Use of Chemical and Isotopic Data to Improve Conceptualization of Groundwater Flow: Lessons from the Middle Rio Grande Basin, NM

The 7,800 km² Middle Rio Grande Basin, located along the Rio Grande Rift in west-central New Mexico, contains up to 4.3 km of, generally, poorly-consolidated basin-fill siliciclastic sediments. Chemical (some 30 major, minor and trace elements), isotopic (²H, ¹⁸O, ³H, ¹⁴C, ¹³C, ³⁴S), and dissolved gas (N₂, Ar, He, CO₂, CFCs, SF₆) data for ground water from 280 wells with depths of 7 to 615 m were used to identify and map ground-water flow of 12 sources of recharge, evaluate ground-water ages, refine the conceptual model of the Santa Fe Group aquifer system, and provide input to aid in calibration of a regional ground-water flow model. A zone of paleowater (>20 ka) was mapped extending north-south through the center of the Basin, and beneath more recent (<10 ka) recharge near Albuquerque and along the Basin's northern margin. Recent recharge occurs along the Rio Grande, Rio Puerco, adjacent mountain fronts, and various arroyos. Recharge temperatures were as much as 10^oC lower in paleowater in northern parts of the Basin than in overlying Holocene ground water; samples in other areas have warm recharge temperatures reflecting infiltration through deep unsaturated zones. The ground-water model, which includes ¹⁴C activities and hydrochemical zones as parameter constraints, reproduces key features of the ground-water flow system, including a trough in the water-table surface in the central part of the Basin and observation of a substantial quantity of paleo Rio Grande water in the aquifer. Model simulations indicate that Basin-scale recharge rates during the Last Glacial Maximum were approximately 10-fold the modern recharge rate. The radiocarbon data indicate modern recharge rates of about 3 cm yr^-1 along the eastern mountain front and beneath the Rio Grande near Albuquerque. Trends in stable isotope composition of paleo mountain-front and Rio Grande water may reflect climate change in the Basin.