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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

tetrachloroethenearrowtrichloroethenearrowcis-1,2-dichloroethenearrowvinyl chloridearrowethene

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-trichloroethanearrow1,2-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.

 


U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/wri004121
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Last modified: Friday, September 16 2005, 04:24:04 PM
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