Collection of Unsaturated-Zone Water Samples for Tracer Analysis: Evaluation of an Inverting Membrane Technique

摘要

As part of the site characterization efforts at Yucca Mountain, Nevada, a series of unsaturatedzonerntracer tests has been performed at nearby Busted Butte. The Phase-2 tracer test was conducted withinrnan instrumented 10 m x 10 m x 7 m in-situ block of vitric tuff. A complex tracer solution containing bothrnreactive and nonreactive tracers was pumped into the block over a period of 27 months. Throughout therntest, thousands of repeated unsaturated-zone porewater samples were collected on sorptive pads attachedrnto pneumatically emplaced inverting membranes and then analyzed for tracer concentration. Partwayrnthrough the experiment, three new boreholes were drilled into the block, and two intercepted the tracerrnplume. The rock core was removed for porewater extraction and analysis, and the boreholes were thenrninstrumented with inverting membranes and sorptive pads. The initial set of porewater-soaked pads wasrnremoved from the boreholes a week after they were emplaced, and the porewater was extracted and analyzed.rnThis paper compares the tracer composition data from the rock cores and the pads to evaluate therneffectiveness of the inverting-membrane collection technique for a variety of tracers. While the sorptivernpads sample only dissolved tracers, rock cores contain both dissolved and sorbed tracer. For nonreactiverntracers, such as bromide and fluorinated benzoic acids, this distinction is immaterial, and the rock and padrndata agree quite well. For reactive tracers, such as lithium, the dissolved tracer concentrations can be estimatedrnfrom rock analyses by dividing by the tracer’s retardation factor; when this correction is applied, thernrock and pad lithium data are also in reasonable agreement.