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Methodology for Estimating Times of Remediation Associated with Monitored Natural Attenuation

By Francis H. Chapelle, Mark A. Widdowson, J. Steven Brauner, Eduardo Mendez III, and Clifton C. Casey



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Water-Resources Investigation Report 03-4057
By Francis H. Chapelle1, Mark A. Widdowson2, J. Steven Brauner2, Eduardo Mendez III3, and Clifton C. Casey3
Prepared in cooperation with the SOUTHERN DIVISION, NAVAL FACILITIES ENGINEERING COMMAND and the NAVAL FACILITIES ENGINEERING SERVICE CENTER
1U.S. Geological Survey, Columbia, South Carolina
2Virginia Polytechnic Institute and State University, Department of Civil Engineering, Blacksburg, Virginia
3Southern Division Naval Facilities Engineering Command, North Charlestion, South Carolina
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Abstract

   Natural attenuation processes combine to disperse, immobilize, and biologically transform anthropogenic contaminants, such as petroleum hydrocarbons and chlorinated ethenes, in ground-water systems. The time required for these processes to lower contaminant concentrations to levels protective of human health and the environment, however, varies widely between different hydrologic systems, different chemical contaminants, and varying amounts of contaminants. This report outlines a method for estimating timeframes required for natural attenuation processes, such as dispersion, sorption, and biodegradation, to lower contaminant concentrations and mass to predetermined regulatory goals in groundwater systems.

   The time-of-remediation (TOR) problem described in this report is formulated as three interactive components: (1) estimating the length of a contaminant plume once it has achieved a steady-state configuration from a source area of constant contaminant concentration, (2) estimating the time required for a plume to shrink to a smaller, regulatoryacceptable configuration when source-area contaminant concentrations are lowered by engineered methods, and (3) estimating the time needed for nonaqueous phase liquid (NAPL) contaminants to dissolve, disperse, and biodegrade below predetermined levels in contaminant source areas. This conceptualization was used to develop Natural Attenuation Software (NAS), an interactive computer aquifers. NAS was designed as a screening tool and requires the input of detailed site information about hydrogeology, redox conditions, and the distribution of contaminants. Because NAS is based on numerous simplifications of hydrologic, microbial, and geochemical processes, the program may introduce unacceptable errors for highly heterogeneous hydrologic systems. In such cases, application of the TOR framework outlined in this report may require more detailed, site-specific digital modeling. The NAS software may be downloaded from the Web site http://www.cee.vt.edu/NAS/

    Application of NAS illustrates several general characteristics shared by all TOR problems. First, the distance of stabilization of a contaminant plume is strongly dependent on the natural attenuation capacity of particular ground-water systems. The time that it takes a plume to reach a steady-state configuration, however, is independent of natural attenuation capacity. Rather, the time of stabilization is most strongly affected by the sorptive capacity of the aquifer, which is dependent on the organic matter content of the aquifer sediments, as well as the sorptive properties of individual contaminants. As a general rule, a high sorptive capacity retards a plume.s growth or shrinkage, and increases the time of stabilization. Finally, the time of NAPL dissolution depends largely on NAPL mass, composition, geometry, and hydrologic factors, such as ground-water flow rates.

    An example TOR analysis for petroleum hydrocarbon NAPL was performed for the Laurel Bay site in South Carolina. About 500 to 1,000 pounds of gasoline leaked into the aquifer at this site in 1991, and the NAS simulations suggested that TOR would be on the order of 10 years for soluble and poorly sorbed compounds, such as benzene and methyl tertiary-butyl ether (MTBE). Conversely, TOR would be on the order of 40 years for less soluble, more strongly sorbed compounds, such as toluene, ethylbenzene, and xylenes (TEX). These TOR estimates are roughly consistent with contaminant concentrations observed over 10 years of monitoring at this site where benzene and MTBE concentrations were observed to decrease rapidly and are approaching regulatory maximum concentration limits, whereas toluene, ethylbenzene, and xylene concentrations decreased at a slower rate and have remained relatively high.

    An example TOR analysis for petroleum hydrocarbon NAPL was performed for the Laurel Bay site in South Carolina. About 500 to 1,000 pounds of gasoline leaked into the aquifer at this site in 1991, and the NAS simulations suggested that TOR would be on the order of 10 years for soluble and poorly sorbed compounds, such as benzene and methyl tertiary-butyl ether (MTBE). Conversely, TOR would be on the order of 40 years for less soluble, more strongly sorbed compounds, such as toluene, ethylbenzene, and xylenes (TEX). These TOR estimates are roughly consistent with contaminant concentrations observed over 10 years of monitoring at this site where benzene and MTBE concentrations were observed to decrease rapidly and are approaching regulatory maximum concentration limits, whereas toluene, ethylbenzene, and xylene concentrations decreased at a slower rate and have remained relatively high.

    An example TOR analysis for chlorinated ethene NAPL was performed at the Kings Bay, Ga., site. NAPL removal action by in situ oxidation was performed here, and the NAS simulations indicated that TOR was highly dependent upon location within the plume (upgradient areas remediate faster than downgradient areas), and the organic carbon content (sorptive capacity) of the aquifer. In general, the NASestimated decreases in chlorinated ethene concentrations at different locations within the Kings Bay plume are roughly consistent with observed decreases over 3 years of monitoring. This comparison, however, also shows that observed patterns of contaminant concentration changes are much more complex than indicated by NAS. This, in turn, illustrates the general principle that hydrologic complexities of ground-water systems are not fully accounted for in simulation tools like NAS, and that TOR estimates made with such tools are inherently uncertain. Thus, although TOR estimates can be useful for evaluating different remediation strategies and goals for particular sites, these estimates should always be verified with site monitoring.
CONTENTS
Abstract
Introduction
  Purpose and Scope
Natural Attenuation of Petroleum Hydrocarbons
  Biodegradation Processes
    Aerobic Oxidation
    Anaerobic Oxidation
    Biodegradation of Methyl Tertiary-Butyl Ether
  Sorption Processes
  Advection and Dispersion
  Volatilization
Natural Attenuation of Chlorinated Ethenes
  Biodegradation Processes
    Reductive Dechlorination
    Aerobic Oxidation
    Anaerobic Oxidation
    Aerobic Cometabolism
  Redox Conditions and the Biodegradation of Chlorinated Ethenes in Ground-Water Systems
    Delineating the Distribution of Redox Processes in Ground-Water Systems
    Organic Carbon Substrates that Support Reductive Biodegradation
    Daughter Products Indicating Biodegradation
  Sorption Processes
  Measuring Biodegradation Rates
  Sources of Uncertainty in Biodegradation Rate Estimates
Time of Remediation Associated with Natural Attenuation
  Summing the Processes that Contribute to Natural Attenuation
  Natural Attenuation Capacity
  Distance of Stabilization
  Time of Stabilization
  Time of Nonaqueous Phase Liquid Dissolution
Time of Remediation Software
  Overview of Natural Attenuation Software
  Data Requirements and Input Using Natural Attenuation Software
Time of Remediation Examples
  Gasoline Nonaqueous Phase Liquid Example: Laurel Bay, South Carolina
  Chlorinated Ethene Example: Kings Bay, Georgia
Summary
References
FIGURES
1. Diagrams showing conceptualization of the time of remediation problem
2. Schematic diagram showing how molecular hydrogen drives reductive dechlorination
3. Graph showing characteristic H2 concentrations associated
    with different terminal electron-accepting processes
4. Flowchart for deducing terminal electron-accepting processes in ground-water systems

5-7. Graphs showing:
    5. Effects of natural attenuation capacity on contaminant concentration
        declines along ground-water flowpaths
    6. Effects of natural attenuation capacity, initial contaminant concentration,
        organic matter content of aquifer material, and contaminant composition on times of stabilization
    7. Time of nonaqueous phase liquid (NAPL) dissolution versus NAPL mass, and NAPL mass
        in differing aquifer thickness
8. Flowchart showing how NAS can be applied to time of remediation problems
9. Initial computer screens showing NAS and the hydrogeology data entry of NAS
10. NAS computer screens of distance and time of stabilization screens, and graphical representation of
     time-of-stabilization calculations
11. NAS computer screens of data input for nonaqueous phase liquid time-of-dissolution calculations,
     and example of time-of-dissolution results
12. Site map, locations of observation wells, and potentiometric surface of the Laurel Bay site, South Carolina
13. NAS computer screens of summary of contaminant data, redox data, and summary of biodegradation rate
     and natural attenuation capacity estimates made from these data for the Laurel Bay site
14. NAS computer screens of summary of time-of-stabilization and distance of stabilization estimates,
15. Graphs showing predicted benzene concentrations using time of stabilization estimates at well MW-8
     plotted versus measured benzene concentrations over time at the Laurel Bay site
16. Graphs showing estimated time of nonaqueous phase liquid dissolution for MTBE, benzene,
     and total BTEX using NAS, and observed source-area contaminant concentration decreases
     in source-area well EX-1 at the Laurel Bay site
17. Map showing location of total chlorinated ethene plume prior to in situ oxidation and locations of
     monitoring wells

18-20. Computer screens showing:
     18. Summary of hydrologic, contaminant, and redox characteristics of the Kings Bay site
     19. Graph depicting discontinuity in biodegradation rates between redox zones, and summary tables
          of estimated biodegradation rates at the Kings Bay site
     20. Summary of time and distance of stabilization estimates for the Kings Bay site
21. Graph showing estimated time of stabilization for plume following nonaqueous phase liquid (NAPL)
     removal and time of NAPL dissolution versus NAPL mass at the Kings Bay site

22-25. Graphs showing observed concentration changes:
     22. At well USGS-3 in the zone of Fenton.s reagent injection, and well KBA-11-13A, which are 160 feet
         downgradient of the injection zone
     23. At wells USGS-9 and USGS-11, which are 380 feet downgradient of the injection zone
     24. Compared to NAS-simulated changes at well KBA-11-13A
     25. Compared to NAS-simulated changes at wells USGS-9 and USGS-11
TABLE
1. Ground-water chemistry constituents used to identify microbial redox processes in ground-water systems, and common methods of analysis

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