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Sampling of monitoring well TW-G along the bank of the Missouri River at
operable unit OU1 just before the well is submerged by the rising river on
May 17, 2002. The large snag on the right is the top of the boat ramp. This well monitors
the downgradient end of the PCE plume in the alluvial aquifer emanating from
the Front Street building about 600 feet southwest of the well.
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| Careful sampling during the drilling of a shallow monitoring well at operable unit OU2
detected the presence of a dense non-aqueous phase liquid (DNAPL) in fractures between
16 and 20 feet deep. Analysis the DNAPL (black fluid) indicated that it was pure
tetrachloroethene (PCE).
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The USEPA cooperated with City of New Haven and the Kellwood Company during the
installation of a new water line to the high school to ensure that contaminants
from OU2 did not affect the water quality in the line. No contaminants were detected
in soils during the installation of the water line or in water from the line after
its installation. |
CABLE TOOL DRILLING RIG
This type of drill rig is used to install wells in areas where contamination is known
or suspected to exist at high levels. Well casing can be installed as the borehole is
being drilled. This technology has been used for more than 100 years, but generally has
been replaced by faster air-rotary methods. Essentially, a heavy bit is suspended on a
cable, raised a small distance, and dropped to break up the rock. A bailer is used every
3 to 5 ft (feet) to remove the rock cuttings.
This method is slow (20 to 40 ft per day) but no air compressors are needed
(potential source of contaminants themselves), and little water is added while
drilling. The lack of additives to the borehole enables U.S. Geological Survey (USGS)
personnel collect high-quality samples every 5 ft of depth--providing a vertical
distribution of tetrachloroethene (PCE) in the bedrock aquifer. In addition, the method is
clean and cuttings can easily be placed in drums for removal.
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This may be not be pretty and it is slow, but it provides for good samples.
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Very fast, but can be messy and difficult to get a vertical profile of contamination.
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AIR ROTARY DRILLING RIG
Nearly all public-and domestic-supply water wells are drilled using air rotary
methods. This type of drill rig is used when there is little to no contamination
is suspected and is used primarily to install the deeper bedrock wells. A large
air driven hammer bit on the end of a hollow string of drill pipe is use to break
up the rock. The compressed air is used to blow the rock cuttings and ground water
up the borehole and onto the surface. Hundreds of cubic feet of air per minute at
several hundred pounds per square inch pressure are required.
Air rotary drilling is fast (hundreds of feet per day) but can be messy. A problem
with the method is the large volume of waste generated during drilling. You may see
large dumpsters or roll-off containers placed next to the drill rig to catch the
drill cuttings. Water is pumped from the containers and disposed in the sanitary
sewer system each day. The large volume of air used strips PCE and other VOCs from
the water making it difficult to determine the vertical profile of PCE
in the bedrock. To avoid this problem, water samples are collected from
the borehole at the beginning of each day.
For deeper holes such as were drilled at the grade school and at the Kellwood site,
an extra compressor (red trailer on the right in top photo) is needed.
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DIRECT-PUSH SOIL SAMPLING AND WELLS
In unconsolidated sediments such as in the Missouri River alluvium at operable unit
OU-1, direct-push technology is used to collect soil and ground-water samples. Clockwise
from the top left corner shows a GeoprobeTM truck-mounted direct-push
drill rig that was used to collect ground-water samples along the Missouri River.
The geoprobeTM pushes threaded steel rods into the ground using a hydraulic
hammer. At the desired depth, a 4-ft long well screen at the bottom of the rods is
opened and a plastic tube inserted from the surface (middle photo).
A small pump is used to remove a water sample from the temporary well, and the screen is
pulled. The process repeated at various depths. Using this technology, the vertical
distribution of PCE in the alluvial aquifer at various locations can be determined.
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Temporary wells allow the vertical profile of PCE to be determined in soft sediments.
Technique used extensively at OU-1 (Riverfront site).
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You are what you eat, and a tree is no different. Technique used extensively at
all operable units.
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TREE CORING
Drilling monitoring wells or collecting subsurface soil samples is costly and can
be a great nuisance to property owners. The USGS has been researching the use of
tree- core samples as a means of determining if subsurface soils or shallow
ground-water at a site are contaminated. The technique is fast and inexpensive.
Trees pull moisture from the ground and with that moisture any dissolved contaminants.
Volatile organic compounds such as PCE also may be absorbed by the trees roots from
soil gases.
A core sample from the tree trunk is placed in a sealed glass vial and analyzed for PCE and other VOCs. The plume of contaminated
ground water in the
alluvium at the Riverfront Site (OU-1) was initially discovered by
analyzing tree core samples.
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