Environmental Research & Technology Division
Technology Development & Applications Group
Perfluorocarbon Tracer Verification Technology
Introduction
 One of the more promising remediation options available to the
Department of Energy (DOE) waste
management community is subsurface barriers. Some of the uses of subsurface barriers
include surrounding and/or containing buried waste, as secondary
confinement of underground storage tanks
, to direct or contain subsurface contaminant plumes and to restrict remediation
methods, such as vacuum extraction, to a limited area. Subsurface barriers are a remediation
option for many of the DOE defense sites including: Sandia
National Laboratories
(SNL), Hanford, Idaho National Engineering
Laboratories (INEL),
Oak Ridge National Laboratories (ORNL) , Fernald,
and Rocky Flats. Barriers are also considered an important remediation option by the
USEPA. Several of the DOE Integrated Demonstrations (MWLID, BWID) and Programs
(ISRIP) are investigating variations of permeation grouting
and jet grouting to emplace grout
barriers. Permeation grouting is plagued by short circuiting of the flow of grout which can
leave large untreated areas. Jet grouting methods require straight boreholes and sufficient
overlap of columns to maintain barrier continuity. Often the borehole wanders or the jet is
partially obstructed by cobble or varying soil types, leaving a gap in the final barrier. Panel
jet grouting may leave gaps between panels and or at the junctions of horizontal and vertical
barrier walls. Panel grouting may be thinner,
and thus more prone to cracking. Additionally, at the time of gel formation separations or
"tears" may occur if localized settling takes place. The ability to verify barrier integrity is
valuable to the DOE, EPA, and commercial sector and will be required to gain full public
acceptance of subsurface barriers as either primary or secondary confinement at waste sites.
It is recognized that no suitable method exists for the verification of an
emplaced
barrier's integrity. Because of the large size and deep placement of subsurface barriers,
detection of leaks is challenging. This becomes magnified if the permissible leakage from the
site is low. Detection of small cracks (fractions of an inch) at depths of 100 feet or more has
not been possible using existing surface geophysical techniques. Compounding the problem of
locating flaws in a barrier is the fact that no placement technology can guarantee the
completeness or integrity of the emplaced barrier.
Brookhaven National Laboratory
(BNL) has developed a host of perfluorocarbon
tracers (PFT). These tracers were originally utilized in atmospheric and oceanographic studies
and have since been applied to a great variety of problems including detecting leaks in buried
natural gas pipelines and locating radon ingress pathways in residential basements. Prior
accomplishments in determining gas pathways in residential basements is extremely applicable
to barrier verification. The residential basements studied are essentially miniature "barriers"
with vertical concrete walls and a horizontal concrete floor. BNL is applying this tracer
technology to barrier verification.
PFTs can be detected at extremely low levels, parts per quadrillion are
routinely
measured. This allows detection of very small breaches in the barrier. A breach can be
located by injecting a tracers on one side of a barrier wall and monitoring that tracer
on the other side
. The injection and monitoring of
the tracers can be accomplished
using conventional low cost monitoring methods such as existing vadose zone monitoring wells
or multilevel monitoring ports, placed using cone penetrometer techniques (e.g. Hydropunch).
The amount and type of tracer detected on the monitoring side of the barrier will determine the
size and location of a breach. It is easy to see that the larger the opening in a barrier the
greater the amount of tracer that transports across the barrier. Locating the breach requires
more sophistication in the tracer methodology. Multiple tracer types can be injected at
different points along the barrier (both vertical and horizontal). Investigation of the spectra of
tracers coming through a breach then gives a location relative to the various tracer injection
points. The key advantages of the BNL PFTs are
multiple tracers,
regulatory acceptance (PFTs have been used in atmospheric, oceanographic, and subterranean
studies), extreme sensitivity, and proven technology with commercial acceptance and use.
PFT technology should allow locating and sizing of breaches on the
order of fractions
of an inch at depth in a subsurface barrier. The technology has regulatory acceptance for other
applications and is used commercially for non-waste management practices (e.g., detecting
leaks in underground power cables). This technology has been used in a variety of soils and
may be applicable to the entire DOE complex as well as commercial waste sites. The major
use of tracers will be to verify placement continuity of a freshly emplaced barrier and to re-
check corrective actions that may be used to seal or repair a breach. It may also be useful to
periodically check a barrier to determine the long term integrity of the walls. This would
certainly be beneficial if a cementitous grout (portland based) barrier were used. Cementitous
grouts are prone to cracking from various degradation modes including wet-dry cycling which
is prevalent at many of the DOE sites (e.g., Sandia and Hanford). Continued use of tracers
would allow determination of containment performance over the life of the barrier.
An additional use for PFTs may be in confirming integrity of waste
disposal vaults.
Many of the agreement states have regulated that low level waste will be disposed of in some
form of an engineered vault. The technology developed for subsurface barriers should directly
transfer to disposal vault validation. Since the predominant failure mode of vaults is some
form of concrete cracking, tracers could be used for the entire lifetime of the vault to
determine when and what performance loss occurs.
Technology Description
Perfluorocarbon tracer (PFT) technology consists of the tracers
themselves, injection
techniques, samplers and analyzers. PFTs have the following advantages over conventional
tracers:
a) Negligible background concentrations of PFTs in the environment.
Consequently, only small quantities are needed:
b) PFTs are nontoxic, nonreactive, nonflammable, environmentally safe (contains
no chlorine), and commercially available;
c) PFT technology is the most sensitive of all non-radioactive tracer technologies
and concentrations in the range of 10 parts per quadrillion of air (ppq) can be
routinely measured;
d) The PFTs technology is a multi-tracer technology permitting up to six PFTs to
be simultaneously deployed, sampled, and analyzed with the same
instrumentation. This results in a lower cost and flexibility in experimental
design and data interpretation. All six PFTs can be analyzed in 15 minutes on a
laboratory based gas chromatograph.
Typically, the PFTs are measured by a capillary adsorbent tracer sampler
(CATS)
which is a small cigarette sized glass tube containing a carbonaceous adsorbent specific for the
PFTs. This sampler can be used dynamically (flowing a sample through the CATS) or
passively (Opening only one end so as to allow the CATS to sample by diffusion). The passive
mode allows a time integrated PFT concentration to be measured in a simple manner. The
CATS are shipped back to the laboratory for PFT analysis. Several real-time PFT analyzers
are available, one which detects four different PFTs per five minutes sample down to the
ambient background of the PFTs in air. Another real time instrument can analyze PFT down
to a part per trillion but cannot speciate the various PFTs.
The two general areas of application for a multi-tracer technology are
a) Transport and dispersion studies, that is, tracers are used to tag a
flowing medium,
such as gases or liquid, so as to determine where that flowing medium is going and how it is
being dispersed in a surrounding matrix. Examples applications are transport and dispersion of
pollutants in air parcels on a continental scale; transport and dispersion of injected fluids in
petroleum reservoirs for enhanced oil recovery and transport and dispersion of air within
homes and commercial buildings for energy efficiency calculations.
b) Leak detection studies, that is, PFTs have been used to locate and
estimate leak rates
in various scenarios. A present application is the location of leaks in urban High Pressure
Fluid Filled (HPFF) electrical cables which is the primary method of transporting electrical
power in urban areas. The dielectric fluid is tagged with a 0.01% concentration of PFTs and is
transported along with the leaking dielectric fluid into the surrounding subsurface environment.
The dissolved PFT then volatizes through the vadose zone and is emitted into the air through
cracks and other openings present in city streets. With the use of PFT samplers and real-time
monitors the leak can be located within 2 feet along a several miles stretch of buried cable.
Another PFT application relevant to this task is the determination of
radon flux into
residential homes through cracks in the basement floor and surrounding subsurface walls. This
can only be achieved with a multitracer technology. One type of PFT source (with a calibrated
emission rate, most often prepared as a permeation device) is buried outside a residence under
study. Several other types of PFTS sources are placed in the basement and the first floor of the
residence. From a time averaged measurement of the various PFT concentrations in the
basement and first floor and a measured radon concentration in these areas it is possible to
calculate a radon flux rate into the home. This technique of using a reference source of PFT to
estimate the source rate of a compound of interest has also been used to estimate;
The source rate of dioxin into a commercial building from surrounding contaminated soil.
The rate of leaking gasoline from underground storage tanks at gasoline stations.
The rate of leaking dielectric fluid from subsurface HPFF cables.
PFTs also have been used to estimate barrier effectiveness in a study on
locating
explosives tagged with PFTs. Various barriers were studied using reference PFT source
techniques and the barrier effectiveness was quantitatively evaluated.
Low detection limits allow detection of very small breaches in the
barrier. Breaches
will be located by injecting a series of PFTs on one side of a barrier wall and monitoring for
those tracers on the other side. The injection and monitoring (through CATS) of the PFTs will
be accomplished through vadose zone monitoring wells and/or multilevel monitoring ports,
placed using cone penetrometer techniques (e.g. Hydropunch). The amount and type of tracer
detected on the monitoring side of the barrier will determine the size and location of a breach.
It is easy to see that the larger the opening in a barrier the greater the amount of tracer that
transports across the barrier. Locating the breach requires more sophistication in the tracer
methodology. Multiple tracer types can be injected at different points along the barrier (both
vertical and horizontal). Investigation of the spectra of tracers coming through a breach then
gives a location relative to the various tracer injection points (see attachment figure 1).
The key advantages of the BNL PFTs are multiple tracers, regulatory
acceptance (PFTs
have been used in atmospheric, oceanographic, and subterranean studies), extreme sensitivity
(most sensitive of the non-radioactive), and proven technology with commercial acceptance
and use.
Program Progress
PFT tracer verification of the cement layer of a
close-coupled barrier has been initiated. This barrier
is part of a demonstration being conducted jointly by BNL and SNL at Hanford.
On Monday Oct. 16, BNL set up the PFT tracer analyzer at Hanford and background sampling
for PFTs was completed, no PFTs were detected. On Tuesday the tracer injection line was
installed beneath the buried tank. Injection of tracers was began at 1330 hrs and later that
afternoon the first samples were taken. PFTs were detected at several locations. Beginning
Friday CATS sampling, in 50 cc aliquots, was begun. The CATS sampling will continue daily for
two weeks.
BNL is
optimistic that the low level uniform readings will allow at least a gross estimate of diffusion
characteristics of the cement grout.
Sampling was completed during the first week of November. The CATS
were returned to
BNL and chemical analysis for PFTs has been completed. Thus far the sampling shows
promising results. Measurable amounts of tracer are seen all around the cement grout layer.
The computer analysis was initiated to evaluate the results. Initial indications show no breaches
in the
barrier and an estimated gaseous diffusion coefficient on the order of 0.001 cm/sec.
This is similar to results found for concrete basements and radon diffusion.
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Last Modified: November 12, 2009
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