Tracer/Time-Lapse Radar Imaging Test At The Boise Hydrogeophysical Research Site

A combined tracer and time-lapse radar imaging experiment was conducted in the unconfined<br>coarse fluvial aquifer at the Boise Hydrogeophysical Research Site in August, 2001. Two tracers<br>(bromide and uranine) were injected to form a plume over a 4-m interval that spanned the contact<br>between hydrostratigraphic units with contrasting permeability. The tracer plume traveled 6.9 m to well<br>B6, passing through well A1 instrumented with 20 sampling zones over a 5-m interval that spanned the<br>injection interval. Radar tomographic data were collected periodically on cross-sectional and<br>longitudinal planes, two of which passed through well A1 for quantitative calibration of radar<br>attenuation tomograms in terms of solute concentration. Pre-test three-dimensional modeling was used<br>to provide estimates of bromide concentration distributions under a variety of scenarios to help optimize<br>(a) radar responses in tomographic planes, and (b) pumping rate from B6 to minimize influence on the<br>plume while ensuring plume passage through A1 and complete breakthrough in two weeks. Bromide<br>breakthrough occurred first and with greatest concentration in the higher permeability unit as expected.<br>Time-lapse cross-hole radar data show plume-related differences in expected regions with time-lapse<br>level runs and time-lapse attenuation-difference tomograms. Uranine breakthrough was significantly<br>diminished and delayed relative to bromide; follow-up experiments suggest biological activity<br>associated with cottonwood roots caused the non-conservative uranine behavior.