NEW INSIGHTS ON SALINIZATION AND PREDEVELOPMENT RECHARGE OF THE RIO GRANDE AQUIFER, EL PASO/JUAREZ AREA

The twin-cities of El Paso and Juarez share the water resources of the Hueco Bolson aquifer and overlying Rio Grande aquifer. Both aquifers span the international border between Mexico and the United States. Salinity in the Rio Grande aquifer varies widely, some parts of the shallow aquifer containing less than 1,000 mg/L total dissolved solids (TDS), other parts of the aquifer exceeding 5,000 mg/L TDS. One sizable part of the "Lower Valley," area, approximately 45 km below El Paso contains very dilute water near the outer edge of the floodplain and very saline water close to the present course of the Rio Grande. Historically it had been thought that the dilute waters in this location were derived from recharge from arroyos that drained proximal parts of the Hueco Bolson. Instead, our hydrogen and oxygen isotope data and carbon-14 data indicate that these dilute waters were derived from pre-dam infiltration of the Rio Grande. These very light and slightly evaporated pre-dam waters (-11.5 del O18) are also relatively young (60 to 90 percent modern carbon), tagging them as runoff waters from snowmelt in Colorado. These isotopically light waters are found up to 110 meters beneath land surface. Prior to Rio Grande rectification and channelization of the mid-1930's, the Rio Grande flowed near the outer edge of the floodplain where these pre-dam, dilute waters are found at depth. Review of drill stem tests from the 1950's, before local development of the Hueco Bolson aquifer indicates a permeable zone at 150 to 230 meters deep that has a lower hydraulic head than the overlying Rio Grande aquifer. When measured in the 1950's, the difference of hydraulic head between the Rio Grande aquifer and this deeper permeable zone was about 15 meters. Data collected in earlier studies suggests that this permeable zone is probably a buried channel of the ancestral Rio Grande. This permeable zone acted as a predevelopment sink for flow that induced recharge from the Rio Grande and Rio Grande aquifer. Thus, we can account for local predevelopment recharge of the Rio Grande aquifer from infiltration of dilute water from the Rio Grande prior to the historic period of channel rectification; and not from recharge from flanking arroyos as had been postulated.