NINETEEN-YEAR HYDROLOGICAL PERFORMANCE OF THE PROTOTYPE HANFORD BARRIER

The Prototype Hanford Barrier (PHB) is an engineered surface cover that consists of components designed to contain, control, and retard the water flow and the migration radioactive contaminants in the subsurface. The PHB was constructed in 1994 and is one of several long-term (with a design life of 1000 years) surface barriers over a waste site in a semi-arid region of the United States. The 80-m by 40-m barrier portion of the PHB consists of a 2-m-thick layer of silt loam resting on a capillary break of coarser materials ranging in sequence from sand to gravel to basalt riprap. The vegetation cover consists of assorted shrubs and annuals typical of the surrounding shrub-steppe environment. From 1995 to 1997, the northern half of the barrier was irrigated so the total amount of water falling on it was three times the average precipitation, while the southern half was exposed under a natural precipitation condition. In 2008, the vegetation on the northern half of the PHB was burned by a controlled fire to investigate the potential impacts of a wild fire on barrier performance. The soil water content within and beneath the 2-m-thick silt loam was monitored with vertical or horizontal neutron logging. The drainage through the barrier and sideslopes were monitored with 12 drainage vaults. The monitoring results and findings since 1994 will be presented and discussed.