Ecohydrology Monitoring and Excavation of Semiarid Landfill Covers a Decade after Installation

closure varies but is generally not expected to exceed more than 30 to 50 years for cases in which institutional Landfill covers are intended to protect buried waste from water control is applied (Suter et al., 1993). However, risks seepage and biointrusion for thirty to thousands of years, yet most cover studies are limited to a few years and do not directly investigate associated with the waste frequently persist beyond innet changes in the soil profile that affect changing landfill performance. stitutional control; hence, the longer-term integrity of We evaluated water balances, vegetation cover, rooting patterns, and landfill covers is of concern. soil profiles of two landfill-cover designs (two plots each) more than a Design of a landfill cover requires consideration of decade after installation at semiarid Los Alamos National Laboratory, several tradeoffs. To keep the wastes dry, the cover is NM, USA: a conventional design of 20 cm of topsoil over compacted designed to minimize seepage. This can be achieved crushed-tuff and an integrated design of 71 cm of topsoil over an in part by storing soil water within the cover and by engineered barrier designed to induce lateral flow (geotextile overmaximizing the subsequent removal of the stored water lying 46 cm of gravel). Water balances for both designs had ~3% of through evapotranspiration. The evaporative component precipitation as seepage; the integrated plots lost 1% of water as of total evapotranspiration can be modified by orienting interflow, probably because the barrier interface had only a 5% slope. The conventional design had a net loss of stored soil water and proporthe cover to maximize incoming solar radiation (e.g., tionally more evapotranspiration than the integrated design. After Nyhan et al., 1997), while the transpiration component more than a decade, (i) vegetation changes included increased biomass can be modified through selection and management of and species diversity on most plots, with proportionally fewer invading plant species on the cover (e.g., Lopez et al., 1988). species and more extensive rooting in the integrated plots; (ii) the Seepage can be reduced further by increasing interflow geotextile was largely unchanged; and (iii) infiltration and subsequent (shallow subsurface lateral flow of water at the interwater penetration occurred primarily via macropores, including root faces between layers) that is then diverted to a location channels and animal burrows. Both cover designs effectively miniaway from the waste via engineering structures (e.g., mized seepage during their initial decade, but observed effects of Khire et al., 2000; Nyhan et al. 1997, 2001). Conversely, environmental processes such as succession and burrowing are exsurface runoff from the cover should be minimized pected to become progressively more important determinants of cover performance over additional decades. through establishment of ground cover sufficient to minimize water erosion, which degrades the integrity of the cover through time. Simultaneously, the cover must minimize biointrusion by plant roots and burrowing aniA strategy for isolating the effects of contamimals. The relative importance of these tradeoffs varies nants in the environment and mitigating associated with climate (e.g., Albright et al., 2004; Khire et al., human and ecological risks is to apply engineered covers 2000). Caution must be used when extrapolating inforover landfills used for disposal of hazardous and municimation from site to site because the seasonal occurrence pal solid waste (Reith and Thomson, 1993; Zornberg et of rain and snow varies with climate and impacts the al., 2003). Landfill covers, which include various combidynamics of the amounts of soil water that can be stored nations of soil layers, engineered barriers, and liners, in the landfill profiles (Gee et al., 1998). are expected to exhibit long-term stability to effectively Development of landfill cover designs appropriate for isolate wastes and to minimize the risks of exposure to arid and semiarid environments is of particular interest the public (Albright et al., 2004; EPA, 1989; Reith and given that hazardous waste disposal sites are numerous Thomson, 1993; Wing and Gee, 1994). The duration in the semiarid to arid western USA (Albright et al., for monitoring and maintenance of landfill covers after 2004; see also Nyhan, 2005 and references therein). Evaluation of alternative cover designs for arid and semiarid D.D. Breshears, School of Natural Resources, Institute for the Study environments has been the focus of several U.S. Departof Planet Earth, and Department of Ecology & Evolutionary Biology, University of Arizona, Tucson, AZ 85721-0043 (present address) and ment of Energy (DOE) sites (e.g., Albright et al., 2004; Earth and Environmental Sciences Division, Los Alamos National Dwyer, 1998; Link et al., 1995; Lopez et al., 1988; Nyhan Laboratory, Los Alamos, NM 87545; John W. Nyhan, Ecology Group, et al., 1989a, 1989b; Nyhan et al., 1990; Nyhan et al., Risk Reduction and Environmental Stewardship Division, Mail Stop 1997), and a few on U.S. Department of Defense (DOD) M887, Los Alamos National Laboratory, NM 87545; David W. Davenport, Los Alamos Technical Associates, 1200 Trinity Drive, Los sites (Hakonson et al., 1994; Paige et al., 1996; Warren Alamos, NM 87544 (present address) and Earth and Environmental et al., 1996;). Most studies have not evaluated cover Sciences Division, Mail Stop J495, Los Alamos National Laboratory, performance for more than a few years (but see AnLos Alamos, NM 87545. Received 8 Feb. 2004. *Corresponding author draski et al., 1995). The cover storage capacity at a site ([email protected]). can be designed to be appropriate for a given site’s Published in Vadose Zone Journal 4:798–810 (2005). precipitation by varying soil and thickness using relaSpecial Section: Los Alamos National Laboratory tively straightforward calculations (Reith and Thomson, doi:10.2136/vzj2004.0038 1993). The more challenging aspects of cover design in© Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA volve determining vegetation effects on hydrology (e.g. 798 Published online August 16, 2005