The United States needs to pour billions of dollars into repairing its pipes and wastewater systems in the coming decades, or it could face drinking water shortages and onerously high sewer rates, the American Society of Civil Engineers said on Tuesday.

In a report it will release later this week, the group found that the gap between needs and anticipated funding for wastewater and drinking water infrastructure will swell to $84 billion by 2020.

"As the U.S. population has increased, the percentage served by public water systems has also increased. Each year new water lines are constructed to connect more distant dwellers to centralized systems, continuing to add users to aging systems," Gregory DiLoreto, its president-elect, told the Senate subcommittee on water of the Environment and Public Works Committee.

"Although new pipes are being added to expand service areas, drinking water systems degrade over time; they must be replaced at the end of their useful life, which ranges from 15 to 95 years," he said.

The demand is growing as the U.S. population increases, the Environmental Protection Agency's director of wastewater management, James Hanlon, told the subcommittee.

"Communities across the country identified the need for $300 billion in wastewater and $335 billion in drinking water infrastructure improvements for capital expenditures over the next 20 years," he said.

On a scale of A through F, where "F" stood for failure, the society gave the U.S. wastewater and drinking water infrastructure a "D-" in 2009.

"Each day, the condition of our water infrastructure results in significant losses and damages from broken water and sewer mains, sewage overflows, and other symptoms of water infrastructure that is reaching the end of its useful life cycle," said Sen. James Inhofe, the most powerful Republican on the Environment and Public Works Committee.

Most of the country obtains water from public systems operated by local authorities, which borrow from the federal government, charge users fees and issue bonds to fund repairs and new construction.

The federal Clean Water and Drinking Water State Revolving Funds have financed more than $111 billion of infrastructure projects since 1987, Hanlon said. The federal stimulus plan in 2009 put nearly $6 billion into 3,214 projects through the revolving funds.

The funds, which make loans and then use the debt repayments to make new loans, are "the basic mechanism for assistance to communities in addressing water quality issues," Joe Freeman, financial assistance division chief for the Oklahoma Water Resources Board, told the subcommittee.

"Currently, funding levels are decreasing while the restrictions and set-asides for those funds are increasing, thus making the program even less sustainable and growing the gap of un-met needs," he said, adding that the EPA's current oversight "stifles innovation and the abilities of states to best respond to local needs."

Nowhere is the struggle to cover the costs of sewer improvements more apparent than in Jefferson County, Alabama. It embarked on a complicated financing scheme to retrofit its sewers that ultimately led the county to file for bankruptcy and pushed taxpayers' monthly sewer bills close to $100.

By 2050, the U.S. Census expects the country's population to have increased by 35 percent, Hanlon told the subcommittee.

"Many of the environmental successes of the past three decades may be overwhelmed by future demands," he said. "These water and wastewater infrastructure challenges will be faced by systems across the country, both in our large and growing urban centers as well as our rural towns."