EFFECTS OF SAMPLE PROCESSING ON CHLORINE-36/CHLORINE RATIOS IN UNSATURATED CRYSTALLINE ROCKS AT YUCCA MOUNTAIN: IMPLICATIONS FOR UNSATURATED ZONE FLOW

Elevated (bomb-pulse) 36Cl/Cl ratios in leachates from unsaturated welded tuff collected near fault zones in underground tunnels at Yucca Mountain, Nevada were interpreted to indicate that water infiltrated from the surface to the tunnel depth (up to 300m) in <50 yr. Given the important implications of these data for evaluating unsaturated-zone transport rates, we conducted laboratory experiments to quantify effects of sample processing method on 36Cl and total Cl release from welded tuff from Yucca Mountain.

Measured 36Cl/Cl ratios were larger for samples left undisturbed during the leach processes (passive leach) when compared to those from samples that were continuously shaken during the leach processes (active leach). Measured 36Cl/Cl for the active-leach sample are inversely correlated with Cl concentrations, suggesting mixing between meteoric and rock components. In several different samples, the active-leach method extracted on average 1.4 times (up to 2.75 times) more Cl than did the passive-leach method. These data indicate that the 36Cl/Cl ratios may be lowered by dilution with rock Cl sources as a result of active leach methods. 36Cl/Cl for 7 of 10 passive-leach samples change little over a 48 hour leach (individual leach samples taken at 0.5, 2, 7, and 48 hrs), with values between 500E-15 and 800E-15 that are consistent with the natural background 36Cl/Cl for meteoric water at Yucca Mountain. 36Cl/Cl for 3 of the passive-leach samples show a decrease in the 36Cl/Cl with time, consistent with early release of a minor bomb-pulse component. These data indicate that laboratory procedures used for earlier and current LANL research have effectively extracted Cl carried in by infiltrated water, with minimal dilution of the meteoric signal by volcanic Cl sources. The data also suggest that leach times <2 hr may be more effective than longer leaches in isolating bomb-pulse 36Cl signature from crystalline rocks.