Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
Drawdown from pumping in the Gallup and Dakota can be large, but is predicted to be localized
to in and near the mine permit aera. Drawdown in the Westwater is modeled as much more
extensive. Figure 43 is a map of the modeled drawdown in the Westwater caused by the Roca
Honda Mine (i.e., the difference between scenarios 2 and 1) at the end of mining (assumed to be
13 years from startup). The map was prepared by the USFS using data from the Intera model.
Contour lines are given for 1,000 feet of drawdown, 100 feet, 10 feet, and 1 foot. The Westwater
cone of depression would continue to expand after mining and dewatering stop and water levels
at the mine begin to recover. This occurs in the Dakota as well, but as its pumping is limited in
time, the effect is much less. The result is that maximum Westwater drawdowns outside the
permit area may occur years after mining.
Figure 44 is a map showing modeled drawdowns 100 years after mining ceases (assumed to be
the year 2125). At this time the Westwater cone is both broad and shallow. As of 2025, the 10-foot
drawdown contour extends 16.6 miles out from the mine and the 1-foot contour extends as far as
17 miles. At this time, the maximum drawdown is still within the permit area and is about 30 feet.
Changes beyond 2125 are not simulated.
Due to the length of time it would take water levels in the Westwater Canyon aquifer to recover to
pre-pumping conditions, the impact of the Roca Honda Mine on this groundwater resource would
be considered significantly adverse.
It is possible to plot drawdown over time for any layer and any grid location within the model
domain. Figures 45–47 provide three such hydrographs; the impact of RHR is seen by comparing
scenario 2 (with the mine) to scenario 1 (existing pumping only). The hydrographs were prepared
by the USFS using data from the Intera model. Year 0 in each graph represents the year
construction begins. Scenario 4 is also shown in these figures and it and scenario 3 are discussed
below under cumulative impacts.
• As indicated in figure 45, Roca Honda Mine dewatering would cause no significant
impact to the San Mateo community well, which draws from the Point Lookout aquifer,
far higher in the stratigraphic column than the Westwater.
• Well 143 is a ranch well about 2 miles west of the mine that may be completed in the
Westwater. (Based on the reported depth of the well, it may be completed in a shallower
unit.) As shown in figure 46, under existing conditions, water levels are simulated as
rising as they recover from past mining. The RHR impact is immediate and large,
reaching about 200 feet of maximum drawdown. Water levels are predicted to recover
after mining ceases.
• As indicated in figure 47, there would be no impact from RHR at Crownpoint, where
community wells also draw from the Westwater but outside the cone of depression of the
mine.
No wells drawing from the Dakota or Gallup were identified that would have significant impacts
from the RHR mine. A possible exception is Well 143, which could be completed in one of those
units; if so, impacts would be much smaller than shown in figure 46.
168 DEIS for Roca Honda Mine, Cibola National Forest