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Model Report


Last Revision Date: 01/21/2014 View as PDF
General Information Back to Top
Model Abbreviated Name:

Model Extended Name:

Model Overview/Abstract:
Stable isotope analyses are often used to quantify the contribution of multiple sources to a mixture, such as proportions of pollutant sources to a waste stream, proportions of food sources in an animal’s diet, etc. Linear mixing models can be used to uniquely partition n+1 sources when there are n isotopic tracers used (e.g. δ13C and δ15N. However, when there are more than n+1 sources, there is no unique solution for source proportions. IsoSource examines all possible combinations of sources, in user-defined increments (e.g., 1%), computes the expected mixture isotopic signatures, and compares them to the observed mixture isotopic signatures. If they match, within a user-specified tolerance, then that set of source proportions is considered a feasible solution. The distribution of such feasible solutions is output from the model. Thus, even though the excess number of sources does not allow for a mathematically unique solution, the range and distribution of feasible contributions to the mixture for each source is empirically determined.
Keywords: Stable isotope analysis, estimating source proportions
Model Technical Contact Information:
Agency Contact
Donald Phillips
Model Homepage: www.epa.gov/wed/pages/models/stableIsotopes/isosource/isosource.htm
Substantive Changes from Prior Version: N/A
Plans for further model development: None

User Information Back to Top
Technical Requirements
Computer Hardware
Compatible Operating Systems
Windows 98 or later
Other Software Required to Run the Model
WinZip or other application to unzip downloaded program file
Download Information
Using the Model
Basic Model Inputs
Isotopic signatures (e.g., atom % or δ) for up to five isotopic tracers (e.g., δ13C and δ15N) for each of up to ten sources and the mixture. (For dietary studies, appropriate isotopic tissue-diet discrimination corrections should be made first.)
Basic Model Outputs
Distributions of multiple solutions for the proportions of each source’s contribution to the mixture.
User Support
User's Guide Available?
On-line documentation and help available in Help menu in the software.
Other User Documents
Phillips DL and Gregg JW (2003) Source partitioning using stable isotopes: coping with too many sources. Oecologia 136: 261-269.
Availability of User Support
e-mail Don Phillips
User Qualifications
Basic familiarity with stable isotope analysis

Model Science Back to Top
Problem Identification
Stable isotope analysis is often used to estimate the proportional contributions of sources to a mixture based on isotopic composition. However, if there are too many sources compared to the number of isotopic tracers used, there is no unique solution. This model empirically determines ranges and distributions of multiple source proportion solutions that satisfy isotopic mass balance.
Summary of Model Structure and Methods
The model is a Visual Basic application. The user supplies information about the isotopic signatures for each source and mixture in the graphical user interface, along with parameter values for Increment and Tolerance, and a title. Source proportion solutions (in specified Increments) that satisfy isotopic mass balance (within the specified Tolerance) are calculated and shown in output files and graphs. One output file contains each individual solution, while another output file shows histogram frequency information for these solutions, along with descriptive statistics for their distributions (mean, minimum, maximum, 1 percentile, 50 percentile, 99 percentile, and standard deviation). Histogram graphs are also displayed that show the distribution of proportion solutions for each source.
Model Evaluation
The model and its constituent equations are described in the peer-reviewed journal publication: Phillips DL and Gregg JW (2003) Source partitioning using stable isotopes: coping with too many sources. Oecologia 136: 261-269.

The code was verified by comparison of results to those from an independent program written in SAS for the same purpose. Sensitivity analyses were performed for the effect of parameter values for source increment and mass balance, as described in Phillips and Gregg (2003).

Key Limitations to Model Scope
The model as currently configured can compute the proportional contributions for up to ten sources using up to five isotopic signatures.
Case Studies
The Phillips & Gregg (2003) paper shown above gives four examples of application (plant water use, nutrient inputs to soils, atmospheric lead sources, and food sources in mink diets). A wide variety of additional examples can be found in 304 other papers that have cited this paper (per Google Scholar, 11/11/2009).

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