UNSATURATED-ZONE POREWATER COLLECTION FOR TRACER ANALYSIS: ASSESSMENT OF AN INNOVATIVE TECHNIQUE

An innovative technique has been used to collect unsaturated-zone porewater samples at Busted Butte, Nevada. Field-scale tracer tests at Busted Butte are being carried out in order to validate conceptual and numerical models of unsaturated-zone transport for the Yucca Mountain Project. The underground facility consists of a 10 m by 10 m by 7 m undisturbed test block instrumented with horizontal injection and collection boreholes. A tracer solution containing reactive and non-reactive tracers was introduced into the injection boreholes. After the tracers traveled through the rock, they were collected using an inverting membrane system. Pads consisting of bundles of three filter paper strips, 4 cm by 28.5 cm, were attached to inflatable membranes. The membranes were emplaced by inversion into collection boreholes and inflated, which pushed the pads against the walls of the borehole. The pads were left in place for 1-2 weeks to imbibe porewater, then removed and taken back to the laboratory where the tracer solution was extracted and analyzed to determine the concentration of the various tracer constituents.

After 20 months of tracer injection, three boreholes were drilled into the tracer plume to collect rock samples for tracer analyses. The holes were then instrumented with inverting membrane collection systems. By extracting tracer from the pads and from the rock we can determine the accuracy with which porewater extracted from the pads reflects the chemistry of the porewater in the rock. This work will enable us to assess the effectiveness of the inverting membrane system of pore water sampling, and thus validate it as a method of collecting porewaters from the unsaturated zone.