Excavation. If during site plan review the Board finds any construction, filling or excavation in excess of 300 square feet, or grading or other alteration of more than 3,000 square feet, on a lot within the proceeding three-year period, the Planning Board shall ensure that such construction, filling, excavation, grading or alteration meets the buffer requirements provided below and will not result in erosion or in wetland, stream or lake pollution from surface or subsurface runoff, before it shall grant approval.