Capillary Barriers: Reflections On Theory, Testing, and Performance.

Walter Gardner’s 1960s film, “How Water Moves in Soil” depicts capillary barriers and  illustrates how water can be stored in elevated amounts in soils underlain by gravels or coarse sands.  Over the past 50 years, the capillary-barrier concept has been applied and tested as a landfill-cover design for arid and semi-arid waste sites. At waste sites, where water-balance considerations are favorable, evapotranspiration (ET) can act as a pump and remove most, if not all the annual precipitation from landfill covers with capillary barriers, thus minimizing drainage into underlying wastes.  At Cadarache, France in the early 70s, a capillary barrier test failed due to excess water applied as rainfall and irrigation.  Failure was also observed at Hill Air Force Base, Utah due to excess winter precipitation. In contrast, numerous tests at the Hanford Site, in 3-m-deep lysimeters and with a full-scale cover test, illustrate how even under simulated 1000-yr storm events, capillary barriers successfully minimized drainage to near-zero (< 0.1 mm/yr) design limits.  We also provide some precautions on the indiscriminate use of ET-capillary barriers.