TCE and 1 2 -DCE Biotransformation Inside a

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 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 (k 1). DCE VC is also a first-order decay reaction with independent decay constant (k 2). The BAZ is surrounded by areas of negligible dechlorination activity.

Anatomy of a 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

Regional Geologic Cross-section Elevation in feet above MSL Canal +1400 ft MSL Vertical exaggeration 5: 1 -2000 ft Site

Phoenix TCE Plume

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

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 C 0 = 3000 µg/L more accurately represents the measured plume 1 – 2. 5 miles downgradient of the BAZ.

1991 Measured Concentrations Domenico Analytical Solution Domenico results for TCE

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 k 1 values ranged from 0. 0008 to 0. 0115 /day. DCE k 2 values ranged from 0. 0002 to 0. 0090 /day. Substrates in these studies were BTEX and petroleum hydrocarbons.

BIOPLUME II Modifications Renamed O 2 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 (k 1) and DCE (k 2). 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 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 square around sources.

BIOPLUME II Model Results First-order constants (k 1, k 2) 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 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 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 C 0 ≈ 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.