Circular 1303

Prepared in cooperation with the Strategic Environmental Research and Development Program

A Framework for Assessing the Sustainability of Monitored Natural Attenuation

U.S. Geological Survey Circular 1303

By Francis H. Chapelle1, John Novak2, John Parker3, Bruce G. Campbell1, and Mark A. Widdowson2

Download PDF file for Circular 1303 (9.3 MB)


The sustainability of monitored natural attenuation (MNA) over time depends upon (1) the presence of chemical/biochemical processes that transform wastes to innocuous byproducts, and (2) the availability of energy to drive these processes to completion. The presence or absence of contaminant-transforming chemical/biochemical processes can be determined by observing contaminant mass loss over time and space (mass balance). The energy available to drive these processes to completion can be assessed by measuring the pool of metabolizable organic carbon available in a system, and by tracing the flow of this energy to available electron acceptors (energy balance). For the special case of chlorinated ethenes in ground-water systems, for which a variety of contaminant-transforming biochemical processes exist, natural attenuation is sustainable when the pool of bioavailable organic carbon is large relative to the carbon flux needed to drive biodegradation to completion.

These principles are illustrated by assessing the sustainability of MNA at a chlorinated ethene-contaminated site in Kings Bay, Georgia. Approximately 1,000 kilograms of perchloroethene (PCE) was released to a municipal landfill in the 1978–1980 timeframe, and the resulting plume of chlorinated ethenes migrated toward a nearby housing development. A numerical model, built using the sequential electron acceptor model code (SEAM3D), was used to quantify mass and energy balance in this system. The model considered the dissolution of non-aqueous phase liquid (NAPL) as the source of the PCE, and was designed to trace energy flow from dissolved organic carbon to available electron acceptors in the sequence oxygen > chlorinated ethenes > ferric iron > sulfate > carbon dioxide. The model was constrained by (1) comparing simulated and measured rates of ground-water flow, (2) reproducing the observed distribution of electron-accepting processes in the aquifer, (3) comparing observed and measured concentrations of chlorinated ethenes, and (4) reproducing the observed production and subsequent dilution of dissolved chloride, a final degradation product of chloroethene biodegradation.

Simulations using the constrained model indicated that an average flux of 5 milligrams per liter per day of organic carbon (CH2O) per model cell (25 square meters) is required to support the short-term sustainability of MNA. Because this flux is small relative to the pool of renewable organic carbon (about 4.7 x 107 milligrams [mg] per model cell) present in the soil zone and non-renewable carbon (about 6.9 x 108 mg per model cell) in an organic-rich sediment layer overlying the aquifer, the long-term sustainability of MNA is similarly large. This study illustrates that the short- and long-term sustainability of MNA can be assessed by:

  1. Estimating the time required for contaminants to dissolve/disperse/degrade under ambient hydrologic conditions (time of remediation).
  2. Quantifying the organic carbon flux to the system needed to consume competing electron acceptors (oxygen) and direct electron flow toward chloroethene degradation (short-term sustainability).
  3. Comparing the required flux of organic carbon to the pool of renewable and non-renewable organic carbon given the estimated time of remediation (long-term sustainability).

These are general principles that can be used to assess the sustainability of MNA in any hydrologic system.




Biological Cycles and the Nature of Sustainability

DDT—An Incomplete Waste-Substrate Cycle

Chlorinated Ethenes in an Oxygenated Aquifer—An Incomplete Waste-Substrate Cycle

Chlorinated Ethenes in an Anoxic Aquifer—A Completed Waste-Substrate Cycle

Mass Balance, Energy Balance, and the Sustainability of Natural Attenuation

Mass and Energy Balance

Mass Balance in Contaminated Ground-Water Systems

Energy Balance in Contaminated Ground-Water Systems

Quantifying Mass and Energy Balance

An Empirical Approach to Mass and Energy Balance

A Deterministic Approach to Mass and Energy Balance

Conceptual Model of the Kings Bay Site

Constructing the Deterministic Model

Constraining the Mass-Balance Model

Rates of Ground-Water Flow

Areal Recharge to the Semi-Confined Aquifer

Mass Balance of NAPL

Mass Balance of Electron Donors and Acceptors

Mass Balance of Dissolved Chlorinated Ethenes

Assessing the Sustainability of Natural Attenuation

NAPL Dissolution and Time of Remediation

NAPL Removal and Time of Remediation

Dissolved Oxygen/Dissolved Organic Carbon Flux: Short-Term Sustainability

Available Organic Carbon and Dissolved Organic Carbon Flux: Long-Term Sustainability

Short-Term and Long-Term Sustainability

Electron Acceptor Depletion and Sustainability



Appendix 1. Description of the Deterministic Model

Appendix 2. Parameters Used to Simulate the Kings Bay Site


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1 U.S. Geological Survey
2 Virginia Polytechnic Institute and State University
3 U.S. Department of Energy

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