Circular 1303
Prepared in cooperation with the Strategic Environmental Research and Development Program
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:
These are general principles that can be used to assess the sustainability of MNA in any hydrologic system.
Abstract
Introduction
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
Conclusions
References
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
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.water.usgs.gov/circ1303
Page Contact Information: Francis H. Chapelle
Page Last Modified: Wednesday, 23-Nov-2016 12:16:36 EST
