HR: 14:40h
AN: H33G-05    [Abstracts]
TI: Buried Paleo-Phreatic Playa is the Primary Source of Salinity in the Rio Grande and Rio Grande Aquifer, El Paso-Juarez Valley
AU: * Hibbs, B J
EM: bhibbs@calstatela.edu
AF: Department of Geological Sciences, California State University, Los Angeles, Los Angeles, CA 90030 United States
AU: Merino, M
H33G-05 AF: Department of Geological Sciences, California State University, Los Angeles, Los Angeles, CA 90030 United States
AB: The Rio Grande alluvial floodplain aquifer between El Paso/Juarez and Fort Quitman, Texas has been intensively tilled and irrigated since 1910. Almost since that time, the Rio Grande and its interconnected alluvial aquifer in this stream reach have been affected by excessive salinity problems. Presently, salinity of surface water and groundwater increases from 800 to 2000 mg/L total dissolved solids (TDS) near El Paso, Texas to 2500 to 5500 mg/L TDS near Fort Quitman, Texas, approximately 115 km downstream. Chloride (Cl) and Chloride/Bromide (Cl/Br) ratios increase downstream, with a marked increase in salinity beginning about 45 km below El Paso. Groundwater sampling in an intensively investigated reach of the floodplain aquifer, extending from 35 to 60 km below El Paso identified two distinct water types. The first water type (17 samples) is a relatively dilute groundwater containing 1000 to 2000 mg/L TDS, 250 to 600 mg/L Cl, and Cl/Br weight ratios of 550 to 900. The second water type (16 samples) is a relatively saline groundwater containing 2000 to 6000 mg/L TDS, 900 to 3500 mg/L Cl, and Cl/Br weight ratios of 1200 to 3500. Recent drilling of nested monitoring wells detected a very saline, silty-clay, water bearing unit at 45 to 80 m bgs, directly below the Rio Grande alluvial deposits. This saline water bearing unit contains about 15,000 mg/L Cl and Cl/Br weight ratios of about 5800. Halide mixing curves and oxygen and deuterium isotopes indicate this saline unit is the primary source of high salinity in the Rio Grande and Rio Grande aquifer. Chlorine isotopes also establish this unit as the mixing source, with a chloride age of about 0.75 m.y. to 1.25 m.y. This is consistent with the timing of the final transformation of the local basin (Hueco Bolson) from a topographically closed and undrained basin, to a through-flowing and drained basin. We interpret this saline unit to be the residual of a closed basin, paleo-phreatic (salt) playa that was buried beneath the contemporary Rio Grande floodplain after the Rio Grande cut through the El Paso/Juarez narrows some 650 to 750 k.y. ago.
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: Fall Meeting 2005