After heavy rains and massive snowmelts, most of the nation's municipal stormwater management systems, which are well over 80 years old, discharge billions of gallons of untreated runoff into streams and rivers annually. These sewer systems -- in cities as diverse as Atlanta, Indianapolis, Miami and Philadelphia -- are under federal compliance orders to substantially upgrade their stormwater infrastructure.

And Philadelphia has a plan. Facing the prospect of spending $8 billion to build a giant underground holding tank to temporarily hold storm runoff, Philadelphia has proposed an alternative compliance plan to the Environmental Protection Agency: The city is looking to replace concrete and asphalt with turf and earth. In effect, Philadelphia proposes to systematically redesign and reinstall water-retaining surfaces over major proportions of public and private property. The city also promises to invest in the restoration of stream habitats and riverfronts. In the end, officials expect the city will reduce carbon dioxide emissions, improve air and water quality, and restore wetlands and other natural habitats. Coupled with the improved recreational opportunities in stream corridors and park areas are increases in property values in urban neighborhoods adjacent to improved parks and greenery.

If approved, this plan may well be the booster rocket to bring green infrastructure into the mainstream.

Permeable hard surfaces, green roofs, rain gardens and catchment parks are all tested applications that slow runoff. Unlike concrete, these surfaces slowly let rain in, allowing it be absorbed, evaporated, filtered and controlled as it moves into the ground and sewer systems, reducing system overloads. A "greened" acre stops 80 to 90 percent of pollution compared to an impervious surface acre.

Many cities are testing and demonstrating a wide range of adaptive interventions, but only Portland, Ore., has integrated the innovations systematically. Should a dense urban center like Philadelphia do so successfully, it could set important national precedent for cities such as Chicago; Memphis, Tenn.; and New York. Key to this strategy's success is its implementation on public-owned land, which constitutes 45 percent of impervious land in the city.

Taken to massive scale as in the Philadelphia proposal, city officials believe they can successfully address the root causes of uncontrolled urban stormwater runoff at its source, before it enters the sewer system.

The conventional "grey" approach to mitigating the damage of untreated urban stormwater runoff is to build giant underground interception tanks that hold the overflow until the sewer system can digest the excess capacity. The cost of these interceptors runs into the billions. Milwaukee, for example, spent $2.3 billion in the 1980s but still failed to fully solve its sewer overload problems.

At their best, these storage tanks offer little value beyond capturing runoff. But with a "greenification" approach, as proposed in Philadelphia, a city could not only comply with the Clean Water Act, but realize other environmental, social and economic benefits.

Whether Philadelphia builds giant holding tanks or goes green, the city faces a multi-billion dollar investment, and the contrasting scenarios offer compelling differences in their relative return on investment. The big interception tank would take 20 years to become operational, and offer single-purpose benefit even when it is completed. Philadelphia's proposal will cost about $1 billion, funded in part by utility increases and stormwater fees, and will include green stormwater infrastructure on publicly controlled spaces, stream corridor restoration and sewer plant upgrades. A significant amount of additional private and other public agency green investments will result from development under the new stormwater regulations, the new stormwater fee structure and other green building projects.

The Environmental Protection Agency is examining closely the complex engineering and environmental calculations that underlie Philadelphia's plan. In the end, it must achieve the objective of reducing Philadelphia's environmental damage to streams and rivers. The city must demonstrate its commitment and capacity to execute the plan with fidelity over many years. But the prospect of tackling a problem at its source, taken to scale for true impact, is an opportunity for an unprecedented "concrete" advance in sustainability in urban centers.