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Staff and consultants say expansion will use modern liners and sentinel well monitoring; groundwater concerns and PFAS remain focal points for commissioners
Summary
Hydrogeologists told the planning commission the proposed expansion will use a composite liner and added leachate controls; the applicant agreed to pre‑development hydrogeologic study and installation of sentinel wells for routine monitoring.
Hydrogeology, groundwater and leachate chemistry were recurring technical concerns in public testimony and at the hearing. New testimony from the project’s hydrogeologic and environmental consultants outlined the following points: the proposed expansion would use a modern composite liner system with leachate collection; a focused hydrogeologic investigation and a network of sentinel wells (sentry wells) will be installed before development of the expansion area; and routine semiannual groundwater monitoring and reporting are proposed to detect any change in groundwater quality.
Consultant John Howard (Geologic Associates) told the commission there is no documented leachate release to groundwater from the lined portions of the site in the last 30 years and that the proposed design should be an improvement over existing conditions because the new liner is more protective and provides additional leachate monitoring capability. He also told commissioners that arsenic detected in groundwater in the area is a naturally occurring condition linked to dissolved oxygen levels in the aquifer; in the consultants’ view a change in groundwater arsenic attributable to construction of the new lined cell is not expected.
Republic's team agreed to install sentinel wells around the expansion area and to include those wells in an ongoing monitoring network. The team proposed a more detailed hydrogeologic baseline study before development and — in response to commissioner questions — indicated willingness to accept a condition that the study and sentry well installation occur prior to construction activities.
On leachate chemistry, consultants said Coffin Butte leachate is similar to that of other municipal solid waste landfills and that leachate sent to the Corvallis wastewater treatment plant has not produced detectable PFAS in the plant’s downstream drinking water (as reported by the city). Consultants also corrected a public statement about current leachate volumes: recent operational figures indicate annual leachate volumes lower than some public assertions (the consultants projected about 1.2 million gallons for the current year, not the 2 million gallons cited in some comments).
Commissioner questions included: whether blasting for excavation could affect local groundwater pathways (consultants said modeled blast fracture influence is conservatively about a 15‑foot radius per borehole), how monitoring would be structured and scheduled, and what steps would follow if monitoring showed an effect on private wells. Consultants said monitoring design would include pumping and aquifer tests where appropriate, and that the applicant would work with nearby residents if the monitoring network suggested off‑site impacts.
No final permit decision was made; the commission requested that the detailed hydrogeologic investigation be a pre‑development condition.
Provenance: Geologic Associates and Republic environmental staff presented groundwater findings and committed to sentinel well monitoring and a focused hydrogeologic investigation.
