Quest For The Perfect Cap: The Prototype Hanford Barrier 15 Years Later-10419

Engineered surface barriers are recognized as a remedial alternative to the removal, treatment and disposal of near- surface contaminants at a variety of waste sites within the DOE complex. One outstanding issue that has impacted stakeholder acceptance of this technology is the limited amount of performance data and the uncertainty in using them to predict long-term performance. In 1994, a 2-ha multi-component barrier was constructed over an existing waste disposal site using natural materials. The 4.5-m thick barrier includes a 1-m thick silt loam surface layer with 15% pea gravel to control erosion as well as a capillary break, an asphaltic concrete layer at the base, and two protective side-slope configurations. The cover was designed to meet a 0.5 mm/yr drainage criterion and is instrumented to monitor stability as well water balance components. A treatability test conducted from 1994-1998 included irrigation at a rate of 480 mm/yr including a simulated 1000-yr return storm each March, in which 68 mm of water was applied over an 8 hr period. Monitoring has been almost continuous for the last 15 yrs and has focused on barrier stability, vegetative cover, evidence of plant and animal intrusion, and the main components of the water balance, including precipitation, runoff, storage, drainage, and deep percolation. The total precipitation received from October 1994 through August 2008 was 3311 mm on the northern half (formerly irrigated), and 2638 mm on the southern, non-irrigated half. Water storage in the fine-soil layer shows a cyclic pattern, increasing in the winter and decreasing in the spring and summer to a lower limit of around 100 mm, regardless of precipitation, in response to evapotranspiration. The functional portion of the barrier and its side slopes are structurally stable. Over the 15 years, only three runoff events have been observed but the 600-mm design storage capacity has never been exceeded. Total percolation ranged from near zero amounts under the soil-covered plots to over 600 mm under the side slopes but an asphalt layer prevented any of this water from reaching the buried waste. A relatively high ground cover of native plants still persists after the initial revegetation although the number of species decreased from 35 in 1994 to 15 in 2009. The vegetative cover, in addition to the silt-loam-gravel admix, proved effective in minimizing erosion but a recent removal of vegetation from the north half resulted in significant soil movement. There is ample evidence of insect and small mammal use suggesting that the barrier is beginning to function like a recovering ecosystem. These data have proven useful in the development of more rigorous methods for evaluating long-term performance and quantifying associated risk and uncertainty using numerical models.