In cooperation with the U.S. Environmental Protection Agency
Assessment of Potential for Natural Attenuation of Chlorinated Ethenes and Ethanes in Ground Water at a Petrochemical Reclamation Site, Harris County, Texas
By Glenn F. Huff, Christopher L. Braun, and Roger W. Lee
U.S. Geological Survey
Water-Resources Investigations Report 00–4121
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Contents
Abstract
Introduction
Purpose and Scope
Hydrogeology
Reductive Dechlorination
Sampling and Analytical Methods
Dissolved Oxygen
Total Iron Species and Iron II
Total Sulfide
Molecular Hydrogen
Methane and Carbon Dioxide
Potential for Natural Attenuation of Chlorinated Ethenes and Ethanes
Spatial Distribution
Redox Conditions
One-Dimensional Fate and Transport Modeling
Conclusions
References
Figures
| 1. | Map showing location of the study area and wells |
| 2. | Lithologic section A–A' showing wells screened in the Numerous Sand Channels Zone (NSCZ) |
| 3. | Maps showing location of sampled wells, water-level-altitude contours, and lines of equal concentrations of selected chlorinated ethenes and ethanes in samples from the wells |
| 4. | Graphs showing simulated and analyzed concentrations of selected chlorinated ethenes and ethanes along the ground-water flowpath |
Tables
| 1. | Properties and constituents measured to evaluate the potential for natural attenuation of chlorinated ethenes and ethanes in ground water |
| 2. | Selected data for wells |
| 3. | Results of analyses for volatile organic compounds in ground-water samples collected during February 8–10, 1999 |
| 4. | Results of analyses for field properties and constituents, inorganic ions, and dissolved organic carbon in ground-water samples collected during February 8–10, 1999 |
| 5. | Simulated first-order decay constants for selected chlorinated ethenes and ethanes in ground water |
VERTICAL DATUM
Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
Abstract
Redox conditions in the Numerous Sand Channels Zone beneath a petrochemical reclamation site in Harris County, Texas, range from sulfate reducing to methanogenic as indicated by the presence of methane in ground water and the range of molecular hydrogen concentrations. Assessment of the potential for reductive dechlorination using BIOCHLOR as a screening tool indicated conditions favoring anaerobic degradation of chlorinated organic compounds in the Numerous Sand Channels Zone. Evidence supporting reductive dechlorination includes apparently biogenic cis-1,2-dichloroethene; an increased ratio of 1,2-dichloroethane to 1,1,2-trichloroethane downgradient from the assumed contaminant source area; ethene and methane concentrations greater than background concentrations within the area of the contaminant plume; and a positive correlation of the ratio of ethene to vinyl chloride as a function of methane concentrations. The body of evidence presented in this report argues for hydrogenolysis of trichloroethene to cis-1,2-dichloroethene; of 1,1,2-trichloroethane to 1,2-dichloroethane; and of vinyl chloride to ethene within the Numerous Sand Channels Zone.
Simulations using BIOCHLOR yielded apparent first-order decay constants for reductive dechlorination in the sequence
tetrachloroethene
trichloroethenecis-1,2-dichloroethenevinyl
chlorideethene
within the range of literature values reported for each compound and apparent first-order decay constants for reductive dechlorination in the sequence
1,1,2-trichloroethane1,2-dichloroethane
slightly greater than literature values reported for each compound along the upgradient segment of a simulated ground-water flowpath. Except for vinyl chloride, apparent rates of reductive dechlorination for all simulated species show a marked decrease along the downgradient segment of the simulated ground-water flowpath.
Evidence for reductive dechlorination of chlorinated ethenes within the Numerous Sand Channels Zone indicates potential for natural attenuation of chlorinated ethenes. Reductive dechlorination of chlorinated ethanes apparently occurs to a lesser extent, indicating relatively less potential for natural attenuation of chlorinated ethanes. Additional data are needed on the concentrations and distribution of chlorinated ethenes and ethanes in individual fine sand intervals of the Numerous Sand Channels Zone. This information, combined with lower minimum reporting levels for future chloroethane analyses, might enable a more complete and quantitative assessment of the potential for natural attenuation at the site.
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