The Phillips & Gregg (2003) paper described the IsoSource model, which was designed to give “the distribution of feasible source contributions based entirely on isotopic constraints when the number of sources precludes a unique solution. Given this information, it is possible to apply other constraints to further reduce the range of contributions from each source.” This was discussed further and several examples were given. As pointed out in the paper, “One flexible way to incorporate such other factors is to give the entire range of solutions using only isotopic constraints, and then extract a subset of the output which also satisfies other non-isotopic constraints.” The document below provides additional details about the mechanics of how this can be accomplished, and works through a detailed example.
For further details, see the Stable Isotope Mixing Models for Estimating Source Proportions page.You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.