Sustainable Covers for Uranium Mill Tailings, Usa: Alternative Design, Performance, and Renovation

The U.S. Department of Energy Office of Legacy Management is investigating alternatives to conventional cover designs for uranium mill tailings. A cover constructed in 2000 near Monticello, Utah, USA, was a redundant design with a conventional low-conductivity composite cover overlain with an alternative cover designed to mimic the natural soil water balance as measured in nearby undisturbed native soils and vegetation. To limit percolation, the alternative cover design relies on a 160-cm layer of sandy clay loam soil overlying a 40cm sand capillary barrier for water storage, and a planting of native sagebrush steppe vegetation to seasonally release soil water through evapotranspiration (ET). Water balance monitoring within a 3.0-ha drainage lysimeter, embedded in the cover during construction, provided convincing evidence that the cover has performed well over a 9-year period (2000– 2009). The total cumulative percolation, 4.8 mm (approximately 0.5 mm yr), satisfied a regulatory goal of <3.0 mm yr. Most percolation can be attributed to the very wet winter and spring of 2004–2005, when soil water content exceeded the storage capacity of the cover. Diversity, percent cover, and leaf area of vegetation increased over the monitoring period. Field and laboratory evaluations several years after construction show that soil structural development, changes in soil hydraulic properties, and development of vegetation patterns have not adversely impacted cover performance. A new test facility was constructed in 2008 near Grand Junction, Colorado, USA, to evaluate low-cost methods for renovating or transforming conventional covers into more sustainable ET covers. INTRODUCTION The U.S. Department of Energy (DOE) Office of Legacy Management is responsible for long-term protection of human health and the environment at the uranium mill tailings disposal cell near Monticello, Utah, USA. (http://www.lm.doe.gov). Many conventional covers designed for uranium mill tailings rely on the low hydraulic conductivity of a compacted soil layer to limit infiltration and percolation. An alternative design approach is to manipulate the cover ecosystem with the goal of enhancing soil water storage and evapotranspiration. The longterm sustainability of the alternative design depends on interactions of the climatology, soil hydrology, and plant ecology of the site. This paper reviews (1) the performance of conventional low-conductivity covers, (2) the environmental setting and design of the Monticello cover, (3) 9 years of water balance and vegetation monitoring results within a 3-ha lysimeter embedded in the Monticello cover during construction, (4) changes in soil morphology and hydraulic properties since construction, and (5) renovation of conventional covers. Results of pilot studies using small monolith soil lysimeters, followed by small engineered soil lysimeters, and then large caisson drainage lysimeters that led to the final cover design for Monticello, were reported previously [1]. CONVENTIONAL AND ALTERNATIVE COVERS Most conventional cover designs for uranium mill tailings include compacted soil layers (CSLs), sometimes called resistive barriers [2], for radon attenuation and to impede percolation into underlying contaminated materials [3]. Although design targets and performance standards for CSLs vary, the typical goal is a saturated hydraulic conductivity less than 1×10 cm/s [4]. Several studies have shown that CSLs in conventional covers for uranium mill tailings, and in similar covers for other applications, often fall short of the lowconductivity targets, often at or shortly after construction, and sometimes by several orders of magnitude [2,5,6,7,8,9,10]. Several reasons are cited: • Unanticipated ecological consequences of designs that encourage biointrusion [11,12,13,14].