Modeling of Engineered Covers for Waste Containment
Bridget Scanlon, principal investigator; Robert C. Reedy
The purpose of this study was to compare water-balance simulation results from 7 different codes, HELP, HYDRUS-1D, SHAW, SoilCover, SWIM, UNSAT-H, and VS2DTI, using 1-3 yr water-balance monitoring data from nonvegetated engineered covers (3 m deep) in warm (Texas) and cold (Idaho) desert regions. The codes evaluated in this study are listed in Table 1. Graphical user interfaces are available for most codes. SHAW has an interface for data input but does not have a postprocessor. Detailed descriptions of the codes can be found in the user's manuals. It is impossible to describe all the attributes of the various codes in this paper; however, many of the attributes related to water-balance modeling for HELP, HYDRUS, SoilCover, and SHAW are described in Wilson et al. (1999).
HELP ((Hydrologic Evaluation of Landfill Performance, Version 3; Schroeder et al., 1994): http://www.wes.army.mil/el/elmodels
HYDRUS1D (Version 3.0; Simunek et al., 1998): http://www.ussl.ars.usda.gov/MODELS/HYDR1D1.HTM
SHAW (Simulation of Heat and Water; Version 2.4; Flerchinger and Saxton, 1989; Flerchinger et al., 1996): http://www.nwrc.ars.usda.gov/models/shaw/index.html
SoilCover (Version 4.1; Wilson, 1990; Wilson et al., 1994; GEO2000, 1997): http://www.geo2000.com
SWIM (Soil Water Infiltration and Movement model; Version 2; Verburg et al., 1996): http://www.clw.csiro.au/products/swim
UNSAT-H (Version 3.0; Fayer, 2000): http://hydrology.pnl.gov/resources.asp
VS2DTI (Variably Saturated 2 Dimensional Transport Interface; Version 1; Healy, 1990): http://water.usgs.gov/software/vs2di.html
Table 1. URL's for the various codes, including references.

Sites
The Texas site is located near Sierra Blanca, which is about 120 km southeast of El Paso, Texas. The site is within the Chihuahuan Desert of Texas. Long-term (30-yr) mean annual precipitation at Sierra Blanca is 320 mm. The site consists of heavily instrumented engineered covers that were installed in the summer of 1997. The surface dimensions of the engineered cover are 34 × 17 m.

The Idaho site is located at the Idaho National Engineering and Environmental Laboratory in southeastern Idaho (Porro, 2001) on the Snake River Plain. Long-term (40-yr) mean annual precipitation is 221 mm. The site consists of a concrete structure containing 10 cells, each of which is 3 m × 3 m × 3 m (four walls and a floor). Replicates of a monolithic soil cover and a capillary barrier cover were constructed in the cells. Data from only one of the monolithic soil cover cells are used in this study.

Codes
This study demonstrates the variability in simulated water-balance components using a variety of codes (listed above in Table 1) on the basis of field monitored data from engineered covers at warm (Texas) and cold desert (Idaho) sites and provides some indication of the expected reliability of simulated water balances. Simulation results from most codes were similar and generally reproduced measured water-balance components at the Texas and Idaho sites. Both sites consisted of unvegetated loam soil.

Simulation of infiltration-excess runoff was a problem for all codes, underscoring the difficulties of representing actual precipitation intensities and of measuring hydraulic conductivity of surficial sediments (as shown by the data from Texas). Drainage is the most critical parameter for evaluation of contaminant transport, engineered covers for waste containment, and groundwater recharge. Drainage could be estimated to within ~ ± 64% by most codes. Outliers for the various simulations could be attributed to the following factors:

· the modeling approach, i.e., water-storage routing versus Richards' equation,
· the upper boundary condition during precipitation and time discretization of precipitation input,
· water retention functions (i.e., van Genuchten versus Brooks and Corey), and
· the lower boundary condition (i.e., unit gradient versus seepage face).

The water storage routing approach does not seem to adequately represent the flow system in semiarid regions. By assuming that gravity is the only driving force and ignoring matric-potential gradients that are often upward in semiarid regions, downward flow is generally overestimated and ultimately results in overestimation of drainage.

The approach used to simulate the upper boundary condition during precipitation is crucial when precipitation is input on a daily or larger time step. Setting PE to zero on rain days (VS2DTI) greatly underestimated evaporation and overestimated drainage. Subtracting PE from precipitation and applying net precipitation or net PE on a daily basis (HYDRUS-1D) had a much lesser impact on simulation results. The best approach is to disaggregate daily precipitation and apply it at a specified rate, allowing PE to occur throughout the rest of the day, as shown by the UNSAT-H simulations.

The impact of water retention functions was demonstrated at the Idaho site, where increased unsaturated hydraulic conductivity based on the Brooks and Corey functions relative to the van Genuchten functions resulted in overestimation of evaporation and underestimation of drainage. In contrast, the input value of residual water content (0 for Campbell function versus > 0 for Brooks and Corey) had little impact on simulation results.

The most appropriate lower boundary condition for simulating wickless lysimeters is a seepage face. Simulations using HYDRUS-1D demonstrated that this boundary condition could be approximated by simulating a thin bottom layer of gravel with a unit gradient boundary condition in codes that use Richards' equation but do not include a seepage face option. However, use of a unit-gradient lower boundary condition alone greatly overestimated drainage. This study demonstrates the usefulness of conducting intercode comparisons to evaluate the reliability of water-balance simulations and to determine important factors controlling water-balance simulation results.

References
Scanlon, B. R., Christman, M., Reedy, R. C., and Gross, Beth, 2002, Intercode comparisons for simulating water balance in an engineered cover, in 2001 International Containment and Remediation Technology Conference, Orlando, Florida, Institute for International Cooperative Environmental Research, Florida State University, Paper ID. No. 148, http://www.iicer.fsu.edu, 3 p.
[PDF]

Scanlon, B. R., Christman, M., Reedy, R. C., Porro, I., Simunek, J., and Flerchinger, G. N., in press, Intercode comparisons for simulating water balance of surficial sediments in semiarid regions: Water Resources Research. [PDF]

Scanlon, B. R., Christman, M., Simunek, J., and Reedy, R. C., 2001, Intercode comparisons for simulating water balance of near-surface soils (abs.), in Eos, v. 82, no. 47, Fall Meeting Supplement, American Geophysical Union, Abstract H12F-10. [PDF]

February 2003