Ecohydrology monitoring and excavation of semiarid landfill covers a decade after installation

Landfill covers are intended to protect buried waste from water seepage and biointrusion for thirty to thousands of years, yet most cover studies are limited to a few years and do not directly investigate net changes in the soil profile that affect changing landfill performance. We evaluated water balances, vegetation cover, rooting patterns, and soil profiles of two landfill-cover designs (two plots each) more than a decade after installation at semiarid Los Alamos National Laboratory, NM, USA: a conventional design of 20 cm of topsoil over compacted crushed-tuff and an integrated design of 71 cm of topsoil over an engineered barrier designed to induce lateral flow (geotextile overlying 46 cm of gravel). Water balances for both designs had ∼3% of precipitation as seepage; the integrated plots lost <1% of water as interflow, probably because the barrier interface had only a 5% slope. The conventional design had a net loss of stored soil water and proportionally more evapotranspiration than the integrated design. After more than a decade, (i) vegetation changes included increased biomass and species diversity on most plots, with proportionally fewer invading species and more extensive rooting in the integrated plots; (ii) the geotextile was largely unchanged; and (iii) infiltration and subsequent water penetration occurred primarily via macropores, including root channels and animal burrows. Both cover designs effectively minimized seepage during their initial decade, but observed effects of environmental processes such as succession and burrowing are expected to become progressively more important determinants of cover performance over additional decades.