SAN DIEGO -- Underneath Qualcomm Stadium in Mission Valley is an aquifer that used to supply San Diegans with precious drinking water.

When a deal was cut to bring in cheaper water from the Colorado River, the aquifer source was forgotten.

Now that other sources of water are drying up, the region is desperate for supplemental sources, including the long-ignored groundwater. There is a potential of pumping two million gallons of water a day from the aquifer below Qualcomm Stadium.

But there's a major problem: pollution.

"The very shallowest groundwater in most urbanized places is contaminated with all the sins of humanity from World War II on," said Wes Danskin, a research hydrologist with the United States Geological Survey.

One of the most persistent pollutants is methyl-tertiary-butyl ether, or MTBE. It was added to gasoline in 1979 to help it burn cleaner. 10News first exposed the chemical's harmful effects 14 years ago. It became an environmental disaster. By the time it was banned in 2004, it had leached into our groundwater -- the result of almost inevitable leaks in fuel storage tanks.

"It's very difficult to catch it, contain it, and also more difficult to remove it from water if you were to try to clean up the water," said John Robertus, recently retired from the San Diego Regional Water Quality Control Board after serving the past 14 years as its executive officer.

Under Mission Valley lies the largest pocket of MTBE pollution in the region, extending over 5,000 feet. It was created by the nearby fuel tank farm along Interstate 15. All of the fuel in the area is distributed by pipes or tanker trucks from the Mission Valley Terminal, whether it's jet fuel for airplanes at Lindbergh Field or gasoline for your local station.

It's been 18 years since the cleanup of the water in the Mission Valley aquifer began. The city of San Diego wants to use the aquifer for drinking water and is suing the tank farm's current owner -- Kinder-Morgan Energy Partners -- for taking so long.

Kinder-Morgan does not agree with the city's assessment. It has been pumping water out of the aquifer, treating it with a sophisticated filtering system and then dumping 500,000 gallons every day into Murphy Canyon Creek, which flows to the San Diego River and out into the ocean.

The city wants Kinder-Morgan to re-inject the treated water back into the aquifer instead of into the creek. The company said that's a mistake; that among other issues, re-injecting could cause the MTBE plume to expand.

The San Diego Regional Water Quality Control Board agreed with the company and asked in a letter to the water department, "Given the current drought condition ... why doesn't the city" take the water?

The written response from the water department argued that the cost to build a temporary system to transport the treated water to the municipal supply "could not be justified to City rate payers."

The letter added, "Local water supply agencies that have been co-opted into relying on well head treatment to deal with a contaminated source have had unsatisfactory results."

Despite the disagreements, the cleanup continues under the tank farm.

Last year, an extension of the plume was found north of Qualcomm Stadium. Nevertheless, the current deadlines set by the regional water board remain the same. The soil is supposed to be cleaned up by Dec. 31, 2010, and the groundwater by Dec. 31, 2013.