Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
groundwater resources in the Cibola National Forest beyond those caused by actions other than
the RHR mine. There would be certain cumulative impacts to groundwater arising from actions
other than the Roca Honda Mine, as discussed subsequently.
Alternative 2
The primary impact to groundwater identified during scoping is drawdown from pumping of
groundwater to allow shaft construction and mining of ore and subsequent recovery of
groundwater levels after mining ceases. Specific impacts of concern include effects of pumping
on offsite wells and springs and changes to water chemistry. Other sources of impacts to
groundwater are related to backfill of mine workings and discharges from surface facilities and
operations.
Impacts of Groundwater Pumping and Recovery
As described in chapter 2, dewatering would be by wells and by underground collection systems,
to an estimated maximum rate of 4,500 gpm (about 10 cfs) from all sources. In assessing impacts
from dewatering, the Forest Service has coordinated with the New Mexico Office of the State
Engineer (NMOSE). RHR has submitted an application to NMOSE for a temporary permit that
seeks authorization for pumping that is equivalent to that considered in this EIS. The State
permitting process involves considerable technical review by the NMOSE; any State approval
would be expected to require mitigation of effects judged significant by NMOSE. The application
has been protested by Acoma Pueblo. As currently scheduled, the State process would not be
expected to produce a final decision until 2014.
Overview of RHR Groundwater Model
To predict dewatering impacts for both the Forest Service and OSE, and for its own planning,
RHR developed a groundwater model (Intera, 2011). Forest Service and NMOSE personnel,
along with the EIS consultants, reviewed this model and provided comments to RHR. These
comments led to substantive revisions of the model, as for example the addition of an explicit
representation of Horace Springs. The most recent version of the model is Intera (2012). The
following attributes indicate the nature of the model and its ability to predict impacts:
• The model uses a framework for the San Juan Basin developed by the USGS (Kernodle,
1996) with many modifications and incorporation of more recent data. It uses the
MODFLOW-SURFACT, a variation of the established MODFLOW computer code.
• Figure 42 is a map of the model area showing the model domain (area covered) and grid.
The model domain extends from the Rio San José on the south to beyond the San Juan
River and into Colorado in the north, a total of 37,000 square miles. The grid has
variable spacing with a maximum of 30,000 feet to a side and in the mine area, cells
which are 330 feet on a side.
• The model is three-dimensional, with 10 layers that represent geologic units from the
Westwater Canyon Member of the Morrison Formation at the bottom, upward to the San
Jose Formation. Table 16 identifies the layers and lists hydraulic conductivies (“Ks”)
used in the model. The horizontal to vertical hydraulic conductivity ratios vary from 1 to
1,000. In the area of the mine, the hydrologic unit containing the Menefee Formation is
the shallowest unit represented, and most of the thickness is in layers 5 and 7. Layer
elevations are specified in the model based on the best available stratigraphic data.
160 DEIS for Roca Honda Mine, Cibola National Forest