Field evidence for strong chemical separation of contaminants in the Hanford Vadose
Water and chemical transport from a point source within vadose zone sediments at Hanford were examined with a leak test consisting of five 3800-liter aliquots of water released at 4.5 m depth every week over a 4-week period. The third aliquot contained bromide, D2O and 87Sr. Movement of the tracers was monitored for 9 months by measuring pore water compositions of samples from boreholes drilled 2-8 m from the injection point. Graded sedimentary layers acting as natural capillary barriers caused significant lateral spreading of the leak water. D2O concentrations >50 percent of the concentration in the tracer aliquot were detected at 9-11 m depth. However, increased water contents, lower d18O values, and geophysical monitoring of moisture changes at other depths signified high concentrations of leak fluids were added where D2O concentrations were <3 percent above background, suggesting limited mixing between different aliquots of the leak fluids. Initially high bromide concentrations decreased more rapidly over time than D2O, suggesting enhanced transport of bromide due to anion exclusion. No significant increase in 87Sr was detected in the sampled pore water, indicating strong retardation of Sr by the sediments. These results highlight some of the processes strongly affecting chemical transport in the vadose zone and demonstrate the significant separation of contaminant plumes that can occur.