Hydrogeology and hydrogeochemistry of a rift-margin aquifer: Effects of basin margin structure and stratigraphy on ground-water resources, Placitas area, New Mexico
LeFEVRE, William, Dept of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, lefevre@nmt.edu; JOHNSON, Peggy, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM 87801; CAMPBELL, Andrew, and McPHERSON, Brian, Dept of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801.
Placitas is located on the eastern margin of the Albuquerque Basin, at the northern termination of the Sandia uplift, northeast of Albuquerque. Two major west-dipping, rift-margin, normal faults of the Rio Grande rift, the San Francisco-Placitas fault zone and the Rio Grande fault, bound the area to the east and west. These, and numerous subsidiary normal faults, cut north-dipping Paleozoic and Mesozoic sedimentary strata where they ramp below Santa Fe Group basin fill.
Ground water in this structurally and stratigraphically complex setting exists in a compartmentalized aquifer system that is recharged along selectively active flow paths that originate in the Sandia Mountains to the south. Ionic and trace element analyses of ground water from wells and springs, stable isotope data, potentiometric data from residential domestic wells, and surface and subsurface geologic data indicate the presence of an assortment of confined and unconfined aquifers with a wide range of water quality and productivity.
Chemical analyses of mountain-front spring water along the Placitas fault zone indicate a common source of calcium-bicarbonate ground water originating in the Madera Group limestone (Penn). Most wells near and down gradient from the Placitas fault zone have calcium and bicarbonate concentrations similar to those of the springs (60-100 ppm calcium, 220-290 ppm bicarbonate), indicating that the main recharge path for the aquifers is through the Madera limestone. Wells completed within the Placitas fault zone commonly exhibit artesian hydrostatic pressure, significantly elevated concentrations of sodium (>400 ppm) and sulfate (>1900 ppm), a relative increase in dissolved silica, and high conductivities (>3000 m mhos/cm). Nearby wells completed adjacent to other subsidiary rift margin faults exhibit similar hydrologic and geochemical characteristics. Such aberrant ground-water chemistries and gradients are interpreted to indicate either a long residence time under confined conditions in Triassic mudstone blocks in the fault zone, or more likely, a second deep-basin source of ground water that flows upward along the rift margin.
Hydrogeology, hydrogeochemistry, stable isotopes, Rio Grande Rift
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