USE OF CARBON-14 AND WATER LEVELS TO CALIBRATE A GROUND-WATER MODEL OF THE MIDDLE RIO GRANDE BASIN

The long-term availability of ground water in the Middle Rio Grande Basin has been addressed recently by the development of ground-water flow models by the U. S. Geological Survey. Carbon-14 data and water levels are currently being used to further improve the latest USGS ground-water model. The current study uses USGS flow, particle tracking, and parameter estimation codes to estimate the hydraulic conductivities of hydrogeologic units and the current and past recharge to the basin. A three-dimensional representation of the geology was discretized into a three-dimensional finite-difference grid of the basin. Hydraulic conductivity zones were assigned on the basis of the geologic model. Observations that were used to calibrate the model included 200 water levels and 200 carbon-14 ages. Basin-margin and tributary recharge values estimated by the model are considerably lower than values used in previous models. These new recharge estimates are only about one-sixth of earlier model estimates, but unlike the earlier models, the current model predicts that recharge also occurs naturally from the Rio Grande. Recent estimates of recharge for the eastern mountain fronts, based on the chloride mass-balance method, were closer to the estimates of this model than to those of previous models. The lower recharge estimates from the current model are also supported by simulated water levels and source-area delineations. In addition to a steady-state model, a transient paleohydrologic model was calibrated to determine whether the carbon-14 data set as a whole contained information about the variability of recharge during the past 30,000 years. The paleohydrologic simulation was set up by simulating a period of 30,000 years, with an independent value of recharge being estimated every 5,000 years. These paleorecharge values were estimated simultaneously with other parameters from the original steady-state model. Although some uncertainty exists in the results, the carbon-14-calibrated model results suggests that recharge to the basin 20,000 years ago was significantly higher than that at present, perhaps by five to six times.