DC Water’s quest to build miles of new tunnels as part of a federally mandated plan to reduce sewage overflows might be at risk if the agency relies solely on ratepayers to pay for it, a new study says.

The study by the Brookings Institution’s Metropolitan Policy Program said the cost of the Clean Rivers project has grown from an estimated $2.2 billion in 2005 to $2.6 billion today, and cost increases to retail, business and household ratepayers are expected to foot the bill.

But the burden will be heaviest, proportionately, on low-income ratepayers. Their payments would double by 2019, the last year of cost projection DC Water provides, the study said. Costs likely would rise again between 2019 and 2025, the project’s projected completion date, to meet water-quality-improvement mandates.

DC Water runs the massive Blue Plains Advanced Wastewater Treatment Plant serving the Washington area. When it rains, its Civil War-era combined sewage overflow system is overwhelmed by rainwater runoff and wastewater, forcing officials to release diluted raw sewage into Rock Creek and the Potomac and Anacostia rivers, creating temporary cesspools.

Clean Rivers is one of the Washington area’s main hopes for cleaning the creek and rivers, where swimming is illegal in the District and eating fish is not advised due to harmful levels of bacteria. Dirty rivers and streams drag down the health of the Chesapeake Bay watershed and its wildlife.

But to make sure the project gets done, DC Water needs help, according to the Brookings study. It advises the Metropolitan Washington Council of Governments to convene stakeholders, including the federal government, to develop a more viable way to endow the project’s funding.

“DC Water has no authority outside of its narrow rate utility and it has no state government to protect its interest,” the study said. “It needs help to ensure that all who will benefit from this expensive and lengthy project pay for it. The federal government has contributed, but not enough.”