Stable O and H isotopes and radioisotopes show that Río Bravo water recharged prior to the construction of Elephant Butte Dam is the principal source of groundwater in the Hueco Bolson aquifer beneath Ciudad Juárez. Mixing between pre-dam and post-dam river water occurs in samples from the aquifer beneath the Río Bravo floodplain, where finite tritium is also present. Along the flanks of the Sierra de Juárez, mixing occurs between pre-dam river water and water derived from the Sierra de Juárez. Carbon-14 content of groundwater generally decreases from northwest to southeast beneath the city; corrected residence times in city supply wells range from post-bomb to 3600 years. Sulfur isotopes in groundwater of pre-dam river water origin and anion ratios (Cl/Br and Cl/SO4) are consistent with mixing of solutes from native Hueco Bolsón groundwater with river water. Stable isotopes serve to identify mixing of 9 percent or more of local water into pre-dam river-derived water. Mixing of smaller amounts of saline Hueco Bolson groundwater into river-derived groundwater can best be identified using anion ratios. The improved understanding of groundwater origins, flow paths, and quality arising from this study is crucial information for future water-supply engineering in Ciudad Juárez and for elucidating preferred pathways for contaminant movement in groundwater.

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