Weathering The Cover Design

Lara McDonald

Your landfill's location can affect its regulatory obligations, according to Subtitle D which recognizes that regional variations may require certain facilities to deviate from its prescribed closure cover design.

Typically, landfill owners construct clay liners to create an impervious barrier to surface moisture infiltration. However, these clay layers are prone to drying and cracking - which encourage, rather than prevent, moisture infiltration.

Alternative covers typically provide a non-erosive path to surface moisture runoff and also act as a monolithic containment structure for infiltration-related moisture. In climates with minimal rainfall, the cover can retain moisture and then respire it into the atmosphere, rather than allowing it to leak into the waste below. This method of moisture management requires that the cover maintain an unsaturated profile, blocking breakthrough and enhancing vegetative respiration appropriate to seasonal and daily temperature variations.

In most cases, an alternative cover design must be qualified with a design study that closely approximates local conditions. Although moisture infiltration is a critical consideration, few instruments are capable of providing easy-to-use, reliable moisture data, acquired in profile, with minimal site impact. One new technology is based on time domain reflectometry (TDR).

TDR data acquisition system configurations measure the contiguous soil moisture profiles necessary to monitor the unsaturated flow. TDR's features include minimal calibration sensitivity to soil chemistry variations, and predictable responses to variations in soil conductivity, chemistry and permeability.

The U.S. Environmental Protection Agency generally has left the evaluation of alternative designs to state agencies, few of whom have monitored - and subsequently approved - these designs. However, California and Arizona have several studies in the works and have approved at least one alternative final cover.

A landfill closed with a Subtitle D cover might be a candidate for alternative cover approval, which would reduce long-term closure maintenance costs. In such cases, a closure permit modification plan, coupled with a moisture infiltration monitoring study, may result in significant savings in planned, long-term monitoring programs.

Smaller landfills may find relief from constructing a Subtitle D cover by developing a closure plan that includes two-year, post-closure monitoring of a final alternative cover that is approved on an interim basis. Here, the preclosure study cost may be eliminated for a landfill operator who cannot afford this study or a Subtitle D cover.

In general, states in regions that climatically afford an alternative cover option are showing the necessary flexibility by enforcing the intent of Subtitle D, rather than the prescribed solution.