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screen. The ratio of productive,
screened well to unscreened riser casing
is many times that of a single vertical
well, and even more favorable compared
to a network of vertical wells.
Further, the horizontal well requires
only one pump, and limited transfer
piping to convey the water to distribution
or treatment facilities. A vertical
well network contains not only a high
ratio of non-screened, non-productive
casing, but each well requires its own
pump, and the water must be conveyed
to a central facility with multiple
trenched pipelines.
Groundwater in
Connection to Surface
Water
For communities adjacent to large
bodies of surface water, such as lakes,
reservoirs or rivers, water may be
obtained directly from the body, or from
multiple shallow wells installed along
the shoreline. More sophisticated solutions
include the Ranney collector type
of well, with radial, horizontal laterals
connecting to a central caisson.
Fisheries agencies are becoming
more demanding in the construction
and ongoing maintenance of intake
weirs that directly pull water from rivers
and lakes, as protection of fish
habitat is becoming an increasingly
contentious issue. These structures are
also prone to damage from storms and
flooding. Vertical wells along shorelines
have similar issues to the wells in
thin aquifers noted above, in that their
productive zones are limited and it
usually requires many wells to achieve
useable volumes. The Ranney collector
system, first developed in the 1930s, is
an effective technology to capture
groundwater in connection with surface
water, but the systems themselves
are generally expensive to construct,
with costs often exceeding $1 million
for a relatively modest system.
Horizontal directional drilling provides
an elegant solution in these scenarios.
One or more horizontal wells
can be drilled from the surface, extending
into the alluvial deposits or shallow
sediments beneath surface water
bodies. These wells take full advantage
of the natural filtering capacity of the
in situ sand and gravel, and protect
the intake from flood damage or scouring.
If additional capacity is required,
multiple radial horizontal wells that
gravity-drain to a central, vertical wet
well can be installed. A Ranney collector
system requires manned jack-andbore
operations at the bottom of the
caisson, with attendant requirements
for dewatering and accommodation
for worker safety. In contrast, since all
of the work for a radial HDD collector
is performed from the surface, the wet
well can be of considerably smaller
diameter than a Ranney collector caisson
and none of the risks and costs of
manned operations at depth are
incurred.
Fractured Bedrock in
Mountainous Regions
Mountainous terrain can pose both
construction and operational challenges
for water suppliers. On steep, sometimes
unstable slopes it can be difficult
to safely place a vertical drill rig in a
position to effectively intercept waterbearing
formations. In some fractured
bedrock aquifers, the water-bearing
joints run vertically, exacerbating the
problem. In other areas, water production
is from landslide masses that are
relatively unstable. In more remote
locations, getting power to the site to
operate pumps is problematic.
Horizontal directional drilling can
provide a solution to many of these
challenges. New advances in steerable
air hammers has enabled the installation
of wells in extremely difficult drilling
conditions in fractured bedrock,
landslide materials and buried talus.
The ability to design a bore path that
intersects known fracture systems can
enhance water recovery. Further, horizontal
directional wells can be drilled
as gravity systems, requiring no pumps
for operation. For smaller water systems
that rely on natural springs as a
source, HDD can be an excellent option
to enhance recovery and maximize
utilization of water rights.
Aquifer Recharge and
Brine Infiltration
Point source discharges of potable
water from water treatment systems are
highly regulated and usually difficult
to site. These include outfalls from
municipal sewage plants, or discharges
from facilities for treating industrial or
remediation site wastewater. Permitting
and design of such discharges must
consider site ecology and wildlife
impacts, erosion control, protection of
the discharge structure from flooding
or severe weather damage, and a host
of other factors.
Subsurface discharge of treated water
can simplify permitting and design
considerations. Horizontal wells can
extend for hundreds of feet within a
receptive aquifer, and can be designed
to disperse high water volumes with
slight impact to surface activities or
ecosystems.
At Sand City, Calif., a 750-foot horizontal
infiltration well installed parallel
to the beach and 50 feet below the
low tide mark handles the entire waste
brine stream from the city’s 300 acrefoot
brackish water desalination plant.
The well is drilled beneath protected
Monterey Bay coastal dunes, which
serve as habitat for endangered bird
and reptile species. The horizontal well
enables the waste brine (at nearly natural
salinity as discharged from the
plant) to mix with natural seawater
beneath the sea floor before entering
the bay, eliminating ecological issues
related to potential salinity imbalances,
and requiring no outfall structures,
which were prohibited in the area.
Conclusion
Although vertical well drilling is a
proven method to tap groundwater
resources, and will continue to be a
standard technology for water supply,
HDD has made significant advances
for water supply in unusual or difficult
scenarios. If conventional drilling
isn’t the answer to your water
supply problem, lateral thinking may
lead to better results.
Michael Lubrecht, L.G., is a Senior Geologist
and Dan Ombalski, PG, is President of
Directed Technologies Drilling.
November/December 2011 Water Utility Infrastructure Management 29