Using geochemical data and aquifer simulation to characterize recharge and groundwater flow in the Middle Rio Grande Basin, New Mexico.

Abstract

Chemical and isotopic data for groundwater from the Middle Rio Grande Basin, New Mexico, were used to (l) delineate hydrochemical zones and their source area to the basin, (2) evaluate paleo-recharge conditions, including recharge from the Rio Grande and mountain fronts, and (3) calibrate a basin-scale groundwater model. Twelve sources of recharge and one zone of groundwater discharge were identified and traced throughout the basin. Geochemical mass-balance calculations, accounting for mixing of surface-water and groundwater sources, evapotranspiration/dilution processes, and water-rock reaction, led to well-defined radiocarbon ages that range from modern to more than 30 ka B.P. in the Santa Fe Group aquifer system. Recharge rates determined from the groundwater model were appreciably lower than previous estimates based on a rainfall-runoff model, but in agreement with recharge estimates determined from a previous investigation using the chloride mass-balance method. The groundwater model reproduces key features of the groundwater-flow system, including a trough in the water-table surface in the central part ofthe basin, and accounts for the observation ofa substantial quantity of paleo Rio Grande water in the aquifer. Results from a 30,000year transient simulation suggest that recharge at the Last Glacial Maximum (LGM) may have been ten times the modern rate, but that recharge following the LGM was about 60 percent of the modern rate. Over the past 30 ka, 62H of Rio Grande water was maximum in the present and at about 15 ka, and minimum at approximately 5 ka and 22 ka, before present. Seasonal shifts in the timing of peak discharge of the Rio Grande andlor changes in the amounts of low-altitude