United Slates
Environmental Protection
Agency
EPA/540/R-08/003
August 2008
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION I
Demonstration Bulletin
Attenuated Anaerobic Dechlorination of Groundwater Using HRCŪ
MACTEC - Harding ESE
Technology Description: MACTEC's Harding
Environmental Science and Engineering Division (Harding
ESE) has designed an in situ Permeable Reactive Barrier
Wall (PRBW) that utilizes Hydrogen Release Compound
(HRCŪ) to treat groundwater contaminated with chlorinated
compounds. HRCŪ is a hydrogen-generating compound
based on a lactic acid/glycerine polyester that is able to
generate free hydrogen under anaerobic conditions. The
bacteria consume the HRCŪ (or hydrolysis products) and
produce the hydrogen needed for dechlorination of
contaminants. This process takes place along with other
anaerobic degradation mechanisms which convert the HRCŪ
to volatile fatty acids such as pyruvic acid, butyric acid,
propionicacid, and acetic acid, which in turn are biodegraded
by methanogenesis to ethane, methane, and carbon dioxide.
HRCŪ is manufactured by Regenesis Bioremediation
Products of San Clemente, California, and is packaged and
shipped in 4.25 gallon PVC buckets, each of which contains
30 pounds of HRCŪ. At room temperature its consistency is
that of a sticky gel (i.e., cold honey). Regenesis advises
heating unopened buckets of HRCŪ in a hot water
(130-170°F) tank to 95°F for 20-30 minutes. When heated to
95°F, the buckets are transferred to a pump hopper and the
desired dose of the fluidized gel is pumped into each well.
By pumping the viscous HRCŪ under pressure into a network
of injection wells (Figure 1), installed perpendicular to the
flow of the groundwater and screened at the anticipated
depth of the plume, HRCŪ can be placed to form a PRBW
perpendicular to the plume.
Although HRCŪ can be used as a stand-alone treatment,
Harding ESE and Regenesis have in the past used a two
staged approach involving Oxygen Release Compound
(ORCŪ) (magnesium peroxide). This oxygen source is
injected into the formation either downgradient of injected
HRCŪ or at a later time. ORCŪ is formulated to convert the
environment to one that is aerobic and oxidative where
daughter products, such as cis-1,2-dichloroethene (DCE) and
vinyl chloride (VC), are more rapidly destroyed than occurs
in anaerobic environments.
The PRBW technology generates essentially no waste
streams above-ground. HRCŪ is reported by Regenesis to be
non-toxic; composed of a polylactate ester (lactic acid) and
food grade magnesium phosphate. Residuals that could be
generated during a full-scale PRBW treatment are
contaminated drill cuttings, purge water, and PPE.
Waste Applicability: According to Harding ESE, their
attenuated anaerobic dechlorination process is applicable to
groundwater plumes contaminated with chlorinated ethenes,
particularly tetrachloroethene (PCE) and trichloroethene
(TCE). Little or no supplementation (nutrients, ammonia,
-------
phosphate) is needed since the soil/groundwater matrix
usually provides enough of these amendments. The soil must
be sufficiently porous to allow groundwater flow through the
PRBWandthegroundwatermustbe in an anaerobic state or
readily convertible to anaerobic conditions (which is helped
by the presence of the HRCŪ hydrogen production capability).
Previous studies have demonstrated that dechlorination is
optimum when the Oxidation/Reduction Potential (ORP) of
the groundwater is in the -100 to -150 millivolt (mV) range;
lower potentials (greater than -150 mV) encourage
methanogenesis and sulfate reduction.
Demonstration Results: A demonstration of Harding
ESE's PRBW was conducted near the Fisherville Mill
brownfields site in South Grafton, MA. The original plan was
to implement a two-stage treatment within a groundwater
plume having relatively high concentrations of TCE. HRCŪ
was to be injected across a section of the plume, followed by
introduction of ORCŪ at a later date, if necessary. However,
when a fire destroyed the existing plant, an alternate off-site
demonstration location was selected.
The evaluation was initiated in June 2000 with the installation
of a series of HRCŪ injection wells and monitoring wells
downgradient of an existing well which was designated as a
"pseudo source" of contamination. The injection wells were
spaced in order to provide overlap of HRCŪ when injected
into the aquifer at 40-50 ft below ground surface (bgs). In this
manner, a PRBWwasto be established perpendiculartothe
flow of the groundwater from the pseudo source well. The
areal extent of the pilot-scale treatment system's injection
and monitoring wells was approximatelyl ,200 ft2.
Two rows of monitoring wells were installed for tracking
HRCŪ and contaminated groundwater. The first row of
monitoring wells was installed 12-15 feet south of the pseudo
source well and were used for early monitoring of the treated
plume (they could also be used as wells where ORCŪ could
be introduced if that became desirable). The second row of
monitoring wells were situated about 30 feet south of the
pseudo well to allow time (and distance) for the HRCŪ to
move into the aquifer and accelerate growth of existing
anaerobic bacteria capable of degrading the chlorinated
ethenes. Additional monitoring wells were located to both
sides of the expected plume and further downgradient to
provide control data on the movement of HRCŪ and
degradation of the chloroethenes.
The primary objective of the demonstration was to determine
the effectiveness of the PRBW in eliminating TCE and its
degradation products from the groundwater plume.
Specifically, the developer claimed that 90% of the samples
collected from the downgradient "critical" wells would meet
the Massachusetts groundwater criteria (GW-1), after a
reasonable period (about 4 months). GW-1 criteria for the
target contaminants are as follows: TCE (5 ppb), cis and
trans-1,2-dichloroethene (70 ppb-combined) and VC (2 ppb).
The demonstration was initially expected to last about 6 to 9
months, which was anticipated to be ample time for
determining if the primary objective had been met. In fact,
the demonstration was extended to over 24 months as TCE
concentrations were found to decrease more slowly than
expected. Water level measurements indicated the project
area to be very flat. Although this would suggest low flow of
water, analysis of various parameters indicated that
movement of HRCŪ and other constituents was occurring.
Concentrations of VOCs and other parameters varied in
adjacent wells throughout the evaluation, possibly due to
non-uniformity in the formation; nevertheless each well
showed similar trends in VOC changes.
After a few months of contact with the HRCŪ there was clear
evidence of conversion of TCE to cis-1,2-DCE (DCE).
Although the treatment did achieve extensive conversion to
DCE (~80%-95%+ in individual critical wells) the GW-1
criteria were not achieved at the downgradient monitoring
wells even though the treatment was allowed to continue for
about two years. In fact, DCE continued to increase for most
of that time before production of VC became apparent; VC
concentrations were still increasing when the project was
considered complete.
The second stage of the investigation, the introduction of
ORCŪ to accelerate the degradation of DCE and VC, was
never undertaken for several reasons. The primary reason
was Harding ESE's expectation that large concentrations of
HRCŪ (relative to TCE concentrations) would enable the
dechlorination to proceed to completion (ethene). Funds and
time were also considerations.
The results of well monitoring are consistent with the
hypothesis that extensive anaerobic dechlorination occurred
in the groundwater as it moved downgradient from the
injection area. Secondary parameters also indicated that
HRCŪ had been forced into the upgradient "source" well,
allowing dechlorination to begin upgradient of the actual
HRCŪ injection wells. Very low groundwater flow gradients
over the two years may have confounded the results.
For Further Information:
EPA Project Manager
Randy Parker
United States Environmental Protection Agency
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
Phone: (513)569-7271
E-mail: de parker.randy@epa.gov
Technology Contact
Willard A. Murray, Ph.D., P.E.
Harding ESE
MACTEC Engineering and Consulting
107 Audubon Road
Wakefield, MA 01880
781-245-6606(Telephone), 781-246-5060(Fax)
wamurray@mactec.com
------- |