Determining the appropriate criteria and designs for hazardous waste landfill covers has spawned much discussion within the environmental remediation arena. Very little reliable comparison of various technologies exists. Researchers at Los Alamos National Laboratory studied the relative hydrologic performance of four landfill cover designs - two capillary barrier designs, one modified EPA RCRA design, and one control cover. Monitoring the fate of natural precipitation for nearly four years showed that the covers with barrier layers more effectively reduced deep percolation than the control cover. Although none entirely eliminated deep percolation, the RCRA cover, incorporating a clay hydraulic barrier, most effectively controlled it. The two capillary barriers reduced deep percolation, but significant amounts were still produced. Over 90 percent of all percolation through the covers, and lateral flow within the covers, occurred during February through May each year, primarily as a result of snowmelt, early spring rains, and low evapotranspiration. The study also showed that gravel mulch surface treatments (70 to 80% ground cover) reduced runoff and erosion. Despite additional shrubs planted on one, the two plots receiving the gravel mulch treatments exhibited equally enhanced amounts of evapotranspiration.

Monolayer evapotranspiration (ET) covers are the baseline method for closure of disposal sites for low-level radioactive waste (LLW), mixed LLW, and transuranic (TRU) waste at the Nevada Test Site (NTS). The regulatory timeline is typically 1,000 years for LLW and 10,000 years for TRU waste. Covers for such waste have different technical considerations than those with shorter timelines because they are subject to environmental change for longer periods of time, and because the environmental processes are often coupled. To evaluate these changes, four analog sites (approximately 30, 1,000 to 2,000, 7,000 to 12,500, and 125,000 years in age) on the NTS were analyzed to address the early post-institutional control period (the youngest site), the 1,000-year compliance period for disposal of LLW, and the 10,000-year period for TRU waste. Tests included soil texture, structure, and morphology; surface soil infiltration and hydraulic conductivity; vegetation and faunal surveys; and literature reviews. Separate measurements were made in plant undercanopy and intercanopy areas. The results showed a progressive increase in silt and clay content of surface soils with age. Changes in soil texture and structure led to a fivefold decline in saturated hydraulic conductivity in intercanopy areas, but no change in undercanopies, which were subject to bioturbation. These changes may have been responsible for the reduction in total plant cover, most dramatically in intercanopy areas, primarily because more precipitation either runs off the site or is held nearer to the surface where plant roots are less common. The results suggest that covers may evolve over longer timeframes to stable landforms that minimize the need for active maintenance.

The Prototype Hanford Barrier (PHB) was constructed in 1994 over an existing waste site as a demonstration. The barrier was tested to evaluate its design and performance at field scale under conditions of enhanced and natural precipitation and no vegetation. Monitoring data demonstrated that the barrier satisfied nearly all objectives over two decades. The PHB far exceeded the RCRA criteria, functioned in Hanford's semiarid climate, limited drainage to well below the 0.5 mm/yr performance criterion, limited runoff, and minimized erosion and bio-intrusion. The exposed subgrade receives protection against erosion, and institutional controls prevent inadvertent human activity at the barrier. Results to date suggest the PHB is very likely to perform for its 1000-year design life. Additional information: 2016 PHB Performance Report and Appendices

An alternative cover design consisting of a monolithic layer of native soil is proposed as the closure path for the Mixed Waste Landfill at Sandia National Laboratories, New Mexico. The proposed design would rely upon soil thickness and evapotranspiration to provide long-term performance and stability, and would be inexpensive to build and maintain. The proposed design is a 3-ft-thick, vegetated soil cover. The alternative cover meets the intent of RCRA Subtitle C regulations in that (a) water migration through the cover is minimized; (b) maintenance is minimized by using a monolithic soil layer; (c) cover erosion is minimized by using erosion control measures; (d) subsidence is accommodated by using a ''soft'' design; and (e) the permeability of the cover is less than or equal to that of natural subsurface soil present. Performance of the proposed cover is integrated with natural site conditions, producing a system performance that will ensure that the cover is protective of human health and the environment. Natural site conditions that will produce this system performance include (a) extremely low precipitation and high potential evapotranspiration; (b) negligible recharge to groundwater; (c) an extensive vadose zone; (d) groundwater approximately 500 ft below the surface; and (e) a versatile, native flora that will persist indefinitely as a climax ecological community with little or no maintenance.

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.

This report summarizes the results of nearly two decades of research that demonstrate the effectiveness of ET caps at the Idaho National Engineering and Environmental Laboratory (INEEL). The products of this research are specific recommendations for construction and maintenance of ET caps at the INEEL to prevent precipitation water from reaching interred wastes at the INEEL. The recommended cap configurations provide a low cost, low maintenance alternative to EPA's recommended RCRA cap and to more complex, highly engineered designs.

A large-scale field demonstration comparing final landfill cover designs has been constructed and is currently being monitored at Sandia National Laboratories in Albuquerque, New Mexico. Two conventional designs (a RCRA Subtitle D Soil Cover and a RCRA Subtitle C Compacted Clay Cover) were constructed side-by-side with four alternative cover test plots designed for dry environments. The demonstration is intended to evaluate the various cover designs based on their respective water balance performance, ease and reliability of construction, and cost. A portion of this project involves the characterization of vegetation establishment and growth on the landfill covers. The various prototype landfill covers are expected to have varying flux rates (Dwyer et al 2000). The landfill covers are further expected to influence vegetation establishment and growth, which may impact site erosion potential and long-term site integrity. Objectives of this phase are to quantify the types of plants occupying each site, the percentage of ground covered by these plants, the density (number of plants per unit area) of plants, and the plant biomass production. The results of this vegetation analysis are presented in this report.

From 2003 to 2009, Sandia National Laboratories (SNL) constructed three ET covers ranging from 2.2 to 4.1 acres in Sandia's Technical Area 3 on Kirtland AFB, south of Albuquerque, NM. The evaporation potential is optimal due to the area's low humidity, generally warm temperatures, and average annual precipitation of 8.72 inches. This presentation notes best practices and lessons learned from SNL's 15 years of experience with ET cover construction, preventive care, and maintenance.

This paper reviews (1) the history and regulatory background of the Monticello Disposal Site, (2) the environmental setting and design of the Monticello cover, (3) the design and installation of a 3-hectare (ha) (7.5-acre) lysimeter embedded in the cover, and (4) water balance and vegetation monitoring results from the lysimeter since 2000.

Surface covers are used to isolate contaminants in hazardous and low-level radioactive sites for time frames ranging from hundreds of years to millennia or more. In the absence of data for such durations, the long-term performance of surface barriers can only be represented with short-term tests or inferred from analogs and modeling. This paper provides evidence of field performance of soil covers for periods up to 17 yr. The results of lysimeter studies from a semiarid site in Washington State show that a cover design known as the Hanford Barrier, which consists of 1.5 m of silt loam above a sand/gravel capillary break, can nearly eliminate drainage. The results were similar if plants were present or not, demonstrating the robustness of the design. Furthermore, reducing the silt loam thickness to 1.0 m (as might occur via erosion), with or without plants, did not lead to drainage. When irrigated to mimic 3x average precipitation conditions, the vegetated Hanford Barrier continued to prevent drainage. Overall, the results showed no loss in performance during the 17 yr of testing. Only when plants were eliminated completely from the 3x precipitation test did drainage occur (rates ranged from 6 to 16 mm yr�1). In a separate test, replacing the top 0.2 m of silt loam with dune sand and reducing the plant cover did not lead immediately to the onset of drainage, but soil matric heads within the silt loam noticeably increased. This observation suggests that dune sand migration onto a surface cover has the potential to reduce a cover's ability to minimize deep drainage.

Relationships of plant cover and soil water content were evaluated for the years 2002-2006 of the Protective Cap/Biobarrier Experiment (PCBE). This experiment is designed to test the effectiveness of alternative evapotranspiration (ET) cap designs for protecting shallowly buried wastes at the Idaho National Laboratory (INL). Comparisons of data from the 1994-2000 and 2002-2006 study periods offer a rare opportunity to examine the dynamic performance of ET caps following episodic drought, because the most significant drought in at least the past 54 years began at the end of the first study period and ended during the second study period. Successful performance of ET caps during climatic fluctuations is crucial to long-term protection of shallowly buried wastes. We did not directly measure water content below the caps. One of the ET caps previously deemed to be most suitable for protection of interred wastes � a cap comprised only of 2 m of topsoil � had among the most frequent and highest levels of water accumulation at the bottom of the cap during 2002-2006. The top performing cap appeared to be one having a layer of cobble at 1 m depth, within 2 m layer of topsoil (�deep biobarrier�) planted with native vegetation. In contrast to the previous report and to our predictions, EPA recommended caps appeared to function well, despite having the least plant cover of all cap types. However, EPA caps generate runoff which must be disposed of properly, and there were several cases of soil moisture below the flexible membrance liner (FML) beneath the cap, indicating cap failure. The findings indicate that simple paradigms of soil-plant water relationships may not be adequate to explain the performance of ET caps. In particular, further research is needed to assess i) how plant cover affects ET, to guide planting strategies, ii) how antecedent moisture affects cap ET following wetting, and iii) how species identity and timing of precipitation affect ET.

The cover of the Lakeview, Oregon, disposal cell relies on a compacted soil layer (CSL) to limit radon escape and water percolation into underlying tailings. The design created habitat favorable for growth of woody plants that sent roots through the CSL. The mean saturated hydraulic conductivity (Ksat) of the CSL, measured at 17 locations, was 3.0 x 10-5 cm s-1, 300 times greater than the design target. The highest Ksat values were measured near the top of the CSL at locations both with and without roots; the lowest Ksat values were measured deeper in the CSL. Water flux meters (WFMs) installed in 2005 to directly measure percolation flux show significant percolation through the cover. Three WMFs began recording percolation in mid-November, 7 days after the start of a prolonged precipitation event, and continued until early June 2006. Percolation flux during this period ranged between 3.1 x 10-5 and 8.5 x 10-5 cm s-1. The cumulative percolation was greater than total precipitation during the period, probably because of a water-harvesting effect. The WFMs were strategically placed in downgradient positions on the cover top slope where water likely accumulated in a sand drainage layer. Routine monitoring at Lakeview shows that the ground water remains protected. LM plans to evaluate potential effects of high percolation rates in covers to ensure that disposal cells remain protective for the long term.