ExampleThree2

The problem to be addressed at this site was the contribution of each party to the total problem. Because station “A” is hydraulically upgradient (ground water flows from station “A” to station “B”), it might appear that station “A” is the source of the problem. To differentiate the two plumes (and possibly a third from station “C” located just to the south), an isotopic study was conducted.
An isotope is one of two or more atoms of the same element that have the same number of protons in their nucleus, but a different number of neutrons. For example, carbon has three isotopes: 12C, 13C and 14C. 12C has 6 protons and 6 neutrons in its nucleus, while 13C has 6 protons and 7 neutrons. 14C has 6 protons, 8 neutrons and is also radioactive.

The isotopic content of specific contaminants in ground water can be assessed through a technology known as gas chromatography with isotope ratio mass spectrometry (GCIRMS). In environmental investigations where hydrocarbons are involved, the ratio of 13C to 12C is commonly used to fingerprint contaminants. Through this technique, values of d13C (the 13C/12C ratio) for a specific compound, such as methyl-tert-butyl ether (MTBE) or benzene, can be determined.

Values of d13C can be used as a proxy for weathering. Bacteria tend to favor the 12C and as biodegradation proceeds, the compound can become enriched in 13C. Accordingly, as weathering becomes more severe, the d13C values increase.
The map above shows the d13C values for MTBE in samples collected from 20 monitoring wells at the two sites (A and B). The d13C values reveal that, beneath station “B”, the MTBE is lighter or contains less 13C and, therefore, could be considered fresh (the d13C values for MTBE are within the range for fresh product). On the other hand, the d13C values for MTBE beneath station “A” are heavier and enriched in 13C.