Effects of basin margin structure and stratigraphy on ground-water recharge in the Placitas area, Northern Albuquerque basin, New Mexico
By Peggy Johnson, William LeFevre, Andrew Campbell, and Brian McPherson
Preliminary data from a ground-water study in progress for Sandoval county by the New Mexico Bureau of Mines and Mineral Resources indicate that faults in the Placitas area behave as both barriers to and conduits for ground-water flow into the northern Albuquerque basin. Placitas is located on the eastern margin of the Albuquerque Basin, at the northern termination of the Sandia uplift, northeast of Albuquerque. Major west-dipping, rift-margin, normal faults, including the San Francisco-Placitas fault zone and numerous subsidiary faults, cut north-dipping Paleozoic and Mesozoic sedimentary strata as 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 through a combination of surface-water and preferential ground-water flow paths originating in the Sandia Mountains to the south. Piezometric data from private wells, preliminary stable isotope, ionic and trace element data from ground and surface water, and surface and subsurface geologic data together indicate the existence of an assortment of confined and unconfined aquifers with a wide range of water quality and productivity, and varying degrees of hydraulic interconnection.
Piezometric data in the vicinity of the Placitas and San Francisco faults generally indicate dramatic declines in fluid potential of 200 to 300 feet across these structures, and numerous springs discharge on the upgradient sides of the faults, thus indicating that the faults attenuate ground-water flow along most of their extent. However, relatively gradual hydraulic gradients do occur across the faults at very isolated locations, pointing to the existence of preferential flow paths and/or recharge zones that channel ground-water flow into the Albuquerque basin.
Stable isotope data from ground water, surface water, and spring discharge are being interpreted relative to a local meteoric water line defined from a network of precipitation collectors located along the north slope of the Sandia Mountains. Variations in ground-water stable isotope ratios (d D of - 119 % to - 87% and d 18O of - 15.0% to - 11.4%) appear to spatially correlate to recharge signatures from precipitation and surface water sources. Isolated regions of anomalously light stable isotope ratios (-119 to -115 % D and -15.0 to -13.6 % 18O), are interpreted to represent areas of paleowater in isolated portions of the Santa Fe Group aquifer.
Chemical analyses of mountain-front spring water along the San Francisco-Placitas fault zone suggest a common source of calcium-bicarbonate ground water originating in the Madera Group limestone (Penn). Ground water from the Santa Fe Group aquifer immediately down gradient of the San Francisco fault has a chemical nature similar to that of the springs and the Madera aquifer wells. A significant change in general and trace element chemistry is detected in ground waters discharging from the Mesozoic-age sediments adjacent to the Placitas fault zone. These ground waters exhibit very high concentrations of sodium (£ 1450 ppm), sulfate (£ 3900 ppm), and dissolved solids (£ 5950 ppm), and elevated concentrations of Fe (£ 940 ppb), Cu (£ 270 ppb), Manganese (£ 370 ppb), and Zinc (£ 110 ppb). Low concentrations of Arsenic (5 to 21 ppb) are generally characteristic of ground water discharging from both the Triassic age Agua Zarca and lower Petrified Forest Formations, as well as certain areas of the Santa Fe Group aquifer. These high TDS, sodium-sulfate ground waters are interpreted to reflect "long" residence times under semi-confined conditions in low permeability Mesozoic-age sediments that are isolated from direct sources of recharge.
Together these various data support a complex pattern of recharge to
the aquifers adjacent to and down-gradient of the San Francisco-Placitas
fault zone. Recharge appears to be moving through the Madera limestone
and across the fault zones via preferential pathways, and is also being
redistributed into the aquifers as line sources of infiltrating surface
water originating from spring discharge and stream flow. Neither piezometric
data nor variations in ionic, trace element, or stable isotope chemistry
support an origin of spring discharge from an alternate, deep-basin source
of ground water that flows vertically upward in the faults. Additional
ground-water sampling for age determination will be undertaken to test
this regional hydrogeologic conceptual model.
New Mexico Bureau of Geology & Mineral Resources
New Mexico Institute of Mining & Technology
801 Leroy Place
Socorro, NM 87801-4796
information: (575) 835-5420
fax: (575) 835-6333
http://geoinfo.nmt.edu
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