The city will hold a groundbreaking ceremony to mark the start of construction of a new park on a former factory site at 45 Walnut St., at 3 p.m. Wednesday. The city recently awarded a $1.01 million contract to T Ford Co. of Georgetown for the project. The first phase of work, set to take about eight weeks, will involve placement of a protective cap over the property to prevent soil contaminants from migrating to the surface, according to Brendan Callahan, the city’s senior planner. The second and final phase will involve creating the park, which will feature an entry plaza with benches, a large grassy area, new trees and bushes, a perimeter walkway, and an elevated boardwalk connecting the walkway to Wallis Street. A wetlands area also will be restored. The 1.3-acre site was acquired by the city in 2009. The property was the site of a tannery from 1917 until the 1940s, and it was then used by New England Sportswear, a manufacturer of leather and suede apparel, until 1990. It has been vacant and unused since. The overall project, which is being funded through state and federal grants, is scheduled for completion in November.