SOURCES OF CHLORIDE ION IN YUCCA MOUNTAIN TUFFS, NEVADA

The presence of chloride ion (Cl) in pore fluids and saturated zone groundwaters in the tuffs at Yucca Mountain, Nevada, has been important for determining rainfall infiltration rates, understanding the origins of pore-fluid salinity, and identifying 'fast track' flow paths in the bedrock where bomb-derived Cl-36 was analyzed. Recent attempts to replicate the early observation of Cl-36 anomalies in the Exploratory Studies Facility (ESF) tunnel have been frustrated by the observation that variable amounts of Cl can be leached from the tuff depending on sample preparation procedures, size of fragments leached, duration of the leach, and possibility of contamination by Cl introduced by tunnel-construction activities.

Chloride in igneous rocks has four main sources: 1) salts or fluids present in fractures, 2) salts or fluids present in intergranular pores, 3) isolated fluid inclusions within mineral grains, and 4) chemically bound Cl in hydrous minerals such as biotite and hornblende. Distinguishing between Cl from fracture walls and in salts or fluids in the rock matrix is difficult because of the high porosity and permeability of the tuff. Standard crush-leach practice tends to extract both fracture-wall and grain-boundary salts and fluids and may dissolve fluid inclusions from broken mineral grains. Long-term leaching may also extract the chemically bound Cl.

Published data show that Yucca Mountain tuffs contain about 170 mg/kg total rock Cl and pore fluids contain about 34 mg/L Cl. Therefore, for the welded Topopah Spring Tuff in the ESF (porosity=8.1 vol. %), pore water Cl contributes only about 1 mg Cl to total rock Cl (i.e. <1%). To determine the relative importance of leachable Cl components in the tuff and more clearly understand Cl-leaching characteristics, Cl has been leached from dry-drilled cores in the ESF for various leach times and fragment sizes. Attempts also have been made to determine the distribution of chemically bound Cl by electron microprobe.