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.