Paper No. 5
Presentation Time: 9:20 AM
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.