IS THERE HORIZONTAL ANISOTROPY IN THE UPPER SANTA FE GROUP?

In a few locations in the Albuquerque Basin there are deep troughs in the Upper Santa Fe Group (USF) groundwater potentiometric surface. One such location is under Kirtland Air Force Base/Sandia National Laboratories (KAFB/SNL). Three possible mechanisms have been postulated for the trough as follows: 1) presence of ancestral Rio Grande (ARG) deposits focusing drawdown from the wellfields in the Northeast Heights, 2) sealing faults to the west and mountains to the east, and 3) horizontal anisotropy. This discussion centers on horizontal anisotropy.

The channel deposits that comprise the more productive portions of the aquifer trend and are more continuous along the direction of ancient-river flow. The geologic conceptual model developed for the area states that the changes in depositional environment will tend to produce long "shoestrings" of higher hydraulic conductivity aquifer material running north-south, with lower hydraulic conductivity material tending to isolate the shoestrings in the east-west direction. Thus, the materials are likely to be horizontally isotropic within a major facies (e.g. channels), but the overall arrangement of the deposits has a definite orientation. Geostatistical analysis of the USF hydraulic conductivity data of Thorn et al. was performed, and found that it was statistically anisotropic, with a major direction of continuity running north-south due to the presence of the channel deposits. Using stochastic theories that relate physical and statistical anisotropy a rough estimate of anisotropy was derived and tested. It was found that introducing horizontal anisotropy improved model agreement.