The technique, which uses water balance covers, offers a more affordable option to smaller communities and operators trying to meet environmental regulations when closing landfills or other contaminated sites.
Rather than the traditional method of covering obsolete landfills or other waste sites with layers of dense clay, plastic liners and dirt fill, water balance covers are a more natural approach that uses evapotranspiration to do the job. Evapotranspiration returns water to the atmosphere through a combination of normal evaporation and transpiration through vegetation.
The technique uses an earthen cover of fine-textured soils that have good absorption capacity topped by native vegetation.
How Water Balance Covers Work
Precipitation soaks the finer soil layer, then evaporates or gets absorbed by plants before reaching the contaminated level below. That natural process — precipitation, evaporation and plant absorption — provides the protective layer.
Water balance covers can be used wherever the rate of evaporation exceeds precipitation. That semi-arid climate exists in the western Great Plains, the Rocky Mountains and the desert Southwest.
While it works great in semi-arid climates, it’s not appropriate for damper climates because more rain falls in these areas than evaporates or transpires back to the atmosphere.
Colorado Leads the Way
Because of its experience working with a massive site cleanup during the past two decades, Colorado has become a pioneer in using water balance covers. The Rocky Mountain Arsenal Superfund site near Denver was one of the first to use this technique on roughly 730 acres of contaminated property.
It took about 10 years to develop, test and implement the cover at the former defense waste site, but once Colorado officials became comfortable with the process, the Department of Public Health and Environmental Hazardous Materials and Waste Management Division developed guidelines to allow water balance covers to be used around the state.
Those guidelines have made it easier for smaller landfill operators to close filled landfill cells at a lower cost while still meeting the standards set by the federal Environmental Protection Agency through the Resource, Conservation and Recovery Act (RCRA).
The Colorado guidelines require users to test their cover soil to obtain the percentages of sand, silt and clay, and plot the results on a U.S. Department of Agriculture textural triangle diagram to obtain the minimum required water balance cover thickness, as shown in the figure to the right. (see figure to the right).
Landfill operators in Colorado can save about 20 percent over the cost of using conventional covers. It’s a win-win for rural communities strapped with tight budgets.
Benefits for Other Western States
The guidelines in Colorado replace the costly sampling and computer modeling required in other Western states where water balance covers could be an attractive alternative.
Colorado allows operators to use a wide range of fine-grained soil types for the cover; other states require using the exact soil type that was used in the modeling and testing at the specific landfill site for the water balance cover. As a result, the process takes longer and is more costly.
Instead of adopting guidelines similar to Colorado’s, other states review the concept on a case-by-case basis for each cover soil type. This can be problematic if the cover soil types vary.
Using Colorado’s straightforward, quick and less expensive approach to installing water balance covers as a model, I see a lot of potential for other states in the region to adopt this concept. Wyoming, for example — a state with similar geography and climate — is exploring the possibility of adopting similar guidelines. Do you have experience working with water balance covers? If so, I’d love to hear about your experience. If this sounds like something that would be beneficial to your community, or if adopting a statewide policy similar to Colorado is appealing to you, feel free to contact me at firstname.lastname@example.org.
Brad Coleman is an environmental and civil engineer with more than 29 years of experience, much of it in Colorado. He specializes in site closures and redevelopment, site remediation, environmental operations, regulator negotiation and financial analysis.
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