39
Radon-222 sample results for MW-04, MW-07, PIEZ-1, PIEZ-2, were below the 4,000 pCi/L
concentration. Results for wells MW-01, MW-06, MW-08, MW-09, MW-10, MW-11, MW-14,
USAF-01, and USAF-02 exceeded the 4,000 pCi/L concentration. The highest radon 222 value
was in MW-14 (32,900 pCi/L). The Vista West sample results exceeded the proposed
community MCL of 300 pCi/L.
Depth to water levels was measured during the sampling events. The information is presented
in Appendix H, Table 9.
5.3.2 Erosion and Surface Water Runoff Control
Methods to prevent erosion and control surface water runoff are included in the Hydrology and
Hydraulics Technical Memorandum (see Appendix D). This is further described above in
Section 4.5.4.4.
5.3.3 Waste Rock Characterization
Waste rock and pit wall surfaces have the potential to impact water resources due to oxidation
and dissolution of a variety of mineral phases under ordinary weathering conditions. RER has
conducted and is continuing geochemical characterization of the waste rock to support
engineering design, permitting, and impact assessment. Characterization of waste rock from the
Bull Hill Resource Area, Whitetail Resource Area, PUG coarse reject, and the Low Grade Ore
Stockpile is in progress. It is anticipated that the material in the Waste Rock Facility will be non-
acid forming due to the low overall deposit acid generation potential (AGP), the existence of
pockets of high acid neutralization potential (ANP) (found primarily in the OxCa and transitional
zones), and the fact that none of the material is currently acid generating based on laboratory
paste pH values.
The mineral deposit is hosted by middle Tertiary (Eocene; 38-50 Ma) alkaline intrusions
consisting of phonolites, trachytes, and associated heterolithic breccias. The intrusions
themselves are crosscut with penecontemporaneous REE-rich carbonatite dikes. The latter, in
particular, impart significant ANP to the deposit due to the abundant carbonate. However, ANP
may be limited for Bull Hill and Whitetail waste material that is located in upper oxidized and
relatively carbonate-free portions of the deposit. The trachyte and phonolite hosts will likely be
able to provide additional long-term neutralization potential via kinetically slow dissolution of
alkaline silicates such as olivine, pyroxene and feldspar group minerals. Studies of other
thorium-REE vein deposits (Armbrustmacher, 1995) suggest that acid rock drainage (ARD) is
generally unlikely, that metal mobility appears limited and that the overall geochemical footprint
of such a deposit is not widely dispersed. ARD is unlikely as most deposits are low in sulfides,
and thus have limited acid generating potential, while the association with carbonatite magmas
and alkaline magmas implies the presence of significant carbonate mineral phases that provide
abundant neutralization potential. Ore and waste dump material is frequently radioactive, but the
thorium bearing minerals are relatively insoluble (even in low pH conditions), which will limit
mobility. Thus, thorium concentrations in natural waters associated with these types of deposits
rarely exceed 1 part per billion (ppb). Soil sediment signatures also confirm that thorium-bearing
phases do not widely disperse and their geochemical haloes are insignificant.
A typical geochemical characterization program for waste rock is phased, with an initial static
phase consisting of acid base accounting (ABA) and short-term leach tests and a follow-up
kinetic phase consisting of humidity cell testing. Existing characterization work is limited to static
testing of Bull Hill waste material, but a humidity cell test program and a full geochemical
characterization program for Whitetail waste rock are also underway. Because humidity cell
tests are designed to optimize sulfide oxidation, acidity generation, metals leaching, and
neutralization potential depletion, and because they are not necessarily appropriate for
