TCE and 1,2-DCE Biotransformation TCE and 1,2-DCE Biotransformation Inside a Biologically Active ZoneInside a Biologically Active Zone

Anthony W. Holder, Philip B. Bedient, and Joseph B. Hughes

Environmental Science and EngineeringRice University, Houston, Texas

Chlorinated SolventsChlorinated 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 BiodegradationTypes of Biodegradation(from Wiedemeier et al., 1996)(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 StudyObjectives 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 ConceptBAZ Concept

A B Biologically AActive ZZone 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.

Contamination(TCE)

DegradationProduct(DCE)

Substrate(Acetone, IPA)

ADVECTIONDISPERSION

BIODEGRADATION

ADVECTIONDISPERSION

BIODEGRADATION

ADVECTIONDISPERSION

BIODEGRADATION

ADVECTIONDISPERSION

ADVECTIONDISPERSION

Biologically Active Zone(BAZ)

Direction of Flow

Anatomy of a BAZAnatomy of a BAZ

Phoenix Site BackgroundPhoenix 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 down- gradient 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

Rid

Canal

Phoenix Mtns.

CamelbackMtns.

NSalt River

F F'

0 1 2(miles)

Grand CanalArizona

Canal

Old CrosscutCanal

PapagoButtes

Regional Geologic Cross-section1300

700

Elevation in feet above MSL

1000 ft

3 miles

Area of detail

Elev

atio

n in

feet

abo

ve M

SL

Vertical exaggeration 5:1

+1400 ft

MSL

-2000 ft

Canal Site

Phoenix TCE PlumePhoenix TCE Plume

Study Facility

Bedrock Ridge

5 µg/L

50 µg/L

100 µg/L

500 µg/L

1000 µg/L

500 µg/L

5 µg/LN

ApproximateModel Boundary

5000 ft

Phoenix Contamination SourcesPhoenix 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 1992 TCE and DCE Centerline ConcentrationsCenterline Concentrations

0

1

2

3

4

5

0 2000 4000 6000 8000 10000 12000 14000

Distance from Source (ft)

Log(Conc, µg/L)

TCE 1,2-DCE

Co-Disposed SubstratesCo-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 SolutionDomenico 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 Domenico Analytical Solution (cont.)(cont.)

Concentrations ≈ 3000 g/L measured just down- gradient 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 1991 Measured Concentrations Domenico Analytical SolutionDomenico Analytical Solution

0

1

2

3

4

5

0 2000 4000 6000 8000 10000 12000 14000Distance from Source (ft)

Log(Conc, µg/L)TCE 1,2-DCEC0 = 3000 C0 = 100,000

Domenico results for TCE

First-Order Rate ConstantsFirst-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 ModificationsBIOPLUME 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 SiteBioplume 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 ResultsBIOPLUME 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 Modeled TCE and 1,2 DCE concentrations for 1991 (from Phoenix site)for 1991 (from Phoenix site)

1 g/L 10 g/L 100 g/L 1000 g/L 10,000 g/L

Modeled vs. Measured TCEModeled vs. Measured TCE(from Bioplume II)(from Bioplume II)

Modeled TCE Concentrations for 1991 Modeled TCE Concentrations for 1991 Assuming No BiodegradationAssuming No Biodegradation

1 g/L10 g/L100 g/L1000 g/L10,000 g/L100,000 g/L

ConclusionsConclusions

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.)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 withC0 ≈ 3000 g/L.

Conclusions (cont.)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.