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Karst survey for proposed 10 Mile Creek solar site found three shallow subsidence features; KCMP to be filed with PSC application
Summary
A Terracon geologist told the board his karst survey found three small subsidence sinks on the proposed 10 Mile Creek solar site; he recommended avoidance, monitoring and remediation measures and said a Karst Construction Management Plan will accompany the project's Public Service Commission filing.
Bob Denton, a senior geologist with Terracon, presented a karst assessment to the County Board and said his desktop and field work for the proposed 10 Mile Creek solar project initially flagged 6—7 suspect features but field verification identified three real, shallow subsidence sinks. "We found 3 real features... very small, very shallow," Denton said, and he told the board that two features in a wet parcel and a third in a different parcel will be avoided in project planning.
Denton explained the karst assessment protocol Terracon used: detailed desktop review (lidar, cave and spring/well data within 1,000 feet, DEMs), field verification, risk and impact assessment, and then avoidance/minimization and remediation measures. He described a five-factor risk-ranking system (parapet shape, open throat, soil raveling, drainage, vegetation) and said the primary karst resource is the aquifer. "Avoidance is the best approach. Minimizing the impact is the second best. Remediate if you have to," Denton said.
On groundwater concerns, Denton and supervisors discussed local nitrate and coliform test results. Denton attributed many nitrate detections to agriculture (manure, fertilizer) and urged careful control of concentrated feed areas and livestock stream access. Supervisors stressed the regional reach of subsurface flows and warned that impacts could affect tens of thousands of residents who rely on shared aquifers.
Denton outlined construction and mitigation approaches the KCMP will include: targeted geophysics and additional borings in a detailed site investigation, grouting/bentonite mixes to seal detected voids where piles intersect voids, graded-filter remediation for sinkholes, and an operations-and-maintenance schedule that can include periodic resurveying and decommissioning requirements. Denton said the KCMP and the detailed investigations will be submitted as part of the project's application to the Public Service Commission.
The board did not vote on the project during the meeting; Denton's survey findings and the county-level discussion will be part of the public record moving into the PSC process.
