The issue of sample archiving and/or preservation of the original support for measurement results can be important in the context of data defensibility. This question capture two distinctly different issues: (1) How does one preserve the original sampled media to allow replicate analyses in the future to validate the original result? (2) How does one ensure the sample remains representative of the decision unit (e.g., barrel, area of contaminated sediment, groundwater point of compliance, etc.) over time?

For ex situ sample analyses, (real-time or traditional) preserving sampled media is straightforward. If sample preservation is a requirement for a program for whatever reason, arrangements can be made for sample archiving that insure the long-term integrity of the sample and that controls chain of custody. For measurements that are in situ, or in other words that measure some key parameter directly for environmental media, the issue becomes one of being able to document the exact measurement location and preserve that location’s state. One must recognize that even if one were able to both exactly identify the location of an original in situ measurement and protect its location, natural variability in environmental conditions and processes would likely lead to different measurement results over time. The real issues in this case, however, are not whether a particular measurement is exactly replicable, but whether one would expect the decision that was drawn based on that measurement to have been different, given the potential range in repeated measurement values over time, and secondly, the length of time over which the field measurement can be expected to be valid. The latter is very site-specific, depending on the nature of the contaminants of concern, the influences of the environmental matrix, and contaminant mobility and fate over time. A related issue is the question of chain-of-custody for sampled media. For field-based methods, the chain-of-custody can be significantly shorter, or even potentially completely eliminated, as compared to traditional fixed-laboratory programs.

How to maintain sample representativeness is a different question and a problem common to both in situ measurement systems and ex situ sample analyses. Particularly for sediments and groundwater, active environmental fate and transport processes virtually guarantee that sampling results will not be representative of the state of the system for any extended period of time. The issue of sample representativeness over time plays to the Triad’s strengths: the ability to quickly and relatively cheaply replicate measurements where there are questions about the current representativeness of historical data sets.