SECONDARY MINERALS AS INDICATORS OF THE DISTRIBUTION OF WATER FLOW IN THE UNSATURATED ZONE, YUCCA MOUNTAIN, NEVADA

One of the key attributes of the proposed high-level radioactive waste repository in the ~500-m-thick unsaturated zone is the limited amount of water that would contact the waste containers. Confirmed natural water seepage has not been observed in the extensive tunnels, drifts, and alcoves excavated within the proposed repository horizon, limiting the predictability of future water seepage into the waste emplacement drifts. However, deposits of calcite and opal present in some open fractures and cavities are at least a partial record of the distribution of past water seepage.

Calcite and opal abundances were determined by visual estimates in a 60-cm wide band along 1350 m of the wall of the East-West Cross Drift; calcite abundances also were measured in cuttings from 500 m of a nearby vertical borehole (USW SD-6) by acid extraction of carbonate. Comparison of vertical borehole and horizontal tunnel distributions of calcite abundances indicates that most of the variations likely are caused by changes in seepage volume with depth. The average interval between secondary mineral deposits surveyed in the East-West Cross Drift increases from 0.8 m to 4 m as the drift penetrates deeper rock units of the proposed repository horizon. This increase in spacing may be either because percolation becomes focused into a smaller number of flow paths or because some water imbibes into the rock matrix or evaporates with depth. The smallest abundances of secondary minerals in the Cross Drift are beneath the crest of Yucca Mountain, an area for which an infiltration model shows the largest infiltration flux. This contrast between infiltration and mineral abundance may indicate that some of the infiltration is redistributed, perhaps by the nonwelded (and unfractured) tuff that overlies the proposed repository horizon. Dating of secondary mineral coatings shows that they formed at slow, relatively uniform, long-term average growth rates during the last 10 m.y. indicating that percolation pathways remained active and that water and gas chemistries remained conducive to calcite and opal precipitation in natural cavities throughout this time. The distribution of calcite and opal may be useful in estimating the spatial distribution of water seepage into the proposed repository.