New Mexico Geological Society Annual Spring Meeting — Abstracts

A quasi three-dimensional model of groundwbter flow was developed for the Mesilla Bolson and the Mesilla Valley stream-aquifer system in south-central New Mexico. The model accounted for two-dimensional horizontal flow in the Santa Fe Group and flood-plain alluvium aquifers, while simulating vertical leakage across aquitard materials that separate the two aquifers. Groundwater-surface water interaction in the Rio Grande Valley via stream seepage, canal losses, drain discharge and evaporation was also simulted.

Steady-state and transient-calibration efforts with the quasi three-dimensional model have helped to develop a better understanding of geologic controls on groundwater movement as well as the major hydraulic processes occurring in the bolson under recent conditions. The groundwater system appears to have been in a state of dynamic equilibrium during the past 25-30 years. Over the long-term, groundwater withdrawals are largely offset by large quantities of recharge contributed by surface water irrigation and inftltration from the Rio Grande River and canals located in the Mesilla Valley. The quantity of mountain front recharge determined from model simulations, although larger than earlier estimates, comprises only a small portion of all sources of subsurface water. Evapotranspiration and pumping account for most discharge from the basin.

Large fluctuations in annual stream and drain flows are observed in the Mesilla Valley. Adequate reproduction of these fluctuations with the model was best accomplished by incorporating a power relationship between stage and flow. Simulation results indicated that groundwater discharge is also a major mechanism of groundwater outflow.