EPA RESPONSE TO GARETH DAVIES COMMENTS ON THE NEAL'S LANDFILL STABILITY EVALUATION
Aug 18, 1999

Cover Letter from Tom Alcamp, EPA

1.Section l. l Site Background.
Mr. Davies states that waste composition and hot spot characterization to determine the final remedy for the site have "little statistical confidence." The Responsiveness Summary previously responded to the same comment when it was made on the proposed plan for source control at the site. To reiterate, the most important factors to consider during a field investigation, aimed at characterizing hot spots in a landfill that will receive a final cap, are the practical aspects of a field investigation, previous waste disposal practices, and generally accepted industry practices to establish a grid spacing over which to collect soil and waste samples. Neal's Landfill contains large quantities of municipal waste mixed with industrial waste. The U.S. EPA placed borings at 100-foot intervals and in some locations at 50-foot intervals. This type of sampling scheme for hot spots is very unusual for sites like Neal's Landfill. For Mr.Davies to state that the site was "clearly not representatively sampled" shows his lack of understanding how the Agency addresses risk and risk reduction at sites containing municipal and industrial waste.

After hot spots are characterized and removed, a final low-permeability cap will be placed over the landfill to prevent direct contact with wastes and minimize continued contaminant leaching to groundwater and surface water. Therefore, the source control remedy, along with future water treatment, sediment removal, and long-term monitoring, will be protective of human health and the environment.

Section 1.2 Objective of this Technical Memorandum.
Mr. Davies states that modification of the landfill surface "might have a deleterious effect" on subsurface groundwater flow. He does not explain his basis for this comment or explain what "deleterious effect" means. CBS has proposed modification of the landfill surface specifically to prevent deleterious effects to the landfill resulting prom and surface water flow. The final remedy proposed by CBS includes (1) removing waste contaminated with polychlorinated biphenyls (PCB) from areas of the site susceptible to backflooding and consolidating the waste under a cap in another area of the site less prone to backflooding, (2) controlling surface water runon and runoff, and (3) allowing the groundwater regime to largely return to its pre-landfill configuration.. These actions are not likely to produce "deleterious" effects. If Mr. Davies refers to the prevention of groundwater infiltration by the presence of the new cap, this effect cannot be considered harmful. In addition, although the existing cap inhibits groundwater recharge, water that can infiltrate the cap could contact PCB-containing waste, thereby contaminating downgradient surface water. The new cap should minimize this harmful situation without significantly affecting the flow of groundwater.

3.Section 3. l . l Native Clay Layer.
Mr. Davies states that soil collapse is the most likely scenario for sinkhole formation at this site and that construction operations should be planned with caution. This issue is addressed in Section 4.3 of the technical memorandum which states that although no guarantees can be made, such a collapse would pose no significant risk of damage to the environment. The collapse of a soil void would probably not result in waste contacting groundwater,which is primarily located beneath the soil-bedrock interface. Further, engineering calculations in the technical memorandum show that collapse of a void whose volume is as large as 75 percent of the total thickness of the overburden may not cause failure of the proposed cap. If such voids exist, they would either be detected during the movement of large machinery across the landfill (which would cause greater loading than expected at any time in the future) or stabilized because the cap would deprive them of the infiltrating water needed for their continued expansion.

4.Section 4 Geological Evaluation.
Mr. Davies points out correctly that CBS has not used the most recent references available on cave formation and conduit development. However, Mr. Davies' conclusion that cave formation and conduit development occur primarily below rather than at the water table would actually indicate reduced risk of bedrock collapse at the site. The deeper the cave or conduit, the greater the bedrock thickness between the ground surface and the conduit or cave, and the greater the thickness of rock, the more stable the site.

Mr. Davies goes on to state that the variable discharge at South Spring is evidence that South Spring by itself is not an adequate monitoring point because the spring may be an overflow feature. South Spring is related to North Spring, which is loNver in elevation and will form part of the monitoring network.

The void size used in the discussion of potential bedrock collapse is only theoretical because all voids encountered during site exploration were significantly smaller. Mr. Davies implies that the assumed void size is too small but does note suggest a more realistic size. The void encountered during the drilling of monitoring well MOO-3 was used as tl1c basis for a worst-case estimate based on the assumption that the vow represents the pathway for the entire volume of groundwater exiting the system. As Mr. Davies points out, this does not imply that the void encountered in MOO-3 is the only conduit pathway beneath the site but rather that if the conduit were as close to the surface as the void in MOO-3, the conduit may still pose no risk of bedrock collapse even under maximum historically observed Groundwater flow conditions. The technical memorandum is clear on this issue.

The low probability of hitting voids during drilling in karst terrain is well known and is the reason that any karst stability analysis must be based on certain assumptions made in light of information known about the site. Although not as much site-specific data are available as desired, assumptions about void size are based on (l) observations made during drilling (MOO-3 reflects the worst conditions found), (2) the fact that no large anomalies were observed during geophysical surveys of the site, and (3) flow volumes of the springs that discharge water from the system. The possible presence of other conduits cannot be denied, but such a possibility is irrelevant to the central argument -- that even if such conduits exist they are unlikely to collapse because of the overarching limestone. Use of Dr. White's charts for calculating the thickness of bedrock above the conduit necessary to maintain stability is certainly appropriate in the technical memorandum, although questions may remain regarding the quality of the bedrock that arches over the conduit (such as whether bedrock is fractured or unfractured).

Mr. Davies states that human activity at the Neal's Landfill site might contribute to soil collapse. This concern is certainly valid and led the U.S. EPA to request CB~i to evaluate the proposed remedy with regard to potential sinkhole development. The proposed final remedy therefore includes measures such as an impermeable cap, surface water controls, and continued site monitoring. All construction in karst areas should include efforts to control the infiltration of surface water because water is required for further development of karst features. To reduce the risk of soil collapse, buildings are built with roof drains directed away from their foundations. To increase the stability of a large area, an impermeable cap is often placed to prevent void development through soil piping. Both of these measures are being implemented at the Neal's Landfill site, with long-term site monitoring to allow early detection of subsidence or contaminant migration.

In his summary, Mr. Davies expresses concern that the landfill waste was not evaluated to determine its consolidation properties. Site-specific data would be ideal; however, CBS has used conservative consolidation assumptions to estimate the consolidation of the waste under the loads to be borne at this site. Tetra Tech's evaluation of engineering assumptions used to prepare the technical memorandum shows these assumptions to be generally consistent with industry practice.