Beauheim 1987 - Waste Isolation Pilot Plant - U.S. Department of ...

freshwater heads in the unnamed lower member of
the Rustler at the contact with the Salado Formation
are generally higher than freshwater heads in the
Culebra, except along the western side of the WlPP
site and in Nash Draw. Freshwater heads in the
Magenta are also higher than freshwater heads in the
Culebra, although the differences between the two
decrease to the west towards Nash Draw. These
observations indicate that over most of the WlPP site
potentials exist for flow upward from the unnamed
lower member to the Culebra, and for flow downward
from the Magenta to the Culebra. These
observations neither support nor contradict the
supposition that precipitation on the surface at the
WlPP site could be recharging the Rustler, and more
particularly, the Culebra.
More recent observations at H-3, H-14, H-16, DOE-2,
and in the WlPP underground facility provide more
detailed insight into potential directions of vertical
fluid movement within the Rustler. Measurements
made by the 5-packer tool in H-16 show that the
static formation pressure of the unnamed lower
member of the Rustler is about 229 psig at a depth of
808 ft (Section 5.2.1), and the static formation
pressure of the Culebra is about 133 psig at a depth
of 712 ft (Section 5.2.2.7). These values confirm
Mercer’s (1983) observation that the potential exists
for flow vertically upwards from the unnamed lower
member to the Culebra, regardless of any
uncertainty in the relative specific gravities of the
Culebra and unnamed member waters.
The highest specific gravity possible for the
unnamed member water is about 1.2, the specific
gravity of a brine saturated with respect to sodium
chloride. With this specific gravity, the 964 elevation
difference between the midpoints of the Culebra and
the unnamed member siltstone could account for
only about 50 psi of the observed 96-psi pressure
difference between these two units at H-16. A lower
value of specific gravity would lead to a larger
residual pressure difference. Thus, the hydraulic
gradient between the unnamed member and the
Culebra at H-16 is definitely upwards.
The upward hydraulic gradient from the unnamed
lower member to the Culebra may have a source
below the Rustler in the Salado Formation. Peterson
et al. (1987) report formation pressures of 1200 to
1500 psig for the Salado near the WlPP facility 21 50 ft
deep. The 1342-ft elevation difference between the
facility and the midpoint of the unnamed lower
member could account for a pressure difference
between the two locations of about 700 psi,
assuming a brine specific gravity of 1.2. Thus, the
residual Salado fluid pressure at the elevation of the
midpoint of the unnamed member would be 500 to
800 psig, considerably higher than the 229 psig
measured in that member. Based on these data, the
vertical hydraulic gradient between the Salado and
the unnamed lower member of the Rustler should be
upward. This discussion assumes, however, that the
distribution of hydraulic properties throughout the
Salado allows the pressures measured at the facility
horizon to be transmitted upward (with some loss
within the Salado) to the base of the Rustler, an
assumption that has yet to be verified by hydraulichead
measurements at different depths within the
Salado. Nevertheless, a potential for a vertical
hydraulic gradient upward from the Salado to the
unnamed lower member of the Rustler clearly exists.
Attempts at measuring the static formation pressure
of the Tamarisk Member between the Magenta and
Culebra failed at DOE-2, H-14, and H-16 because of
low permeabilities and associated long pressurestabilization
times. Tamarisk pressures are
expected, however, to be intermediate between
those of the Magenta and Culebra.
Recent measurements of static formation pressures
for the Magenta and Culebra at H-14, H-16, and DOE-
2 show similar vertical hydraulic gradients. At H-14,
the pressure at the midpoint of the Magenta is 6 to
16 psi higher than the pressure at the midpoint of the
Culebra (Sections 5.2.4.1 and 5.2.2.5), while at H-16,
the Magenta pressure is 1 psi higher than the
Culebra pressure (Sections 5.2.4.2 and 5.2.2.7), and
at DOE-2, the Magenta pressure is 3 psi lower than
the Culebra pressure (Beauheim, 1986).
Considering the elevation differences of 109 to 124 ft
between the Magenta and Culebra at those locations,
vertical hydraulic gradients must be downward from
the Magenta towards the Culebra, regardless of the
specific-gravity values used for Magenta and Culebra
waters. Thus, these recent measurements of vertical
hydraulic gradients agree with Mercer’s (1983)
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