Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
Water Levels in the Westwater
There are two water levels of importance in this and other artesian aquifers. The first is the level
at which water in the aquifer will be encountered; this is the top of the actual aquifer material,
which for the Westwater Canyon in the permit area varies from about 1,600 feet to more than
2,700 feet beneath the land surface. The second is the level to which water will rise in wells that
reach the aquifer; within the permit area, the Westwater potentiometric surface (i.e., the level to
which water will rise in a well under artesian pressure from below) is on the order of 800 to 900
feet above the top of the aquifer. Prior to uranium mining in the region, the water level elevation
in the permit area was probably above 6,500 feet MSL, and may have been much higher (e.g., see
figure 9-5 in the BDR).
Several aspects of water levels and flow directions are of particular interest.
The artesian pressure in the Westwater aquifer might indicate a potential for water to move
upward under natural conditions. However, potentiometric levels in the overlying Dakota aquifer
appears to be slightly higher than the Westwater Canyon, and that in the Gallup Sandstone is at
least a few hundred feet higher. In contrast, levels in the San Andres-Glorieta aquifer are less,
indicating flow in undeveloped conditions would be downward into the Westwater from above
and toward the San Andres below. However, the low permeability of the rocks above and below
the Westwater would be expected to keep actual interaquifer flow to a minimum.
A possible exception to the conclusion is suggested by GMRC (1979a), which noted that the
Westwater Canyon and the Dakota Sandstone locally were hydraulically connected in the Mt.
Taylor mine area. The possibility of such an interconnection in the Roca Honda permit area was
assessed in the Baseline Data Report, because the faulting mapped within the permit area and
vicinity conceivably could provide a pathway for aquifer fluids through the intervening Brushy
Basin Member (Morrison) aquitard. Borehole geophysical data collected in the permit area
indicate that the Brushy Basin Member is as much as 279 feet thick. A detailed reanalysis of the
drill logs of holes drilled through the Brushy Basin Member into the Westwater Canyon in 2007
and 2010 showed that the Brushy Basin Member is composed almost entirely of mudstones. The
aquifer test conducted in 2010 using S-4 as the pumping well, “demonstrated that faulting within
the permit area does not appear to provide a conduit for vertical groundwater movement” (RHR,
2011a). Note that this is not the same as concluding that faulting would not have any impact on
such movement.
The limited available data on potentiometric levels in Westwater Canyon in and near the permit
area indicate a slope of the potentiometric surface and, thus, a direction of groundwater flow to
the northeast. This is consistent with Morrison water level elevation maps in Hagan and others
(2001) and the Baseline Data Report. At a greater distance from the outcrop, flow is toward the
east and southeast and the Rio Grande (Lyford et al., 1980, figure 2). More data to evaluate this
gradient may become available upon construction of RHR’s monitoring well network.
Dewatering of the Westwater Canyon Member for the purpose of underground mining lowered
the potentiometric surface in the local area of uranium mines during the 1970s, but water levels
have substantially recovered since mining ceased. Simulations from RHR’s model and results
from the model of Carpenter and Shomaker (1998) indicate that in the permit area, the Westwater
Canyon water levels were lowered by more than 400 feet, and that at least 100 feet of residual
134 DEIS for Roca Honda Mine, Cibola National Forest