Performance evaluation of a field-scale surface barrier

Surface barriers, designed to limit recharge and biointrusion, have been identified as a critical component in long-term management of buried wastes. However, surface barrier technology remains largely unproven at the field scale. Recently, a prototype of a field-scale (2.5 ha), vegetated, capillary surface barrier was constructed over a waste zone at the semiarid Hanford Site in southeast Washington. The barrier is instrumented to measure the components of water balance under ambient and elevated precipitation scenarios on soil and rock-covered plots. The barrier also allows for the evaluation of two protective side slope configurations, and the monitoring of flow around and under a low permeability asphalt layer. The first 2 yr of testing were unusually wet, with precipitation more than twice the long-term annual average of 160 mm. Even with an imposed irrigation treatment of 480 mm yr(-1), including a simulated 1000-yr storm event each year, there was no drainage from the soil covered plots. This demonstrates the effectiveness of vegetated capillary barriers in an arid environment. Each year, plants used all available water, independent of precipitation treatment, reducing soil water storage to the same lower limit by the end of summer. The soil was wettest during spring, but water storage never exceeded 450 mm in the 2-m thick soil layer, which was designed to store 600 mm. The efficiency of ET was consistently higher on the ambient treatment, suggesting a susceptibility of native plant species to high levels of precipitation. No water has penetrated the low-permeability asphalt layer, although an unprotected section of the toe showed a potential for underflow. While there was no difference in total drainage from the irrigated side slopes over the last 2 yr, the nonirrigated basalt slope drained 55% less water than the gravel. Side slope drainage also showed a seasonal dependence, with the gravel draining more than the basalt in winter and less in the summer. Drainage rates and volumes appear to be controlled by advective airflow.