Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
precipitation on exposed bedrock aquifers or from seepage from runoff from adjoining highlands.
A relatively small proportion of the recharged water is modeled to originate from shallow aquifers
near the Chuska Mountains and on Mt. Taylor. The steady state groundwater is modeled as
discharging to streams, including normally dry streambeds.
Table 19 compares current water balance conditions simulated by the model to the predevelopment
state. The water balance in the basin is simulated to change only slightly during the
83-year (1930-2012) historical period, even though 1,174,000 acre-feet of water was pumped
from the basin. Pumping resulted in a net decline of 1,035,000 acre-feet in the amount of water
stored in the basin. The remaining difference was made up of a 138,000 acre-foot increase in the
input from rivers. The increased input from rivers includes 83,000 acre-feet that infiltrated along
San Mateo Creek downstream of the mine, which originated as water discharged to the stream
from mine dewatering operations (e.g., from Ambrosia Lake).
The model predicts that the pattern of future changes will be similar to the historic pattern. Table
20 summarizes the water balance calculated by the model in scenarios 1 and 2. Scenario 2
includes RHR dewatering.
The model produces a good balance, in that the total inputs and total outputs of water are within
2,000 acre-feet of one another in all cases (an error of 0.3 percent or less). Under scenario 2, RHR
would pump 80,659 acre-feet over the 113-year period of the simulation, all within the first 13
years. As a result, the amount of groundwater in storage would decline by an equivalent amount
(slightly different due to the small model error). Ninety-seven percent of the change in storage is
predicted to occur in the Westwater Member of the Morrison Formation, the rest from other
formations including the Gallup, Dakota, and Brushy Basin. The model is not capable of
simulating any possible effects on the underlying Recapture.
In the model results, water remaining in storage redistributes after pumping stops. Specifically,
while water levels at the mine recover, there is an expansion in the cone of depression outward
that continues to the end of the simulation, and presumably beyond. Impacts on the flows into or
out of the model domain are small, and predicted values are considered to have a a low level of
reliability. Qualitiatively, the prediction that there would be an increase of river impacts over time
is considered probable, as is the expectation that changes in external conditions (such as from
climate change) would not effect the predictions. Overall the model shows that the water removed
by the Roca Honda Mine is a mined resource which for practical purposes represents a permanent
loss of stored groundwater, primarily from the Westwater.
Table 19. San Juan Basin water balance: pre-development vs. current condition
166 DEIS for Roca Honda Mine, Cibola National Forest