Temporal variation of oxygen isotope ratios (δ18 O) in drinking water: Implications for specifying location of origin with human scalp hair

Abstract Previous work suggests that δ18 O values of human hair can be used to constrain the region-of-origin of unknown individuals, but robust assessments of uncertainties in this method are lacking. Here we assess one source of uncertainty – temporal variation in the δ18 O value of drinking water – using a monthly tap water survey of δ18 O to develop geospatial models (i.e., maps) of the intra-annual variation (seasonality) in tap water δ18 O for the contiguous USA. Temporal variation in tap water δ18 O was correlated with water-supply type, and was related to geographic patterns of precipitation δ18 O seasonality and water residence time. The maps were applied in a Bayesian framework to identify the geographic origin of an unidentified woman found in Utah, based on measured δ18 O of scalp hair. The results are robust in specifying parts of the western USA as the most likely region-of-origin. Incorporation of tap water δ18 O seasonality in the analysis reduces the precision of geographic assignments, but other sources of uncertainty (e.g., spatial interpolation uncertainty) have an equal or larger effect.