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Model Report


Last Revision Date: 08/31/2009 View as PDF
General Information Back to Top
Model Abbreviated Name:

Model Extended Name:

Model Overview/Abstract:
Natural attenuation, a remediation strategy that relies on intrinsic physical, chemical, and biological processes to decrease contaminant concentrations, is gaining widespread acceptance in aquifer restoration efforts. The potential for successful remediation by natural attenuation depends on the fate of the organic constituents of the spilled product. These compounds can dissolve in ground water, adsorb to subsurface sediments, volatilize and diffuse through the unsaturated zone, or undergo chemical and biological reactions. Volatilization and biodegradation near the water table are two processes that can contribute significantly to the natural attenuation of volatile organic compounds (VOCs) in shallow ground water. R-UNSAT, a computer model designed for quantifying rates of volatilization and biodegradation of organic compounds near the water table, was developed and documented by the U.S. Geological Survey (USGS) and is available to the public. R-UNSAT also can be applied to other unsaturated-zone transport problems that involve gas diffusion, such as radon migration, and the deposition of compounds from the atmosphere to shallow ground water.

R-UNSAT is designed to simulate reactive, multispecies transport in a layered unsaturated zone with variable moisture content. Two-dimensional (axisymmetric) model equations are solved by using a finite-difference algorithm. Analytical solutions for simulating one dimensional, vertical transport of a nonreactive species in a homogeneous unsaturated zone also are provided. Gas transport is based on Fick's Law of diffusion; therefore, application of R-UNSAT is inappropriate if gas-phase advection is significant (for example, in soil-vapor extraction applications). R-UNSAT can be applied, however, to simulate advective transport of compounds in the aqueous phase associated with constant ground-water recharge. R-UNSAT accounts for equilibrium partitioning of mass among immiscible (the petroleum-product source), solid, aqueous, and gaseous phases. The immiscible phase is assumed to be stationary and is modeled as a boundary condition.

Model Technical Contact Information:
Arthur L. Baehr
U.S. Geological Survey - USGS
609) 771-3902

The model was developed and is maintained by the U.S. Geological Survey (USGS).

Model Homepage: http://nj.usgs.gov/toxics/models.html Exiting the EPA Site

User Information Back to Top
Technical Requirements
Computer Hardware
486 PC with 128 KB RAM and 100 MB ROM (minimum)

Any mainframe with a Fortran compiler

Compatible Operating Systems
Windows 95, 98, or later
Other Software Required to Run the Model
Download Information
The model is publicly available from U.S. Geological Survey (USGS). exit EPA
Using the Model
User Support
User's Guide Available?
Provides: (1) an abstract or abstract equivalent in the introduction, (2) schematic/diagram showing model structure and interaction of model components, (3) equations, equation solution methodologies, and related simplifying assumptions, (4) example input/output files, (5) input and output variable documentation including definitions, units, temporal/spatial dimensions, temporal/spatial resolution options, and (if applicable such as with FORTRAN) format, and (6) guidance on selecting and/or estimating values and/or distributions for input variables (including guidance on calibration and selecting default values and/or distributions).
User Qualifications
User needs only moderate level of technical education and/or modeling experience.

Model Science Back to Top
Summary of Model Structure and Methods
Simulates effects of chemical mixture characteristics on the chemical and/or environmental properties of individual components.

Simulates effects of chemical mixture characteristics on the environmental fate/transport of individual components.

The model uses flexible (user-defined) units.

The model provides mathematical guidance to assist the user when selecting modeling parameters.

Governing equations are solved numerically.

Model Evaluation
Substantial model testing (evaluation) has been performed.

Model has been internally and externally peer reviewed.

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