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This study investigates the relationship between O and H isotopes in samples of Mexican hair and drinking water. The purpose of this study was twofold. First, we wanted to quantify the relationship between isotopes in Mexican hair and tap water to understand the impact of water stress and differing socioeconomic status on accurate predictions of drinking water. Second, we wanted to determine whether currently existing semi mechanistic models could accurately represent the relationship between hair and tap water. For this study, we used a subset of paired (N = 62) human hair and (N = 76) tap water samples. Isotope values in tap water spanned a range from –11.4 ‰ to –4.3 ‰ and –79.1 ‰ to –22.5 ‰ for δ18O and δ2H, respectively. Isotope values in hair ranged from +9.5 ‰ to +16.1 ‰ and –90.8 ‰ to –53.7 ‰ for δ18O and δ2H, respectively. The most depleted δ18O and δ2H hair values came from individuals in the state of Morelos. For our modern Mexican population, positive correlations between isotopes in hair and water were not significant, with correlation coefficients of r = 0.61 (p = 0.05) and r = 0.60 (p = 0.06) for 18O and 2H respectively. Error-in-variables regression yielded linear fits that were somewhat better for 2H relative to 18O: δ18Oh = 0.183 [± 0.132] δ18Otw + 15.7 [± 0.9] ‰ (r2 = 0.23); δ2Hh = 0.181 [± 0.076] δ2Htw – 64.0 [± 3.0] ‰ (r2 = 0.34). In short, data from this Mexican population do not exhibit the strong relationships between isotope values of 18O and 2H in tap water and hair that have been characteristic of other populations studied to date. Given the economic stratification of this region and the poor correlation between hair and water samples, we chose to consider the possibility that l – the fraction of the diet derived from local sources –and fs – the fraction of non- exchangeable H in keratin that was fixed in vivo–are local, rather than global, parameters for this population. We estimated a different value of l and fs for each location. Given the anticipated importance of the non-local dietary contribution, we treated the isotopic content of non-local food and the offset parameters for predicting isotopes in locally derived food as tuning parameters and compared the results with the parameters based on the American supermarket diet. We found that, although O and H isotopes in water and hair maintain similar geographic distributions, O and H isotopes in tap water explain only a small part of the variation observed in hair samples. In comparison to the standard American supermarket diet, the Mexican estimates for non-local diet and local diet offsets predict regional distributions of l and fs that cleanly segregate urban areas from rural towns.