Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
Specific yield in the model is 0.1 for all layers except 4, 6 (NE), 7, and 9 for which the
value is 0.05. Specific storage values are in the range 10 -5 to 10 -6 .
• The bottom layer in the model, layer 10, represents the Westwater Canyon Member of the
Morrison Formation. The model bottom is set at the contact between the Westwater and
its underlying regional aquitard, the Recapture Member of the Morrison Formation. The
model does not represent impacts to the Recapture, although mining and dewatering will
penetrate that unit to a limited degree. The model also does not simulate effects in deeper
units, including the San Andres-Glorieta unit which is an important regional aquifer and
source of water for Horace Springs.
• Aquifer properties used in the model were determined from published studies specific to
the San Juan Basin from generalized literature relationships, from site-specific aquifer
tests performed by RHR, and from model calibration.
• The model was constructed to represent a steady-state period prior to 1930, transient
conditions from 1930 through 2012, and predictions of future conditions from 2013
through 2125 (13 years of mine construction and operation and the subsequent 100
years). The transient period included pumping for dewatering from uranium mines in the
Ambrosia Lake area through 1986 at rates in the range of 7,000–12,000 gpm, 2,000–
4,000 gpm in the Church Rock area, up to 4,500 gpm from the Mt. Taylor Mine, and up
to 800 gpm from the Johnny M. Mine.
• Recharge simulated in the model occurs primarily at higher elevations and especially at
mountain fronts. Recharge related to discharges from past mine operations is
incompletely simulated (e.g., while prior discharge impacts are simulated downstream of
the RHR mine, there is no simulation of historic effects of discharge of water from the
Mt. Taylor mine to San Mateo Creek in the area of San Mateo). This could cause historic
drawdowns to be overpredicted in the shallowest units. See below for a discussion of
pumping assumed to occur in the future.
• Consistent with how Intera interprets the geology, the volcanic core under Mt. Taylor,
Horace Mesa, and Mesa Chivato is simulated as a low permeability zone east of the mine.
The USFS technical review of the model considered that this could overstate the effect of
Mt. Taylor as a barrier to propogation of pumping effects to the southeast (e.g. toward
Horace Springs). As a result of the review, Intera conducted sensitivity simulations in
which the barrier effect of the volcanic core was reduced. These and other sensitivity
evaluations indicated that the model results primarily respond to the amount of pumping
simulated, with details such as reasonable variations in permeability having a lesser effect
on simulated drawdown. Intera’s sensitivity analysis did not assess the effects on
simulated springflow changes or streamflow depletions. The permeability values for
aquifers under Mt. Taylor, Horace Mesa, and Mesa Chivato are orders of magnitude
below values in surrounding areas and are not supported by any direct evidence. Whether
the values are sufficiently conservative may require further consideation.
• Model construction should allow prediction of streamflow impacts to the San Juan River,
Rio San José. and Rio Puerco. However, model outputs have unrealistic variability and
do not provide a reliable quantification of impact.
DEIS for Roca Honda Mine, Cibola National Forest 161