Hydrogeology and Water Resources of the Placitas Area, Sandoval County, New Mexico
The Placitas area straddles the northern flank of the Sandia Mountains and the eastern margin of the Albuquerque Basin of the Rio Grande Rift. The area is geologically complex. Major west-dipping, rift-margin, normal faults, including the San Francisco-Placitas fault zone and numerous subsidiary faults, cut Paleozoic and Mesozoic sedimentary strata that ramp northward below Santa Fe Group basin fill. Faults in the Placitas area behave as both barriers to and conduits for ground-water movement. Surface and subsurface geologic data, well hydrographs, and stable isotope, ion chemistry and trace element data from ground and surface water delineate an assortment of confined and unconfined aquifers with a wide range of water quality and productivity, and varying degrees of hydraulic interconnection, recharge, and residence time. The Placitas area is divided into three major conceptual hydrologic systems: 1) the mountain hydrologic system; 2) the Mesozoic ramp; and 3) the Albuquerque Basin. In general, large supplies of ground water are not available in the mountain system or in the Mesozoic ramp.
The major hydrostratigraphic unit in the mountain system, the Madera Group limestone, forms a dual-porosity, fractured carbonate aquifer with moderate aquifer potential. Ground-water flow is concentrated along discrete fractures, fracture systems, or bedding planes, thus availability of ground water is highly variable and dry holes are relatively common. On a regional scale, the Madera Group limestone possesses very high transmissivity and relatively low storage. Limestone outcrops in the Sandia Mountains form a major ground-water recharge area fed by snowmelt, winter-spring precipitation, and surface water from Las Huertas Creek and other drainages. This recharge water possesses a characteristic water chemistry distinguished by major ions of calcium and bicarbonate, a low TDI concentration (<310 mg/l), temperature less than 16 oC, a high dissolved oxygen content (³6 mg/l), no significant trace elements, and 2H/H and 18O/16O ratios similar to local precipitation. Maps of the spatial distribution of TDI, major anions, temperature, dissolved oxygen, and stable isotope ratios identify pathways for ground-water movement, recharge, and hydraulic interconnection or isolation of aquifers.
Ground water in the Mesozoic ramp is limited to compartmentalized sandstone aquifers in the Triassic Agua Zarca Formation, the Jurassic Entrada and uppermost Triassic Petrified Forest Formation, the Westwater Canyon and Jackpile Sandstone members of the Morrison Formation, and the Cretaceous Dakota, Hosta Dalton, and Point Lookout Formations. Rotation of strata to northeast dips of 30 to 65 degrees created subvertical strip aquifers. Each aquifer is stratigraphically isolated by aquitards of mudstone, shale, and siltstone, and by lateral discontinuities produced by north-south faults. Many of the aquifers exhibit elevated temperature (up to 25 oC), low dissolved oxygen (<1 ppm), and high concentrations of sodium (£1450 ppm), sulfate (£3900 ppm), dissolved ions (£5950 ppm), iron (£600 ppb), copper (£270 ppb), manganese (£ 580 ppb), and zinc (£1300 ppb). Low concentrations of arsenic (5 to 21 ppb) are typical of ground water from the Triassic Agua Zarca and lower Petrified Forest Formations. This hydrogeochemistry reflects long residence times under semi-confined conditions in low permeability sediments isolated from active recharge. Ground waters with a recharge signature are located near streams and arroyos, along the Caballo-Pomecerro faults, and in permeable units in contact with the Madera Group limestone.
Age determinations on 12 ground-water samples using radiocarbon, tritium, and stable isotopes indicates a variation in ground-water residence time ranging from modern to fossil water over 35,000 years old. Locations with a modern isotopic signature are adjacent to Las Huertas Creek, Arroyo Agua Sarca, and the Caballo-Pomecerro fault. Active recharge to aquifers down-gradient of the San Francisco-Placitas fault zone appears to occur through the Madera Group limestone and across the fault zones via cross-cutting faults, hydraulically continuous permeable units, and as surface water originating from spring discharge and runoff.
New Mexico Bureau of Geology & Mineral Resources
New Mexico Institute of Mining & Technology
801 Leroy Place
Socorro, NM 87801-4796
information: (575) 835-5420
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http://geoinfo.nmt.edu
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