The operator of a local landfill is offering the city a 266-acre conservation easement to offset its displacing about two acres of wetlands and vernal pools as part of a major expansion of the waste disposal area. Mitigating the effects on wetlands and vernal pools is a required part of the plan to double the capacity of Juniper Ridge Landfill , and Casella Waste Systems is prepared to create a conservation easement near the landfill, officials say.