The δ²H and δ¹⁸O of tap water from 349 sites in the United States and selected territories

Because the stable isotopic compositions of hydrogen (δ²H) and oxygen (δ¹⁸O) of animal (including human) tissues, such as hair, nail, and urine, reflect the δ²H and δ¹⁸O of water and food ingested by an animal or a human and because the δ²H and δ¹⁸O of environmental waters vary geographically, δ²H and δ¹⁸O values of tap water samples collected in 2007-2008 from 349 sites in the United States and three selected U.S. territories have been measured in support of forensic science applications, creating one of the largest databases of tap water δ²H and δ¹⁸O values to date. The results of replicate isotopic measurements for these tap water samples confirm that the expanded uncertainties (U = 2μ_c) obtained over a period of years by the Reston Stable Isotope Laboratory from δ²H and δ¹⁸O dual-inlet mass spectrometric measurements are conservative, at ±2‰ and ±0.2 ‰, respectively. These uncertainties are important because U.S. Geological Survey data may be needed for forensic science applications, including providing evidence in court cases. Half way through the investigation, an isotope-laser spectrometer was acquired, enabling comparison of dual-inlet isotope-ratio mass spectrometric results with isotope-laser spectrometric results. The uncertainty of the laser-based δ²H measurement results for these tap water samples is comparable to the uncertainty of the mass spectrometric method, with the laser-based method having a slightly lower uncertainty. However, the δ¹⁸O uncertainty of the laser-based method is more than a factor of ten higher than that of the dual-inlet isotoperatio mass spectrometric method.