Transcript TCE and 1,2-DCE Biotransformation Inside a Biologically

TCE and 1,2-DCE Biotransformation
Inside a Biologically Active Zone
Anthony W. Holder, Philip B. Bedient, and
Joseph B. Hughes
Environmental Science and Engineering
Rice University, Houston, Texas
Chlorinated Solvents
Chlorinated solvents (including PCE and TCE) are
a major concern for a number of U.S. industries.
 Generally resistant to biodegradation under aerobic
subsurface conditions.
 TCE can biodegrade under anaerobic conditions in
groundwater to form 1,2-DCE, vinyl chloride, or
ethene.
 Anaerobic biodegradation requires a substrate.

Types of Biodegradation
(from Wiedemeier et al., 1996)

Type I - microbial substrates added along with
chlorinated solvents
– biodegradation is generally rapid and dehalogenation is
promoted

Type II - microbial substrates exist naturally, in
lower levels than in Type I
– biodegradation/dehalogenation rates slower than Type I

Type III - low substrate availability
– nearly non-existent biodegradation/dehalogenation
Objectives of Study
Develop a mechanism to account for the rapid
decline in concentrations near a source, followed by
a slow decline throughout the off-site plume.
 Incorporate biologically active zone (BAZ) into
Bioplume II.
 Model the biodegradation process for chlorinated
hydrocarbons at an actual field site.
 Test the model against measured field data near the
source and in the plume.

BAZ Concept
 A Biologically Active Zone is a zone where
significant co-disposal of substrates has occurred,
and significant dechlorination is taking place.
 Inside the BAZ, TCE 1,2-DCE is approximated
by a first-order decay reaction (k1).
 DCE VC is also a first-order decay reaction with
independent decay constant (k2).
 The BAZ is surrounded by areas of negligible
dechlorination activity.
Anatomy of a BAZ
Direction of Flow
Contamination
(TCE)
ADVECTION
DISPERSION
BIODEGRADATION
Degradation
Product
(DCE)
ADVECTION
DISPERSION
BIODEGRADATION
Substrate
(Acetone, IPA)
ADVECTION
DISPERSION
BIODEGRADATION
ADVECTION
DISPERSION
ADVECTION
DISPERSION
Biologically Active Zone
(BAZ)
Phoenix Site Background
Detection of TCE in wells (1981) led to the
discovery of three distinct plumes of chlorinated
solvents.
 A 5+ mile long shallow plume extends downgradient from former industrial use of TCE,
acetone, and isopropyl alcohol.
 1,2-DCE plume follows TCE plume; DCE is higher
than TCE near source, much lower downgradient.

Plan view of Phoenix site
Phoenix Mtns.
0
1
2
(miles)
Camelback
Mtns .
Arizona
Gran d Cana l
Cana l
Old Crosscu t
Cana l
F'
F
Papago
Butte s
N
Rid
Canal
Salt
River
Regional Geologic Cross-section
Elevat ion in feet above MSL
Elevation in feet above MSL
Site
Canal
130 0
+1400 ft
70 0
Area of detai
1 0 0 0 ft
MSL
Vertical exaggeration
5:1
-2000 ft
3 mile s
l
Phoenix TCE Plume
N
/L
5 µg
g/ L
µ
0
100
50 µg/L
50
g/L
µ
0
500 µg/L
100 µg/L
Study Facility
5000 ft
Approximate
Model Boundary
5 µg/L
Be drock Ridge
Phoenix Contamination Sources
Over 400,000 kg of TCE disposed of or spilled on
site between 1957 and 1974.
 DCE was never used at the site.
 Most chemicals were deposited in a dry well along
with large quantities of acetone and isopropyl
alcohol (IPA).
 Off-site monitoring showed no acetone or IPA.
 Maximum TCE levels were 307,000 g/L.

1992 TCE and DCE
Centerline Concentrations
5
TCE
Log(Conc, µg/L)
4
1,2-DCE
3
2
1
0
0
2000
4000
6000
8000
10000
Distance from Source (ft)
12000
14000
Co-Disposed Substrates
Along with the TCE, over 200,000 gallons of IPA
and almost 100,000 gallons of acetone were
disposed at the site.
 The equivalent substrate mass necessary to convert
all the TCE to VC was compared with the substrate
mass disposed.
 Enough substrate was co-disposed to consume over
10 times the total TCE disposed at the site.

Domenico Analytical Solution
Analytical solution to vertical plane source with
constant concentration (Domenico et al., 1982).
 Approximates the concentrations downgradient
from a source assuming no biodegradation.
 Without a BAZ, the source concentration would be
≈ 300,000 g/L, which produces concentrations of
≈ 100,000 µg/L about 1 – 2.5 miles downgradient.
 Actual downgradient concentrations are ≈ 500 –
1000 µg/L.

Domenico Analytical Solution
(cont.)
Concentrations ≈ 3000 g/L measured just downgradient of BAZ.
 Domenico solution was also applied starting at the
downgradient edge of the BAZ.
 The Domenico solution using C0 = 3000 µg/L more
accurately represents the measured plume 1 – 2.5
miles downgradient of the BAZ.

1991 Measured Concentrations
Domenico Analytical Solution
5
Domenico results for TCE
Log(Conc, µg/L)
4
3
2
TCE
C0 = 3000
1
1,2-DCE
C0 = 100,000
0
0
2000
4000
6000
8000
10000
Distance from Source (ft)
12000
14000
First-Order Rate Constants
Several first-order studies were presented at the
USEPA’s “Symposium on Natural Attenuation of
Chlorinated Organics in Ground Water,” 1996.
 TCE k1 values ranged from 0.0008 to 0.0115 /day.
 DCE k2 values ranged from 0.0002 to 0.0090 /day.
 Substrates in these studies were BTEX and
petroleum hydrocarbons.

BIOPLUME II Modifications
Renamed O2 and HC  TCE and DCE.
 Changed biodegradation code to remove TCE from
the system and add an equivalent amount of DCE.
 Model with two spatially variable first-order decay
constants for TCE (k1) and DCE (k2).
 Kept track of difference in molecular weights of
TCE and DCE.
 Tracked the production of VC for mass balance
calculations.

Bioplume II Model of Phoenix Site
5000 ft  5000 ft sub-area modeled using modified
Bioplume II.
 Uplifted bedrock downgradient of source affects
groundwater flow and transport.
 Site is characterized by very high hydraulic
conductivity (5  10-2 cm/sec).
 BAZ modeled as 500 ft  500 ft square around
sources.

BIOPLUME II Model Results
First-order constants (k1, k2) of 0.01/day for TCE
and DCE provided best results.
 Concentrations near source ≈ 100,000 µg/L.
 Concentrations downgradient ≈ 1,000 µg/L.
 75 – 85% of the TCE converted to VC or ethene.
 Without the BAZ, concentrations > 100,000 µg/L
modeled downstream.
 With a uniform decay constant, the plume
disappears less than a mile downstream.

Modeled TCE and 1,2 DCE concentrations
for 1991 (from Phoenix site)
1 g/L
10 g/L
100 g/L
1000 g/L
10,000 g/L
Modeled vs. Measured TCE
(from Bioplume II)
Modeled TCE Concentrations for 1991
Assuming No Biodegradation
1 g/L
10 g/L
100 g/L
1000 g/L
10,000 g/L
100,000 g/L
Conclusions
Dechlorination in the subsurface near the source
occurs where co-disposed substrates are available,
fostering microbial growth.
 Rapid reduction of contaminant concentration near
the source indicates the presence of a BAZ.
 In the BAZ, the TCE diminishes rapidly (from over
100,000 to 3000 g/L).

Conclusions (cont.)
Off-site, the slow reduction of concentrations
indicates the absence of dechlorination.
 The off-site concentration profile can be
approximated with the Domenico solution.
 Downgradient, the plume resembles the constant
source results from the Domenico solution with
C0 ≈ 3000 g/L.

Conclusions (cont.)
Bioplume II model shows both the rapid decline in
concentrations across the BAZ and the slow
dispersive decline in concentrations downgradient
of the BAZ.
 In a BAZ, a significant portion (> 80%) of the
disposed solvent can be degraded.
 It is important to recognize and properly model
different biodegradation mechanisms, including
areas of rapid dechlorination.
