Two different types of waste covers, an asphalt cap and an evapotranspiration (ET) cover, have been used to contain a contaminated site at Technical Area (TA) 49 at the Los Alamos National Laboratory (LANL). In 1961, a 2.5- to 20-cm-thick asphalt cap and gravel and clay cover was constructed to isolate contamination of surface soils associated with subcritical hydronuclear safety experiments conducted at TA-49 between 1959 and 1961. The asphalt cap remained in place for 37 yr until 1998 when it was replaced with an ET cover. Cracks and subsidence features in the asphalt cap were periodically filled during that time. The thickness of the ET cover is 2.1 m at its center, tapering to zero thickness at its edges. The ET cover is instrumented with three neutron logging access tubes, and four time domain reflectometry (TDR) probes, and covered by a galvanized steel mesh bio-intrusion layer. Soil moisture monitoring was conducted immediately after the removal of the asphalt cap in 1998, and continuously since 1999 within the ET cover, providing a unique comparison of the performance of these cap and cover types at a semiarid site. Soil moisture monitoring results indicate that the soils within and beneath the ET cover have been drying since its installation, and infiltration of precipitation and snowmelt has been minimized. The replacement of the asphalt cap with the ET cover has significantly decreased shallow subsurface moisture contents. With continued growth of vegetation and increased transpiration, the site should continue its return to native conditions where net infiltration and associated groundwater recharge rates are <7 mm yr−1.

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