What is the trend I might expect?
The trend really depends on the mechanism of degradation. Figure 6 presents the most typical causes for a change in the isotopic ratio, or del, of VOC’s in ground water. Figure 6. The most typical things to cause a change in the del, or isotopic ratio, and the direction of that change As we have said above, degradation causes the remaining VOC’s to get heavier. So if you have a system where there is no longer a source, there is no NAPL and there is only degradation, then the situation is simple. The only thing that changes the del is degradation and CSIA is a direct indicator of the extent of that degradation. That situation often describes MTBE releases or TCE co-metabolism. However, when there is a source, when there is NAPL, or when there is both degradation and production (as is the case with TCE at DNAPL sites where there is reductive dechlorination of both the PCE and the TCE) the situation is much more complex. At those sites CSIA can still be a very powerful tool, but qualitati
