A 15-year case launched to protect Plymouth’s Eel River appears to have come to an end, after the plaintiffs missed a deadline to appeal a state court ruling allowing a permit for the town’s sewer treatment plant to stand.

The suit was originally filed in 1997 by the Eel River Watershed Association to force state environmental regulators to take stronger action against ground-water pollution from the new sewer treatment plant the town of Plymouth was building within the watershed.