Executive Summary:

"Review of Sandia National Laboratories Evapotranspiration Cap Closure Plans for the Mixed Waste Landfill" - by Tom Hakonson, Ph.D., Environmental Evaluation Services, LLC.

Conclusions:

" Buried waste can be mobilized to the ground surface through plant roots and animals and insect burrowing can dramatically increase infiltration of water into landfill with covers as thick as those proposed;
" Vertical transport of contaminants to the ground surface by biota may be small on a short time scale, but over many decades these processes may become dominant in mobilizing buried waste;
" The long-term consequences of biointrusion into low level waste landfills located in arid areas estimated that doses to humans resulting from biological transport were as high as doses calculated from a human intrusion scenario (Pacific Northwest Laboratory);
" One of the more important deficiencies in Sandia National Lab's (SNL) closure plan proposed for the MWL is the assumption that vertical and horizontal transport of contaminants resulting from biological processes is not an important contributor to exposure pathways;
" Both cap designs (Dwyer et, al., SNL Environmental Restoration group) do a credible job of analyzing the evapotranspiration (ET) cover, and in the reviewer's opinion both cap designs will provide adequate protection of contaminants to ground water assuming the site is diligently monitored and maintained throughout the post closure monitoring period while assuming the surface pathway proves to be unimportant in contributing doses to humans;
" Under the right conditions the roots of ALL types of vegetation have the ability to extend several meters into the soil and transport contaminants to the surface.
" While an ET cap can minimize soil moisture it can contribute to vapor phase transport of volatiles;
" SNL's conclusion that waste has not been mobilized to the ground surface by animals is poorly supported as it is: 1) based on soil samples taken (in part) from areas of landfill recently backfilled; 2) sampling was coarse in resolution; 3) samples were non-random in space; and 4) samples purposely did not include disturbed areas created by burrowing animals;
" Once contaminants are transported to ground surface a complex distribution process occurs that can result in widespread transport of contaminants across the landfill surface to offsite areas;
" Human intrusion scenarios should take a conservative approach such as the loss of institutional controls under a subsistence farmer scenario;
" Changes in climate can radically affect the integrity of cap;
" SNL's proposed plan to use a neutron moisture gage (NMG) are vague on how the monitoring data will be used to conclude that percolation is or is not occurring. NMG is labor intensive (data must be downloaded and managed); NMG must be calibrated to soil (difficult when layered soils are involved); reliable measurements are limited to volumetric water contents above 5%; NMG integrates moisture content over a relatively large area making it difficult to pinpoint the specific zone depth being interrogated; NMG provides instantaneous estimates of soil moisture so that measuring after precipitation is critical; NMG should not be used as an early warning system (see page 50 for detailed review).
" Little or no planning has been done on the post-closure phase of Mixed Waste Landfill (MWL) closure and there is no contingency plan should the ET cap not perform as predicted;

Recommendations:

1. Any post closure plan should provide measurements on all possible migration pathways that include vadose zone transport, soil sampling for surface contamination, and biological transport;
2. Soil surveys should be required in undisturbed areas closed early in the landfill operation with a comprehensive long-term sampling program after MWL is closed consisting of sampling of surface soils and biota;
3. A comprehensive sampling plan should be required that reflects the inventory of contaminants in landfill, not just tritium;
4. The use of biointrusion barriers to keep animals from burrowing into landfills has had mixed reviews in terms of effectiveness; a wire mesh type barrier proposed by Dwyer et.al. is the best choice for the MWL although it will not keep ants and other insects from burrowing into the landfill;
5. A contingency plan should be developed and incorporated into the post closure plan in the event the cap fails and/or contaminants are found to be migrating;
6. A financial assurance mechanism should be established for monitoring, maintenance, and contingency costs based on NMED requirements for closure and post-closure plans under RCRA: a) evaluation of the effectiveness of the cover at specific time intervals; b) monitoring; c) remedial measures that include excavation and removal of landfill contents should a significant problem be revealed through monitoring;