Town committee asks EPA/consultant to address bentonite wall design, requests monitoring details

Town of Concord 2229 Main Street Site Committee members asked the U.S. Environmental Protection Agency and the site consultant to provide clearer answers and monitoring plans for a proposed bentonite wall described in the EPA 60% remedial design package and Maximus’ December 2 response to EPA comments.

Committee members said the main question is whether the bentonite or other wall mix above the water table could crack or dry out over time and whether the design and monitoring would detect and address loss of integrity. Ray (committee member) agreed to draft formal comments and circulate them to the committee for submission to town staff by the deadline noted in the meeting materials.

The committee’s discussion focused on three technical points: the potential for bentonite and related materials to dry and crack above the water table, the proposed change in embedment depth into bedrock (from five feet to three feet in the 60% design), and what kinds of monitoring could confirm long-term integrity if cracking occurred. Several members said they have seen bentonite or concrete desiccate in monitoring wells over decades and asked that the consultant explain how the selected mix addresses that risk.

Committee members noted that Maximus' December 2 response to EPA’s October 2 letter addresses the wall mix design but that the committee had not yet reviewed that response in detail. Members recommended asking Maximus and EPA to show how they will monitor wall integrity (the committee discussed noninvasive monitoring ideas such as remote sensing or radio-frequency methods, but did not recommend a specific technology). They agreed to ask for an answer before or as part of the 95% design submission rather than waiting until final design.

Pam (committee member) and others emphasized that the committee’s role is to raise technical questions and request further explanation rather than prescribe specific engineering solutions. Kevin (committee member) noted that the concern is greater above the water table; below the water table the risk of desiccation is lower. Len (committee member) and Fred (committee member) described prior field experience that led to the concern.

Ray said he would draft a short comment letter capturing the committee’s concerns about the wall mix and the change in bedrock embedment depth, circulate it to committee members for review, and submit it to town staff to forward to Maximus and EPA in time for the January/February comment window. The committee agreed to keep the question active and to expect a response in subsequent design submittals (the 95% design was cited as the next major submittal).