Bathwater could be an untapped source of forensic DNA, according to experiments using samples from both tests and real forensic investigations.
While victims in aquatic deaths can usually be identified, “determining whether other individuals were present at the scene can be much more challenging, especially when there are no witnesses,” explains Mitsuyo Machida, a forensic scientist from Tokyo Women’s Medical University. Her research was motivated by the question, “Can DNA recovered from water provide information about who had been present?”
To evaluate the feasibility of recovering DNA from bathwater, researchers analyzed samples collected from the baths of volunteers after 1–10 min of immersion (PLOS One 2026, DOI: 10.1371/journal.pone.0345878). After simple preparation, samples underwent short tandem repeat (STR) profiling, a common forensic method that generates a genetic fingerprint based on the number of repeated sequences in specific regions of DNA.
In general, prior to 1 min of immersion, DNA only from previous bathers was detectable. After 1 min, DNA quantity jumped sharply, and after 2 min, so did the proportion of DNA matching the profile of the current bather. Samples collected after 10 min of immersion yielded complete STR profiles for most volunteers. But there was significant individual variation in the amount of DNA shed.
The method was also tested with water taken from real forensic cases. Human DNA was found in samples taken from bathtubs, harbors, a swimming pool, and a river, but only the bathtub samples contained sufficient DNA for STR analysis, yielding three profiles, two complete and one partial.
The effects of environmental and interpersonal factors on DNA persistence and degradation need further study, but the findings suggest that water samples could provide supplementary information to aid forensic investigations, Machida says. “For example, detecting DNA from an unexpected individual, or observing the absence of an expected contributor, might offer clues about possible copresence or inconsistencies in statements.”
2026 American Chemical Society
