Tilcon wants to expand its quarry into 72 acres of New Britain-owned watershed. The land is environmentally sensitive and protected by state law, but the water department contends that the benefits of Tilcon’s plan could outweigh the destruction of forest, vernal pools and animal habitats. Tilcon wants to lease the land from New Britain for 35 to 40 years. Nobody is saying how much the company would pay, but it offered $15 million for a roughly similar proposal 11 years ago. In addition, Tilcon would give New Britain, Plainville and Southington a total of 291 acres as open space.