June 2011

A Natural Harmony

Conventional RCRA Subtitle D landfill covers include a composite hydraulic barrier in the cover system. However, many landfills use an alternative water balance cover that employs water storage mechanisms instead of a low-conductivity barrier.

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Native vegetation, most notably prairie grass, offers an appropriate and cost-effective choice for any landfill cover system that includes a vegetated surface layer, including those with a plastic membrane and those that have a water balance cover.

The authors believe there are opportunities to save money and create long-term environmental benefits at many sites by considering the application of water balance covers and the appropriate native vegetation for stability and water management.

The purpose of this article is twofold: 1) to help the reader better understand the applicability of native vegetation on nearly all landfill covers, and 2) to understand the applicability of basic water balance covers. These are separate subjects with considerable overlap and significant potential for landfills throughout North America.

Native Vegetation: Addressing Misconceptions
The benefits of native vegetative cover systems are significant enough that one might assume they would have been adopted nearly universally. However, significant misunderstandings and a lack of native vegetation expertise within the landfill-design community have limited their use. Some of the common misconceptions are discussed below.

Misconception 1: The roots of native grasses go so deep that they will penetrate a landfill cover system that includes plastic layer and/or compacted clay, thus compromising the performance/integrity of the cover system.

Technical findings from scientific studies—Most studies evaluating different plant species (trees, shrubs, grasses, wildflowers, etc.) in landfill caps with active outgassing and gas collection systems have found that roots do not penetrate soils with high methane concentrations. Other studies have shown roots to penetrate compacted clay barriers, which develop preferential flow paths due to natural processes including wet-dry cycles, freeze-thaw cycles, and biointrusion by plant roots and burrowing fauna.

Misconception 2: A landfill cover of native vegetation must be burned periodically, which is incompatible with a landfill’s gas extraction system because of fire hazard and damage to external infrastructure.

Technical findings from scientific studies—Although we understand the Illinois EPA is contracting prescribed burns for burning prairie vegetation on several landfill caps—and we know it can be done safely without damage to infrastructure—it’s not necessary to burn native vegetation systems planted on caps. In fact, simple mowing is enough to allow for the annual inspection needs of a cap and also to remove the buildup of litter and duff.

Misconception 3: Native seed mixes are difficult to specify correctly for local conditions, are overly expensive compared with a regular highway mix, and are difficult to establish.

Technical findings from scientific studies—The initial purchase of native seed mixes may be slightly more expensive than your typical highway mix, but over time these plants are substantially lower maintenance and have far better soil holding capacity. Erosion risks and costs for slope repair as a result of failures and erosion are greatly reduced. And, of course, you can always develop native species mixes with cost-conscious selection of species that would be priced comparable to highway mixes.

Misconception 4: Regulators in our state have never been supportive of native vegetative covers.

Technical findings from scientific studies—Regulators typically have been very supportive of native vegetation covers. The problem has been that the design community hasn’t proposed native vegetation covers, so there aren’t many good examples. Most design-engineering firms focus very little on the vegetation. It’s often their last concern because it is not their area of expertise. So they use an accepted mix pre-specified for highway use, simply because they and the agencies are familiar with that.

Misconception 5: Such burrowing animals as groundhogs and foxes are particularly attracted to native vegetative covers, resulting in greater problems and costs than less-hospitable landfill cover of cool season grasses.

Technical findings from scientific studies—Burrowing animals are just as attracted to cool-season highway mix covers as they are native vegetation. In fact, in many locations, the very palatable clover and cool season grasses in a typical highway mix are more desirable to nuisance wildlife than native species. And really, in both situations, such burrowing animals represent a very manageable risk.

Misconception 6: Whatever will ultimately grow on a landfill cover will grow. Whether it is native or exotic is up to nature, and nature will figure out what works best on its own.

Technical findings from scientific studies—Plant species will grow if given the proper conditions. To establish a native vegetation cover, you need to understand the species and the timing of when germination best works. Far too many attempts to establish native species on landfills have been seeded way too late in the growing season. From Illinois to the West, on the high and dry environment of a landfill, it’s imperative that seeding be done significantly earlier in the year to take advantage of spring moisture. Otherwise what you’ll get are invasive, noxious weeds and less than successful plantings of natives.

Misconception 7: There are no construction cost savings for a native vegetation cover system, nor are there savings in the type or amount of soils used in the cover system.

Technical findings from scientific studies—Construction costs for water balance covers are typically much less than for conventional clay caps. Clay must often be hauled long distances and must also be heavily compacted in thin lifts and at a water content that is optimal for compaction. By contrast, soil for water balance covers is most often locally available and is usually placed in thicker lifts with much less compactive effort. Maintenance cost savings are realized due to the relative ease in repairing areas of differential settlement, and due to increased slope stability and reduced erosion. Several studies have shown that successful revegetation of landfills and mine-waste sites can often be achieved utilizing relatively thin layers of topsoil and soils with a wide range of textures and chemistries. When drainage into the waste is an important criterion, soil water storage capacity is critical, and the cover soils and thickness must be appropriate.

A Look at the Real Story of Native Vegetation
Native vegetation covers are compatible with nearly all landfill cover systems, often save construction costs, certainly save long-term maintenance costs, enhance the appearance of an unnatural landform, diversify the ecology of the property and are often much appreciated by neighbors.

Native vegetative covers may not be common in our landfill industry because of the above misconceptions, a lack of experience with them by landfill designers and managers, and a lack of expertise by engineers tasked to specify landfill cover vegetation. Expertise is a significant component of specifying a native vegetation cover because native species are not as opportunistic and fast-growing as exotic weedy and highway mix species. But these preferred native species could be easily well established by ecological consultants and landscape designers who understand their germination and management requirements.

When used in combination with the special capping designs and techniques of a water balance cover system, such an alternative landfill cap can provide multiple, long-term benefits to the landfill owner/operator.

Water Balance Covers: Potential for Expansion
Water balance landfill cover systems have been used throughout North America where the performance criterion for such a system is equivalence to a compacted clay cover system in terms of water flux through the system. These systems are also referred to as evapotranspiration covers, or ETs.

The big difference in this alternative style of cap is that water balance caps are designed to hold and evaporate rainwater through plants growing on the cap, thereby transferring the water directly back to the atmosphere. The amount and type of substrate and rooting medium is designed to provide the storage capacity to hold the rainfall events and prevent excessive drainage. There is enough soil and enough of an evaporative draw by native plants, sunlight heating, and internal heat generated from within the landfill to return the stored moisture to the atmosphere.

This strategy has potential to produce a predetermined quantity of percolation—including design criteria that would produce no percolation at some sites—and a water balance system doesn’t need a hermetically sealed cap with associated management and regulatory costs.

Water balance landfill covers have been constructed in dry as well as humid regions of the country, with most being used in the drier climates. As our industry explores the future of landfills, the effort warrants a thorough look at the water balance issue for environmental and cost-saving reasons.

Evaluating Alternatives
There is cause to re-evaluate these water balance covers as landfill owners consider bioreactor landfills, landfill stabilization plans, and the merits of native vegetation. While the hydraulic barrier in traditional designs is intended to exclude all drainage, water balance covers can be designed as a control valve intended to allow a prescribed drainage.

Should we ask: What are the very long-term benefits/problems of having waste covered with the geomembrane-based covers now being used? Would a ‘degradable plastic’ layer, in conjunction with a water balance cover, make sense? Should a geomembrane layer be used, if our goal is to stabilize the waste in a landfill?

The application of water balance covers is limited to sites with appropriate performance criteria and where regulations and regulators will consider a cover system without a geomembrane. When the regulatory evaluation process emphasizes a particular material parameter instead of performance, then a water balance cover is not an approvable alternative cover. Water balance covers are achievable only where there are appropriate and clearly understood performance goals, without prescriptions tailored to a specific material.

Water balance covers allow more drainage in wetter climates (they have been installed in places like Iowa and Georgia) but in these areas they are applied to achieve a certain performance criteria. The main design criterion of a water balance cover consists of matching the water storage capacity of the cover to the storage requirements and performance criteria for the site.

The soil storage capacity is a function of the unsaturated (water storage) characteristics of the soil and the soil thickness of the cover. Very coarse soils and very fine, clayey soils often have poor water storage properties. Loamy soils typically are a better choice for both water storage properties and vegetative success.

The specific soils for a cover project should be analyzed by a laboratory procedure such as that described in ASTMD 6836 (Standard Test Methods for Determination of the Soil Water Characteristic Curve for Desorption). Simple calculations for a preliminary determination of required and available storage can be found in “Water Balance Covers for Waste Containment: Principles and Practice” (Albright et al, 2010). Specific aspects of a cover design and any required sensitivity analysis should be evaluated using a numerical model that uses the Richards equation to determine movement of water in the cover. The HELP model, while adequate for conventional covers, is not appropriate for application to water balance covers.

Design costs for a water balance cover can be more than for a conventional cover, due to the more complex system that controls the water balance. Major construction costs depend on the availability of suitable soil, hauling distances, soil placement, and revegetation with the design plant community. However, water balance covers are often far less expensive to construct than conventional designs, due to the costs avoided in placing compacted clay and geomembranes and the additional QA/QC costs associated with clay and membrane placement.

In Conclusion
To ensure stability and functionality of the water balance cap (and, for that matter, all landfill caps), care must be taken to match the characteristics of the specific plant community to the site, climate, and soil and water management properties. Prairie species from Kansas, for example, may not thrive in the wetter climes of Michigan. And native seed propagated in Wisconsin may not do well in eastern Washington. But if local species and regional genetics are selected for the vegetation cover—and if they are species adapted to the high-and-dry conditions on an elevated landfill cap—long-term stability and erosion protection will follow.

Looking ahead, as the cost of oil and overall capping operations increases, and as our societal environmental goals continue to evolve with more depth and breadth, landfill owners who are willing to seek alternative capping solutions will find multiple benefits in the application of water balance caps and native plant covers. Increasingly, higher costs for construction and maintenance of artificial liners will make water balance/native vegetation covers very attractive in more ways that just aesthetics.

Author's Bio: William H. Albright, Ph.D., is research hydrogeologist for the Desert Research Institute.

Author's Bio: Steve Apfelbaum is owner and principal ecologist with Applied Ecological Services Inc.