Describe how isotopic fingerprinting can be used for source attribution in environmental forensics and provide a basic example.

• A. Distinct isotopic ratios (e.g., nitrate 15N/14N, sulfate 34S/32S) link contamination to specific sources; example: distinguishing fertilizer-derived nitrate from septic system nitrate in groundwater.
• B. Isotopic fingerprinting uses concentration levels to identify sources.
• C. Isotopic fingerprints cannot distinguish sources due to identical isotopic ratios across sources.
• D. It is used only for dating samples, not source attribution.

Answer: (A)

Isotopic fingerprinting relies on distinctive isotope ratios that act like a signature for where a contaminant comes from. Different sources and the environmental processes they undergo imprint characteristic heavy-to-light isotope ratios, such as nitrate with 15N/14N or sulfate with 34S/32S. By measuring these ratios in environmental samples and comparing them to known source signatures, you can attribute contamination to specific sources. For example, nitrate in groundwater can originate from synthetic fertilizer or from septic systems, and these sources often produce different 15N/14N and 18O/16O patterns in the nitrate. Using multiple isotopes together strengthens discrimination, especially in mixtures, more so than concentration data alone. While isotopes can be used for dating in other contexts, here the emphasis is on linking the contaminant to its source through its isotopic signature.