Comparison of the Water Balance of an Asphalt Cover and an Evapotranspiration Cover at Technical Area 49 at the Los Alamos National Laboratory

Abstract

test experiments were conducted is now referred to as Material Disposal Area (MDA) AB. The areas within Two different types of waste covers, an asphalt cap and an evapo-MDA AB where the underground experiments were transpiration (ET) cover, have been used to contain a contaminated site at Technical Area (TA) 49 at the Los Alamos National Laboratory conducted are referred to as Areas 1, 2, 2A, 2B, 3, and (LANL). In 1961, a 2.5-to 20-cm-thick asphalt cap and gravel and 4. These Areas are also referred to as Potential Release clay cover was constructed to isolate contamination of surface soils Sites (PRSs) (LANL, 1992; Levitt et al., 2003). Refer associated with subcritical hydronuclear safety experiments conducted to Fig. 1 for the location of TA-49 and LANL within at TA-49 between 1959 and 1961. The asphalt cap remained in place New Mexico, and refer to Fig. 2 for the locations of the for 37 yr until 1998 when it was replaced with an ET cover. Cracks Areas and PRSs within TA-49. and subsidence features in the asphalt cap were periodically filled During the drilling of a new test shaft in 1960, transduring that time. The thickness of the ET cover is 2.1 m at its center, uranic materials were encountered from an adjacent tapering to zero thickness at its edges. The ET cover is instrumented shaft, resulting in contamination of surface soils. The with three neutron logging access tubes, and four time domain reflecarea of contamination, within Area 2, was covered by tometry (TDR) probes, and covered by a galvanized steel mesh biointrusion layer. Soil moisture monitoring was conducted immediately Published in Vadose Zone Journal 4:789-797 (2005).

Conceptual Models of Vadose Zone Flow and Transport beneath the Pajarito Plateau, Los Alamos, New Mexico

“…Background water content profiles measured in four 37‐m boreholes (Fig. 11) and a 210‐m borehole (Levitt et al, 2005) illustrate the site's dry background conditions. Water content of the tuff below about 3 m is <10%.…”

“…Volumetric Water Content in four boreholes for background conditions at TA‐49 near MDA AB (Levitt et al, 2005). …”

“…The site was then regraded, capped with an ET cover, and revegetated. From 2000 through 2004, monitoring has shown slow drying in the upper 6 m of the soil layer beneath the ET cover (Levitt et al, 2005). Water contents at 12‐m to 18‐m depths show a slight increase in time (Levitt et al, 2005), as predicted, because of the steep water content profile that existed before removal of the asphalt is relaxing.…”

Moisture Monitoring at Area G, Technical Area 54, Los Alamos National Laboratory

“…The occurrence of roads, asphalt paving, and structures overlying closed waste pits can be seen in Figure 7. Neeper et al (1996), Rofer et al (1997), and Levitt et al (2005) describe the effects of an asphalt cover over MDA AB at TA-49 at LANL. These studies conclude that the asphalt cover causes increased infiltration by all but eliminating ET, but allowing runoff and focused infiltration through cracks in the asphalt.…”

The Hydrogeology of Los Alamos National Laboratory: Site History and Overview of Vadose Zone and Groundwater Issues

“…These results can be used to help assess the appropriateness of ET covers for other sites and for optimizing cover design. Levitt et al (2005b) compare water content monitoring results from a simple asphalt surface cover and an ET cover at the same site. Their results show that simply paving over a site with asphalt can create an adverse condition in terms of maintaining low water contents below the cover.…”