Less sewage, please. (Linda Davidson/The Washington Post)

It’s not that the tunnels won’t get built, it’s that the current funding streams to float the bonds issued to build the tunnels will (a) raise consumer water bills to wildly unpopular levels while (b) crowding out other funding needs, starting with the routine replacement of aging water and sewer pipes.

So the Brookings report suggests pressing the feds and suburban jurisdictions for additional help in paying for the project. It’s a fine suggestion, but as a practical matter, a difficult prospect.

There’s another possibility to trim the tunnel costs, discussed at some length in the report. That’s to lessen the amount of runoff that makes it into the city sewer system by making sure more of it gets absorbed into the earth or otherwise evaporates first.

That’s the upshot of what are typically called “low impact development” or “green infrastructure” efforts, and D.C. Water has proposed spending as much as $30 million on a pilot effort to show that stuff like green roofs, permeable alleys and rain gardens prevent sewage outflows. If so, the utility would like to modify the court agreement mandating the tunnels to make the second and third tunnels smaller and less costly.

D.C. Water is not venturing a figure on the possible savings — they’re more interested at the moment in touting side benefits of low impact development, like jobs and clean air — but it’s easy to see many tens of millions of dollars saved, if not more.

Problem is, the environmental advocates on the other side of the tunnel agreement remain skeptical of the low impact development plan. They are understandably hesitant to give up the guaranteed prevention of sewage outflows for a speculative reduction in runoff that would require years of delay.

Leaders of 16 environmental groups signed a letter late last month saying, essentially, that low impact development is a good idea and all, but they’re not convinced it’s a proven way to handle a serious sewage problem. Just today, Earthjustice — the legal group behind the lawsuit that forced the tunnel deal — told the Environmental Protection Agency it will “strongly oppose” any attempt to change the agreement to swap tunnel capacity for green infrastructure.

Nevertheless, D.C. Water is forging ahead, seeking permission from the EPA to move forward with the demonstration project. (Philadelphia was recently given permission to proceed with their own efforts.)

“It’s too early to report on any progress on those discussions, but they are ongoing and we’re still optimistic,” said Alan Heymann, a D.C. Water spokesman.