What are isotopic tracers used for in site characterization?

• A. To differentiate sources, age of contamination, and hydrologic processes affecting transport.
• B. To measure pH of groundwater.
• C. To identify microbial species in soil.
• D. To calibrate instruments.

Answer: (A)

Isotopic tracers are used to distinguish where contamination comes from, estimate when it was released, and reveal how groundwater moves and transports contaminants. Different sources carry unique isotopic signatures, so measuring isotope ratios in the plume lets you match the contamination to its source. Isotopes with known decay or production histories let you date the release or estimate groundwater travel times. Observing how isotopic compositions shift as water mixes and moves helps reveal recharge, mixing, and transport pathways, clarifying the hydrologic processes at work. This is different from measuring pH, identifying microbial species, or calibrating instruments, which involve pH probes, microbiology methods, or calibration routines rather than tracing water masses and contaminant ages.