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a Pacific Northwest National Laboratory, Richland, WA
b University of Wisconsin, Madison, WI
c Desert Research Institute, Reno, NV
glendon.gee{at}pnl.gov
Key Words: ET, evapotranspiration
Landfill covers relying on a balance between soil water storage and evapotranspiration (ET) as the primary means to control drainage were described in a 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 semiarid environments (Hauser et al., 2001; Madalinksi et al., 2003). Scanlon et al. (2005) presented a summary of their studies in Texas and New Mexico, demonstrating an acceptable performance of ET covers in minimizing drainage under their test 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 during the past 20 yr store more water than surface barriers that have uniform profiles.
Although the findings in Scanlon et al. (2005) are positive, there appears to be a growing and almost blind sentiment that ET covers are the cure-all for landfills in arid and semiarid settings. We offer here a few words of caution. Many examples show that short-term (generally <10 yr) testing and our current models may not adequately predict long-term cover performance (Hakonson et al., 1994; Gee et al., 1998; Albright et al., 2004; Benson et al., 2004, 2005). The shortcomings of ET covers and some cautionary notes are made in the following discussion.
Higher Than Anticipated Drainage Rates
Albright et al. (2004) cited a definitive example of ET covers (specifically capillary barriers) that were designed for semiarid Los Alamos, NM that did not work when applied at Ogden, UT, a cooler semiarid site. In contrast, a capillary barrier designed for the Hanford Site in Richland, WA worked remarkably well at the Ogden, UT site. The Los Alamos ET capillary barriers tested at Ogden were well vegetated and had cover thicknesses equal to or greater than those tested at Los Alamos, yet significant drainage occurred through the Los Alamos ET capillary barrier systems. Drainage occurred from the Ogden covers because the soil water storage capacities of the Los Alamos ET capillary barriers were insufficient to store water that infiltrated the cover as a result of snowmelt events in early spring. During the first 3 yr of testing alone, the Los Alamos ET capillary barriers each drained about 350 mm of water. In contrast, a Hanford Barrier design (an ET capillary barrier with more than 500 mm of water storage capacity) constructed at the Ogden site has yet to drain after more than 10 yr of testing (Albright et al., 2004). Excessive drainage has also been observed in more arid climates at test sites in Sacramento (Albright et al., 2004) and Altamont, CA (Benson et al., 2004, 2005), where monolithic covers are being tested. The excessive drainage from these ET covers in semiarid and arid settings should be of concern to those regulating ET cover system at landfills. All of these covers were monitored with very large lysimeters that contained a coarse geocomposite drainage layer for collecting drainage, so, the lysimeters were treated as capillary barrier-type covers with a sharp textural break at the base of the cover. As discussed by Albright et al. (2004), even more drainage may occur in a full-scale landfill application than was observed in the lysimeters because the only textural contrast in a full-scale application is the break between the base of the cover and the coarser solid waste below. For this reason, the lysimeter data represent a conservative estimate of the actual drainage for a given cover design.
Cover Design Generalizations
Scanlon et al. (2005) made what appears to be an unqualified statement about cover thickness requirements. They concluded that "a 1-m-thick ET cover underlain by a capillary barrier should be adequate to minimize drainage to 
1 mm yr–1 in these arid and semiarid settings." This is a significant overstatement if it applies to most or all ET covers used for waste containment in "arid and semiarid regions in the southwestern USA" (as implied in the title of their paper). For example, soil type and water storage have a major impact on the effectiveness of ET covers, and the expected performance must be considered in the context of the soil resources available for a given project. A 1-m-thick ET cover may be perfectly suitable with some fine-textured soils, and inadequate with other fine-textured soils. Moreover, general conclusions regarding cover performance in the southwestern USA should not be drawn from observations from two field sites. For example, Benson et al. (2004, 2005) reported that excessive drainage from a 1-m-thick ET cover in Altamont CA, a semiarid site in nearby southern California, USA. If the concluding statement was intended to apply only to the test sites in Texas and New Mexico, and specifically to the soils used at these sites, then the statement should have been more explicit to avoid confusion.
Failure to draw definitive conclusions about the importance of soil type and the corresponding soil water storage can be problematic. As an example, consider the capillary barrier described in Albright et al. (2004) that is located in semiarid Marina, CA (average annual precipitation, P, = 466 mm and annual precipitation/potential evapotranspiration, P/PET, ratio = 0.46) (Fig. 1 ). The capillary barrier consisted of 1.22 m of clayey sand overlying 0.3 m of clean medium sand and was constructed on a 4:1 slope. The clayey sand used for the storage layer contained 30 to 40% fines (particles finer than the no. 200 sieve), making the soil nearly ideal for a storage layer. Despite having a storage layer thickness in excess of 1 m, the cover transmitted 63 mm yr–1, on average, during the 5-yr test period. In fact, the storage layer at this semiarid site probably needs to be at least 1.5 m thick to maintain drainage of <1 mm yr–1. In contrast, a similar but thinner capillary barrier (0.6 mm storage layer of nonplastic silt over sand) located in cooler subhumid Polson, MT (P = 380 mm, P/PET = 0.58) transmitted <1 mm of total percolation for the same test period.
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In summary, general inferences about suitable cover designs should not be made. Each cover must be designed on a site-specific basis with due consideration for the soils that are available and the climate in which the cover will function. Covers much thicker than 1 m may be needed at some sites, whereas covers thinner than 1 m may be acceptable at other sites.
Effect of Wetter Climate
As precipitation increases, the probability of net infiltration increases, particularly under conditions that simulate true climate change. The tests in Texas and New Mexico with excess irrigation water under conditions of dry soils in a dry climate should not be construed to be tests of true climate change and may not reflect actual barrier performance under a wetter climate. In wetter environments, potential ET conditions and water storage conditions would be markedly different. Wetter conditions usually occur with cooler weather, considerably less potential ET, and significantly less available water storage (because soils are wetter), thus increasing the probability of excessive drainage. Test under naturally wetter climate may be required for validation of ET-cover performance. For this reason, tests of the Hanford Barrier were performed both under irrigated (3 times the average precipitation) conditions at Richland, WA and under naturally wetter (3 times) conditions at Ogden, Utah (Gee et al., 1998; Albright et al., 2004). Albright et al. (2004) demonstrated conclusively that in wetter climates ET covers perform poorly. This is, in part, due to the lack of redundancy in the cover design, specifically the lack of a subsurface resistive layer to limit water intrusion into underlying wastes. The lack of redundancy is the Achilles heel of ET covers.
Practical Limitations of Capillary Barriers
Another factor to consider is that capillary barriers can be uneconomical and their performance somewhat unpredictable. The construction cost for a capillary barrier can be significantly higher than that for a conventional cover system constructed with geosynthetics because of the material quality requirements needed to produce a distinct capillary break. The higher costs of capillary barriers have been clearly demonstrated at the New Mexico site in papers cited by Scanlon et al. (2005). High costs of capillary barriers have been cited as at least one reason for considering ET-alternative soil covers (Madalinski et al., 2003). Costs of fill materials vary widely and are often tied to transportation costs. Processed coarse-grained materials used to form capillary breaks can be 5 to 10 times more expensive than on-site unprocessed fill materials used for the storage layer, particularly if they must be trucked to the job site. In addition, numerous reports in the literature show that capillary barriers when wetted sufficiently give rise to flow instabilities, making it difficult to predict travel times of contaminants to groundwater (Hillel, 1987; Steenhuis et al., 1991; Selker et al., 1992; DiCarlo et al., 1999; Walter et al., 2000). For sloping capillary barriers, flaws in construction give rise to points where water can accumulate and "finger" preferentially through the barrier, a condition that generally is not considered in conventional water balance modeling or design computations. Covers with slopes of 25% or more are common for solid waste landfills. Because all of the Texas and New Mexico tests were conducted on relatively flat surfaces, it is not known how construction flaws in sloping covers might affect performance of the capillary ET barriers under the tested climate. These uncertainties, coupled with added costs, can make ET covers with capillary barriers much less attractive than often considered. In addition, there are concerns about the long-term viability of capillary breaks due to factors such as biota intrusion and differential settlement, both of which are common at solid waste landfills.
Modeling and Plant Dynamics
Scanlon et al. (2005) pointed out that computer models can fail to capture the true dynamics of the ET cover system and that most models rely on empirical inputs such as root water uptake factors and crop or vegetation coefficients. We agree and suggest that unless the dynamics of the vegetation are known a priori, drainage predictions will be uncertain. For example, consider the water balance data shown in Fig. 2 , which are from a monolithic ET cover tested in semiarid Sacramento, CA (P = 434 mm, P/PET = 0.33). In three of the 5 yr of monitoring, the vegetation did not remove all of the stored water, resulting in large quantities of drainage (as much as 106 mm yr–1) during the following wet season (Albright et al., 2004). In contrast, model predictions made during design suggested that the vegetation would remove the stored water each growing season. A similar observation was reported by Benson et al. (2004, 2005) for the monolithic ET cover tested at the site near Altamont, CA that was mentioned previously (P = 358 mm, P/PET = 0.31). The vegetation on the cover did not remove the stored water after a wet winter, and a large quantity of percolation (64 mm) was transmitted during the following winter season. What is lacking in current water balance models is a way to know when to incorporate the plant dynamics or to assume typical phenology when such detailed and site specific data are lacking. Short-term changes in plant phenology, responsible for the large shifts in water storage in Sacramento and Altamont, were totally unexpected and currently cannot be predicted. We doubt that these changes would have been predicted by any of the models currently in development that consider dynamic changes in the vegetation in response to climatic variations. Fayer et al. (1992), Fayer and Gee (1997), and Tyler et al. (1999) also demonstrated that for landfill cover tests, differences in soil wetting and drying must be considered, so appropriate inclusion of soil hysteretic properties, along with plant parameters, may be necessary in the calibration process to accurately predict drainage rates. More research is needed on these topics before predictions can be made with confidence.
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From the cases we have cited, there should be sufficient concern by regulators and designers to suggest a cautious approach when permitting ET covers. This is not meant to imply that ET covers should be rejected outright by regulators or designers. In specific cases ET covers may function satisfactorily. However, care must be used in their design to ensure that site-specific conditions are properly considered. For ET covers that may be used in arid and semiarid settings, the assumptions and methods used during the design should be well documented, and the water storage capacity of the cover directly measured, in the field under actual wetter-climate conditions, to properly estimate safety factors and ensure long-term performance. Also, more effort is needed to calibrate and verify computer models before such model predictions can be accepted as reliable estimates of actual drainage from field-scale ET covers. Finally, soil properties, particularly those that affect water storage of ET covers, cannot be ignored. This may require accounting for soil hysteresis because of the periodic wetting and drying of field soils.
REFERENCES
This article has been cited by other articles:
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  B. R. Scanlon, R. C. Reedy, K. E. Dwyer, and S. F. Dwyer Response to "Comments on 'Evaluation of Evapotranspirative Covers for Waste Containment in Arid and Semiarid Regions in the Southwestern USA'" Vadose Zone J., May 26, 2006; 5(2): 813 - 814. [Full Text] [PDF]
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