ISOTOPES ( O, H, TRITIUM) AS INDICATORS OF WATER MOVEMENT NEAR THE INTERNATIONAL BOUNDARY IN THE EL PASO (TEXAS) -CIUDAD JUAREZ(CHIHUAHUA) METROPOLITAN AREA

Stable oxygen and hydrogen isotopes and tritium indicate three distinctive types of groundwater in the Hueco Bolson aquifer (HBA) beneath El Paso and Cd. Juarez. Stable isotope data are listed below as (delta¹⁸O ‰, deltaD ‰). Type 1 is modern Rio Grande river water, (-8.5, -70) to (-7.0, -64), strongly evaporated and containing finite tritium. Type 2 is "pre-dam" Rio Grande water, less evaporated (-11.9, -92) to (-10.0, -80) and with tritium below detection, consistent with recharge of river water prior to 1915 when Elephant Butte Dam in New Mexico was completed. Both kinds of river water have recharged the HBA near the Rio Grande. Type 3 is water from local precipitation (-11.0, -78) to (-9.0, -62), which plots on the global meteoric water line, and has recharged the HBA from mountains on the west side of the basin. In El Paso north of the flood plain, production wells yield type 1 water. On the flood plain in El Paso, production and monitoring wells yield type 2 water, which has infiltrated to a depth of 330 m near downtown, or mixtures of types 1 and 2 in the upper 100 m 10 km southeast of downtown. In Cd. Juarez, type 2 water has infiltrated the HBA to a depth of 200m, and has mixed with type 3 water near the Sierra Juarez and with type 1 water near the Rio Grande. The data indicate the following flow directions in pre-development times: type 2 water moved downward and southeast from the Rio Grande near the city centers; type 3 water moved south from the Texas section of the HBA, but did not mix significantly with type 2 water above 330 m (downtown El Paso) or above 200 m (Cd. Juarez). The indicated flow of type 3 water is at variance with aspects of hydrological modeling which predicted cross-border flow of type 3 water to replenish the HBA beneath Cd. Juarez. Post-development pumping has generated cones of depression beneath both cities, affecting flow paths and leading to the infiltration of type 1 water near the Rio Grande in Cd. Juarez, and the mixing of water types 1 and 2 in the flood plain southeast of the city centers.