Field Evidence for Strong Chemical Separation of Contaminants in the Hanford Vadose Zone

Abstract

Water and chemical transport from a point source within vadose zone sediments at the Hanford Site in Washington State were examined with a leak test consisting of five 3800‐L aliquots of water released at 4.5‐m depth every week over a 4‐wk period. The third aliquot contained bromide, D2O, and 87Sr. Movement of the tracers was monitored for 9 mo by measuring pore water compositions of samples from boreholes drilled 2 to 8 m from the injection point. Graded sedimentary layers acting as natural capillary barriers caused significant lateral spreading of the leak water. Shortly after injections were completed, D2O was found at the 9‐ to 11‐m depth at levels in excess of 50% of the tracer aliquot concentration, while sediment layers with elevated water content at the 6‐ to 7‐m depth had less than 3% of the D2O tracer concentration, suggesting deep penetration of the D2O tracer and limited mixing between different aliquots of 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.