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Geoprobe®: The Right Tools for Investigation of Uranium in Drinking Water
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| Day One at the Clarks, NE, site. The Geoprobe® R&D team used a 6625CPT machine to collect Hydraulic Profiling Tool (HPT) logs near the two
public water supply wells. Steve Knobbe (left), Project Engineer, and Richard Holmgren, Direct Image® Specialist, work at the HPT1 location. |
A few years back, the Nebraska Department of Health and Human Services began testing all of the water
supply wells across Nebraska to be sure they were in compliance with the new uranium-mass regulation promulgated
by the U.S. EPA. This new regulation established a maximum contaminant level (MCL) of 30 ug/L for uranium
in drinking water. Tom Christopherson, Program Manager for Water Well Standards of DHHS, soon learned
that the old public water supply (PWS) wells in the small farming community of Clarks in central Nebraska
were contaminated with uranium at concentrations between 100 to 200 ug/L. This was bad news for the
approximately 375 residents of Clarks. After a difficult search, they installed two test wells about 1.5
miles northeast of town, and both wells were nondetect for uranium. Two new PWS wells
were installed adjacent to the test wells, and initial pumping and testing found that the
new North well was yielding water with uranium concentrations about 30 ug/L. After initial testing was done by the University of Nebraska - Lincoln, it became
apparent that a modified, low-speed pumping program was not going
to correct the problem. It was then that Tom Christopherson asked
if Geoprobe Systems® would demonstrate direct push methods at the
site, and assist the agency in understanding the cause of the elevated
uranium problem at the Clarks well field.
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| Site sketch map of Clarks, NE, public water supply wells, test wells, HPT
logs, and prepacked screen monitoring wells installed during the project.
Green diamonds mark HPT log locations. Solid circles mark DP well locations
screened at different depths to study changes in groundwater chemistry. |
Led by Wes McCall, Geoprobe® Environmental Geologist, a team
of Geoprobe® Research and Development Engineers mobilized equipment
to Clarks in September and performed a 4-day expedited investigation
at the new well field. According to Wes, “Going to sites such as this allows the
Geoprobe® R&D team to test new techniques and tooling in ‘real world’ applications.
Not only was this a great site to test our new tooling, but we were also able to help a small
community in need of safe drinking water.”
Using the Hydraulic Profiling Tool (HPT), five HPT logs were collected across the well field site. A Geoprobe®
6625CPT machine and the HPT system were used to gather information on the local hydrostratigraphy at depths approaching
120 feet.
The first log
(HPT1) was
obtained near
the South Public
Water Supply
(PWS) well. “HPT logs allowed
us to ‘see’
subsurface information,”
Wes
explained, “and
helped us to plan
the placement
of monitoring
wells during the
next steps in the
project.” The
logs revealed the
presence of several
fine-grained
layers or lenses
separating thicker sand zones. Also shown on the first log (next page) is the construction of the 4-inch diameter test well
and the new 12-inch diameter South PWS well, based on drillers’ logs. The North PWS well and test well were constructed
similarly.
On Day Two, another team arrived for monitoring well installation using direct push
(DP) methods. “Based on the information displayed in the HPT logs,” Wes said, “0.75-
in. ID prepacked screen wells were installed at selected depths adjacent to the HPT1 log
location (A-Group wells).” Five-foot prepacks were installed in the aquifer between the
fine-grained lenses which were identified by the HPT logs.
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The Clarks South PWS well field is located in the flood plain of the Platte River in Central Nebraska. The Village Board of
Clarks worked with the local grocery store owner to install a reverse osmosis filtration system to provide “treated” bottled
water to the residents that wished to participate. The restaurant and convenience store also installed similar systems. |
During this process, the R&D team used the new Model 8040DT machine to
advance 2.25 in. OD rods up to 118-feet deep. A new Model GS2250 grout machine
performed bottom-up tremie grouting with a 25 percent solids bentonite slurry. A second set of prepacked screen wells (B-Group) was installed beside
the HPT2 location and at some distance from both of the
PWS wells. Because of the 8040DT’s capabilities, eight of
the prepacked wells were installed in two days. The fifth
shallow well at HPT2 was installed the following morning.
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Tom Christy, Geoprobe® Vice President, (left)
explains downhole information displayed realtime
on the FC5000 Field Instrument to Dana
Peterson, with Tagge Engineering Consultants
in Holdrege, NE, as HPT logs are taken near the
Clarks South PWS well. Tagge Engineering was
investigating options for overcoming the high
levels of uranium in the PWS wells. Richard
Holmgren (far left) operated the 6625CPT
machine as the logs were collected. |
One-half-inch PE tubing and tubing bottom check
valves were used for the initial development of the 0.75-in.
prepacked screen wells. The 12-volt Geoprobe® Electric
Actuator powered the check valve system to develop the
wells up to 115 ft. deep. The check valves were moved
across the screen interval to surge and purge between five
and twenty gallons from each well. After the initial development
significantly lowered the turbidity, mechanical bladder
pumps were installed to perform low-flow purging and sampling.
Electric actuators also powered the bladder pumps
at flow rates between approximately 150 ml/min to 300
ml/min. Flow from the pumps was directed through a small
flow cell equipped with a YSI556 Multi-Parameter Probe to
monitor water quality parameters (DO, ORP, pH, Specific
conductance and temperature). Turbidity was monitored
periodically with a Cole Parmer turbidity meter. Turbidity
was below 10 (NTU) in all of the wells prior to sampling.
DHHS personnel collected samples for several cations,
anions, and trace elements, including uranium, from each
of the newly-installed prepacked screen wells. “Since one of
the 4-in. test wells had shown elevated uranium from earlier
sampling,” Wes said, “both of these wells were sampled for
all of the analytes and uranium using mechanical bladder
pumps and the electric actuators.”
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| (l to r) Wes McCall, Geoprobe®
Environmental Geologist, reviews HPT logs and
monitoring well installation plans with Tom
Christopherson, Tony Martinez, and Bob Byrkit,
all with the Nebraska Department of Health and
Human Services in Lincoln, NE. (The bottle of
suntan lotion in the foreground was a coveted
commodity under the hot September sun.) |
Samples from all nine of the DP installed prepacked
screen wells and the two 4-in. test wells were collected by DHHS personnel and submitted to the Nebraska State
Laboratory for analysis. According to a brief summary of
the chemistry for the A-Group wells, which were installed
near the South PWS well, the Specific conductance, sodium
(Na), sulfate (SO4), selemium (Se) and uranium (U) all
tended to decrease with depth. Wes explained, “However,
the results for the South 4-in. test well, near
the A-Group DP wells, appeared anomalous. Although the test well was screened
deep in the aquifer, it had chemistry similar
to the shallow DP wells (A4 & A3), and
was out of equilibrium with the deep DP
wells (A1 & A2) screened at similar depths.
Based on HPT log information and well
data, the gravel packed annulus of the old
4-in. test well was behaving as a conduit
for shallow groundwater to move down to
depth in the aquifer, especially when the
PWS well pumps were activated.” Probably
most obvious is the uranium in the South
4-in. test well. “Data indicates that the
uranium levels in the South test well are 168
ug/L, which is about five times higher than
anything observed in the nearby A-Group
wells for uranium,” Wes added. “This is
definitely not in equilibrium with the A1
and A2 DP wells screened at similar depths,
which are both non-detect for uranium.”
 Results for the B-Group wells show
that Specific conductance, Na, SO4, and Se
again tend to decrease with depth. However,
uranium concentrations are distinctly
higher in the B4 and B3 wells ... up to 376
ug/L. According to Wes, “It appears that
these two zones in the aquifer are probable
sources for the uranium observed in the South 4-in. test well and the PWS wells.” By observing the
construction information of the South PWS well, the filter
pack extends up to approximately 60 ft. below grade. This
means that the filter pack for this well directly intersects the
B3 zone of the aquifer with the high uranium concentrations. “It becomes apparent,” he said, “that both the filter-packed annulus of the original test wells and the extended
filter pack of the PWS wells allow for movement of groundwater
from shallow zones of the aquifer downward into the
well screen.”
It’s important to note that all of the field investigation
activities described here took place in less than a week onsite.
That’s nine prepacks set to depths up to 110 feet for a
total of 609 feet of prepack installation.
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| Brent Kejr, Geoprobe® Product Engineer, uses 2.25 in.OD rods under the Geoprobe® 8040DT
machine during the monitoring well installation process. Some of the casings were driven to 118
feet depending upon the information obtained from the HPT logs. |
“This was a good project to demonstrate the ability of
the new 8040DT machine,” noted Tom Christy, Geoprobe® Vice President. “The installations went faster and
smoother than anything we could have achieved with our
other model probe machines.”
Tom also noted that this project sets a good example for
the use of direct push logging data. “We sent the HPT unit
out there a day ahead of time. The logs we got were very informative;
showing us clay and fine-grained zones that previous investigations of the site by drilling just were not able to
pick up. We knew exactly where to set our prepacks to get a
good, multi-level picture of the distribution of
uranium. As a result, all of the prepacks developed
and yielded good samples for analysis.
Based on knowledge of the geology, the
contamination is not caused by human activity
but rather the uranium occurs naturally in the
sediments of the aquifer. There is also some
uranium in the soil outside your window and
in the soil most everywhere on the planet. And
actually the U in the soil outside your window
may be 10 to 100 times the concentration in
the drinking water the people in the community
of Clarks were drinking, Wes explained.
Ultimately, the uranium in the aquifer
sediments below Clarks comes from the sedimentary
rocks in eastern Wyoming and also from
the granites in the rocky mountains. The rivers erode
the sedimentary rocks and granites, and carry the sand, silt,
clay, and gravels down the Platte River ... along with the
uranium inside of them. These sediments now make up the
aquifer materials in the Platte River and Village of Clarks
water supply aquifer.
According to Wes, the real problem is exposure to the
uranium. “We don’t eat the soil, and we usually don’t put
your dirty hands in your mouth,” he said. “We usually
don’t inhale much of it either, even on dry windy days in the
central plains. So we don’t actually get ‘exposed’.”
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David Jundt, Water Supply Specialist with the Nebraska Department of
Health and Human Services, collects a sample for uranium from the South
4-in. test well. A Geoprobe® Electric Actuator draws samples through the
downhole Mechanical Bladder Pump which were then taken to the NEDHHS
lab for analysis. |
Unfortunately
for
the people in
Clarks, the
small amount
of uranium
in the
sediments in
their aquifer
(probably
only a few
parts per
million) is
dissolving a
little in the
groundwater
due to the
groundwater
chemistry in
some parts of the aquifer. This dissolved uranium gets pulled
in by the pump in their supply wells and it gets to their
water tap at home. “The residents of Clark drink the water or cook with the water,” Wes said, “and so ingest it and are ‘exposed’. It seems
the primary concern
with uranium is not its
radioactive decay in the
body (though that’s not
good), but the effects
Uuranium has on our
kidneys as a heavy
metal causing damage
to the organ.”
Currently, the
community of Clarks
is having the test wells
properly abandoned
and is evaluating corrective measures for the new PWS wells
to come back into compliance with the 30 ug/L uranium
MCL. Wes concluded, “While we’re more familiar investigating
problems with man-made contaminants, like gasoline
or TCE in our aquifers, this project demonstrates that naturally-occurring materials, such as uranium or arsenic, can
be a significant problem.” He also stated that this project
revealed that the same direct push methods the industry has
applied for the investigation of other contaminants works
well to understand the presence and distribution of naturally
occurring analytes. “It’s easy to imagine how differently the
PWS well design and construction would have progressed,” Wes concluded, “if the data from the small DP wells had
been available before the project was started.”
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| Wes McCall, Geoprobe® Environmental Geologist and
Project Coordinator, checks formation collapse through
the ID of the well casing prior to grouting the well. |
Although finding uranium in your drinking water isn’t
good news for the people in Clarks, having the ability to test
new products on sites such as this is beneficial to the Geoprobe® R&D team. It also has a positive impact on
the industry. New technology and product improvements
to better the environment are a direct result of
being able to test equipment and new ideas. It’s also a
good reminder of why the direct push method is good
for both the environment and for those who work in
this industry. Although the levels of uranium were
high in the groundwater, at no time was the site team
exposed to contaminants or were there hazards to their
safety. There were no waste cuttings generated at the
site which eliminated the need for handling, storage, sampling,
analyzing, transporting, or disposing of the contaminated
waste ... both a risk to health and a costly investment.
For information on uranium and other radionuclide
regulations go to www.epa.gov/safewater/radionuclides/basicinformation.html. Additional information about this
investigation and the specific field methods
applied and equipment used is available
by contacting Wes McCall at Geoprobe
Systems®, 1-800-436-7762 or at mccallw@geoprobe.com.
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