To anyone who has watched storm water rush through a man-made ditch to the sea, the idea seems blessed with more than its share of common sense.

Why not find a way to capture those uncounted millions of gallons of rain water and use them for something?

The state likes the idea, too, and has joined federal water experts in a $400,000 study to see how difficult it would be to use storm water runoff. The Department of Land and Natural Resources and the U.S. Department of the Interior's Bureau of Reclamation want to see if the same reclamation technology in use nationwide could be applied here.

Increased demand for water in a state with finite underground aquifers is at the core of the project, said Neil Fujii, the drought coordinator for the state Commission on Water Resource Management.

"Over 99 percent of our drinking water is ground water," he said. "Any way we can reduce the withdrawals from our aquifers protects our resources."

The amount of storm water that could be gained is staggering.

"It's millions of gallons for sure," Fujii said. "It may be billions."

But there are serious and costly problems that must be solved first, including how the water would be used, which determines how clean it needs to be, and how much could be captured during a storm and where it would be stored, Fujii said.

On the Mainland, some cities collect the water in reservoirs that contain millions of gallons, treat it to drinking water quality and then pump it into local underground reservoirs, he said.

"When the dry season comes around, they pump it back out," he said.

A smaller-scale reclamation project would involve methods of directing runoff at shopping centers, parking lots and similar buildings. The goal is to get the water onto grassy areas and, finally, to the aquifer below.

"Usually what they do now is channel the water off the parking lot and into a storm drain and then into the ocean," Fujii said. "They have been doing that for years, but what people are doing now is looking at ways to actually keep this water on site. Maybe put in some infiltration basins."

The 21-month study, which is being done by the Honolulu office of Brown and Caldwell Consultants, would have to determine if purifying this potentially toxic runoff is too costly.

"Hopefully they will tell us what kind of water quality we can expect from typical sources," Fujii said.

The study also will recommend which reclamation methods would work best in Hawai'i and explore how they can be used on O'ahu's dry 'Ewa Plains, one of the fastest growing areas in the state.

Because most storm water runoff ends up in the ocean — unfiltered and often contaminated with hazardous liquids — it can have a significant impact on coastal ecosystems. The U.S. Environmental Protection Agency considers untreated runoff a critical issue worthy of stiff penalties. Last year it reached a $52 million settlement with the state Department of Transportation for runoff that polluted nearshore waters on O'ahu, Maui and Kaua'i.

"Look at the theory of what they want to do," said Dean Higuchi, spokesman for the EPA office in Honolulu. "It protects resources for your drinking water. It could help recharge water sources. And it prevents non-point source pollution."

University of Hawai'i professor of civil and environmental engineering Clark Liu said storm runoff is generally cleaner than wastewater reclaimed in some cities for irrigation and industrial use. But it brings with it a level of unpredictability, he said. When will the heavens open and how heavy will it be?

"When we talk about storm runoff, that occurs only after a heavy rainstorm," he said. "The amount is very big when it occurs. You have a lot of water. Instead of treatment then, the main concern is storage. You have to have something that can store it, something big."

And it can be costly to purify a huge volume of water, he said.

"If those two things can be resolved, treatment and storage, then this is feasible," Liu said.

Reach Mike Gordon at mgordon@honoluluadvertiser.com.