Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
DBSA (2001) reported on water quality from the Mt. Taylor Mine. The TDS was on the order of
800 mg/l, with elevated sulfate. Arsenic was found above the current SDWA standard, and some
high selenium concentrates were reported, although the average was below the SDWA standard.
Dissolved uranium was found about 0.9 mg/l, above the SDWA standard of 0.03 mg/l.
Dakota Sandstone
Within the permit area, the Dakota Sandstone has an average thickness of about 50 feet. The top
of the Dakota is an elevation of about 5,600 to 5,400 feet. The Dakota Sandstone near the permit
area was not considered an aquifer in Sheet 1 of Stone et al. (1983), but rather as “Locally an
aquifer or contains aquifer.” RHR reports “significant” quantities of groundwater in the Dakota
during drilling of its deep monitor wells in Section 16, and expects to have to pump Dakota
groundwater at up to 144 gpm for up to 12 months during advancement of the production shaft
(RHR, January 2011 Revision 1; Intera, 2011b).
Five wells completed in the Dakota were identified in the RHR well inventory; none were
sampled and none are included in the ongoing monitoring program. The closest to the permit area
is No. 139, KMC LMD-1, in Section 17 of T13N, R8W (no OSE file number provided). Reported
water level elevation in this well was 6,308 feet in late 1981; it was 6,400.6 feet in October of
2010. Two other wells identified were associated with the defunct Johnny M Mine. The BDR
speculates that a few wells may produce from the Dakota for stock watering north of the RHR
permit area; No. 129, the Polvadera well, is the only Dakota well identified in the inventory north
of the permit area.
Absent site-specific data, water quality in the Dakota is assumed to be typical of what is found
elsewhere, which would make it poorer than that in the underlying Westwater Canyon Member of
the Morrison (Stone et al., 1983). Brod and Stone (1981) and Kelley et al. (1980) report that
water in the Dakota is typically of the sodium-sulfate type, with TDS in the range of 600 to 1,400
mg/L.
Stone et al. (1983) note that pre-development heads in the Dakota may have been more than 200
feet higher than in the underlying Morrison (Westwater Canyon) aquifer, and suggest that the
persistence of such differences is an indication that relatively low vertical permeability exists in
the confining unit between the two.
Overlying the Dakota Sandstone is the Mancos Shale, which in other areas of the basin contains
productive sandstones in the middle and lower parts, including at the Mt. Taylor Mine. However,
no producible quantities of groundwater in the Mancos were found in the three monitor wells
drilled by RHR in Section 16 that penetrated the Mancos Shale section on the way to Westwater
Canyon. This indicates that the Mancos acts as a confining bed, as is common in many areas.
Gallup Sandstone
In the permit area, the Gallup Sandstone is composed of two sandstone units with a total
thickness approximating 85 feet. They are separated by the Pescado Tongue of the Mancos Shale,
approximately 20 feet thick. The top of the Gallup is at an elevation of about 6,700 to 6,900 feet
in Section 16 of the permit area and, as confirmed by the “first water” drilling, this is expected to
be the first significant aquifer beneath Section 16 (RHR, 2011b: Groundwater Monitoring Plan,
January).
136 DEIS for Roca Honda Mine, Cibola National Forest