Cast iron and steel pipes and valves integral to the tunnel system that provides 60 percent of the water to eastern Massachusetts communities served by the Massachusetts Water Resources Authority are in such poor condition that the system’s overseers have avoided engaging them due to fears of a catastrophic failure.

Authority officials have been aware of the system’s problems for years and now appear poised to try to reach agreement this year or in early 2017 on long-term plans to create redundant water flow systems that would enable them to address the infrastructure vulnerabilities associated with tunnel networks that are 46, 53, and 66 years old and run beneath communities just north and south of downtown Boston.

“They need to be maintained and right now we have no ability to bring the tunnels down to maintain them,’’ MWRA Executive Director Fred Laskey told the News Service in an interview Wednesday afternoon. “If something went wrong and there was a leak that developed, it would be catastrophic.’’

Among the big questions before the authority board, which plans to focus on the problem at a special meeting on Thursday, are the details of a plan to create a redundant water tunnel system, the timeline for getting that system in place, the costs associated with the work, and the impacts on customer rates and affected communities.

The system’s three Metropolitan Tunnels are the 5.4-mile City Tunnel, built in 1950 and running from Weston under Newton to Chestnut Hill; the 7-mile City Tunnel Extension, built in 1963 and running under Brighton, Cambridge, Somerville, and Medford to the Malden line; and the 6.4-mile Dorchester Tunnel, which was built in 1976 and stretches from Chestnut Hill under Brookline to Morton Avenue in Dorchester.

The three tunnels come together at Shaft 7 at Chestnut Hill.

Authority officials believe there is a “low risk of failure’’ in the concrete-lined, deep rock tunnels and concrete vertical shafts that link the tunnels to the surface. The shafts are located in Weston, Chestnut Hill, Allston, Somerville, Malden, West Roxbury, and Dorchester.

Valve reliability for the Metropolitan Tunnels is a concern,’’ authority officials wrote in documents that MWRA Board members have been reviewing in anticipation of a meeting Thursday morning in Marlborough devoted entirely to the situation.

Authority staff have explored 30 options for tunnel redundancy and are recommending a “ preferred alternative’’ to the board. The alternative calls for a new 4.5-mile northern tunnel from Weston under Waltham to the Belmont line. A new 9.5-mile southern tunnel would run from Weston under Newton and Brookline to Lower Mills in Dorchester.

The estimated midpoint cost of the northern tunnel is $472 million; the midpoint of the southern tunnel: $1 billion. The combined $1.475 billion cost of the tunnels corresponds to an estimated 17-year construction schedule and includes allowances for a 30 percent contingency and 4 percent annual construction cost escalation.

Staff is recommending that work start first on the northern tunnel.