The Walsh Construction Co., under a contract awarded by the Massachusetts Water Resources Authority, this month is beginning construction of a 20 million gallon water storage facility and pump station in Stoneham. The underground concrete tanks are being constructed behind the site of the former Boston Regional Medical Center on Woodland Road. When completed, the tanks will provide storage for six communities, including Chelsea, Malden, Medford, Everett, and Somerville, and backup for 21 communities, including Stoneham, Wakefield, and Winchester, in case of an emergency. During construction, the north entrance to the medical center campus on Woodland Road will be open for use by construction vehicles only. The $49.3 million project is expected to be completed in the fall of 2014.