward slope of well with respect to water table
(Adapted from Effective Air Delivery from a Horizontal Sparging Well (2001) by Paul D. Lundegard, Brent Chaffee,
and Doug LaBrecque with permission of the National Ground Water Association.)
When considering horizontal wells for AS and SVE, there are several things to keep in mind. Because
groundwater levels fluctuate seasonally, SVE wells should be placed above the seasonal high
groundwater elevation to avoid damaging the SVE system by sucking water into the system (Doherty et
al. 2000; DTD 2004). One source recommends placing SVE wells at least 6-8 feet above the seasonal
high groundwater elevation (Strong 2016). Alternatively, a dual-phase extraction system could be used,
in which vacuum suction extracts contaminated groundwater, hydrocarbon vapor, and separate-phase
petroleum product from the subsurface.
Several sources have noted that clogging of well screen slots can be a problem for horizontal AS wells,
especially when the slots are very small (Butler and Mott-Smith 2011: DTD 2004). Slot sizing should be
based on design principles for naturally developed wells (Driscoll 2014). With correct slot sizing, fines
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intrusion is generally limited, but the potential for well redevelopment to address biological or chemical
fouling should be considered, especially for pumping and injection wells (Strong 2016).
Hot Air Or Steam Injection
Some remediation systems inject hot air or steam into the subsurface to volatilize contaminants, such as
diesel and fuel oil, that do not readily volatilize at normal temperatures. To avoid problems with oil
buildup in the extraction wells, the engineer must design the system to ensure it can deal with
condensate in the removal wells (DTD 2004). Methods of handling condensate accumulation issues in
piping systems include:
	Designing wellheads with union or flanged connections for ease of redevelopment, well screen
cleaning, vacuum removal of accumulated oils,
	Designing surface-to-surface wells to allow pressure flushing of well screen by using surfactants,
compressed air, etc.,
	Oversizing well screen diameter to accommodate buildup of condensate.
Bioremediation
HRWs have been used for bioremediation. For example, HRWs were used to inject oxygen gas for the
biostimulation of poly aromatic hydrocarbons (PAHs) degradation at a former wood treatment facility
(Mott-Smith et al. 2011). HRWs were also used to inject a mixture containing bacteria, enzymes, and
oxygen at a leaking underground storage tank site (Clark and Laughlin 2000). See Air Sparging And Soil
Vapor Extraction section about ensuring uniform air delivery.
Chemical Injection
HRWs have been used for injecting chemicals "to either reduce the toxicity of contaminants or to
prevent their migration" (DTD 2004). Some of these injections are one-time events, so the "wells
typically require less extensive well development than longer-term treatment alternatives" (DTD 2004).
A 2008 remedial action used horizontal wells to inject permanganate for in-situ chemical oxidation of
trichloroethylene (TCE) in groundwater (USACE 2010). Chapter XIII of the EPA's How to Evaluate
Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan
Reviewers describes using chemical oxidation at leaking underground storage tank sites.
Corrective action plan reviewers should be aware that because the quantities of oxidants used are
typically greater in horizontal wells than in vertical wells, permitting and notification requirements in the
National Fire Protection Association's NFPA 430: Code for the Storage ofLiguid and Solid Oxidizers may
be triggered when HRWs are used for in-situ chemical oxidation.
KemoV'.. . '..Non-Aqueous Phase	!
Leaking underground storage tank sites frequently have contaminants present as LNAPL which is often
concentrated near the water table interface. Changes in seasons and weather cause the water table to
fluctuate, which results in LNAPL accumulating above or below the water table; this is known as the
smear zone.
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HRWs can be used to pump LNAPL from the subsurface or to sparge LNAPL. For example, a large plume
of aviation fuel was removed by three HRWs at the Marine Corps Air Station at Cherry Point, North
Carolina (Collins 2014). Another cleanup project captured a gasoline plume by drilling a horizontal
collection well to intersect vertical fractures; a vertical recovery well was drilled into the low point of the
horizontal well to pump out recovered gasoline and contaminated groundwater (Carlisle et al. 2002).
At sites with highly variable water table elevations, horizontal wells may be less useful than vertical
wells for LNAPL removal (DTD 2004). At these sites, consider using a sloped well or nested well
configuration with a dual-phase extraction system when attempting to remove LNAPL with a horizontal
well. A dual-phase system will extract groundwater or LNAPL when the water table is high and soil
vapor when the water table is low (DTD 2004).
Plume Containment
HRWs have been used to contain the migration of groundwater plumes by withdrawing groundwater
(Lubrecht 2012; Walters 1999/2000) or air sparging (Sequino et al. 2011). Installing several horizontal
extraction wells, one above the other, perpendicular to the direction of groundwater flow, can help
contain a groundwater plume. By installing wells at various elevations, this configuration would also
help address the concern that a lowering water table could leave a horizontal extraction well stranded
above the water table (DTD 2004).
Injection Of Treated. Water
HRWs can be used to infiltrate treated water back into the subsurface (DTD 2004). Horizontal wells may
reduce "the potential for local groundwater mounding or other adverse changes in groundwater
gradient associated with shallow or deep vertical injection wells" because horizontal wells often have
longer screen lengths than do vertical wells, and these screens distribute the injected volume over a
longer lateral distance in the aquifer (Lubrecht 2012).
Sampling
Some sampling techniques have been developed for use with HDD equipment; however, HDD sampling
is not yet widespread (DTD 2004). Limited soil sampling can be accomplished using HDD equipment, but
there are difficulties in keeping the borehole open and retrieving the sample. Some groundwater
sampling can also be conducted using HDD equipment. (DTD 2004). New equipment for sampling and
site characterization is being developed for use with HDD methods to take advantage of HDD's ability to
reach areas not accessible vertically. For example, Ahn et al. (2006) describe the Smart Subsurface
Horizontal Investigation Probe (SSHIP) that combines an array of miniature sampling tubes, a logging
tool, a miniature resistivity probe, and a miniature cone penetrometer. The SSHIP is controlled from the
surface and can transmit data in real-time over a distance of up to 300 meters.
A proprietary technology developed in 2015, allows the placement of multiple multi-purpose wells
within a single horizontal borehole, allowing collection of separate samples from different points along
the borepath (EN Rx 2015). With this technology, each well is screened in a different location, and the
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well screens are separated from each other by bentonite and grout as shown in Exhibit A-5; the wells
can be used for treatment, sampling, or a combination of the two within a single borehole.
Exhibit A-5. Segmented Horizontal Remediation Wells
ELECTRICAL PIPES
SEWER DRAIN
WATER TABLE
INDIVIDUAL WELL SECTIONS
(Reprinted from EN Rx, Inc. website (2015) with permission of EN Rx, Inc.)
Drilling A Horizontal Remediation Well
Planning Activities
Weigh various factors when considering and planning a HRW including (Bardsley 2014a; Wampler
undated):
	Surface and near-surface constraints
	Treatment objective
	Site geotechnical conditions
	Depth
	Borehole configuration
	Allowable bending radius of drill pipe and well materials
	Locations of underground utilities
	Setback distance required
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At some sites, surface constraints, for example a gas station, a tank farm, an active runway, or business
operation, may be the primary factor driving the decision to use HRWs. As such, the bore path should
be planned to avoid interfering with these types of surface features.
As with vertical wells, understanding the site's geology and developing an accurate conceptual site
model are important. At sites where the conceptual model has not yet been well-defined, a pre-design
investigation should be conducted using vertical test borings at specified intervals along the proposed
bore path. Soil samples should be collected from these borings for grain-size determinations, which are
critical for well screen selection. The exploratory borings should be grouted to prevent inadvertent
return of drilling fluid. The North American Society for Trenchless Technology's (NASTT) Horizontal
Directional Drilling Good Practices Guidelines provides advice on how to conduct a site investigation; see
Chapter 4 of NASTT 2008.
HRWs can also be installed via trenching, rather than drilling at shallower depths, depending on soil type
and stability. However, trenching is impractical for deeper applications, such as those deeper than
about 20 feet, and for wells to be screened in the saturated zone, due to the increased costs for
dewatering and shoring.
Horizontal wells installed at relatively shallow depths typically use walkover navigation methods to
determine the location of the drillhead. These battery-powered transmitters are currently inadequate
for depths greater than 80-100 feet. Advanced navigation methods for depths exceeding 80-100 feet
are more expensive than walkover locators as discussed in Locating Technologies section (Directional
Technologies. Inc. (DTI) 2013; DTD 2014a. 2014b).
HRWs are advanced to the desired depth at a shallow angle, approximately 10 to 20 degrees from
horizontal, so the entry and exit bores may need to be located a significant horizontal distance away
from the treatment area, potentially on a neighboring parcel. This may entail requesting access from
adjacent property owners. Horizontal wells typically require approximately 5 feet of setback for every
foot of depth. If the well is to be screened at a location 50 feet below a building, the entry point would
be located about 250 feet away from the building. If the well is a surface-to-surface well, the exit bore
would be about 250 feet away from the other side of the building. The 5:1 setback is a rule of thumb;
HRW installers may be able to achieve shorter setbacks depending on the borehole diameter, geology,
and type of drill rod (DTI 2013. Strong 2016). The necessary setback distance may be a limiting factor
when deciding whether HRWs are practical at a given site, taking into account surrounding land use and
access.
A borehole's radius of curvature defines how tightly it curves as shown in Exhibit A-6. A longer radius of
curvature puts less stress on the drilling equipment and well materials, but requires a longer borehole
and larger setback distances (Kaback 2002). Directional drilling may be more difficult in geologic layers
that contain cobbles or boulders, because these may deflect the drill bit away from the desired
borepath.
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Exhibit A-6. Borehole Radius Of Curvature
Gradual Radius
Tight Radius
Precautions To Take When Planning A Horizontal Remediation Well
Some researchers have cautioned that certain horizontal wells could "provide a conduit for
contaminated groundwater to be drawn into the well, conveyed a large distance, and injected into an
uncontaminated region of an aquifer" (Steward and Jin 2001). Installing the well parallel to
potentiometric isocontours or perpendicular to groundwater flow direction, can help prevent
preferential pathway concerns. The corrective action plan reviewer should ensure the plan explains
what precautions will be taken to guarantee that the proposed HRW will not act as a preferential
pathway to spread contamination. Possible precautions are listed below:
	Wells should not be screened across both contaminated and uncontaminated areas.
	Wells should not perforate competent confining layers where possible, to avoid creating a
conduit for contamination of naturally protected saturated zones.
Performance demonstration via groundwater flow or contaminant transport modeling may be needed
to alleviate concerns.
Prior to any excavation or boring, drillers should notify the local one call center; www.call811.com for
contact information. The one call center will send companies to mark the locations of underground
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utilities; however, depth information is not usually provided. It is the responsibility of the HDD driller to
determine the exact locations of utilities, in order to avoid damaging them (NASTT 2008; Wampler
undated).
As with any construction project, the corrective action plan reviewer should ensure plans are in place to
mitigate noise and traffic impacts, (NASTT 2008). The corrective action plan reviewer should also ensure
a safety plan is in place and addresses safety issues relevant to HDD (NASTT 2008):
	Staying clear of overhead power lines when moving drill rigs and handling drill pipe;
	Verifying location of underground utilities, such as electric cables, gas lines, communication
cables, sewer pipes; and
	Following procedures if the drill strikes an underground utility.
juipnieiit
In horizontal directional drilling, a drill rig, also known as a boring machine, advances the drill rod
through the ground, while the drill path is being controlled using one of several locating techniques. The
drill rig then pulls the drill string out of the borehole. The sizes of drill rigs are rated based on how many
pounds of thrust or pullback capability and torque they have. HDD drill rigs used for environmental
applications are grouped into (DTD 2004; NASTT 2008):
	Small rigs: less than 40,000-50,000 pounds of thrust or pullback capacity, used for drilling less
than 700 feet as shown in Exhibit A-7; and
	Large rigs: greater than 40,000-50,000 pounds of thrust or pullback capacity, used for drilling
700-2,000 feet as shown in Exhibit A-8.
Small rigs usually have all needed components integrated on the rig, making them more mobile. Small
rigs are also able to operate in a much smaller work area (NASTT 2008, Table 3-1). Large rigs can install
longer wells, but their setup is more complicated due to their use of separate units for power, drilling
fluid mixing, and drill rod handling. Large rigs use a crane to load drill rods, whereas small rigs have
integrated automated rod loaders (DTD 2004).
Drill pipe, also known as drill stem and drill rod is hollow and comes in various diameters from about 1
inch to over 6 inches, and segment lengths, 6 feet to over 30 feet (DTD 2004; NASTT 2008). Most drill
pipe is made of steel manufactured specifically to withstand the various forces experienced during HDD.
These forces include twisting, due to drilling; compression, due to thrusting; tension, due to pullback;
and bending, due to following the desired bore path (DTD 2004).
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Exhibit A-7. Smaller HDD Drill Rigs
(Reprinted from Directed Technologies Drilling, Inc. website (2015) with permission of Directed Technologies
Drilling, Inc.)
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Exhibit A-S. Larger HDD Drill Rigs
(Top photo reprinted from Directed Technologies Drilling, Inc. website (2015) with permission of Directed
Technologies Drilling, Inc. Bottom photo reprinted from Horizontal Environmental Drilling 101 - An Introduction to
the Means and Methods for Horizontal Environmental Well Installation (20141 by David Bardsley with permission of
Directed Technologies Drilling, Inc.)
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The drill-head is located at the leading end of the drill string. The drill-head includes the drill bit and a
cavity housing the electronic locating equipment, as shown in Exhibit A-9. Drilling fluid pumped down
the drill pipe travels through the cavity to cool the electronics and the drill bit (DTD 2004). Drill bits are
available in various styles and sizes; the type of drill bit is selected based on the characteristics of the
formation being drilled, as shown in Exhibit A-ll. The most common is the duckbill bit, shown in
Exhibits A-9 and A-10. The angle of the drill bit allows operators to steer; when the drill pipe is pushed
forward without rotation, the angle of the bit causes the borehole to advance at an angle. Other types
of drill bits include carbide bits, conical bits, and toothed bits (DTD 2004).
Exhibit A-9. Typical Drillhead (With Cavity Open)
(Reprinted from Horizontal Directional Drilling Good Practices Guidelines (2008) by the HDD Consortium with
permission of NASTT. Copyright 2008 by NASTT. All rights reserved.)
Exhibit A-10. Typical Drillhead
(Reprinted from Drill Pipe Inc. website (2015) with permission of Drill Pipe Inc.)
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Exhibit A-ll. Drill Bit Types
Drill Bit Type
Application
Slant-Face Bits
Flat spade
Bent spade
Clay
Sand
Organic soils
Modified spade
Hard ground conditions
Rock bits
Soft to medium rock
Hardpan
Rotary Rock Bits
Milltooth tri-cone
Soft rock
Tungsten carbide inserts (TCI)
tri-cone with sealed bearings
Medium to hard rock
Drag bit
Soft rock
Percussive Bits
Eccentric flat-faced with carbide
button inserts
Soft to hard rock
Carbide button slant-face
Soft to hard rock
Hardpan
Carbide button round face
Soft to hard rock
(Source: NASTT 2008)
Drilling Tools
Several types of drilling tools are used for directional drilling: compaction tools, jetting tools, downhole
mud motors, and air hammers.
The drilling tools used most commonly for HRWs are compaction tools, such as the duckbill bit as shown
in Exhibits A-9 and A-10 and the toothed bit displayed, in Exhibit A-12. Compaction tools work by
scraping away the geologic material to form a borehole, much like a wood chisel. The bit is inclined at
an angle to the drill pipe, so when the driller applies thrust to the drill pipe without rotation, the
borehole turns in the direction of the bit, as shown in Exhibit A-13. When the driller rotates the drill
pipe, the borehole advances in a straight line (Bardsley 2014a; DTD 2004; Kaback 2002). Compaction
tools are most useful in small boreholes, shallow depths of less than 50 feet, and unconsolidated
formations (Kaback 2002). Compaction tools cause reduced permeability surrounding the borehole
(Kaback 2002), which must be addressed during the later back-reaming step.
Jetting tools use hydraulic pressure to drill the borehole. The hydraulic jet comes out of a bent housing
or from a port on a drill bit attached to a bent subassembly often called a bent sub. Due to the bend,
the hydraulic jet comes out at an angle to the axis of the drill pipe. When the drill pipe is not being
rotated, the angle of the jet causes boring to proceed in that direction, as shown in Exhibit A-14. When
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drilling in a straight line, the driller rotates the drill pipe, which prevents the angled jet from establishing
a preferred direction (Bardsley 2014a; Kaback 2002).
Exhibit A-12. I oothed Drill Bit
- S *

ma
(Reprinted from Directional Technologies, Inc. website (2015) with permission of Directional Technologies, Inc.)
Exhibit A-13. Steering The Borehole
Left: Driller has stopped rotation to steer the bit to the right
Right: Drill pipe is being rotated to drill a straight borehole
(Reprinted from Horizontal Wells for Groundwater Remediation (2013) by Garry Van Heest with permission of
Directional Technologies, Inc.)
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Exhibit A-14. Use Of Jetting Tool To Drill A Curve
a	b	c
a: Rotary drilling to drill a straight section
b: Hydraulic jetting without rotation to jet a pocket
c: Drillhead advanced into pocket and rotary drilling resumed
Downhole mud motors are a more complex type of drilling tool used to drill in rock or very hard
formations, as shown in Exhibit A-15 (DTD 2004; NASTT 2008). Drilling fluid is forced through the mud
motor, which causes the motor, and the drill bit attached to the motor to rotate. Percussive drilling
systems, such as air hammers, are another type of drilling tool used to bore through rock or very hard
formations (DTD 2004; NASTT 2008).
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Exhibit A-15. Mud Motor And Hydraulic Jetting Tools
Motor assembly

%b

Rock Bit
Mud Motor
PWD Sub/ Non-Mag Collar (ParaTrack-2
Orienting Sub pressure module & probe inside)
Jetting assembly
Rock Bit Extension Sub Bent Sub/ Non-Mag Collar (ParaTrack-2
Orienting Sub pressure module & probe inside)
(Reprinted from Prime Horizontal website (2016) with permission of Prime Horizontal. Illustration copyright of
Prime Horizontal Ltd.)
Two Ways Of Installing Horizontal Wells: Surface-To-Surface And Blind
Drilling
A HRW can be installed using either both an entry hole and an exit hole known as a surface-to-surface,
continuous, or double-ended well; or by using only an entry hole known as a blind well. To install a
surface-to-surface well, the drill enters the ground, descends to the desired depth, traverses the desired
distance, rises to the surface, and exits the ground. The drill rig is then used to pull the well casing into
place through the exit hole. To install a blind well, the drill enters the ground, descends to the desired
depth, traverses the desired distance, and is then withdrawn from the borehole. The drill rig is then
used to push the well casing into place through the entry hole. With both types of wells, soil cuttings
and drilling fluids are removed from the well during the well development process; contaminated soil
and fluids must be handled accordingly; collected, characterized, treated, and properly disposed as
necessary, see Drilling Fluids section.
Surface-To-Surface/Continuous Wells
Surface-to-surface drilling is a common way to install HRWs. Surface-to-surface HRWs have achieved
depths greater than 200 feet and lengths over 2,850 feet (Bardsley 2014a). In this method, once the
drillhead re-emerges from the ground at the exit point, as shown in Exhibit A-16, it is removed from the
drill pipe and a back reamer is attached to the drill pipe, as seen in Exhibit A-17. The reamer is then
pulled back through the borehole to enlarge it to accommodate the well materials. There are different
types of reamers suitable for different soil conditions (NASTT 2008):
	Compaction reamers can be used in clays, silts, sands, and cobbles;
	Mixing reamers can be used in clays and sands; and
	Hole openers can be used in hard soil and rock.
Usually the well materials - well casing and screen, are attached to the reamer and is pulled into place
directly behind the reamer, as shown in Exhibits A-18, A-19. The well materials can be attached to the
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back reamer using a swivel so that the well materials are not forced to rotate during pullback. The
reamer alone can also be pulled through the borehole one or more times prior to pulling through the
well materials to make pullback easier; this is known as pre-reaming (DTD 2004; NASTT 2003. Pre-
reaming may be useful when the well materials have a large diameter or low tensile strength (NASTT
2008; Wampler undated).
Exhibit A-16. Drillhead Emerging From Ground
(Reprinted from Horizontal Environmental Well Handbook (2004) by Directed Technologies Drilling with permission
of Directed Technologies Drilling, Inc.)
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Exhibit A-17. Process Of Drilling Surface-To-Surface Well




Drill Bit
f l 	m I
Pilot Hole Drilling
Drill Pipe

Reamer
Drill Pipe
Pre-Reaming (optional)
Swivel
Reamer
Pullback
Well Casing
Exit Side of Borehole
is Cased or Properly
Abandoned
Completed Well
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Exhibit A-18. Back Reamer (Right) Attached To Well Casing (Left) Prior To Pullback
(Reprinted from Directed Technologies Drilling, Inc. website (2015) with permission of Directed Technologies
Drilling, Inc.)
Exhibit A-19. Back Reamer (Right) Attached To Well Casing (Left) After Fullback
(Reprinted from "Tracer Wire System Importance for HDD" (2013) by Lee Dester with permission of Copperhead
Industries, LLC)
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Blind Wells
Blind wells are useful when there is no place to locate an exit hole (Sequino 2014; Van Heest et al. 2013).
Blind wells have been installed at depths greater than 200 feet and with lengths of over 1,500 feet
(Bardsley 2014a). Blind wells cost less than surface-to-surface wells, but have greater uncertainty of
successfully installing the well materials. They are used with more confidence where the geology is free
of rocks and cobbles.
Two common ways to install a blind well are by completing as an open-hole or using a larger diameter
hollow-stem drill casing that encloses the well material. In an open-hole completion, the drillhead
enters the ground, descends to the target depth, and is then withdrawn from the borehole, as shown in
Exhibit A-20. A reamer is then attached to the drill pipe and is pushed into the borehole to enlarge it.
The well materials are then pushed into the borehole. Prior to this step, a carrier casing can be installed
in the curved portion of the borehole to prevent the well materials from digging into the side of the
borehole (DTD 2004). In some geological materials (such as gravels, cobbles, poorly consolidated sands,
and swelling clays) it may be difficult to maintain an open borehole while pushing the well material into
the borehole. In these situations, using a larger diameter hollow-stem drill casing surrounding the well
material may be useful. Also, commonly used well materials may not be able to withstand the
compression forces caused when pushing the well materials into the borehole. Steel is better able to
withstand compression, but is much more expensive than polyvinyl chloride (PVC) or high-density
polyethylene (HDPE) (Bardsley and Ombalski 2014).
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Exhibit A-20. Process Of Drilling Blind Well (Open-Hole Completion)
Dr Bit
Pilot Hole Drilling
Reaming
Well Installation
Well Casing
Well
Screen
Completed Well
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An example of this larger hollow stem drilling technique is described by Bardsley and Ombalski (2014).
It is a proprietary method called knock off technology, in which a large-diameter drill pipe is used with
the well materials already inside the drill pipe. Once the desired borehole location is reached, the drill
bit detaches from the drill pipe, the drill pipe is withdrawn from the borehole, and the well materials
remain in place.
Locating Technologies
The ability to know precisely where the drillhead is located underground is vital to the successful
installation of a HRW. Several locating technologies are used in HDD:
	Walkover iocators
	Remote magnetic systems
	Gyroscopic steering tools
The type of locating technology used for a particular HRW project is selected "based on bore path,
interference risk, depth and cost" (Bardsley 2014a).
Walkover Locators
Walkover locators are the type of locating technology most often used in HRW installation. A battery-
operated transmitter, called a sonde, in the drillhead transmits wireless radio signals that are received
by a handheld receiver carried by a crew member on the surface, as shown in Exhibits A-21, A-22. These
signals inform the driller about the drill-head's location, depth, and orientation, allowing the driller to
adjust the drilling process to keep the drillhead on the desired bore path (Bardsiev 2014a; DTD 2004;
DTD 2014a; NASTT 2008). The orientation information includes pitch or the angle with respect to
horizontal and roll or the rotation of the drillhead.
Exhibit A-21. Walkover Locator Receiver In Use
(Reprinted from Horizontal Environmental Well Handbook (2004) by Directed Technologies Drilling with permission
of Directed Technologies Drilling, Inc.)
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Exhibit A-22. Walkover Locator (Cross-Section)
s	/
Walkover locators have several limitations:
	Walkover locators can be difficult to use at sites with excessive electromagnetic interference for
example, interference from reinforced concrete, buried pipelines or drums, power lines,
cathodic protection systems, or industrial motors (DTD 2014a; Sequino 2014).
	Walkover iocators cannot be used when the boring depth is too great for the signal to reach the
surface. The maximum depth at which walkover locators can be used has been reported as
about 80 feet (DTD 2014a) to 100 feet (DTI 2013). Although signal strength can be an issue,
replacing the battery power with an electrical wire running through the drill pipe can boost the
signal, allowing walkover locators to function for deeper boreholes and at sites with
electromagnetic interference (DTD 2014a).
	If the battery powering the sonde becomes depleted during drilling, then the drill pipe must be
withdrawn from the borehole in order to replace the battery (DTD 2004). Therefore, field
personnel should be aware of manufacturers' specifications about battery life expectancy in
order to proactively maximize drilling efficiency by changing batteries as necessary.
Remote Magnetic Systems
Remote magnetic guidance systems have a sensor in the sonde that senses its location and orientation
relative to a magnetic field, either the Earth's natural magnetic field or an induced magnetic field (DTD
2014a). Most remote magnetic systems create an artificial magnetic field using a coil of wire placed on
the ground surface around the borepath; the sensor in the sonde senses its location within this magnetic
field (DTD 2014a). The remote magnetic guidance system sends the drillhead's orientation information,
including pitch and roll, to the driller through a wire within the drill pipe. The wire also supplies power
to the sonde, so battery life is not a concern. Remote magnetic guidance systems are useful in areas
where interference makes walkover locators inaccurate and for long and deep borings where battery life
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is a concern for walkover locators (DTD 2004; DTD 2014a: DTI 2013; Finnsson 2005). Remote magnetic
guidance systems can be used at depths of up to 200 feet (DTD 2014a).
Remote magnetic guidance systems are more expensive to use than walkover locators because of the
labor needed to lay out and survey the coil and to maintain the wire connection (DTD 2004; DTD 2014a;
DTI 2013). Every time an additional iength of drill pipe is added to the drill string, the drilling team must
re-establish the wire connection. This can either be done manually by wire splicing or by using special
drill pipe with cable embedded within the pipe (DTD 2004; Finnsson 2005). There are additional costs
for the remote magnetic guidance system as well as the use of specialty drill pipe.
Gyroscopic Steering Tools
Gyroscopic steering tools are a highly accurate type of HDD locating technology that do not require any
wireless communication or surface access, as shown in Exhibit A-23. They use optical gyroscopes and
accelerometers to track the drill-head's position and orientation (DTD 2014b). All of the information is
transmitted to the driller through a wire in the drill pipe. Gyroscopic steering tools are useful for very
deep boreholes or where interference is too great to use either walkover or remote magnetic guidance
systems (DTD 2004; DTD 2014b; Kaback 2002; Lubrecht 2011; NASTT 2008). However, using a
gyroscopic steering tool is much more expensive than the other locating technologies because it
requires specialized technicians, a large drill rig, and large drill rods (DTD 2004; DTD 2014b).
Exhibit A-23. Gyroscopic Steering Tool
(Reprinted from Horizontal Technology, Inc. website (2015) with permission of Horizontal Technology, Inc.)
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Drilling Fluids
HDD drill rigs pump drilling fluid down the borehole while drilling to maintain borehole stability, remove
cuttings, cool the drill-head and bit, and lubricate the well materials to be installed (Bardsley 2014a; DTD
2004; Kaback 2002). Selecting an appropriate drilling fluid is crucial for a successful HRW installation.
Using an improper type or density of drilling fluid may seal the pore spaces in the soil around the
borehole, reducing the ability of the HRW to function as intended (Bardsley 2014b). To prevent loss of
porosity, some cleanup managers limit the density of drilling fluid that can be used, or require the use of
biodegradable drilling fluids (Bardsley 2014b).
Bentonite-based drilling fluid is typically used in general HDD applications. It transports cuttings well
and seals the borehole, keeping the drilling fluid from escaping into the surrounding soil. However,
bentonite's effectiveness at sealing borehole walls can be a disadvantage when installing a remediation
well, which must extract or inject materials from or into the subsurface (Wampler undated). Therefore,
bentonite-based drilling fluid must be removed from the borehole during the back-reaming process or
during well development (Kaback 2002); see the Well Development section.
Several manufacturers have developed drilling fluids specifically for HRWs. Bio-polymers are an
alternative type of drilling fluid that biodegrade after the drilling process to avoid reducing the soil's
permeability (Bardsley 2014a; Mott-Smith et al. 2011; Carlisle et al. 2002; Doesburg 2013; DTD 2010).
The biodegradation occurs naturally within a few days or can be degraded in one day by adding an
enzyme (DTD 2010; Lubrecht 2011). Bio-polymers are more expensive than bentonite (DTD 2010). Care
should be taken to ensure that the bio-polymer's breakdown products do not adversely affect the
chemistry of the subsurface.
At some HDD projects, drilling fluid is recycled by collecting fluid as it comes out of the borehole,
removing drill cuttings from the fluid, and reusing the fluid in the drilling process (DTD 2004; Lubrecht
2012). Recycling the drilling fluid reduces the volume of fluid that must be produced and later disposed
of (Bardsley 2015; DTD 2004; Lubrecht 2012). However, at cleanup sites, potential contamination of the
drilling fluid may make it preferable not to recycle the fluid (DTD 2004). Small HDD rigs usually do not
recycle drilling fluid (DTD 2004). Drilling fluids often contain additives to improve performance (Bardsley
2015; Wampler undated). For example, surfactants can be added to prevent clays from sticking to the
drill pipe (Wampler undated). A corrective action plan proposing to use HRWs should include a disposal
plan for used drilling fluid and cuttings, as well as ensure additives in the drilling fluid will not adversely
affect the chemistry of the subsurface.
For surface-to-surface horizontal wells, if the entry hole and the exit hole are at different elevations,
drilling fluid management becomes a concern (Lubrecht 2011). It may be advisable to avoid drilling
uphill and prevent drilling fluid from spilling out of the entry hole (Wampler undated).
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Containing Contamination
Depending on the nature of the contamination at the site and at the location of the planned borehole,
the corrective action plan should include plans for dealing with contaminated drilling fluid and
contaminated drilling equipment (Bardslev 2015; DTD 2004). HDD drill rigs wipe mud off the exterior of
the drill pipes as they are withdrawn from the borehole. If additional decontamination is needed,
specialized drill pipe cleaners that use steam are available (DTD 2004). If drilling fluid encounters
contamination and is reused, then the interiors of the drill pipes may need to be decontaminated.
Sometimes during HDD, drilling fluids escape from the ground at unexpected places. These occurrences
are called inadvertent returns, or daylighting, as shown in Exhibit A-24. Inadvertent returns can occur
due to hydrofractures called frac-outs, existing preferential pathways, such as soil fissures, loose gravel,
building foundations; or other causes (Bardslev 2014a; NASTT 2008). Inadvertent returns are more likely
to occur under these conditions (Bardslev 2014a):
	High mud weight and pump pressure
	Boring near the surface typically within 10 feet
Corrective action plans that propose to use a HRW should include a plan for dealing with inadvertent
returns. Bardsley (2015) recommends the following precautions:
"Supplies necessary to handle the IR [inadvertent return] must be on site and, often, the IR plan
may require one member of the crew to continuously walk the bore path to observe and report
any inadvertent returns. The IR plan also must fully detail plans in case the drilling fluid appears
in the stream, wetland or river being drilled."
Use hay bales, silt fencing, sand bags, berms, and vacuum trucks to contain or capture drilling fluid spills
(NASTT 2008).
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Exhibit A-24. Inadvertent Returns During HDD
(Reprinted from Horizontal Environmental Drilling 101 -An Introduction to the Means and Methods for Horizontal
Environmental Well Installation (2014) by David Bardsley with permission of Directed Technologies Drilling, Inc.)
Casing And Well Screen Selection
Well design is one of the most challenging aspects of HRW installation and project success. Well design
is highly variable and is based on multiple factors, including design flow rates, length of the screen, total
depth, total well length, soil types, and others.
Horizontal well casing materials are similar to those used for vertical wells (Bardsley 2014a):
	HDPE
	Stainless steel
	Carbon steel
	PVC
	Fiberglass
Several general rules apply when selecting casing materials. HDPE is used where design necessitates
that significant curving or bending radius is needed. Stainless or carbon steel may be used for high
pressure or temperature applications, or where installation requires higher tensile strength. PVC is
typically used for shorter or shallower installations.
In general, greater stresses are placed on the well materials during installation of horizontal wells than
conventional vertical wells. The well materials are either pulled or pushed into long, curving boreholes,
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creating high tension or compression, and bending (DTD 2004; Kaback 2002). The pullback strength of
HDD rigs greatly exceeds the tensile strength of some casing materials (DTD 2004). The casing material
must also be able to withstand the curvature of the borepath. Potential damage includes abrading the
well screen during installation, causing reduced permeability, or partial breaking of the well screen,
causing the well to admit formation materials (DTD 2004). If the damage is severe, the HRW may not
function as intended. Horizontal wells may also be subject to crushing forces from the overlying soils "if
poorly-consolidated material is being penetrated or the installation is very shallow, particularly if the
casing is longitudinally slotted" (DTD 2004). HRW designers must take into account these stresses when
selecting well materials and screen types (NASTT 2008, sections 5.4.6 and 5.4.7).
As for all wells, the diameter of the riser pipe must be large enough to accommodate the submersible
pump needed for operation. This is especially important to keep in mind for HRWs because they
typically have smaller diameter casings than conventional vertical wells (Soukup 2016).
Proper well screen selection is also crucial for ensuring a well's effectiveness. Well screen selection is
highly site-specific and depends on many variables; therefore, the design and selection of the well
screen should not be based on untested assumptions. The well screen must be designed to withstand
installation stresses, carry out the desired remediation function, and prevent excessive infiltration of
sediment into the well. Due to the complexity of well screen design, some HRW drilling firms
subcontract well screen design to a materials specialist. Exhibit A-25 presents a typical well screen.
Hydrogeologic software is also available to aid in designing the well screen (Van Heest et al. 2013). For
horizontal SVE wells, a zone well screen design can be used, with more or larger openings at the end of
the screen that is farther from the pump; this helps make the flow uniform by equalizing the pressure
along the length of the well screen (Doherty et al. 2000).
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Exhibit A-25. Well Screen In HDPE Pipe
(Reprinted from Horizontal Wells for Groundwater Remediation (2013) by Garry Van Heest with permission of
Directional Technologies, Inc.)
Conventional well construction techniques call for installing a filter pack to fill the space between the
well casing and the borehole, which improves the well's function by preventing clogging. Installing a
filter pack in a horizontal well is more difficult than for a conventional vertical well because the
horizontal orientation of the well means that a filter pack cannot be poured in place by gravity methods
as is done for conventional vertical wells (DTD 2004). Therefore, it is common for HRWs to have no filter
pack installed; such naturally developed wells have the well screen in direct contact with the geologic
formation (Soukup 2016; Strong 2016). Naturally developed wells work best in coarse, well-graded
formations; natural well development is more challenging in fine, uniform material. At sites with fine,
uniform material, the well screen slot size must be carefully chosen; openings slightly too small will
reduce well yield, and openings slightly too large will cause long-term infiltration of sand and thus
increase maintenance (Soukup 2016). Natural well development may also be practical for some types of
HRWs where clogging is less likely, such as SVE wells and some injection wells (DTD 2004).
Several manufacturers produce pre-packed well screens specifically for horizontal wells, in which a well
screen is manufactured with a filter already in place (DTD 2004). These pre-packed filters are relatively
fragile and susceptible to damage during installation; they must be able to withstand the bending,
dragging, and other forces imposed during weli installation. Pre-packed weil screens also require larger
boring diameters.
Sealing The Well
After the well materials have been installed in the borehole, the annulus (the space between the riser
pipe and the borehole) must be sealed to prevent surface water from entering the well and, for injection
wells, to prevent the injected materials from escaping to the surface along the riser pipe (DTD 2004;
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Lubrecht 2011). For surface-to-surface wells, both ends of the well need to be sealed. Horizontal wells
are typically sealed by pumping cement grout down the borehole using tremie pipes (Exhibit A-26).
Prior to pumping the cement grout into the borehole, it may be necessary to block the annular space to
prevent the cement grout from flowing down to the screened portion of the well casing. Various
materials can be used to block the annular space, including bentonite, sand, polyurethane grout, and
pneumatic packers (DTD 2004, pp. 18-20).
Exhibit A-26. Schematic Of Horizontal Well Construction
Grout Seal
Well
Casing
Well Screen
Horizontal well ends can be designed to be either aboveground or underground; underground well ends
are placed within a vault. If using a vault, the well's angle and configuration of the vault must allow all
necessary tools and instruments to enter the well (DTD 2004; Lubrecht 2011).
Well Development
After the well materials have been inserted in the borehole and the well has been sealed, well
development must take place. One purpose of this step is to remove the soil particles that usually get
caught in the well screen openings during horizontal well installation (DTD 2004; Lubrecht 2011).
Another purpose of well development is to remove remaining drilling fluid from the well, borehole, and
surrounding soil (Bardsley 2014b).
Developing horizontal wells is different from developing conventional vertical wells. Horizontal wells
have much longer screens, and these screens are pushed or pulled for long distances through the
borehole during installation (DTD 2004). Therefore, developing a horizontal well can take much longer
than a conventional vertical well (Kaback 2002).
Two methods for well development are overpressuring and jetting. Overpressuring the well works by
capping one end of the well and pumping water into the well at high pressure. The hydraulic pressure
forces the clogging soil particles out of the well screen openings. The overpressuring technique is
inexpensive and simple, but does not work as well with clayey soils and long well screens. In addition, in
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screens longer than 200 feet, the pressure may not be great enough to clear all of the openings. The
overpressuring technique can be augmented by using inflatable packers to isolate one section of the
well screen at a time; each section is then overpressured in turn, rather than overpressuring the entire
length of the screen at one time (DTD 2004).
Jetting is the second method commonly used to develop horizontal wells. Jetting entails pushing or
pulling a hydraulic jet through the well screen to dislodge soil particles from the well screen openings, as
shown in Exhibit A-27. Jetting a well takes longer and is more expensive than overpressuring the well
but is typically more effective in removing fines from the annular space and the slots. Jetting is
recommended in wells where overpressuring may not be effective for example in clayey soils or in wells
using long well screens (DTD 2004).
Exhibit A-27. Hydraulic Jet Used For Horizontal Well Development
(Reprinted from Horizontal Environmental Well Handbook (2004) by Directed Technologies Drilling with permission
of Directed Technologies Drilling, Inc.)
Operation And Maintenance
Depending on their use and the subsurface conditions, HRWs typically require less frequent
redevelopment than do vertical wells, because it takes more time for the longer well screens of HRWs to
be rendered ineffective by clogging. But depending on the geologic formation's particle sizes and the
well screen selected, naturally developed HRWs with no filter pack, may experience long-term
infiltration of sand and silt, requiring recurring maintenance (Soukup 2016).
Measuring the water level and drawdown is more difficult in a horizontal well due to the angled riser.
The contractor must devise a means of manually recording the distance to static level down the riser
pipe and converting it to the elevation of the groundwater surface relative to mean sea level, To
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accomplish this, the borehole's trigonometry needs to be defined using detailed well construction logs
with boring angles and casing lengths. To promote accurate well gauging, use results from manual well
gauging and pressure transducers to calibrate and refine each other (Soukup 2016).
The angled risers of HRWs also require selecting pumps that are capable of operating at an inclined
angle without excessive wear on the motor and impellers (Soukup 2016).
Conclusion
HRWs, like vertical wells, can be used with a variety of cleanup methods. The choice of horizontal wells
or vertical wells depends on site-specific conditions and the remediation technology deployed. The
main advantages of HRWs are:
	Improved access at sites with surface or near-surface obstructions
	Improved contact efficiency for elongated, relatively shallow plumes
	Reduced site impact
	Reduced operation and maintenance
Disadvantages include:
	Their relatively high initial cost
	The increased complexity of planning, installing, and developing a horizontal well compared to a
conventional vertical well
	Limited number of drilling subcontractors with environmental experience
	Limited acceptance and familiarity from the environmental remediation industry and
environmental regulators
Depending on site-specific considerations, HRWs may be a viable and cost-effective option to consider
during corrective action planning.
October 2017
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