Geochemical Characterization of Ground-water Flow in the Santa Fe Group Aquifer System, Middle Rio Grande Basin, New Mexico
Chemical and isotopic data were obtained from ground water and surface water throughout the Middle Rio Grande Basin (MRGB), New Mexico, and supplemented with selected data from the U.S. Geological Survey (USGS) National Water Information System (NWIS) and City of Albuquerque water-quality database in an effort to refine the conceptual model of ground-water flow in the basin. The ground-water data collected as part of this study include major- and minor-element chemistry (30 elements), oxygen-18 and deuterium content of water, carbon-13 content and carbon-14 activity of dissolved inorganic carbon, sulfur-34 content of dissolved sulfate, tritium, and dissolved atmospheric gases including nitrogen, argon, helium, chlorofluorocarbons, and sulfur hexafluoride from 288 wells and springs in parts of the Santa Fe Group aquifer system. The surface-water data collected as part of this study include monthly measurements of major- and minorelement chemistry (30 elements), oxygen-18 and deuterium content of water, chlorofluorocarbons, and tritium content at 14 locations throughout the basin. Additional data include stable isotope analyses of precipitation and of ground water from City of Albuquerque production wells collected and archived from the early 1980's, and other data on the chemical and isotopic composition of air, unsaturated zone air, plants, and carbonate minerals from throughout the basin.
The data were used to identify 12 sources of water to the basin, map spatial and vertical extents of ground-water flow, map water chemistry in relation to hydrogeologic, stratigraphic, and structural properties of the basin, determine radiocarbon ages of ground water, and reconstruct paleo-environmental conditions in the basin over the past 30,000 years.
The data indicate that concentrations of most elements and isotopes generally parallel the predominant north to south direction of ground-water flow. The radiocarbon ages of dissolved inorganic carbon in ground water range from modern (post-1950) to more than 30,000 years before present, and appear to be particularly well defined in the predominantly siliciclastic aquifer system. Major sources of water to the basin include (1) recharge from mountains along the north, east and southwest margins (median age 5,000-9,000 years); (2) seepage from the Rio Grande and Rio Puerco (median age 4,000-8,000 years), and from Abo and Tijeras Arroyos (median age 3,000- 9,000 years); (3) inflow of saline water along the southwestern basin margin (median age 20,000 years); and (4) inflow along the northern basin margin that probably represents recharge from the Jemez Mountains during the last glacial period (median age 20,000 years). Water recharged from the Jemez Mountains during the last glacial period occurs at the water table in the central part of the basin and beneath younger recharge along the Rio Grande and the northern mountain front.
In some parts of the basin, boundaries between hydrochemical zones appear to be near major faults that may affect ground-water flow. However, in other parts of the basin, such as along the east side of Albuquerque near the Sandia Fault zone, ground-water flow appears to be unaffected by major faults. Upward leakage of saline water occurs along some faults and can be a source of salinity and elevated arsenic concentrations in some ground water.
A trough in the modern and predevelopment water table west of Albuquerque is centered along a zone of predominantly late Pleistocene age water through the center of the basin and is flanked and overlain along the trough boundary by water that infiltrated from the Rio Puerco on the west and the Rio Grande to the east. It is suggested that the groundwater trough is a relatively recent transient feature of the Santa Fe Group aquifer system.
At Albuquerque, a distinct north-south boundary in deuterium content of ground water marks the division between recharge from the eastern mountain front and that from the Rio Grande. Water beneath approximately two-thirds of the City of Albuquerque is predominantly of Rio Grande origin infiltrated from areas north of the city.
The stable isotope data for ground water in the vicinity of Albuquerque indicate little movement of ground water in response to withdrawals from public supply wells during the past 20 years, even though in places the modern water table has fallen as much as 140 feet below the predevelopment potentiometric surface. Small shifts over the past 20 years in stable isotope composition of water discharged from public supply wells along the boundaries between the West- Central zone (paleowater) and Central zone (Rio Grande water) west of the Rio Grande, and along the boundary between the Central zone and Eastern Mountain Front zone east of the Rio Grande indicate local areas where paleowater of Rio Grande origin is beginning to move west and east in response to groundwater pumping.
Age gradients from piezometer nests range from 0.1 to 2 yr cm-1 and indicate a recharge rate of about 3 cm yr-1 for recharge along the eastern mountain front and infiltration from the Rio Grande near Albuquerque. There has been appreciably less recharge along the eastern mountain front in areas both north and south of Albuquerque.
The d2H isotopic composition of water, and recharge temperatures based on dissolved N2 and Ar data, were interpreted in conjunction with the radiocarbon age to improve understanding of water source, and mechanism and timing of recharge. The N2-Ar recharge temperatures vary widely throughout the MRGB. The minimum recharge temperature for a particular hydrochemical zone appears to be near the mean annual temperature, and the maximum recharge temperature approaches that of ground water beneath the deep (greater than 300 feet) unsaturated zones. The dissolved gas recharge temperatures demonstrate cases of both focused (cold recharge temperatures) and diffuse flow (warm recharge temperatures) recharge mechanisms in the predominantly semi-arid MRGB. During the last glacial period, water recharged to the West-Central zone varied widely in stable isotope composition and recharge temperature, indicating the occurrence of both diffuse and focused recharge of low- and high-altitude precipitation. The range in d2H of West-Central zone waters from the last glacial maximum (LGM, approximately 18,000 radiocarbon years before present, B.P.), indicates recharge that occurred over a 4,000-foot range in altitude. Ground water in the Central zone was recharged by direct infiltration from the Rio Grande and apparently records surface-water temperature and stable isotopic composition at the time of recharge. The dissolved N2-Ar data indicate that the average temperature of water infiltrated from the Rio Grande varied by only about ± 1 degree Celsius (°C) from the modern mean annual temperature (13.6°C) at Albuquerque over the past 27,000 years. Rio Grande water was coldest (12.7 ± 1.4°C) during the period 15,000-27,000 years B.P., and warmest (14.5 ± 1.4°C) during the period 5,000-9,000 years B.P., and averaged 13.0 ± 2.2°C during the past 5,000 years. Together, the stable isotope data and dissolved gas recharge temperatures indicate that in the past, the timing of the spring runoff of northern New Mexico and southern Colorado snowmelt varied, coming late into early- to mid-summer during cold periods and overlapping, in part, with the summer monsoon season (currently July- October). During warm periods, such as modern times, the peak discharge of the Rio Grande occurred in mid- to late spring in advance of the summer monsoon season.
Recharge temperatures from approximately 20,000 radiocarbon years ago were as low as 3.2°C, as recorded in the dissolved gas composition of water recharged north of the basin, and 8.1°C along the eastern mountain front. During the last 5,000 years, the d2H isotopic composition of eastern mountain front recharge has decreased about 7 per mil. This decrease indicates an average cooling of about 1.4°C following the mid-Holocene warm period. Over the same time span, the d2H isotopic composition of Rio Grande water increased approximately 6 per mil, consistent with a shift in season of peak snowmelt into the beginning of the summer monsoon season.
The d13C isotopic composition of dissolved inorganic carbon in ground water is remarkably constant throughout most of the basin indicating a nearly constant historical predominance of C4 over C3 plants. However, recent recharge along the basin margins indicates a rather abrupt increase in C3 plant abundance during the past 1,000 years, and perhaps even more recently than 1,000 years, as recorded in depleted d13C isotopic compositions of dissolved inorganic carbon (DIC).
This study demonstrates the benefits of obtaining a diverse and extensive chemical and isotopic dataset when characterizing hydrochemical processes in ground-water systems, retrieving historical environmental records from ground water, and/or refining conceptual models of ground-water system.