The rehabilitation and replacement of a major water pipeline in Lower Mills enters its toughest phase on Monday, Aug. 1. That phase includes shutting down a portion of Washington Street where Jersey barriers will go up at the intersection of Dorchester Ave. and Adams Street, according to the Massachusetts Water Resources Authority, which is overseeing the project.

Barriers will also be placed on Washington Street at River Street, leaving that section of the roadway closed to traffic from Monday mornings through early Thursday evenings. Normal traffic flow is expected Fridays through Sundays, extending into September.

According to the MWRA, “Residents parked within the parking area alongside Spuckies Pizza and in behind will be able to exit and enter the parking area at all times. Movable barriers will be in position to allow for this 24 hours a day. When we begin on Monday, vehicles will have to exit taking a left towards the River Street side and the entrance to gain access to the parking area will be on that side as well. At some point this will be shifted in the other direction and everyone will be notified.”

The entire $14.5 million project, which started in April and is expected to last until November, also includes the installation of a 24-inch pipe at the entrance of the Baker Chocolate Factory condominiums occurring at the same time.

Constructed in the early 1900s, the pipeline distributes water for Milton, Quincy, and the southeast corner of Boston.

According to the MWRA, some sections of the pipeline are old and functioning at 50 percent capacity because of rust build-up and pocking along the pipeline’s walls. The agency says the project will extend the lifespan of the pipeline by 50 years and provide a back-up in case of a major water main break.