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SOURCE PARTITIONING USING STABLE ISOTOPES: COPING WITH TOO MANY SOURCES
Citation:
Phillips, D L. AND J W. Gregg. SOURCE PARTITIONING USING STABLE ISOTOPES: COPING WITH TOO MANY SOURCES. Presented at Application of Stable Isotope Techniques in Ecological Studies Conference, Flagstaff, AZ, April 29-May 1, 2002.
Description:
Stable isotopes are increasingly being used as tracers in environmental studies. One application is to use isotopic ratios to quantitatively determine the proportional contribution of several sources to a mixture, such as the proportion of various pollution sources in a waste stream. In general, the proportional contributions of n+1 different sources can be uniquely determined by the use of n different isotope system tracers (e.g., d13C, d15N, d18O) with linear mixing models based on mass balance equations. Often, however, the number of potential sources exceeds this number, which prevents finding a unique solution of source proportions. What then can be done in these situations? While no definitive solution exists, we propose a method that is informative in determining bounds for the contributions of each source. In this method, all possible combinations of source proportions are examined (in increments of 1% or 2%), and the predicted isotopic signatures for the mixture are computed and tested for compatibility with the mass balance equations. Any combinations that result in predicted mixture isotopic signatures within a small tolerance (e.g. 0.1 l) of the observed signatures are considered to be feasible solutions. For each source, the range of proportions in the set of feasible solutions gives the minimum and maximum possible contribution the source made to the mixture. We applied this method to a number of environmental studies in which stable isotope tracers were used to quantify the relative magnitude of multiple sources. These examples include: (a) atmospheric inputs of lead pollution from different geographic areas; (b) geologic, atmospheric, and oceanic inputs of nutrients to the Hawaiian Islands; (c) CO2 flux from various organic carbon pools in the soil; and (d) dietary analyses of carnivores.