A newsletter about soil, sediment, and graundwater characterization and remediation technologies
Technology
News & Trends
EPA 542-N-12-004 \ Issue No. 60, August 2012
This issue of Technology News & Trends highlights the use of permeable reactive barriers (PRBs) to treat
groundwater plumes containing organic or inorganic contaminants. Each featured article describes work
y the U.S. Environmental Protection Agency (EPA) National Risk Management Research Laboratory to
evaluate long-term performance of PRBs constructed of carbon-rich organic materials, zero-valent iron,
or other substrates. Monitoring results indicate that PRB performance can change significantly over time
due to factors such as reactive media depletion, fluctuations in rainfall, or a site's hydrologic conditions.
FEATURED ARTICLES
Performance Monitoring: Evaluating a Wheat Straw PRB for Nitrate
Removal at an Agricultural Operation
Contributed by Stephen R. Hutchins. Ph.D., and Richard T. Wilkin. Ph.D., U.S. EPA National Risk
Management Laboratory
The U.S. EPA Office of Research and Development's National Risk Management Research Laboratory
(NRMRL) is conducting long-term monitoring of a permeable reactive barrier (PRB) for remediation of
groundwater contaminatecTwith nitrate from a now-closed swine concentrated animal feeding operation
(CAFO) in Oklahoma. Extensive groundwater contamination by both nitrate and ammonium arose from a
leaking waste lagoon during seven years of operation. The selected remediation strategy involved
installing an interception trench barrier for recovery and subsequent above-ground treatment of
ammonium through use of an evaporation basin. A PRB containing commercial wheat straw as the
reactive matrix was installed for in situ treatment of nitrate resulting from nitrification of the lagoon's
ammonium plume and also from excessive land application of lagoon effluent.
The PRB was constructed in 2003 in a shallow aquifer consisting of fractured sandstone with interspersed
clay and sand lenses that are underlain by a thick layer of shale at approximately 25 to 50 feet below
ground surface (bgs). A trench approximately 840 feet long and 4 feet wide was excavated to intercep
the contaminant plume at 8 to 18 feet bgs. Approximately 4,500 square and 300_round bales of wheat
straw were emplaced in the trench. The rate or qroundwatu
feet/day.
: groundwater flow through the PRB is estimated at 500
Technology News & Trends
August 2012 Issue
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Figure 1. Aerial view of PRB, wells, and surface features at the Oklahoma CAFO
site.
The PRB was not constructed on level terrain. As a result, much of it slopes sharply toward a center
drainage area between access wells NB1 and NB3 (Figure 1). A seep in this drainage area emanates from
the top surface of the PRB and consequently acts as a seasonal discharge point. Because the PRB matrix
is 500 to 4,000 times more permeable than the aquifer matrix, there is a significant potential for
longitudinal flow within the PRB toward this seep.
In 2004, NRMRL researchers established transects across the PRB at two locations, each with fully
screened 2-inch PVC wells upgradient, within, and downgradient of the PRB (Figure 2). Although the goal
was to place three wells inside the PRB at each transect, uncertainties regarding the location of the PRB
boundary led to the final position of well T2B being slightly upgradient of the PRB at transect #2.
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PRB at transect #2.
Technology News & Trends
August 2012 Issue
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To provide vertical resolution of water quality within the PRB, cluster wells were also installed at transect
#2 using 0.25-inch polyethylene tubing with 6-inch-long stainless steel well screens to a depth of 14 to
23 feet bgs (Figure 2). Quarterly monitoring of the transect wells for multiple parameters began in 2004;
this schedule was scaled back to annual sampling in 2008 and continues to the present. During the
nine-year study period, groundwater elevations fluctuated by as much as 4-6 feet due to weather
extremes, which included droughts in 2004 and 2011 and excessive rainfall in 2007. These conditions
also affected PRB performance.
Denitrification occurred rapidly within the PRB soon after construction. Although monitoring of the
transect wells did not begin until 2004, analysis of samples from the five PRB access wells taken four
months after start-up revealed an actively reducing environment. Reducing conditions included an
oxidation-reductionjbotential of -63 to -98 millivolts and pH of 3.7 to 4.5, with high organic carbon (total
organic carbon (TOC) of 1,030 to 3,920 milligrams per liter [mg/L]) and no detectable nitrate (< 0.004
mg/L nitrate-nitrogen). Ammonium production, however, was substantial (36 to 250 mg/L ammonium
ion-nitrogen). One source of ammonium could be the decomposition of organic nitrogen in the straw.
Another potential source is the dissimilatory nitrate reduction to ammonium (DNRA), which is common
among fermentative bacteria and would be expected under conditions of low oxygen, low nitrate, and
high TOC.
Over time, the high ammonium concentrations dissipated. Based on transect well data, monitoring results
showed that denitrification was occurring at both transect locations and the PRB chemistry generally
began working as expected. After two years of PRB operation, subsidence was observed along the barrier
length. Subsidence worsened with time to the point that ground surface elevations dropped as much as 5
feet at the centerline along much of the PRB, especially in sections northeast of the seep. The subsidence
was attributed primarily to extensive decomposition of the straw matrix, and the decision was made to
not replenish this material. Excessive rainfall likely exacerbated this condition in 2007, when the PRB
began collecting runoff and creating a temporary infiltration gallery along much of its length.
These events are evident in the water chemistry at transect #1 (Figure 3). For the first four months,
nitrate was generally absent within the PRB and excess TOC existed for denitrification. The nitrate
concentrations dropped slowly in the dpwngradient well TIE and then unexpectedly rose. This trend may
be due to a greater component of longitudinal flow within the PRB at this location toward the seep,
thereby providing partial isolation of this well location. It is also possible that ammonium initially
produced by DNRA migrates dpwngradient of the PRB and, as the groundwater slowly aerates, is
converted to nitrate through nitrification.
Transect #1 - Nitrate
Transect #1 -TOC
Jun03 JunOS Jun07 Kin 09 Am 11
Jun03 JunQS Jun07 Jun09 Junll
Figure 3. Transect #1 groundwater nitrate and TOC profiles in upgradient (T1A), within-PRB (T1B, T1C, T1D)
and downgradient (T1E) wells.
The effect of excessive rainfall in 2007 can be seen in the water chemistry data; infiltrating runoff caused
substantial outflow from the PRB in both directions, drastically lowering nitrate levels in both upgradient
well T1A and downgradient well TIE (Figure 3). The event also appears to have brought TOC into the PRB
at this location, although TOC levels subsequently dropped rapidly and continued to decline. The
decrease in TOC corresponds with an increase of nitrate within the PRB, which has led to substantial
failure of the PRB at this location.
Technology News & Trends
August 2012 Issue
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Transect #2 - Nitrate
Transect #2-TOC
Jun03 Am 05 Am 07 Am 09 Am 11
Jun03 JunQS Jun07 JunOT Jun 11
F/gure 4. Transect #2 groundwater nitrate and TOO profiles in upgradient (T2A, T2B), within-PRB (T2C, T2D),
and downgradient (T2E) wells.
In contrast, the PRB continues to perform effectively at transect #2. Denitrification is quite active, and
nitrate concentrations have remained below 0.2 mg/L nitrate-nitrogen in the downgradient well T2E since
2007 (Figure 4). Unlike transect #1, TOC persistedfor a longer time within the PRB at this location. This
persistence may be attributed to the downward slope of the PRB in both directions at this point, which
reduces longitudinal flow along the PRB length and lowers the infiltration components, consequently
preserving more of the straw matrix. The high rainfall event in 2007 also caused outflow from the PRB in
both directions at this transect; however, recovery time for well T2A was much longer than for well T2B.
Overall results of monitoring since 2004 indicate that the PRB is beginning to fail, especially in areas
where longitudinal flow and infiltration of rainfall runoff may have rapidly depleted TOC. Because
replenishment of the straw matrix or backfill of the subsided areas is not expected, it is unclear whether
the PRB performance will outlast the anticipated reduction in nitrate mass influx. The high levels of
ammonium initially produced within the PRB are an additional concern, since the amount of ammonium
transported to areas beyond the PRB is unknown. For the past seven years, ammonium plumes have
been observed at two downgradient wells (MW34 and MW35) with levels of ammonium ion-nitrogen
peaking at 10 to 13 mg/L and slowly dropping to current levels of 2 to 4 mg/L.
NRMRL plans to continue monitoring performance of the PRB as long as possible. Discussions are pending
between responsible parties and regulatory personnel on what additional steps, if any, need to be
undertaken to bring this site to appropriate closure status.
Performance Monitoring: Evaluating an Organic Carbon-Limestone PRB
for Treatment of Heavy Metals and Acidity
Contributed bv Ralph Ludwia. Ph.D.. Richard T. Wilkin. Ph.D.. Steve Acres, and Randall Ross. Ph.D., U.S.
EPA National Risk Management Research Laboratory; Katrina Hiaains-Coltrain, U.S. EPA Region 6
Since 2004, researchers from the U.S. EPA National Risk Management Research Laboratory (NRMRL)
have annually evaluated performance of an organic carbon-limestone permeable reactive barrier (PRB)
system installed in 2003 by EPA Region 6 at the Delatte Metals Superrund site in Ponchatoula, Louisiana.
Based on early results from testing of two pilot-scale PRBs, a full-scale PRB was installed adjacent to the
pilot PRBs to fully intercept and treat a low-pH, heavy metal groundwater plume prior to entry into
nearby surface water. As constructed, the full-scale PRB contained approximately 67% composted cow
manure and 33% limestone and measured 1.8 meters wide, 4-5 meters deep, and over 300 meters long.
The most recent (2011) round of hydrologic and chemical data shows sustained effective treatment of
the target contaminant plume by the collective PRB system (two pilot-scale PRBs plus one full-scale
PRB), but also shows signs of possible depletion of the reactive media in one of the former pilot-scale
PRBs.
Technology News & Trends
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Figure 1. GeoProbe® deployment on a swamp
buggy to install monitoring wells.
The site was used previously for battery recycling. Onsite activities included spent lead-acid battery
demolition and lead smelting of battery lead plates to produce lead ingots. Initial remedial actions
involved excavation, treatment, and orfsite disposal or approximately 32,400 cubic meters of impacted
soil. A large, low pH (<3.2), high acid-generating (high acidity) groundwater plume containing significant
concentrations of heavy metals remained and continued to discharge to a creek. Cumulative yearly
average lead (Pb), cadmium (Cd), and nickel (Ni) concentrations as high as 80.9 micrograms per liter
(ug/L), 451 ug/L, and 408 ug/L, respectively, nave been measured in wells immediately upgradient of
the PRB.
The primary objective of installing the PRB system was to raise the pH and remove heavy metals, such
as lead ana cadmium, from impacted groundwater to the extent possible by promoting microbially
mediated sulfate reduction and limestone dissolution. Operation of the two pilot-scale PRBs for
approximately one year helped evaluate candidate organic carbon-limestone matrices. One consisted of
the 2:1 cow manure and limestone gravel mixture while the other contained a reported 1:1:1 mixture of
cow manure, wood chips, and limestone gravel. Each pilot-scale PRB measured 1.8 meters wide, 4.3
meters deep, and 30 meters long. The mixture of 2:1 manure and limestone was ultimately selected for
full-scale application due to its better removal of heavy metals during the brief pilot study. After
construction of the full-scale PRB was complete, monitoring wells were installed in multiple transects
across the collective PRB system (Figure 1).
Figure 2. Configuration of PRB system at the
Delatte Metals site showing locations of transects
sampled; orange segment represents former
pilot-scale PRB containing 1:1:1 cow manure,
wood chips, and limestone gravel, and green
segment represents former pilot-scale PRB
containing 2:1 cow manure and limestone (as
used in full-scale PRB depicted in yellow).
One transect through each of the two pilot PRBs and two transects through the full-scale PRB are
monitored annually. Transect TEPA-1 traverses the former pilot PRB consisting of the 1:1:1 mixture of
manure, wood chips, and limestone that now comprises the southern end of the PRB system (Figure 2).
Transect TEPA-2 traverses the former pilpt-scale PRB consisting of the 2:1 manure/limestone mixture
also used in the full-scale PRB. The remaining two transects (TEPA-5 and TEPA-6) traverse the full-scale
PRB at locations where target contaminant concentrations entering the PRB were initially observed to be
Technology News & Trends
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highest.
The annual monitoring program involves hydrologic measurements and groundwater sampling. The
hydrologic measurements include collection of groundwater elevation data at wells within ancf
surrounding the PRB system. Hydraulic conductivity within and adjacent to the PRB is estimated using
slug testing in approximately 70 wells. Resulting data indicate that both the PRB and native materials
upgradient of the PRB are highly heterogeneous.
The hydraulic heads along the PRB suggest the majority of the groundwater plume is flowing through the
PRB system. The average seepage velocity of water moving through the PRB has been calculated to be
between 3 centimeters per day (cm/d) and 6 cm/d. Due to the significant heterogeneity of native
materials, however, significant uncertainty about this range exists. Some groundwater mounding has
been observed upgradient of the PRB, indicating the average hydraulic conductivity within the PRB at
some locations may be lower than that of the surrounding aquifer. Slug testing within the PRB at various
locations has indicated hydraulic conductivity values spanning at least an order of magnitude.
Cumulative Yearly Average
pH (upgraded!)
p.H (wiltlln PRB)
Pb (UMFadent)
Pb (within PRB)
% Pb remwed
Cd (upgra dent)
Cd (within PRB)
% Cd removed
Ni (upgradant)
Ni (within PRB)
% Ni r«mov*d
Acidity: (upqradent)
Alkalinityr (wrthn PRB)
3-17
5.96
16.4
0.343
87.8
297
0.041
100
192
32.0
83,3
874
499
3.09
6.32
14.4
0.482
96.7
461
0.022
100
342
28.6
61.3
1383
755
3.10
6.46
80.9
0.775
99.0
220
0.090
100
277
11.9
96.7
1983
923
2.99
6.44
78,1
0.308
996
268
0.048
100
408
20.5
96.0
4308
1092
3.11
5.41
44.0
0.213
995
273
0.048
100
129
493
61.8
591
257
2011 Data
2.97
6.43
12.3
0.092
99.3
403
0.049
100
332
47.3
85.8
1257
340
3.07
6.43
17.0
0.194
96.9
55.0
0.171
98.7
236
19.0
91.9
ieie
515
2.90
6.54
14.1
0.235
98.3
23.8
0.149
99.4
404
30.1
92.5
3COO
867
Table 1. Cumulative average yearly concentrations (pH, akalinity, acidity
since 2004; Cd and Pb since 2006, Ni since 2008) and 201 1 average
concentrations upgradient and within PRB system; Pb, Cd, and Ni
measured in pg/L, acidity measured in mg/L CaC£>equivalents; and
alkalinity measured in mg/L
Groundwater sampling data have shown effective treatment of targeted contaminants along all four
transects. Flow through the PRB system is confirmed by significant PRB-induced geochemical changes on
the downgradient side of the PRB. For example, data collected in 2011 from wells approximately 15
meters downgradient (near the creek's edge) show pH values exceeding 6.1 and alkalinity values
exceeding 400 milligrams per liter (mg/L) as calcium carbonate (CaCQ) along all four transects. This
contrasts with significantly lower pH values (as low as 2.97) and acidity values as high as 3,030 mg/L (as
CaCOs equivalents) in groundwater entering the PRB during 2011.
-1,0
2005 2006 2007 2008 2009 2010 2011
Figure 3. Seven-year trends in groundwater pH at
monitoring transects in the PRB's upgradient half.
Technology News & Trends
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More recent pH measurements along TEPA-1 may indicate the first signs of reactive media depletion in
the former pilot-scale PRB consisting of the 1:1:1 mixture of cow manure, wood chips, and limestone
gravel. Specifically, pH within its upgradient half ajong TEPA-1 (Figure 3) has gradually decreased over
the past few years, suggesting the treatment media at this location is no longer adequately neutralizing
the low pH groundwater entering the PRB. Treatment in the downgradient half along this transect,
however, remains effective. A possible cause pf the reduced treatment performance in the upgradient
half may be depletion of the manure and inability of the wood chips to adequately compensate for the
depletion.
Monitoring of PRB performance by NRMRL at this site will continue annually, pending availability of
funding. Future activities will focus on evaluating whether or not the PRB system will outlast the
groundwater plume.
Performance Monitoring: Bromide Tracer Test and CSIA Characterize
Treatment of TCE in Mulch Biowall
Contributed by John Wilson, Ph.D., U.S. EPA, Hai Shen, U.S. DOE
The Air Force Center for Engineering and the Environment, in collaboration with the National Nuclear
Security Administration, conducted a bromide tracer test and a compound-specific isotope analysis
(CSIA) investigation at the Altus Air Force Base (AFB) Landfill 3 (a RCRA Corrective Action Site) in
Oklahoma to characterize the remedial performance of a mulch biowall. The bromide tracer helped
delineate the groundwater flow pattern and established a field-scale rate constant for removal of
trichloroethene (TCE) in the biowall to better understand the residence time of TCE. CSIA subsequently
was used to identify the carbon isotope ratio (L3C/12C or 513C) of TCE to determine the role degradation
plays in reducing TCE concentrations.
Landfill 3 was used for disposal of hazardous materials, including TCE, from aircraft maintenance
activities and industrial operations from 1956 to 1965. The gypsum-rich soil at the site consists of
approximately 5 feet of clayey silt underlain by weathered, silty clay extending to the top of dense shale
approximately 23 to 35 feet below ground surface (bgs). Depth to groundwater within the unconfined
shallow aquifer varies seasonally from 6 to 12 feet bgs, with groundwater flowing to the southeast. TCE
and its daughter productp/s-l,2-dichloroethene (cis-l,2-DCE), were the most prevalent chlorinated
volatile organic compounds (VOCs) detected in the groundwater. Prior to biowall installation, TCE and
c/s-l,2-DCE concentrations as high as 8,000 micrograms per liter (ug/L) and 1,800 ug/L, respectively,
were measured.
Groundwater ' i
Flow • !« •
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biowall : i
(nol drawn '. ,
~" - \\ * 10m
Figure 1. Plan view showing
the biowall (dashed lines) and
the monitoring network.
Technology News & Trends 7 of 13 EPA 542-N-12-004 | Issue No. 60
August 2012 Issue clu-in.org/newsletters
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A mulch biowall composed of shredded tree mulch (48%), cotton gin compost (10%),
(42%) was installed in June 2002 to demonstrate enhanced in situ bioremediation or1
and river sand
: VOCs and to
address dissolved TCE discharges to an adjacent creek. The 455-foot-long, 1.5-foot-wide, and
24-foot-deep biowall was constructed to intercept the groundwater profile. The biowall does not extend to
the top of the shale because the maximum depth entrenching tools could reach at the time of installation
was 25 ft bgs. In addition, because the biowall was designed as a pilot, it does not intercept the entire
width of the TCE plume. As a result, migration of groundwater underneath the biowall trench or around
the edges of the biowall is possible. Figure 1 provides a layout of the biowall relative to groundwater flow
and the location of monitoring wells.
The geochemistry of groundwater within the biowall significantly changed within one month of biowall
installation. Changes providing conditions favorable for the reductive dechlorination of TCE and DCE
included a reduction of sulfate and production of ferrous iron, hydrogen sulfide, and methane. The
oxidation-reduction potential (ORP) decreased from about 100 millivolts (my) in upgradient wells to
about -300 mV in wells within the biowall, indicating highly reducing conditions. Over time, the ORP
downgradient of the biowall also decreased to about -lOOmV. The 2,800 mg/L total organic carbon (TOC)
concentration within the biowall was very high compared to 5 mg/L upgradient; TOC within the biowall
decreased over time due to consumption of the plant-released organic compounds by various
microorganisms. Sampling confirmed presence of Dehalococcoides, organisms important for reductive
dechlorination, within and 29.5 feet downgradient of the biowall.
upgradient
10 JO 3D 40 50 «0 70 80 90 100 110 120 130 I4D ISO
Distance Along Transect North to South (Feet)
Within Biowall
10 20 30 40 50 6D 70 80 80 100 110 120 130 1HO ISO
Distance Along Transect North to South (feel)
Downgradient
0 10 20 30 40 50 SO 70 BO 80 100 110 120 120 I4G 150
Distance Along Transect North to South (feel)
Figure 2. Contaminant concentrations in August
2006 showed increasedcis-1,2-DCE and VC
Technology News & Trends
August 2012 Issue
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concentrations within the biowall and downgradient
as TCE concentrations decreased, suggesting
occurrence of reductive dechlorination.
Local concentrations of TCE significantly decreased within one rranth of biowall installation, from
approxjmately 6,044 ug/L (46 micromoles per liter [uM]) upgradient of the biowall to an average of
1,313 Ig/L (10 uM) within the biowall. By 2006, upgradient TCE concentrations decreased to a maximum
of 1,182 ug/L (9 uM) likely due to attenuation. Upgradient c/s-l,2-DCE and vinyl chloride (VC)
concentrations fluctuated from near zero to just above 1200 ug/L both before and after biowall
installation.
Within the biowall and at downgradient wells, TCE concentrations declined to an average of 53 ug/L (0.4
T
-------
The presence of a preferential flow pathway causes differences in residence time of groundwater in the
biowall, which result in vertical variations in contaminant degradation within the biowall matrix. When
water enters the biowall from the transmissive zone, its flow path diverges. Water moving directly across
the biowall has the shortest residence time. Water moving along less direct paths has a longer residence
time within the biowall (between 10 and 26 days) and therefore has more time in contact with biowall
matrix materials.
CSIA was performed in September 2007 to help determine whether the reduction of TCE was the result
of biological transformation rather than dilution or sorption. In cases where biotic or abiotic degradation
is occurring, the parent compound TCE becomes enriched in13C (less negative 513C value) in
downgradient groundwater when compared to source area groundwater. Since bonds between lighter
isotopes are easier to break, degradation reactions occur faster for compounds containing lighter 12C
isotope than those containing heavier 13C. The degree of enrichment or fractionation depends on the
specific reaction mechanisms taking place during degradation.
During the CSIA, groundwater samples from wells within the biowall as well as 30 feet upgradient, and
30 feet downgradient were analyzed for the presence of chlorinated volatile organic compounds, and gas
chromatography and isotope ratio mass spectrometry were used to determine the 513C for TCE.
TCE in groundwater samples was measured at 550 ug/L upgradient of the biowall and 58.4 ug/L
downgradient of the biowall but was not detected within the biowall. The TCE fpC value upgradient of
the biowall was -26.0T
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\
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Figure 1. Maps showing arsenic plume extent, location
of PRB, and monitoring well locations around and in
the PRB.
Ongoing studies at the East Helena Superfund site near Helena, Montana, show that a permeable reactive
barrier (PRB) containing zero-valent iron (ZVI) can be used effectively to treat arsenic-contaminated
groundwater in a complex hydrogeologic environment. The PRB was installed at this former lead
smelting facility in June 2005 on a pilot-scale basis. At the time of construction completion, the PRB
contained approximately 175 tons of ZVI filings in a trench approximately 6 feet wide, 46 feet deep, and
30 feet long. [See the November 2005 issue of Technology News and Trends for more information about
the installation.] Studies demonstrate that detailed geochemical and hydrogeologic data, including
flux-based analysis, are critical to evaluating the PRB's efficacy for both trivalent and pentavalent
arsenic. Six years of monitoring also have revealed shortcomings of the hanging-wall design used for
PRB construction.
The monitoring network encompasses approximately 40 groundwater sampling points at 17
1-inch-diameter wells, nine 2-inch wells, four 4-inch wells, and five multiport wells (Figure 1).
Groundwater samples have been collected and analyzed after 1 month, 4 months, 12 months, 15
months, 25 montns, 39 months, 51 months, and 63 months of PRB installation. Concurrent to each
sampling event since 2006, hydraulic gradients were estimated from water table elevation
measurements. Field measurements of hydraulic conductivity were made using single-well pumping tests
and pneumatic slug testing.
The concentration of arsenic in groundwater entering the PRB averages about 25 milligrams per liter
(mg/L). Arsenic concentrations within the PRB have generally been below 0.05 mg/L. Of 120 groundwater
samples collected from within the PRB, arsenic concentrations in 26 samples exceeded 0.050 mg/L, 94
were at or below 0.050 mg/L, and 43 were at or below the 0.010 mg/L maximum contaminant level for
arsenic (Figure 2). The highest arsenic concentrations appear to be restricted to deeper wells within the
PRB, which likely reflects areas with fast groundwater seepage velocities and/or where a lower volume of
ZVI was emplaced due to partial collapse of the trench. Arsenic within the PRB is present in both the
pentavalent and trivalent states.
Technology News & Trends
August 2012 Issue
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t-
t
1 2"
1 -
0-
u pqr.iriicnt As concentration
A A
A
A
U il ft
A
1
25 mg/L
-
-
A
I j;
A
i
D 5 10 15 2O 25 30 3S 10 15 50 55 60 65 70
Months of Operation
Figure 2. Plot showing trends in arsenic
concentre- tions within the PRB over time.
The pH and Eh within the PRB show trends that are expected as water reacts with the ZVI. For example,
the pH of grqundwater entering the PRB ranges from 6.1 to 6.5, whereas the pH of groundwater within
the PRB has increased to as high as 10.8. Similarly, the Eh of groundwater entering the PRB ranges from
about 130 to 220 millivolts (mV) whereas Eh values within the PRB range from about 140 mV to -380 mV.
The lowest Eh values (highly reducing) were recorded within the first four months of operation; after
about one year and continuing through the fifth year of operation, Eh values of groundwater within the
PRB increased to a range more typical for a ZVI system (100 to -250 mV) and have since remained
relatively static. Limited changes in pH and Eh in other portions of the PRB can be attributed to high
velocity and short residence time of the groundwater.
Geochemical parameters of groundwater within the PRB have generally reflected the intended
interactions with ZVI, although unexpectedly high concentrations of dissolved organic carbon (DOC) were
measured over the first four months of PRB operation. This was attributed to carbon remaining from guar
bioslurry used during PRB construction to create a hanging wall that would stabilize the trench. The
residual bioslurry also caused difficulty in filtering samples (using 0.45-micrometer disc filters) during the
first two sampling events and generated a somewhat foul odor. After 12 months, however, DOC
concentrations decreased to near background levels (mean value of 0.9 mg/L). After 15 months, DOC
concentrations in the PRB were indistinguishable from concentrations upgradient of the PRB (mean value
0.5 mg/L).
Water level measurements indicate that the PRB has not affected the water table elevation, which varied
seasonally by about 1 meter between June 2002 (three years prior to PRB installation) and September
2010 (five years after PRB installation). Overall, the water table has remained below the top of the ZVI
across the PRB, as desired in order to capture the vertical extent of the target plume. Negligible evidence
of grqundwater mounding has been observed upgradient of the PRB, which suggests that the PRB is not
significantly impeding groundwater flow.
Results of pneumatic slug tests conducted 5 and 16 months after PRB completion suggest that corrosion
and mineral precipitation have had no impact on the areaT
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results indicate that groundwater is moving beneath the PRB and transporting arsenic across the plane of
the PRB. In the upper region of the aquifer where groundwater is moving through the PRB, an estimated
99% arsenic removal is achieved.
Arsenic mass flux entering the PRB ranges from 0.5 to 16.9 grams/day/meter2 with a maximum value at
a depth of about 40 feet bgs. Above 40 feet bgs, the flux of arsenic downgradient of the PRB ranges
from 0.01 to 2 grams/day/meter2. Arsenic mass flux is significantly reduced downgradient of the PRB
from the groundwater table to a depth of about 40 feet bgs. Deeper in the aquifer, where emplacement
of ZVI was incomplete, there is no indication that arsenic transport is being impacted.
Based on the original design estimates, replenishment of the ZVI would be needed in 2015 in order for
the PRB to remain effective. EPA's Region 8 office is now determining suitability of a full-scale PRB that
could include the replenished pilot-scale structure. Full-scale implementation would require emplacement
of ZVI at a greater depth that extends to the underlying confining (ash tuff) layer. The design also would
need to consider the high flux zone below 40 feet bgs and the full width of the arsenic plume.
CLU-IN Website: Permeable Reactive Barriers, Permeable Treatment
Zones, and Application of Zero-Valent Iron
This remediation technology area of CLU-IN provides an overview and compendium of guidance
materials, application reports, and training or other tools for designing, installing, and monitoring PRBs.
Federal Remediation Technologies Roundtable (FRTR): Cost and
Performance Case Studies
The FRTR maintains a searchable database currently containing 21 case studies on remediation demonstration projects
involving PRBs as the primary technology.
Interstate Technology & Regulatory Council (ITRC): Permeable Reactive
Barrier: Technology Update
This 2011 technical/regulatory guidance compiled by the ITRC provides updated information about designing and
using PRBs to treat contaminated groundwater.
Upcoming Training: Permeable Reactive Barrier: Technology Update
The ITRC offers Internet-based training open to the public on October 11, 2012. Online registration for
the 1 hour and 15-minute class opens in early September.
1 tnis newsletter as a means or disseminating userui information regarping innovative ana alternative treatment
technologies and techniques. The Agency does not endorse specific technology vendors.
Contact Us:
Suggestions for articles in upcoming issues of Technology News and Trends may be submitted to
John Quander via email at iBEHHililileiHiHBli^M
Past issues of the newsletter are availa"
httrj://www.clu-in.ora/products/newsltrs/tnandt/
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August 2012 Issue
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EPA 542-N-12-004 | Issue No. 60
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