USING GEOCHEMICAL DATA TO TRACE AND DATE GROUND WATER RECHARGE IN PARTS OF THE SANTA FE GROUP AQUIFER SYSTEM, MIDDLE RIO GRANDE BASIN, NEW MEXICO

Chemical and isotopic data for water from 275 wells sampled in parts of the Santa Fe Group aquifer system, Middle Rio Grande Basin, New Mexico, define 12 sources of recharge and a zone of ground-water discharge for the basin. The data include major- and minor-element chemistry (30 elements), oxygen-18 and deuterium content of water, carbon-13 and carbon-14 content of dissolved inorganic carbon, sulfur-34 content of dissolved sulfate, tritium, and dissolved atmospheric gases including chlorofluorocarbons and sulfur hexafluoride. Contours drawn on concentrations of most elements and isotopes generally parallel the predominant north to south direction of ground-water flow. The radiocarbon ages of DIC in ground water range from modern to more than 30 ka. Major sources of water to the basin include (1) recharge from mountains along the north, east and southwest (median age 5-9 ka); (2) seepage from the Rio Grande and Rio Puerco (median age 4-8 ka), and from the Abo and Tijeras Arroyos (median age 3-9 ka); (3) inflow of saline water along the southwest basin margin (median age 20 ka); and (4) inflow along the northern basin margin that probably represents recharge from the Jemez mountains during the last glacial period (median age 20 ka). This latter source 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. Chlorofluorocarbon and tritium-helium-3 data indicate that recharge has occurred within the past 30 years in parts of the inner valley of the Rio Grande and along the Sandia Mountains. A distinct north-south boundary in deuterium content beneath Albuquerque, NM marks the division between recharge from the eastern mountain front and from the Rio Grande. Radiocarbon ages indicate that recharge rates during the last glacial period were approximately 6 times greater than during the Holocene.