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Title: Multiple-Year Water Balance of Soil Covers in a Semiarid Setting

Abstract

Surface covers are used to close 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 such 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 years. The results of lysimeter studies from a semi-arid site in Washington State show that a soil cover of 1.5 m of silt loam above a sand/gravel capillary break can 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 precipitation conditions, the vegetated Hanford Barrier continued to prevent drainage. Overall, the results showed no diminution in performance during the 17 years of testing. Only when plants were eliminated completely from the 3x precipitation test did drainage occur (rates ranged from 6 to 16 mm/yr). In a separate test, replacing the top 0.2 m of silt loam with dunemore » 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.« less

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
881086
Report Number(s):
PNNL-SA-43141
Journal ID: ISSN 0047-2425; JEVQAA; 830403000; TRN: US200612%%753
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Environmental Quality, 35(2):366-377; Journal Volume: 35; Journal Issue: 2
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; DESIGN; DRAINAGE; LOAM; LYSIMETERS; PERFORMANCE; PRECIPITATION; SAND; SILT; SIMULATION; SOILS; TESTING; THICKNESS; WATER

Citation Formats

Fayer, Michael J., and Gee, Glendon W. Multiple-Year Water Balance of Soil Covers in a Semiarid Setting. United States: N. p., 2006. Web. doi:10.2134/jeq2004.0391.
Fayer, Michael J., & Gee, Glendon W. Multiple-Year Water Balance of Soil Covers in a Semiarid Setting. United States. doi:10.2134/jeq2004.0391.
Fayer, Michael J., and Gee, Glendon W. Wed . "Multiple-Year Water Balance of Soil Covers in a Semiarid Setting". United States. doi:10.2134/jeq2004.0391.
@article{osti_881086,
title = {Multiple-Year Water Balance of Soil Covers in a Semiarid Setting},
author = {Fayer, Michael J. and Gee, Glendon W.},
abstractNote = {Surface covers are used to close 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 such 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 years. The results of lysimeter studies from a semi-arid site in Washington State show that a soil cover of 1.5 m of silt loam above a sand/gravel capillary break can 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 precipitation conditions, the vegetated Hanford Barrier continued to prevent drainage. Overall, the results showed no diminution in performance during the 17 years of testing. Only when plants were eliminated completely from the 3x precipitation test did drainage occur (rates ranged from 6 to 16 mm/yr). 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.},
doi = {10.2134/jeq2004.0391},
journal = {Journal of Environmental Quality, 35(2):366-377},
number = 2,
volume = 35,
place = {United States},
year = {Wed Mar 01 00:00:00 EST 2006},
month = {Wed Mar 01 00:00:00 EST 2006}
}
  • The results from several field experiments on methods to control soil erosion, biointrusion, and water infiltration were used to design and test an enhanced landfill cover that improves the ability of the disposal site to isolate buried wastes. The performance of the improved cover design in managing water and biota at the disposal site was compared for 3 yr with that obtained from a more conventional design that has been widely used in the industry. The conventional cover design consisted of 20 cm of sandy loam topsoil over 108 cm of a sandy silt backfill, whereas the improved design consistsmore » of 71 cm of topsoil over a minimum of 46 cm of gravel, 91 cm of river cobble, and 38 cm of sandy silt backfill. Each plot was lined with an impermeable liner to allow for mass balance calculation of water dynamics. Results over a 3-yr period, including 2 wet yr, demonstrated that the improved design reduced percolation of water through the landfill cover by a factor of >4 over the conventional design. This decrease in percolation was attributed to a combination of increased evapotranspiration from the plant cover and the effect of a capillary barrier embedded in the enhanced cover profile in diverting water laterally in the cover. The field data are finally discussed in terms of its usefulness for waste management decisions to be made in the future for both new and existing landfills at Los Alamos, NM, and at other semiarid waste disposal sites.« less
  • Hydrologic data measured from two earthen final cover test sections constructed on actual landfill final covers are presented with predictions made using two water balance models (HELP and UNSAT-H). Both test sections sere constructed as traditional resistive barriers comprised of a compacted fine-grained layer overlain by a vegetated surface layer. Hydrologic and meteorological data including precipitation, air temperature, solar radiation, relative humidity, wind speed, and wind direction were collected at each test section for three years. Percolation, overland flow, and soil water content were monitored continuously. Predictions of the water balance were made using the water balance models HELP andmore » UNSAT-H. In general, HELP overpredicted percolation, sometimes significantly, and UNSAT-H slightly underpredicted percolation. However, both models captured the seasonal variations in overland flow, evapotranspiration, soil water storage, and percolation. UNSAT-H captured these variations more accurately than HELP.« less
  • Landfill covers are critical to waste containment, yet field performance of specific cover designs has not been well documented and seldom been compared in side-by-side testing. A study was conducted to asses the ability of landfill final covers to control percolation into underlying waste. Conventional covers employing resistive barriers as well as alternative covers relying on water-storage principles were monitored in large (10 x 20), instrumented drainage lysimeters over a range of climates at field sites in the United States. Surface runoff was a small fraction of the water balance (0-10%, 4% on average) and was nearly insensitive to themore » cover slope, cover design, or climate. Lateral drainage from internal drainage layers was also a small fraction of the water balance (0-5%, 2.0% on average). Average percolation rates for the conventional covers with composite barriers (geomembrane over fine soil) typically were less than 12 mm/yr (1.4% of precipitation) at humid locations and 1.5 mm/yr (0.4% of precipitation) at arid, semiarid, and subhumid locations. Average percolation rates for conventional covers with soil barriers in humid climates were between 52 and 195 mm/yr (6-17% of precipitation), probably due to preferential flow through defects in the soil barriers. Average percolation rates for alternative covers ranged between 33 and 160 mm/yr (6 and 18% if precipitation) in humid climates and generally less than 2.2 mm/yr (0.4% of precipitation) in arid, semiarid, and subhumid climates. One half (five) of the alternative covers in arid, semiarid, and subhumid climates transmitted less than 0.1 mm of percolation, but two transmitted much more percolation (26.8 and 52 mm) than anticipated during design. The data collected support conclusions from other studies that detailed, site-specific design procedures are very important for successful performance of alternative landfill covers.« less
  • Landfill covers relying on a balance between soil water storage and evapotranspiration (ET) as the primary means to control drainage have been described in the recent paper by Scanlon et al. (2005) in the Vadose Zone Journal. These so-called "ET Covers" have been receiving considerable interest in the past few years as economically viable cover systems for landfills in arid and semi-arid environments (Hauser et al. 2001, Madalinksi et al. 2003) Scanlon et al. (2005) have provided a summary of their studies in Texas and New Mexico, demonstrating an acceptable performance of ET covers in minimizing drainage under their testmore » conditions. Further, they illustrate with both measurement and modeling that capillary barriers (fine soils over coarse soils) similar in concept to those previously built and tested at Los Alamos, NM and Hanford, WA over the past 20 years, store more water than surface barriers that have uniform profiles.« less
  • Numerical water-balance modeling of store-and-release soil covers for hypothetical mine tailings was conducted using the Hydrologic Evaluation of Landfill Performance (HELP) and SoilCover models. The objective of the modeling was to compare the utility of both models in a semi-arid environment. Although values for input parameters were chosen to make simulations as identical as possible between models, differences in model solution methods and discretization led to different water-balance predictions. Specifically, SoilCover predicted less percolation than HELP, because HELP uses simplified water-routing algorithms which may over predict infiltration and under predict subsequent evapotranspiration. Since SoilCover explicitly solves physically based governing equationsmore » for heat and water flow, its predictions more accurately represent the water balance in semi-arid regions where evapotranspiration dominates, HELP can only conservatively predict percolation in dry environments.« less