Oxygen isotope variation in drinking water in Oaxaca, Mexico and its implications for forensic provenancing efforts

Forensic anthropologists have used oxygen isotopic analyses (δ¹⁸O) in recent decades because of its ability to help estimate probable regions of origin of unidentified individuals based on isotopic composition of bodily tissues. Prior research has found that drinking water is geographically patterned and that tap water is often representative of drinking water in assessing these links between human tissues and geography. Researchers have begun applying the method in a variety of global contexts. This research critically assesses the assumption that tap water is isotopically representative of drinking water in Oaxaca, Mexico. Drinking water samples from Oaxaca were analyzed and compared with predicted δ¹⁸O values from a previously constructed tap water isoscape of Mexico. Hair samples from Oaxaca were also analyzed to compare against various water sources. Tap water δ¹⁸O values do not reliably reflect drinking water δ¹⁸O in sampled regions of Oaxaca, Mexico. Further, the models relating hair keratin to drinking water, built on this assumption, fail to hold predictive power. With an incomplete understanding of tap water’s ability to represent drinking water isotopically, researchers need to study the role of other key factors in δ¹⁸O values. If the individual’s being identified with δ¹⁸O are from non-Western communities, forensic practitioners run the risk of incorrectly predicting region of origin because of the method’s assumptions. Recognizing the varying sociocultural realities of the communities forensic practitioners seek to aid is paramount as we grow and develop our methods moving forward.