SIMULATION OF A LONG-TERM AQUIFER TEST CONDUCTED NEAR THE RIO GRANDE, ALBUQUERQUE, NEW MEXICO

A long-term aquifer test was conducted near the Rio Grande in Albuquerque during January and February 1995 using 22 wells and piezometers at nine sites, with the City of Albuquerque Griegos 1 production well as the pumped well. Griegos 1 discharge averaged about 2,330 gallons per minute for 54.4 days. A three-dimensional finite-difference ground-water-flow model was used to estimate aquifer properties in the vicinity of the Griegos well field and the amount of infiltration induced from the Rio Grande and riverside drains as a result of pumping during the test. The model was initially calibrated by trial and error, then was re-calibrated using a nonlinear least-squares regression technique.

Not all aquifer properties could be estimated using nonlinear regression because of model insensitivity to some aquifer properties at observation locations. These non-estimated properties were fixed at values consistent with the types of sediment present. Of the properties estimated, hydraulic conductivity of the upper part of the Santa Fe Group was estimated to be 12 feet per day, the vertical to horizontal anisotropy ratio was estimated to be 1:82, and specific storage was estimated to be 1.2 x 10^-6 per foot of aquifer thickness. The overall sum of squared errors between the observed and simulated drawdowns was 87 feet squared -- a significant improvement compared to the 130 feet squared obtained by trial and error calibration alone.

At the end of aquifer-test pumping, induced infiltration from the Rio Grande and riverside drains was simulated to be 13 percent of the total amount of water pumped. The remainder was water removed from aquifer storage. After pumping stopped, induced infiltration continued to replenish aquifer storage. Simulations estimated that 5 years after pumping began, 58 to 72 percent of the total water pumped was replenished by induced infiltration from the Rio Grande surface-water system.